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Found 882 entries in the Bibliography.
Showing entries from 751 through 800
| 2014 |
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Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr < Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr ≃ Ωe/2. This paper uses spacecraft observations and two-dimensional particle-in-cell (PIC) simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from HOPE instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three-component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi-electrostatic upper-band chorus, and that the hot component drives the electromagnetic lower-band chorus; the gap at \~ Ωe/2 is a natural consequence of the growth of two whistler modes with different properties. Fu, Xiangrong; Cowee, Misa; Friedel, Reinhard; Funsten, Herbert; Gary, Peter; Hospodarsky, George; Kletzing, Craig; Kurth, William; Larsen, Brian; Liu, Kaijun; MacDonald, Elizabeth; Min, Kyungguk; Reeves, Geoffrey; Skoug, Ruth; Winske, Dan; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020364 |
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The Evolving Space Weather System - Van Allen Probes Contribution The overarching goal and purpose of the study of space weather is clear - to understand and address the issues caused by solar disturbances on humans and technological systems. Space weather has evolved in the past few decades from a collection of concerned agencies and researchers to a critical function of the National Weather Service of NOAA. The general effects have also evolved from the well-known telegraph disruptions of the mid-1800\textquoterights to modern day disturbances of the electric power grid, communications and navigation, human spaceflight and spacecraft systems. The last two items in this list, and specifically the effects of penetrating radiation, were the impetus for the space weather broadcast implemented on NASA\textquoterights Van Allen Probes\textquoteright twin pair of satellites, launched in August of 2012 and orbiting directly through Earth\textquoterights severe radiation belts. The Van Allen Probes mission, formerly the Radiation Belt Storm Probes (RBSP, http://vanallenprobes.jhuapl.edu), were renamed soon after launch to honor the discoverer of Earth\textquoterights radiation belts at the beginning of the space age, the late James Van Allen (the spacecraft themselves are still referred to as RBSP-A and RBSP-B). The Van Allen Probes (Mauk et al., 2012 and other team contributions in the same special issue of Space Science Reviews, 2012) are one part of NASA\textquoterights Living With a Star (LWS, http://lws.gsfc.nasa.gov) program formulated to advance the scientific understanding of the connection between solar disturbances, the resulting heliospheric conditions and their effects on the geospace and Earth environment. Zanetti, L.; Mauk, B.; Fox, N.J.; Barnes, R.J.; Weiss, M.; Sotirelis, T.S.; Raouafi, N.-E.; Kessel, R.; Becker, H.; Published by: Space Weather Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014SW001108 |
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We present in situ observations of a shock-induced substorm-like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm-like electron injections with energy of 30\textendash500 keV were observed in the region from L\~5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90\textdegree phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is \~14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex-like flow structure-driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L\~3 to 6 on the nightside, while the time profile of the wave is different from dayside cases. Hao, Y.; Zong, Q.-G.; Wang, Y.; Zhou, X.-Z.; Zhang, Hui; Fu, S; Pu, Z; Spence, H.; Blake, J.; Bonnell, J.; Wygant, J.; Kletzing, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020023 energetic particles; interplanetary shock; magnetotail ULF wave; poloidal and toroidal mode; Van Allen Probes; wave-particle interactions |
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March 2013 provided the first equinoctial period when all of the instruments on the Van Allen Probes spacecraft were fully operational. This interval was characterized by disturbances of outer zone electrons with two timescales of variation, diffusive and rapid dropout and restoration [Baker et al., 2014]. A radial diffusion model was applied to the month-long interval to confirm that electron phase space density is well described by radial diffusion for the whole month at low first invariant <=400 MeV/G, but peaks in phase space density observed by the ECT instrument suite at higher first invariant are not reproduced by radial transport from a source at higher L. The model does well for much of the month-long interval, capturing three of four enhancements in phase space density which emerge from the outer boundary, while the strong enhancement following dropout on 17-18 March requires local acceleration at higher first invariant (M = 1000 MeV/G vs. 200 MeV/G) not included in our model. We have incorporated phase space density from ECT measurement at the outer boundary and plasmapause determination from the EFW instrument to separate hiss and chorus loss models. Li, Zhao; Hudson, Mary; Jaynes, Allison; Boyd, Alexander; Malaspina, David; Thaller, Scott; Wygant, John; Henderson, Michael; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020359 |
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The Comprehensive Inner Magnetosphere-Ionosphere Model Simulation studies of the Earth\textquoterights radiation belts and ring current are very useful in understanding the acceleration, transport, and loss of energetic particles. Recently, the Comprehensive Ring Current Model (CRCM) and the Radiation Belt Environment (RBE) model were merged to form a Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model. CIMI solves for many essential quantities in the inner magnetosphere, including ion and electron distributions in the ring current and radiation belts, plasmaspheric density, Region 2 currents, convection potential, and precipitation in the ionosphere. It incorporates whistler mode chorus and hiss wave diffusion of energetic electrons in energy, pitch angle, and cross terms. CIMI thus represents a comprehensive model that considers the effects of the ring current and plasmasphere on the radiation belts. We have performed a CIMI simulation for the storm on 5\textendash9 April 2010 and then compared our results with data from the Two Wide-angle Imaging Neutral-atom Spectrometers and Akebono satellites. We identify the dominant energization and loss processes for the ring current and radiation belts. We find that the interactions with the whistler mode chorus waves are the main cause of the flux increase of MeV electrons during the recovery phase of this particular storm. When a self-consistent electric field from the CRCM is used, the enhancement of MeV electrons is higher than when an empirical convection model is applied. We also demonstrate how CIMI can be a powerful tool for analyzing and interpreting data from the new Van Allen Probes mission. Fok, M.-C.; Buzulukova, N; Chen, S.-H.; Glocer, A.; Nagai, T.; Valek, P.; Perez, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.910.1002/2014JA020239 inner magnetosphere; magnetosphere-ionosphere coupling; ring current; Radiation belts; Van Allen Probes |
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The effects of magnetic fields on photoelectron-mediated spacecraft potential fluctuations Previously, we have experimentally studied photoelectron-mediated spacecraft potential fluctuations associated with time-dependent external electric fields. In this paper, we investigate the effects of magnetic fields on such spacecraft potential fluctuations. A magnetic field is created above the UV-illuminated surface of a spacecraft model to alter the escape rate of photoelectrons. The packet of the observed potential oscillations becomes less positive with increasing magnetic field strength because more of the emitted photoelectrons are returned to the surface. As a result, the photoelectric charging time is increased, corresponding to a decrease in the response frequency of the photoemitting surface. The amplitude of the potential oscillations decreases when the response frequency becomes lower than the electric field oscillation frequency. A test particle simulation is validated with the laboratory experiments and applied to estimate the photoelectron escape rate from the Van Allen Probes spacecraft, showing that the photoelectron current is reduced by as much as 30\% when magnetic field strength is 1200 nT. Based on our laboratory results and computer simulations, we discuss the effects of magnetic fields on the spacecraft potential fluctuations observed by the Van Allen Probes. Wang, X.; Malaspina, D.; Hsu, H.-W.; Ergun, R.; M., Hor\; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.910.1002/2014JA019923 chorus waves; magnetic fields; photoelectrons; spacecraft potential fluctuations; Van Allen Probes |
