Biblio

Found 12 results
Filters: Author is Malaspina, D. M.  [Clear All Filters]
2018
Authors: Jaynes A. N., Ali A. F., Elkington S R, Malaspina D. M., Baker D N, et al.
Title: Fast diffusion of ultra-relativistic electrons in the outer radiation belt: 17 March 2015 storm event
Abstract: Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event‐specific and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultra‐relativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher‐frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave‐driven diffusion. We examine this fast diffusion rate alon. . .
Date: 09/2018 Publisher: Geophysical Research Letters DOI: 10.1029/2018GL079786 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL079786
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2016
Authors: Califf S., Li X, Wolf R. A., Zhao H., Jaynes A. N., et al.
Title: Large-amplitude electric fields in the inner magnetosphere: Van Allen Probes observations of subauroral polarization streams
Abstract: The subauroral polarization stream (SAPS) is an important magnetosphere-ionosphere (MI) coupling phenomenon that impacts a range of particle populations in the inner magnetosphere. SAPS studies often emphasize ionospheric signatures of fast westward flows, but the equatorial magnetosphere is also affected through strong radial electric fields in the dusk sector. This study focuses on a period of steady southward interplanetary magnetic field (IMF) during the 29 June 2013 geomagnetic storm where the Van Allen Probes observe a region of intense electric fields near the plasmapause over multiple consecutive outbound duskside passes. We show that the large-amplitude electric fields near the equatorial plane are consistent with SAPS by investigating the relationship between plasma sheet ion and. . .
Date: 05/2016 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA022252 Available at: http://doi.wiley.com/10.1002/2015JA022252
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Authors: Usanova M. E., Malaspina D. M., Jaynes A. N., Bruder R. J., Mann I. R., et al.
Title: Van Allen Probes observations of oxygen cyclotron harmonic waves in the inner magnetosphere
Abstract: Waves with frequencies in the vicinity of the oxygen cyclotron frequency and its harmonics have been regularly observed on the Van Allen Probes satellites during geomagnetic storms. We focus on properties of these waves and present events from the main phase of two storms on 1 November 2012 and 17 March 2013 and associated dropouts of a few MeV electron fluxes. They are electromagnetic, in the frequency range ~0.5 to several Hz, and amplitude ~0.1 to a few nT in magnetic and ~0.1 to a few mV/m in electric field, with both the wave velocity and the Poynting vector directed almost parallel to the background magnetic field. These properties are very similar to those of electromagnetic ion cyclotron waves, which are believed to contribute to loss of ring current ions and radiation belt electro. . .
Date: 09/2016 Publisher: Geophysical Research Letters Pages: 8827 - 8834 DOI: 10.1002/grl.v43.1710.1002/2016GL070233 Available at: http://onlinelibrary.wiley.com/doi/10.1002/2016GL070233/abstract
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2015
Authors: Jaynes A. N., Lessard M. R., Takahashi K., Ali A. F., Malaspina D. M., et al.
Title: Correlated Pc4-5 ULF waves, whistler-mode chorus and pulsating aurora observed by the Van Allen Probes and ground-based systems
Abstract: Theory and observations have linked equatorial VLF waves with pulsating aurora for decades, invoking the process of pitch-angle scattering of 10's keV electrons in the equatorial magnetosphere. Recently published satellite studies have strengthened this argument, by showing strong correlation between pulsating auroral patches and both lower-band chorus and 10's keV electron modulation in the vicinity of geosynchronous orbit. Additionally, a previous link has been made between Pc4-5 compressional pulsations and modulation of whistler-mode chorus using THEMIS. In the current study, we present simultaneous in-situ observations of structured chorus waves and an apparent field line resonance (in the Pc4-5 range) as a result of a substorm injection, observed by Van Allen Probes, along with groun. . .
Date: 07/2015 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA021380 Available at: http://doi.wiley.com/10.1002/2015JA021380
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2014
Authors: Zhao H., Li X, Blake J B, Fennell J. F., Claudepierre S G, et al.
Title: Characteristics of pitch angle distributions of 100 s keV electrons in the slot region and inner radiation belt
Abstract: The pitch angle distribution (PAD) of energetic electrons in the slot region and inner radiation belt received little attention in the past decades due to the lack of quality measurements. Using the state-of-art pitch-angle-resolved data from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes, a detailed analysis of 100 s keV electron PADs below L = 4 is performed, in which the PADs is categorized into three types: normal (flux peaking at 90∘), cap (exceedingly peaking narrowly around 90∘) and 90∘-minimum (lower flux at 90∘) PADs. By examining the characteristics of the PADs of ~460 keV electrons for over a year, we find that the 90∘-minimum PADs are generally present in the inner belt (L < 2), while normal PADs dominate at .L ~3.5. . .
