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Found 6 entries in the Bibliography.
Showing entries from 1 through 6
| 2015 |
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Laboratory studies of nonlinear whistler wave processes in the Van Allen radiation belts Important nonlinear wave-wave and wave-particle interactions that occur in the Earth\textquoterights Van Allen radiation belts are investigated in a laboratory experiment. Predominantly electrostatic waves in the whistler branch are launched that propagate near the resonance cone with measured wave normal angle greater than 85\textordmasculine. When the pump amplitude exceeds a threshold ~5 x10^6 times the back- ground magnetic field, wave power at frequencies below the pump frequency is observed at wave normal angles (~55\textordmasculine). The scattered wave has a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Occasionally, the parametric decay of a lower hybrid wave into a magnetosonic wave and a whistler wave is simultaneously observed with a Tejero, E.; Crabtree, C.; Blackwell, D.; Amatucci, W.; Mithaiwala, M.; Ganguli, G.; Rudakov, L.; Published by: Physics of Plasmas Published on: 08/2015 YEAR: 2015 DOI: 10.1063/1.4928944 Electrostatic Waves; magnetic fields; Nonlinear scattering; Plasma electromagnetic waves; Whistler waves |
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In the present paper, we investigate the trapping of relativistic electrons by intense whistler-mode waves or electromagnetic ion cyclotron waves in the Earth\textquoterights radiation belts. We consider the non-resonant impact of additional, lower amplitude magnetic field fluctuations on the stability of electron trapping. We show that such additional non-resonant fluctuations can break the adiabatic invariant corresponding to trapped electron oscillations in the effective wave potential. This destruction results in a diffusive escape of electrons from the trapped regime of motion and thus can lead to a significant reduction of the efficiency of electron acceleration. We demonstrate that when energetic electrons are trapped by intense parallel or very oblique whistler-mode waves, non-resonant magnetic field fluctuations in the whistler-mode frequency range with moderate amplitudes around 3-15 pT (much less intense than the primary waves) can totally disrupt the trapped motion. However, the trapping of relativistic electrons by electromagnetic ion cyclotron waves is noticeably more stable. We also discuss how the proposed approach can be used to estimate the effects of wave amplitude modulations on the motion of trapped particles. Artemyev, A.; Mourenas, D.; Agapitov, O.; Vainchtein, D.; Mozer, F.; Krasnoselskikh, V.; Published by: Physics of Plasmas Published on: 08/2015 YEAR: 2015 DOI: 10.1063/1.4927774 Cyclotron resonances; magnetic fields; Particle fluctuations; Plasma electromagnetic waves; Whistler waves |
| 2014 |
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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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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 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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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 large range of field and particle interactions are involved in this physics from large-scale ring current ion and magnetic field dynamics to microscopic kinetic interactions of whistler-mode chorus waves with energetic electrons. To measure these kinds of radiation belt interactions, NASA implemented the two-satellite Van Allen Probes mission. 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 include AC electric and magnetic fields from 10Hz to 400 kHz. Published by: Published on: 01/2014 YEAR: 2014 DOI: 10.1109/USNC-URSI-NRSM.2014.6928090 Magnetic field measurement; magnetic fields; Magnetic flux; Van Allen Probes |
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