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New Twists in Earth\textquoterights Radiation Belts In 1958, an early satellite, Explorer I, made the discovery that Earth is enshrouded in belts of extraordinarily high-energy, high-intensity radiation. Now called the Van Allen belts, after the researcher who led that satellite mission, these rings are known to wax and wane in intensity, for reasons that are still being investigated. Satellites now criss-cross these belts, so understanding what influences them has dire implications for communications and other technologies in our modern age. Solar storms and space weather can pump them up, making the radiation zones around Earth immensely more dangerous for days or even weeks on end. The author has been involved with instruments on the dual Radiation Belt Storm Probes satellites that were launched on August 30, 2012, into Earth orbit to study the two known Van Allen belts, and the team was lucky to catch a large solar flare a few days after launch. Amazingly, the particles impacted by this solar storm settled into a new configuration, showing an extra, temporary third belt between the other two. This new research shows that the belts can reconfigure their structure and density quickly, on the range of seconds to hours. Other observed events are showing researchers the interplay between the belt\textquoterights own internal magnetic acceleration of particles and the influx of energy from space weather. The results may not only help to protect satellites but can also be extrapolated to understand what\textquoterights happening on other planets inside and outside our Solar System. Published by: American Scientist Published on: 09/2014 YEAR: 2014 DOI: 10.1511/2014.110.374 |
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Energetic particle transport into the inner magnetosphere during geomagnetic storms is responsible for significant plasma pressure enhancement, which is the driver of large-scale currents that control the global electrodynamics within the magnetosphere-ionosphere system. Therefore, understanding the transport of plasma from the tail deep into the near-Earth magnetosphere, as well as the energization processes associated with this transport, is essential for a comprehensive knowledge of the near-Earth space environment. During the main phase of a geomagnetic storm on March 17th 2013 (minimum Dst ~ -137 nT), the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on the Van Allen Probes observed frequent, small-scale proton injections deep into the inner nightside magnetosphere in the region L ~ 4 \textendash 6. Although isolated injections have been previously reported inside geosynchronous orbit, the large number of small-scale injections observed in this event suggests that, during geomagnetic storms injections provide a robust mechanism for transporting energetic ions deep into the inner magnetosphere. In order to understand the role that these injections play in the ring current dynamics, we determine the following properties for each injection: i) associated pressure enhancement, ii) the time duration of this enhancement, iii) and the lowest and highest energy channels exhibiting a sharp increase in their intensities. Based on these properties, we estimate the effect of these small-scale injections on the pressure buildup during the storm. We find that this mode of transport could make a substantial contribution to the total energy gain in the storm-time inner magnetosphere. Gkioulidou, Matina; Ukhorskiy, A.; Mitchell, D.; Sotirelis, T.; Mauk, B.; Lanzerotti, L.; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/2014JA020096 Geomagnetic storms; Ion injections; ring current; Van Allen Probes |
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Simulation of Van Allen Probes Plasmapause Encounters We use an E \texttimes B-driven plasmapause test particle (PTP) simulation to provide global contextual information for in situ measurements by the Van Allen Probes (RBSP) during 15\textendash20 January 2013. During 120 h of simulation time beginning on 15 January, geomagnetic activity produced three plumes. The third and largest simulated plume formed during enhanced convection on 17 January, and survived as a rotating, wrapped, residual plume for tens of hours. To validate the simulation, we compare its output with RBSP data. Virtual RBSP satellites recorded 28 virtual plasmapause encounters during 15\textendash19 January. For 26 of 28 (92\%) virtual crossings, there were corresponding actual RBSP encounters with plasmapause density gradients. The mean difference in encounter time between model and data is 36 min. The mean model-data difference in radial location is 0:40\textpm0:05 RE. The model-data agreement is better for strong convection than for quiet or weakly disturbed conditions. On 18 January, both RBSP spacecraft crossed a tenuous, detached plasma feature at approximately the same time and nightside location as a wrapped residual plume, predicted by the model to have formed 32 h earlier on 17 January. The agreement between simulation and data indicates that the model-provided global information is adequate to correctly interpret the RBSP density observations. Goldstein, J.; De Pascuale, S.; Kletzing, C.; Kurth, W.; Genestreti, K.; Skoug, R.; Larsen, B.; Kistler, L.; Mouikis, C.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/2014JA020252 observations; plasmasphere; residual plume; simulation; Van Allen Probes |
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The trapping of equatorial magnetosonic waves in the Earth\textquoterights outer plasmasphere We investigate the excitation and propagation of equatorial magnetosonic waves observed by the Van Allen Probes and describe evidence for a trapping mechanism for magnetosonic waves in the Earth\textquoterights plasmasphere. Intense equatorial magnetosonic waves were observed inside the plasmasphere in association with a pronounced proton ring distribution, which provides free energy for wave excitation. Instability analysis along the inbound orbit demonstrates that broadband magnetosonic waves can be excited over a localized spatial region near the plasmapause. The waves can subsequently propagate into the inner plasmasphere and remain trapped over a limited radial extent, consistent with the predictions of near-perpendicular propagation. By performing a similar analysis on another observed magnetosonic wave event, we demonstrate that magnetosonic waves can also be trapped within local density structures. We suggest that perpendicular wave propagation is important for explaining the presence of magnetosonic waves in the Earth\textquoterights plasmasphere at locations away from the generation region. Ma, Q.; Li, W.; Chen, L.; Thorne, R.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Reeves, G.; Henderson, M.; Spence, H.; Published by: Geophysical Research Letters Published on: 09/2014 YEAR: 2014 DOI: 10.1002/2014GL061414 magnetosonic waves; Van Allen Probes; wave excitation; wave propagation |
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Acceleration and loss driven by VLF chorus: Van Allen Probes observations and DREAM model results For over a decade now we have understood the response of the Earth\textquoterights radiation belts to solar wind driving are a delicate balance of acceleration and loss processes. Theory has shown that the interaction of relativistic electrons with VLF whistler mode chorus can produce both energization through momentum diffusion and loss through pitch angle diffusion. Recent results from the Van Allen Probes mission has confirmed observationally that chorus can produce both acceleration and loss. The Van Allen Probes satellites are able to measure all the critical particle populations and wave fields with unprecedented precision and resolution but only at the two spacecraft locations. Those spatially-localized observations can be extended globally using three-dimensional diffusion codes such as the DREAM model. We will discuss some of the recent Van Allen Probes observations that firmly demonstrate local acceleration by chorus and losses due to chorus-produced pitch angle scattering (as well as outward radial diffusion). We will look at observational evidence for the complex chain of processes that inject a \textquotedblleftseed population\textquotedblright, generate chorus, and ultimately drive radiation belt acceleration and loss. We will also discuss how local satellite observations can be generalized to simulate global dynamics using data-driven input and boundary conditions. RW1/J/IEIE0175/0001 Reeves, G.; Spence, H.; Henderson, M.; Tu, W.; Cunningham, G.; Chen, Y.; Blake, J.; Fennell, J.; Baker, D.; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929879 |
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Calculation of whistler-mode wave intensity using energetic electron precipitation The energetic electron population measured by multiple low-altitude POES satellites is used to infer whistlermode wave amplitudes using a physics-based inversion technique. We validate this technique by quantitatively analyzing a conjunction event between the Van Allen Probes and POES, and find that the inferred hiss wave amplitudes from POES electron measurements agree remarkably well with directly measured hiss waves amplitudes. We also use this technique to construct the global distribution of chorus wave intensity with extensive coverage over a broad L-MLT region during the 8\textendash9 October 2012 storm and demonstrate that the inferred chorus wave amplitudes agree well with conjugate measurements of chorus wave amplitudes from the Van Allen Probes. The evolution of the whistler-mode wave intensity inferred from low-altitude electron measurements can provide real-time global estimates of the wave intensity, which cannot be obtained from in-situ wave measurements by equatorial satellites alone, but are crucial in quantifying radiation belt electron dynamics. Li, W.; Ni, B.; Thorne, R.; Bortnik, J.; Green, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929965 Electron traps; Energy measurement; Plasma measurements; Van Allen Probes |