Date: 11/2014 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020386 Available at: http://doi.wiley.com/10.1002/2014JA020386
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Authors: Malaspina D. M., Ergun R. E., Sturner A., Wygant J R, Bonnell J W, et al.
Title: Chorus waves and spacecraft potential fluctuations: Evidence for wave-enhanced photoelectron escape
Abstract: Chorus waves are important for electron energization and loss in Earth's radiation belts and inner magnetosphere. Because the amplitude and spatial distribution of chorus waves can be strongly influenced by plasma density fluctuations and spacecraft floating potential can be a diagnostic of plasma density, the relationship between measured potential and chorus waves is examined using Van Allen Probes data. While measured potential and chorus wave electric fields correlate strongly, potential fluctuation properties are found not to be consistent with plasma density fluctuations on the timescales of individual chorus wave packets. Instead, potential fluctuations are consistent with enhanced photoelectron escape driven by chorus wave electric fields. Enhanced photoelectron escape may result i. . .
Date: 01/2014 Publisher: Geophysical Research Letters Pages: 236 - 243 DOI: 10.1002/2013GL058769 Available at: http://doi.wiley.com/10.1002/2013GL058769
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Authors: Wang X., Malaspina D. M., Hsu H.-W., Ergun R. E., and M. Horányi.
Title: The effects of magnetic fields on photoelectron-mediated spacecraft potential fluctuations
Abstract: 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 bec. . .
Date: 09/2014 Publisher: Journal of Geophysical Research: Space Physics Pages: 7319 - 7326 DOI: 10.1002/jgra.v119.910.1002/2014JA019923 Available at: http://doi.wiley.com/10.1002/jgra.v119.9http://doi.wiley.com/10.1002/2014JA019923
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Authors: Baker D N, Jaynes A. N., Li X, Henderson M G, Kanekal S G, et al.
Title: Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations
Abstract: The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth's 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 ± 0.5). Thi. . .
Date: 03/2014 Publisher: Geophysical Research Letters Pages: 1351 - 1358 DOI: 10.1002/2013GL058942 Available at: http://doi.wiley.com/10.1002/2013GL058942
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Authors: Baker D N, Jaynes A. N., Hoxie V C, Thorne R M, Foster J. C., et al.
Title: An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts
Abstract: Early observations1, 2 indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies3, 4 showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary5, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location6. Re. . .
Date: 11/2014 Publisher: Nature Pages: 531 - 534 DOI: 10.1038/nature13956 Available at: http://www.nature.com/doifinder/10.1038/nature13956
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Authors: Malaspina D. M., Andersson L., Ergun R. E., Wygant J R, Bonnell J W, et al.
Title: Nonlinear Electric Field Structures in the Inner Magnetosphere
Abstract: 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 m. . .
Date: 08/2014 Publisher: Geophysical Research Letters DOI: 10.1002/2014GL061109 Available at: http://doi.wiley.com/10.1002/2014GL061109
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Authors: Zhao H., Li X, Blake J B, Fennell J. F., Claudepierre S G, et al.
Title: Peculiar pitch angle distribution of relativistic electrons in the inner radiation belt and slot region
Abstract: 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° 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° minimum PADs is that off 90° electrons are preferentially heated by chorus waves just out. . .
Date: 04/2014 Publisher: Geophysical Research Letters Pages: 2250 - 2257 DOI: 10.1002/2014GL059725 Available at: http://doi.wiley.com/10.1002/2014GL059725
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Authors: Wang X., Malaspina D. M., Ergun R. E., and M. Horányi.
Title: Photoelectron-mediated spacecraft potential fluctuations
Abstract: Electric field fluctuations such as those due to plasma waves in Earth's magnetosphere may modulate photoelectrons emitted from spacecraft surface, causing fluctuations in spacecraft potential. We experimentally investigate such photoelectron-mediated spacecraft potential fluctuations. The photoelectric charge of a spacecraft model is found to increase with increasing applied electric field as more photoelectrons escape the spacecraft model surface and dissipates with a decrease in the electric field through collection of ambient plasma electrons. When the applied electric field is driven to oscillate at a frequency lower than the response frequency of the spacecraft model, the surface potential follows the electric field oscillations. The spacecraft model maintains an approximately consta. . .
Date: 02/2014 Publisher: Journal of Geophysical Research: Space Physics Pages: 1094 - 1101 DOI: 10.1002/2013JA019502 Available at: http://doi.wiley.com/10.1002/2013JA019502
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