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Chorus-driven acceleration of radiation belt electrons in the unusual temporal/spatial regions Cyclotron resonance with whistler-mode chorus waves is an important mechanism for the local acceleration of radiation belt energetic electrons. Such acceleration process has been widely investigated during the storm times, and its favored region is usually considered to be the low-density plasmatrough with magnetic local time (MLT) from midnight through dawn to noon. Here we present two case studies on the chorus-driven acceleration of radiation belt electrons in some \textquotedblleftunusual\textquotedblright temporal /spatial regions. (1) The first event recorded by the Van Allen Probes during the nonstorm times from 21 to 23 February 2013. Within two days, a new radiation belt centering around L=5.8 formed and gradually merged with the original outer belt. The corresponding relativistic electron fluxes increased by a factor of up to 50, accompanied by strong chorus waves. The quasi-linear STEERB model, including the local acceleration of detected chorus waves, can basically reproduce the observed 0.2\textendash5.0 MeV electron flux enhancement at the center of new belt. These results clearly illustrate the importance of chorus-driven local acceleration during the nonstorm times. (2) The second event observed by the Van Allen Probes in the duskside (MLT\~18) region on 2 October 2013. The quasi-linear diffusion analysis of STEERB code shows that, even in the duskside region with large ratio between the electron plasma frequency and the electron gyrofrequency, the detected intense (\~0.5 nT) chorus waves can still effectively accelerate radiation belt electrons. These results clearly exhibit the broader effective acceleration regions than usually estimated, at least for this one example. Su, Zhenpeng; Xiao, Fuliang; Zheng, Huinan; Zhu, Hui; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929875 |
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Comparison of Energetic Electron Intensities Outside and Inside the Radiation Belts The intensities of energetic electrons (~25 \textendash 800 keV) outside and inside Earth\textquoterights radiation belts are reported using measurements from THEMIS and Van Allen Probes during non-geomagnetic storm periods. Three intervals of current disruption/dipolarization events in August, 2013 were selected for comparison. The following results are obtained. (1) Phase space densities (PSDs) for the equatorially mirroring electron population at three values of the first adiabatic invariant (20, 70, and 200 MeV/G) at the outer radiation belt boundary are found to be one to three orders of magnitude higher than values measured just inside the radiation belt. (2) There is indication that substorm activity leads to PSD increases inside L = 5.5 in less than 1 hr. (3) Evidence for progressive inward transport of enhanced PSDs is found. (4) Reductions and enhancements in the PSDs over L-shells from 3.5 to 6 are found to occur rapidly in ~2 \textendash 3 hrs. These results suggest that (1) continual replenishments are required to maintain high levels of PSD for electrons at these energies, and (2) inward radial transport of these electrons occurs in a fast time scale of a few hrs. T. Y. Lui, A.; Mitchell, D.; Lanzerotti, L.; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014JA020049 Dipolarization; energetic electrons; Radiation belts; substorm; Van Allen Probes |
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We present first results from a study of the plasma instability mechanism responsible for the small-scale (\~10 m) ionospheric density irregularities commonly observed by the Super Dual Auroral Radar Network (SuperDARN) HF radars in the vicinity of Sub Auroral Polarization Streams (SAPS) during periods of geomagnetic disturbance. A focus is placed on the mid-latitude region of the ionosphere over North America where recent expansion of the SuperDARN network allows for extensive direct comparisons with total electron content (TEC) measurements from a dense network of ground-based GPS receivers. The TEC observations indicate that high-speed SAPS channels and the associated small-scale irregularities are typically located within the mid-latitude ionospheric trough. The Millstone Hill Incoherent Scatter Radar (ISR), operating in campaign mode in support of the NASA Van Allen Probes mission, provided measurements of F region ion/electron density, velocity, and temperature suitable for identifying potential mechanisms of plasma instability during a SAPS event that extended over 12 hours of magnetic local time (MLT) on 2 February 2013. Previous work has indicated that the density gradients associated with the poleward wall of the mid-latitude trough can produce small-scale irregularities due to the gradient drift instability during quiet periods by cascade from larger-scale structures. In this study we demonstrate that the gradient drift instability is a viable source for the direct generation of the small-scale irregularities observed by SuperDARN radars in the mid-latitude ionosphere during geomagnetically disturbed conditions. Thomas, Evan; Yan, Jingye; Zhang, Jiaojiao; Baker, Joseph; Ruohoniemi, Michael; Hoskawa, Keisuke; Erickson, Philip; Coster, Anthea; Foster, John; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929853 |
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Generation of Unusually Low Frequency Plasmaspheric Hiss It has been reported from Van Allen Probe observations that plasmaspheric hiss intensification in the outer plasmasphere, associated with a substorm injection on Sept 30 2012, occurred with a peak frequency near 100 Hz, well below the typical plasmaspheric hiss frequency range, extending down to ~20 Hz. We examine this event of unusually low frequency plasmaspheric hiss to understand its generation mechanism. Quantitative analysis is performed by simulating wave ray paths via the HOTRAY ray tracing code with measured plasma density and calculating ray path-integrated wave gain evaluated using the measured energetic electron distribution. We demonstrate that the growth rate due to substorm injected electrons is positive but rather weak, leading to small wave gain (~10 dB) during a single equatorial crossing. Propagation characteristics aided by the sharp density gradient associated with the plasmapause, however, can enable these low frequency waves to undergo cyclic ray paths, which return to the unstable region leading to repeated amplification to yield sufficient net wave gain (>40 dB) to allow waves to grow from the thermal noise. Chen, Lunjin; Thorne, Richard; Bortnik, Jacob; Li, Wen; Horne, Richard; Reeves, G.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Blake, J.; Fennell, J.; Published by: Geophysical Research Letters Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014GL060628 Chorus; Generation; Plasmaspheric Hiss; Ray Tracing; Van Allen Probes |
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For over a decade, incoherent scatter radar observations of the mid and auroral-latitude ionosphere combined with ground based GPS observations of total electron content (TEC) have been used to study the intense storm enhanced density (SED) plumes that form over the Americas during major geomagnetic storms [1]. Magnetic field mapping of the ionospheric observations to magnetospheric heights revealed close correspondence between the SED and plasmasphere erosion plumes observed from space in EUV imagery by the IMAGE satellite [2]. During the current solar cycle the global distribution of GPS receivers used in creating the TEC maps and movies has increased significantly providing near-continuous two-dimensional coverage of TEC morphology and dynamics over much the northern hemisphere mid and high-latitude region. The dynamics and structure of the outer reaches of the plasmasphere, the plasmasphere boundary layer, are driven by coupling to overlying magnetospheric processes. To first order, cold plasma redistribution proceeds such that plasma parcels at ionospheric heights and at the apex of a magnetic field line move together in the E \texttimes B direction maintaining their magnetic field alignment. In this sense the TEC structure and dynamics imaged in the ionosphere projects along the magnetic field providing an image of the plasmaspheric configuration. The recently launched Van Allen Probes twin satellites (RBSP-A \& RBSP-B) are in near-equatorial orbits well suited for studies of phenomena at the apex of field lines threading the plasmasphere boundary layer. The RBSP instrumentation includes in situ electric field, density, ion composition, magnetic field, plasma wave, and full particle pitch angle and energy spectral information from <1 eV to 10s of MeV for ions and electrons. We use ground based TEC mapping to create 2-D images of the plasmasphere during transits of the RBSP and Themis spacecraft. We intercompare the dynamic changes in the plasmasphe- e configuration with the detailed in situ observations. We image and observe the transition from quiet plasmasphere, to erosion plume formation and development, to recovery. The RBSP spacecraft provide quantitative measurements of ion composition and erosion flux within the plume and the mapping between low and high altitudes facilitates intercomparisons between ionospheric and magnetospheric characteristics and phenomena. Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929943 magnetic fields; Magnetic resonance imaging; Magnetosphere; Van Allen Probes |
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The Sub-Auroral Polarization Stream (SAPS) is a geospace boundary layer phenomenon associated with the interaction of the warm plasma of the magnetospheric ring current with the cold ions and electrons of the outer plasmasphere [1]. Driven by ring current enhancements during magnetospheric disturbances, SAPS location, intensity, and characteristics are greatly influenced by the underlying ionosphere. Strong M-I coupling by means of field-aligned currents creates a high-speed (>1000 m/s) westward plasma flow channel in the ionosphere at pre-midnight/post-noon local times which is readily observable by incoherent scatter [2] and HF radars and in plasma drift observations by low-altitude spacecraft (e.g. DMSP). The fast ionospheric flows generate E-region irregularities providing for additional diagnostics using coherent backscatter techniques [3]. SAPS plays a significant role in the redistribution of cold plasma through the geospace system at both ionospheric and magnetospheric heights. Where the SAPS flow channel overlaps the mid-latitude ionosphere and outer plasmasphere, streams of cold plasma are carried westward and sunward as plumes of storm enhanced density (SED) in the ionosphere and as plasmasphere erosion plumes at high altitude. Ground-based maps of GPS total electron content (TEC) serve to visualize the spatial extent and evolution of the SAPS and SED. Mapping these features to magnetospheric altitudes along magnetic field lines permits direct intercomparison with in situ spacecraft observations. The recently launched Van Allen Probes twin satellites (RBSP-A \& RBSP-B) are in near-equatorial orbits well suited for studies of the SAPS and related phenomena at the apex of field lines threading the plasmasphere boundary layer. Simultaneous near magnetic field aligned observations of SAPS at DMSP altitude (\~800 km) and by RBSP-A at \~20,000 km show close correspondence of SAPS location and characteristics between the ionosphere and- magnetosphere. In highly elliptical orbits with apogee near 5.5 Re, the RBSP spacecraft often spend hours at a time skimming the outer plasmasphere within the SAPS region. A great variety of wave phenomena are observed. Here we describe long-duration large amplitude (+/- 5 mV/m) electric field oscillations with 3 min\textendash5 min period seen in the magnetospheric equatorial plane within the SAPS/erosion plume region. Foster, John; Erickson, Philip; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929852 |
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Nonlinear Electric Field Structures in the Inner Magnetosphere Van Allen Probes observations are presented which demonstrate the presence of nonlinear electric field structures in the inner terrestrial magnetosphere (< 6 RE). A range of structures are observed, including phase space holes and double layers.These structures are observed over several Earth radii in radial distance and over a wide range of magnetic local times. They are observed in the dusk, midnight, and dawn sectors, with the highest concentration pre-midnight. Some nonlinear electric field structures are observed to coincide with dipolarizations of the magnetic field and increases in electron energy flux for energies between 1 keV and 30 keV. Nonlinear electric field structures possess isolated impulsive electric fields, often with a significant component parallel to the ambient magnetic field, providing a mechanism for non-adiabatic wave-particle interactions in the inner magnetosphere. Malaspina, D.; Andersson, L.; Ergun, R.; Wygant, J.; Bonnell, J; Kletzing, C.; Reeves, G.; Skoug, R.; Larsen, B.; Published by: Geophysical Research Letters Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014GL061109 |
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Observations of the inner radiation belt: CRAND and trapped solar protons Measurements of inner radiation belt protons have been made by the Van Allen Probes Relativistic Electron-Proton Telescopes as a function of kinetic energy (24 to 76 MeV), equatorial pitch angle, and magnetic L shell, during late-2013 and early-2014. A probabilistic data analysis method reduces background from contamination by higher energy protons. Resulting proton intensities are compared to predictions of a theoretical radiation belt model. Then trapped protons originating both from cosmic ray albedo neutron decay (CRAND) and from trapping of solar protons are evident in the measured distributions. An observed double-peaked distribution in L is attributed, based on the model comparison, to a gap in the occurrence of solar proton events during the 2007 to 2011 solar minimum. Equatorial pitch angle distributions show that trapped solar protons are confined near the magnetic equator, but that CRAND protons can reach low-altitudes. Narrow pitch angle distributions near the outer edge of the inner belt are characteristic of proton trapping limits. Selesnick, R.; Baker, D.; Jaynes, A.; Li, X.; Kanekal, S.; Hudson, M.; Kress, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014JA020188 |
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The physics of the creation, loss, and transport of radiation belt particles is intimately connected to the electric and magnetic fields which mediate these processes. A key wave-particle interaction important to both acceleration and loss in the radiation belts is the of whistler-mode chorus interacting with energetic electrons. To measure this important radiation belt interaction, the two-satellite Van Allen Probes mission utilizes one of the most complete sets of measurements ever made in the inner magnetosphere. As part of the mission, the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) investigation is an integrated set of instruments consisting of a tri-axial fluxgate magnetometer (MAG) and a Waves instrument which includes a tri-axial search coil magnetometer (MSC). These wave measurements allow sophisticated diagnosis of a variety of features important for whistlermode chorus including wave normal direction, direct waveform capture of the full electric and magnetic vector fields to investigate individual chorus elements, and determination of the key background parameters of plasma density and background magnetic field. Examples are shown of these measurements as well as progress on understanding aspects of chorus emission such as the gap at one-half the electron cyclotron frequency, comparison with the electron measurements in the energy range important for generation of chorus emission, and comparison with electrons that are energized by chorus. Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929872 Instruments; Magnetic field measurement; magnetic fields; Magnetometers; Magnetosphere; Van Allen Probes |
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Prompt energization of relativistic and highly relativistic electrons during a substorm interval On 17 March 2013, a large magnetic storm significantly depleted the multi-MeV radiation belt. We present multi-instrument observations from the Van Allen Probes spacecraft Radiation Belt Storm Probe A and Radiation Belt Storm Probe B at \~6 Re in the midnight sector magnetosphere and from ground-based ionospheric sensors during a substorm dipolarization followed by rapid reenergization of multi-MeV electrons [1]. A 50\% increase in magnetic field magnitude occurred simultaneously with dramatic increases in 100 keV electron fluxes and a 100 times increase in VLF wave intensity. Chorus is excited following the injection of low-energy (1\textendash30 keV) plasma sheet electrons into the inner magnetosphere [2]. During the 17 March substorm injection, cold plasma that had circulated into the nightside magnetosphere from the dayside ionosphere-plasmasphere contributed to an energetic (50 keV) electron population involved in chorus-mode wave amplification [3]. The high-energy tail (>100 keV) of the injected electrons and the intense VLF waves provide a seed population and energy source for subsequent radiation belt energization. The observed electron flux behavior is striking in its large increases over short intervals. As seen by RBSP-A at L* \~ 4.5 highly relativistic (>2MeV) electron fluxes increased immediately at the time of the substorm injection and strong chorus enhancement. At RBSP-B, at apogee at substorm onset, observed in the \~5 h separation between L* = 4.0 crossings, 3.60 MeV highly relativistic electron fluxes increased by a factor of 56, while 4.50 MeV flux increased by an even larger factor of 95. The 17 March multipoint observations indicate the significant role that substorm processes can play in creating a seed population of 100 keV electrons and VLF chorus wave enhancements that can lead to a prompt energization of relativistic and highly relativistic electrons in the region outside the plasm- pause. Foster, John; Erickson, Philip; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929876 Magnetic flux; Magnetosphere; Van Allen Belts; Van Allen Probes |
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Radiation belt electron acceleration by chorus waves during the 17 March 2013 storm Local acceleration driven by whistler-mode chorus waves is suggested to be fundamentally important for accelerating seed electron population to ultra-relativistic energies in the outer radiation belt. In this study, we quantitatively evaluate chorus-driven electron acceleration during the 17 March 2013 storm, when Van Allen Probes observed very rapid electron acceleration up to multi MeV within \~15 hours. A clear peak in electron phase space density observed at L* \~ 4 indicates that the internal local acceleration process was operating. We construct the global distribution of chorus wave intensity from the low-altitude electron measurements by multiple POES satellites over a broad L-MLT region, which is used to simulate the radiation belt electron dynamics driven by chorus waves. Our simulation results show remarkable agreement with the observed electron PSD near its peak in timing, energy dependence, and pitch angle distribution, but other loss processes and radial diffusion may be required to explain the differences in observation and simulation at other locations away from the PSD peak. Our simulation results suggest that local acceleration by chorus waves is likely to be a robust and repetitive process and plays a critical role in accelerating radiation belt electrons from injected convective energies (\~ 100 keV) to ultra-relativistic energies (multi MeV). Thorne, R.; Li, W.; Ma, Q.; Ni, B.; Bortnik, J.; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929882 |
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Radiation belt losses observed from multiple stratospheric balloons over Antarctica Relativistic electrons, trapped by Earth\textquoterights magnetic field, have received increasing attention since increasing numbers of commercial and research spacecraft traverse regions of high radiation flux. The Van Allen probes were launched into Earth\textquoterights radiation belts in September 2012, making comprehensive measurements of charged particle fluxes and electromagnetic fields, with the objective of a better understanding of the processes that modulate radiation belt fluxes. Because losses of radiation belt electrons to Earth\textquoterights atmosphere are very difficult to measure from high altitude spacecraft, a balloon-based program, consisting of campaigns in January 2013 and 2014, was funded to measure losses in conjunction with the Van Allen probes mission. We present results from both balloon campaigns, which succeeded in maintaining an array of balloons over Antarctica, achieving spacecraft conjunction measurements, and viewing several periods of disturbed magnetospheric activity. Measurements from a balloon platform uniquely allows loss measurements for several hundred seconds from the same location, and therefore illuminate the role of slow magnetic field variations in radiation belt losses. The coincident measurement of radiation belt losses by the balloon array provides vital information for understanding flux changes at geosynchronous altitudes, giving a means to distinguish true losses from lossless transport away from the spacecraft. McCarthy, Michael; Millan, Robyn; Sample, John; Smith, David; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929960 Extraterrestrial measurements; Loss measurement; Magnetosphere; Van Allen Probes |
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Wave-particle interactions in the Earth\textquoterights Van Allen radiation belts are known to be an efficient process of the exchange of energy between different particle populations, including the energetic radiation belt particles. The whistler mode waves, especially chorus, can control the radiation belt dynamics via linear or nonlinear interactions with both the energetic radiation belt electrons and lower energy electron populations. Wave vector directions are a very important parameter of these wave-particle interactions. We use measurements of whistlermode waves by the WAVES instrument from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) onboard the Van Allen Probes spacecraft covering the equatorial region of the Earth\textquoterights magnetosphere in all MLT sectors, and a large database of measurements of the STAFF-SA instrument onboard the Cluster spacecraft, covering different latitudes for a time interval of more than one solar cycle. Multicomponent measurements of these instruments are a basis for the determination of statistical properties of the wave vector directions defined by two spherical angles with respect to the direction of the local magnetic field line. We calculate the probability density functions and probability density functions weighted by the wave intensity for both these angles. This work receives EU support through the FP7-Space grant agreement no 284520 for the MAARBLE collaborative research project. Santolik, O.; Hospodarsky, G.; Kurth, W.; Averkamp, T.; Kletzing, C.; Cornilleau-Wehrlin, N.; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929880 Atmospheric measurements; Magnetic field measurement; Van Allen Probes |
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Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (towards the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 hours is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0-nm emission by 630.0-nm emission from a different (non-thermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0-nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emission from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of mid-latitude storm-time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecrafts, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions. Makela, Jonathan; Harding, Brian; Meriwether, John; Mesquita, Rafael; Sanders, Samuel; Ridley, Aaron; Castellez, Michael; Ciocca, Marco; Earle, Gregory; Frissell, Nathaniel; Hampton, Donald; Gerrard, Andrew; Noto, John; Martinis, Carlos; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014JA019832 geomagnetic storm response; thermospheric winds; Van Allen Probes |
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Thermal electron acceleration by localized bursts of electric field in the radiation belts In this paper we investigate the resonant interaction of thermal ~10-100 eV electrons with a burst of electrostatic field that results in electron acceleration to kilovolt energies. This single burst contains a large parallel electric field of one sign and a much smaller, longer lasting parallel field of the opposite sign. The Van Allen Probe spacecraft often observes clusters of spatially localized bursts in the Earth\textquoterights outer radiation belts. These structures propagate mostly away from thegeomagnetic equator and share properties of soliton-like nonlinear electron-acoustic waves: a velocity of propagation is about the thermal velocity of cold electrons (~3000-10000 km/s), and a spatial scale of electric field localization alongthe field lines is about the Debye radius of hot electrons (~5-30 km). We model the nonlinear resonant interaction of these electric field structures and cold background electrons. Artemyev, A.; Agapitov, O.; Mozer, F.; Krasnoselskikh, V.; Published by: Geophysical Research Letters Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014GL061248 Radiation belts; thermal electrons; Van Allen Probes; Wave-particle interaction |
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Boom and bust for radiation belt high-energy electron populations Launched on 30 August 2012, the twin Van Allen probes constitute the first dedicated mission in decades to study the Earth\textquoterights radiation belts. The sensor-laden spacecraft follow a nearly equatorial orbit, which gives them a complete view of the full range of radiation belt processes. In a new study, Baker et al. lay out some of the surprising results unveiled by the crafts\textquoteright first year in orbit. Published by: Eos, Transactions American Geophysical Union Published on: 07/2014 YEAR: 2014 DOI: 10.1002/eost.v95.2810.1002/2014EO280021 |
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The mechanisms for accelerating electrons from thermal to relativistic energies in the terrestrial magnetosphere, on the sun, and in many astrophysical environments have never been verified. We present the first direct observation of two processes that, in a chain, cause this acceleration in Earth\textquoterights outer radiation belt. The two processes are parallel acceleration from electron-volt to kilovolt energies by parallel electric fields in time-domain structures (TDS), after which the parallel electron velocity becomes sufficiently large for Doppler-shifted upper band whistler frequencies to be in resonance with the electron gyration frequency, even though the electron energies are kilovolts and not hundreds of kilovolts. The electrons are then accelerated by the whistler perpendicular electric field to relativistic energies in several resonant interactions. TDS are packets of electric field spikes, each spike having duration of a few hundred microseconds and containing a local parallel electric field. The TDS of interest resulted from nonlinearity of the parallel electric field component in oblique whistlers and consisted of \~0.1 msec pulses superposed on the whistler waveform with each such spike containing a net parallel potential the order of 50 V. Local magnetic field compression from remote activity provided the free energy to drive the two processes. The expected temporal correlations between the compressed magnetic field, the nonlinear whistlers with their parallel electric field spikes, the electron flux and the electron pitch angle distributions were all observed. Mozer, S.; Agapitov, O.; Krasnoselskikh, V.; Lejosne, S.; Reeves, D.; Roth, I.; Published by: Physical Review Letters Published on: 07/2014 YEAR: 2014 DOI: 10.1103/PhysRevLett.113.035001 |
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Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013 We report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe-A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Σh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 \textpm 0.01, 0.15 \textpm 0.02, and 0.07 \textpm 0.02 during the three wave events, respectively. On Van Allen Probe-B, this difference never exceeds 0. Compared to linear theory (Σh > Sh), the waves are only excited for elevated thresholds. Zhang, J.-C.; Saikin, A.; Kistler, L.; Smith, C.; Spence, H.; Mouikis, C.; Torbert, R.; Larsen, B.; Reeves, G.; Skoug, R.; Funsten, H.; Kurth, W.; Kletzing, C.; Allen, R.; Jordanova, V.; Published by: Geophysical Research Letters Published on: 06/2014 YEAR: 2014 DOI: 10.1002/2014GL060621 |
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Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10-3nT2/Hz) in the low-latitude region outside of L=5. Such chorus waves are found to be generated by the substorm-injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long-term evolution of radiation belt electron fluxes may need further investigations in future. Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; He, Zhaoguo; Shen, Chao; Shen, Chenglong; Wang, C.; Liu, Rui; Zhang, Min; Wang, Shui; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.610.1002/2014JA019919 |
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We report large directional anisotropies of >60 MeV protons using instrumentation on the Van Allen Probes. The combination of a spinning satellite and measurements from the Relativistic Proton Spectrometer instruments that are insensitive to protons outside the instrument field of view together yield a new look at proton radial gradients. The relatively large proton gyroradius at 60 MeV couples with the radial gradients to produce large (maximum ~10:1) flux anisotropies depending on (i) whether the proton guiding center was above or below the Van Allen Probes spacecraft and (ii) the sign of the local flux gradient. In addition to these newly measured anisotropies, below ~2000 km we report a new effect of systematically changing minimum altitude on some proton drift shells that further modulates the anisotropy caused by the atmosphere. This discovery may offer a new way of monitoring changes to the loss of inner belt protons into the Earth\textquoterights atmosphere. Mazur, J.; O\textquoterightBrien, T.; Looper, M.; Blake, J.; Published by: Geophysical Research Letters Published on: 06/2014 YEAR: 2014 DOI: 10.1002/grl.v41.1110.1002/2014GL060029 |
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Radiation belt electron acceleration by chorus waves during the 17 March 2013 storm Local acceleration driven by whistler-mode chorus waves is fundamentally important for accelerating seed electron populations to highly relativistic energies in the outer radiation belt. In this study, we quantitatively evaluate chorus-driven electron acceleration during the 17 March 2013 storm, when the Van Allen Probes observed very rapid electron acceleration up to several MeV within ~12 hours. A clear radial peak in electron phase space density (PSD) observed near L* ~4 indicates that an internal local acceleration process was operating. We construct the global distribution of chorus wave intensity from the low-altitude electron measurements made by multiple Polar Orbiting Environmental Satellites (POES) satellites over a broad region, which is ultimately used to simulate the radiation belt electron dynamics driven by chorus waves. Our simulation results show remarkable agreement in magnitude, timing, energy dependence, and pitch angle distribution with the observed electron PSD near its peak location. However, radial diffusion and other loss processes may be required to explain the differences between the observation and simulation at other locations away from the PSD peak. Our simulation results, together with previous studies, suggest that local acceleration by chorus waves is a robust and ubiquitous process and plays a critical role in accelerating injected seed electrons with convective energies (~100 keV) to highly relativistic energies (several MeV). Li, W.; Thorne, R.; Ma, Q.; Ni, B.; Bortnik, J.; Baker, D.; Spence, H.; Reeves, G.; Kanekal, S.; Green, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Blake, J.; Fennell, J.; Claudepierre, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.610.1002/2014JA019945 |
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Chorus acceleration of radiation belt relativistic electrons during March 2013 geomagnetic storm The recent launching of Van Allen probes provides an unprecedent opportunity to investigate variations of the radiation belt relativistic electrons. During the 17\textendash19 March 2013 storm, the Van Allen probes simultaneously detected strong chorus waves and substantial increases in fluxes of relativistic (2 - 4.5 MeV) electrons around L = 4.5. Chorus waves occurred within the lower band 0.1\textendash0.5fce (the electron equatorial gyrofrequency), with a peak spectral density \~10-4 nT2/Hz. Correspondingly, relativistic electron fluxes increased by a factor of 102\textendash103 during the recovery phase compared to the main phase levels. By means of a Gaussian fit to the observed chorus spectra, the drift and bounce-averaged diffusion coefficients are calculated and then used to solve a 2-D Fokker-Planck diffusion equation. Numerical simulations demonstrate that the lower-band chorus waves indeed produce such huge enhancements in relativistic electron fluxes within 15 h, fitting well with the observation. Xiao, Fuliang; Yang, Chang; He, Zhaoguo; Su, Zhenpeng; Zhou, Qinghua; He, Yihua; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2014 YEAR: 2014 DOI: 10.1002/2014JA019822 |
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In situ observations of Pc1 pearl pulsations by the Van Allen Probes We present in situ observations of Pc1 pearl pulsations using the Van Allen Probes. These waves are often observed using ground-based magnetometers, but are rarely observed by orbiting satellites. With the Van Allen Probes, we have seen at least 14 different pearl pulsation events during the first year of operations. These new in situ measurements allow us to identify the wave classification based on local magnetic field conditions. Additionally, by using two spacecraft, we are able to observe temporal changes in the region of observation. The waves appear to be generated at an overall central frequency, as often observed on the ground, and change polarization from left- to right-handedness as they propagate into a region where they are resonant with the crossover frequency (where R- and L-mode waves have the same phase velocity). By combining both in situ and ground-based data, we have found that the region satisfying electromagnetic ion cyclotron wave generation conditions is azimuthally large while radially narrow. The observation of a similar modulation period on the ground as in the magnetosphere contradicts the bouncing wave packet mechanism of generation. Paulson, K.; Smith, C.; Lessard, M.; Engebretson, M.; Torbert, R.; Kletzing, C.; Published by: Geophysical Research Letters Published on: 04/2014 YEAR: 2014 DOI: 10.1002/2013GL059187 |
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Global statistics of the amplitude distributions of hiss, lightning-generated, and other whistler mode waves from terrestrial VLF transmitters have been obtained from the EXOS-D (Akebono) satellite in the Earth\textquoterights plasmasphere and fitted as functions of L and latitude for two geomagnetic activity ranges (Kp<3 and Kp>3). In particular, the present study focuses on the inner zone L∈[1.4,2] where reliable in situ measurements were lacking. Such statistics are critically needed for an accurate assessment of the role and relative dominance of each type of wave in the dynamics of the inner radiation belt. While VLF waves seem to propagate mainly in a ducted mode at L\~1.5\textendash3 for Kp<3, they appear to be substantially unducted during more disturbed periods (Kp>3). Hiss waves are generally the most intense in the inner belt, and lightning-generated and hiss wave intensities increase with geomagnetic activity. Lightning-generated wave amplitudes generally peak within 10\textdegree of the equator in the region L<2 where magnetosonic wave amplitudes are weak for Kp<3. Based on this statistics, simplified models of each wave type are presented. Quasi-linear pitch angle and energy diffusion rates of electrons by the full wave model are then calculated. Corresponding electron lifetimes compare well with decay rates of trapped energetic electrons obtained from Solar Anomalous and Magnetospheric Particle Explorer and other satellites at L∈[1.4,2]. Agapitov, O.; Artemyev, A.; Mourenas, D.; Kasahara, Y.; Krasnoselskikh, V.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.410.1002/2014JA019886 Inner radiation belt; Van Allen Probes; Wave-particle interaction |
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Journal Special Collection Explores Early Results From the Van Allen Probes Mission The processes governing the charged particle populations in the radiation belts encircling Earth have been the subject of intense interest and increasing concern since their discovery by James Van Allen and his team more than 50 years ago [Baker et al., 2013]. Intense interest continues because we still do not know how the various processes work in concert to enhance, remove, and transport particle radiation. Concern is ongoing because the Van Allen radiation belts pose hazards to astronauts and our ever-growing fleet of spacecraft with increasingly sensitive components. Mauk, Barry; Sibeck, David; Kessel, Ramona; Published by: Eos, Transactions American Geophysical Union Published on: 04/2014 YEAR: 2014 DOI: 10.1002/eost.v95.1310.1002/2014EO130007 |
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Space weather forecasting critically depends upon availability of timely and reliable observational data. It is therefore particularly important to understand how existing and newly planned observational assets perform during periods of severe space weather. Extreme space weather creates challenging conditions under which instrumentation and spacecraft may be impeded or in which parameters reach values that are outside the nominal observational range. This paper analyzes existing and upcoming observational capabilities for forecasting, and discusses how the findings may impact space weather research and its transition to operations. A single limitation to the assessment is lack of information provided to us on radiation monitor performance, which caused us not to fully assess (i.e., not assess short term) radiation storm forecasting. The assessment finds that at least two widely spaced coronagraphs including L4 would provide reliability for Earth-bound CMEs. Furthermore, all magnetic field measurements assessed fully meet requirements. However, with current or even with near term new assets in place, in the worst-case scenario there could be a near-complete lack of key near-real-time solar wind plasma data of severe disturbances heading toward and impacting Earth\textquoterights magnetosphere. Models that attempt to simulate the effects of these disturbances in near real time or with archival data require solar wind plasma observations as input. Moreover, the study finds that near-future observational assets will be less capable of advancing the understanding of extreme geomagnetic disturbances at Earth, which might make the resulting space weather models unsuitable for transition to operations. Posner, A.; Hesse, M.; St. Cyr, O.; Published by: Space Weather Published on: 04/2014 YEAR: 2014 DOI: 10.1002/swe.v12.410.1002/2013SW001007 Assessment; Space Hardware; SWx Forecasting; Van Allen Probes |
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The relativistic electrons in the inner radiation belt have received little attention in the past due to sparse measurements and unforgiving contamination from the inner belt protons. The high-quality measurements of the Magnetic Electron Ion Spectrometer instrument onboard Van Allen Probes provide a great opportunity to investigate the dynamics of relativistic electrons in the low L region. In this letter, we report the newly unveiled pitch angle distribution (PAD) of the energetic electrons with minima at 90\textdegree near the magnetic equator in the inner belt and slot region. Such a PAD is persistently present throughout the inner belt and appears in the slot region during storms. One hypothesis for 90\textdegree minimum PADs is that off 90\textdegree electrons are preferentially heated by chorus waves just outside the plasmapause (which can be at very low L during storms) and/or fast magnetosonic waves which exist both inside and outside the plasmasphere. Zhao, H.; Li, X.; Blake, J.; Fennell, J.; Claudepierre, S.; Baker, D.; Jaynes, A.; Malaspina, D.; Kanekal, S.; Published by: Geophysical Research Letters Published on: 04/2014 YEAR: 2014 DOI: 10.1002/2014GL059725 |
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Quantifying the radiation belt seed population in the 17 March 2013 electron acceleration event We present phase space density (PSD) observations using data from the Magnetic Electron Ion Spectrometer instrument on the Van Allen Probes for the 17 March 2013 electron acceleration event. We confirm previous results and quantify how PSD gradients depend on the first adiabatic invariant. We find a systematic difference between the lower-energy electrons ( Boyd, A.; Spence, H.; Claudepierre, S.; Fennell, J.; Blake, J.; Baker, D.; Reeves, G.; Turner, D.; Published by: Geophysical Research Letters Published on: 04/2014 YEAR: 2014 DOI: 10.1002/2014GL059626 |
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RadFET Dosimeters in the Belt: the Van Allen Probes on Day 365 Van Allen Probes A and B, launched more than a year ago (in August 2012), carried 16 p-channel metal-oxide-semiconductor Radiation-sensitive Field Effect Transistors (RadFET)s into an orbit designed by NASA to probe the heart of the trapped-radiation belts. Nearly 350 days of in situ measurements from the Engineering Radiation Monitor (ERM) (1) demonstrated strong variations of dose rates with time, (2) revealed a critical correlation between the ERM RadFET dosimeters and the ERM Faraday cup data on charged particles, and (3) permitted the mapping of the belts by measuring variation with orbit altitude. This paper provides an update on early results given in a NSREC2012 paper along with details and discussion of the RadFET dosimetry data analyzed . Holmes-Siedle, A.; Goldsten, J.; Maurer, R.; Peplowski, P.; Published by: IEEE Transactions on Nuclear Science Published on: 04/2014 YEAR: 2014 DOI: 10.1109/TNS.2014.2307012 |
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Simulations from our newly expanded ring current-atmosphere interactions model with self-consistent magnetic field (RAM-SCB), now valid out to 9 RE, are compared for the first time with Van Allen Probes observations. The expanded model reproduces the storm time ring current buildup due to the increased convection and inflow of plasma from the magnetotail. It matches Magnetic Electron Ion Spectrometer (MagEIS) observations of the trapped high-energy (>50 keV) ion flux; however, it underestimates the low-energy (<10 keV) Helium, Oxygen, Proton, and Electron (HOPE) observations. The dispersed injections of ring current ions observed with the Energetic particle, Composition, and Thermal plasma (ECT) suite at high (>20 keV) energy are better reproduced using a high-resolution convection model. In agreement with Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) observations, RAM-SCB indicates that the large-scale magnetic field is depressed as close as \~4.5 RE during even a moderate storm. Regions of electromagnetic ion cyclotron instability are predicted on the duskside from \~6 to \~9 RE, indicating that previous studies confined to geosynchronous orbit may have underestimated their scattering effect on the energetic particles. Jordanova, V.; Yu, Y.; Niehof, J.; Skoug, R.; Reeves, G.; Kletzing, C.; Fennell, J.; Spence, H.; Published by: Geophysical Research Letters Published on: 04/2014 YEAR: 2014 DOI: 10.1002/2014GL059533 |
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Van Allen Probes observations of direct wave-particle interactions Quasiperiodic increases, or \textquotedblleftbursts,\textquotedblright of 17\textendash26 keV electron fluxes in conjunction with chorus wave bursts were observed following a plasma injection on 13 January 2013. The pitch angle distributions changed during the burst events, evolving from sinN(α) to distributions that formed maxima at α = 75\textendash80\textdegree, while fluxes at 90\textdegree and <60\textdegree remained nearly unchanged. The observations occurred outside of the plasmasphere in the postmidnight region and were observed by both Van Allen Probes. Density, cyclotron frequency, and pitch angle of the peak flux were used to estimate resonant electron energy. The result of ~15\textendash35 keV is consistent with the energies of the electrons showing the flux enhancements and corresponds to electrons in and above the steep flux gradient that signals the presence of an Alfv\ en boundary in the plasma. The cause of the quasiperiodic nature (on the order of a few minutes) of the bursts is not understood at this time. Fennell, J.; Roeder, J.; Kurth, W.; Henderson, M.; Larsen, B.; Hospodarsky, G.; Wygant, J.; Claudepierre, J.; Blake, J.; Spence, H.; Clemmons, J.; Funsten, H.; Kletzing, C.; Reeves, G.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL059165 |
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We expanded our previous work on L* neural networks that used empirical magnetic field models as the underlying models by applying and extending our technique to drift shells calculated from a physics-based magnetic field model. While empirical magnetic field models represent an average, statistical magnetospheric state, the RAM-SCB model, a first-principles magnetically self-consistent code, computes magnetic fields based on fundamental equations of plasma physics. Unlike the previous L* neural networks that include McIlwain L and mirror point magnetic field as part of the inputs, the new L* neural network only requires solar wind conditions and the Dst index, allowing for an easier preparation of input parameters. This new neural network is compared against those previously trained networks and validated by the tracing method in the International Radiation Belt Environment Modeling (IRBEM) library. The accuracy of all L* neural networks with different underlying magnetic field models is evaluated by applying the electron phase space density (PSD)-matching technique derived from the Liouville\textquoterights theorem to the Van Allen Probes observations. Results indicate that the uncertainty in the predicted L* is statistically (75\%) below 0.7 with a median value mostly below 0.2 and the median absolute deviation around 0.15, regardless of the underlying magnetic field model. We found that such an uncertainty in the calculated L* value can shift the peak location of electron phase space density (PSD) profile by 0.2 RE radially but with its shape nearly preserved. Yu, Yiqun; Koller, Josef; Jordanova, Vania; Zaharia, Sorin; Friedel, Reinhard; Morley, Steven; Chen, Yue; Baker, Daniel; Reeves, Geoffrey; Spence, Harlan; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2013JA019350 |
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On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event On 30 September 2012, a flux \textquotedblleftdropout\textquotedblright occurred throughout Earth\textquoterights outer electron radiation belt during the main phase of a strong geomagnetic storm. Using eight spacecraft from NASA\textquoterights Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes missions and NOAA\textquoterights Geostationary Operational Environmental Satellites constellation, we examined the full extent and timescales of the dropout based on particle energy, equatorial pitch angle, radial distance, and species. We calculated phase space densities of relativistic electrons, in adiabatic invariant coordinates, which revealed that loss processes during the dropout were > 90\% effective throughout the majority of the outer belt and the plasmapause played a key role in limiting the spatial extent of the dropout. THEMIS and the Van Allen Probes observed telltale signatures of loss due to magnetopause shadowing and subsequent outward radial transport, including similar loss of energetic ring current ions. However, Van Allen Probes observations suggest that another loss process played a role for multi-MeV electrons at lower L shells (L* < ~4). Turner, D.; Angelopoulos, V.; Morley, S.; Henderson, M.; Reeves, G.; Li, W.; Baker, D.; Huang, C.-L.; Boyd, A.; Spence, H.; Claudepierre, S.; Blake, J.; Rodriguez, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013JA019446 dropouts; inner magnetosphere; loss; Radiation belts; relativistic electrons; Van Allen Probes |
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Drastic variations of Earth\textquoterights outer radiation belt electrons ultimately result from various competing source, loss, and transport processes, to which wave-particle interactions are critically important. Using 15 spacecraft including NASA\textquoterights Van Allen Probes, THEMIS, and SAMPEX missions and NOAA\textquoterights GOES and POES constellations, we investigated the evolution of the outer belt during the strong geomagnetic storm of 30 September to 3 October 2012. This storm\textquoterights main phase dropout exhibited enhanced losses to the atmosphere at L* < 4, where the phase space density (PSD) of multi-MeV electrons dropped by over an order of magnitude in <4 h. Based on POES observations of precipitating >1 MeV electrons and energetic protons, SAMPEX >1 MeV electrons, and ground observations of band-limited Pc1-2 wave activity, we show that this sudden loss was consistent with pitch angle scattering by electromagnetic ion cyclotron waves in the dusk magnetic local time sector at 3 < L* < 4. At 4 < L* < 5, local acceleration was also active during the main and early recovery phases, when growing peaks in electron PSD were observed by both Van Allen Probes and THEMIS. This acceleration corresponded to the period when IMF Bz was southward, the AE index was >300 nT, and energetic electron injections and whistler-mode chorus waves were observed throughout the inner magnetosphere for >12 h. After this period, Bz turned northward, and injections, chorus activity, and enhancements in PSD ceased. Overall, the outer belt was depleted by this storm. From the unprecedented level of observations available, we show direct evidence of the competitive nature of different wave-particle interactions controlling relativistic electron fluxes in the outer radiation belt. Turner, D.; Angelopoulos, V.; Li, W.; Bortnik, J.; Ni, B.; Ma, Q.; Thorne, R.; Morley, S.; Henderson, M.; Reeves, G.; Usanova, M.; Mann, I.; Claudepierre, S.; Blake, J.; Baker, D.; Huang, C.-L.; Spence, H.; Kurth, W.; Kletzing, C.; Rodriguez, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2014JA019770 |
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We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of >0.7 MeV electrons nor reductions in Van Allen Probe 90\textdegree pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90\textdegree. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2\textendash8 MeV) electron loss but which is confined to pitch angles below around 45\textdegree and not affecting the core distribution. Usanova, M.; Drozdov, A.; Orlova, K.; Mann, I.; Shprits, Y.; Robertson, M.; Turner, D.; Milling, D.; Kale, A.; Baker, D.; Thaller, S.; Reeves, G.; Spence, H.; Kletzing, C.; Wygant, J.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL059024 |
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Event-specific chorus wave and electron seed population models in DREAM3D using the Van Allen Probes The DREAM3D diffusion model is applied to Van Allen Probes observations of the fast dropout and strong enhancement of MeV electrons during the October 2012 \textquotedblleftdouble-dip\textquotedblright storm. We show that in order to explain the very different behavior in the two \textquotedblleftdips,\textquotedblright diffusion in all three dimensions (energy, pitch angle, and L*) coupled with data-driven, event-specific inputs, and boundary conditions is required. Specifically, we find that outward radial diffusion to the solar wind-driven magnetopause, an event-specific chorus wave model, and a dynamic lower-energy seed population are critical for modeling the dynamics. In contrast, models that include only a subset of processes, use statistical wave amplitudes, or rely on inward radial diffusion of a seed population, perform poorly. The results illustrate the utility of the high resolution, comprehensive set of Van Allen Probes\textquoteright measurements in studying the balance between source and loss in the radiation belt, a principal goal of the mission. Tu, Weichao; Cunningham, G.; Chen, Y.; Morley, S.; Reeves, G.; Blake, J.; Baker, D.; Spence, H.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL058819 |
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The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth\textquoterights radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 \textpm 0.5). This reveals graphically that both \textquotedblleftcompeting\textquotedblright mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession. Baker, D.; Jaynes, A.; Li, X.; Henderson, M.; Kanekal, S.; Reeves, G.; Spence, H.; Claudepierre, S.; Fennell, J.; Hudson, M.; Thorne, R.; Foster, J.; Erickson, P.; Malaspina, D.; Wygant, J.; Boyd, A.; Kletzing, C.; Drozdov, A.; Shprits, Y; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL058942 |
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Radial diffusion comparing a THEMIS statistical model with geosynchronous measurements as input The outer boundary energetic electron flux is used as a driver in radial diffusion calculations, and its precise determination is critical to the solution. A new model was proposed recently based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) measurements to express the boundary flux as three fit functions of solar wind parameters in a response window that depend on energy and which solar wind parameter is used: speed, density, or both. The Dartmouth radial diffusion model has been run using Los Alamos National Laboratory (LANL) geosynchronous satellite measurements as the constraint for a one-month interval in July to August 2004, and the calculated phase space density (PSD) is compared with GPS measurements, at magnetic equatorial plane crossings, as a test of the model. We also used the PSD generated from the Shin and Lee model as constraint and examined it by computing the error relative to the LANL geosynchronous spacecraft data-driven run. The calculation shows that there is overestimation and underestimation at different times; however, the direct insertion of the statistical model can be used to drive the radial diffusion model generally, producing the phase space density dropout and increase during a storm. Having this model based on a solar wind parameterized data set, we can run the radial diffusion model for storms when particle measurements are not available as input. We chose the Whole Heliosphere Interval as an example and compared the result with MHD/test-particle simulations, obtaining better agreement with GPS measurement using the diffusion model, which incorporates atmospheric losses and an initial equilibrium radial profile. Li, Zhao; Hudson, Mary; Chen, Yue; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2013JA019320 outer boundary; radial diffusion; Radiation belt; Van Allen Probes |