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Found 55 entries in the Bibliography.

Showing entries from 51 through 55


Characteristics of pitch angle distributions of 100 s keV electrons in the slot region and inner radiation belt

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 90o), cap (exceedingly peaking narrowly around 90o) and 90o-minimum (lower flux at 90o) PADs. By examining the characteristics of the PADs of ~460 keV electrons for over a year, we find that the 90o-minimum PADs are generally present in the inner belt (L < 2), while normal PADs dominate at .L ~3.5 - 4. In the region between, 90o-minimum PADs dominate during injection times and normal PADs dominate during quiet times. Cap PADs appear mostly at the decay phase of storms in the slot region and are likely caused by the pitch angle scattering of hiss waves. Fitting the normal PADs into sinnĪ± form, the parameter n is much higher below L = 3 than that in the outer belt and relatively constant in the inner belt but changes significantly in the slot region (2 < L < 3) during injection times. As for the 90o-minimum PADs, by performing a detailed case study, we find in the slot region this type of PAD is likely caused by chorus wave heating, butthis mechanism can hardly explain the formation of 90o-minimum PADs at the center of inner belt.

Zhao, H.; Li, X.; Blake, J.; Fennell, J.; Claudepierre, S.; Baker, D.; Jaynes, A.; Malaspina, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 11/2014

YEAR: 2014     DOI: 10.1002/2014JA020386

energetic electrons; Inner radiation belt; Pitch angle distribution; plasmasphere; Slot region; Van Allen Probes; Wave-particle interaction

Interactions of energetic electrons with ULF waves triggered by interplanetary shock: Van Allen Probes observations in the magnetotail

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

THEMIS measurements of quasi-static electric fields in the inner magnetosphere

We use four years of THEMIS double-probe measurements to offer, for the first time, a complete picture of the dawn-dusk electric field covering all local times and radial distances in the inner magnetosphere based on in situ equatorial observations. This study is motivated by the results from the CRRES mission, which revealed a local maximum in the electric field developing near Earth during storm times, rather than the expected enhancement at higher L shells that is shielded near Earth as suggested by the Volland-Stern model. The CRRES observations were limited to the dusk side, while THEMIS provides complete local time coverage. We show strong agreement with the CRRES results on the dusk side, with a local maximum near L =4 for moderate levels of geomagnetic activity and evidence of strong electric fields inside L =3 during the most active times. The extensive dataset from THEMIS also confirms the day/night asymmetry on the dusk side, where the enhancement is closest to Earth in the dusk-midnight sector, and is farther away closer to noon. A similar, but smaller in magnitude, local maximum is observed on the dawn side near L =4. The noon sector shows the smallest average electric fields, and for more active times, the enhancement develops near L =7 rather than L =4. We also investigate the impact of the uncertain boom-shorting factor on the results, and show that while the absolute magnitude of the electric field may be underestimated, the trends with geomagnetic activity remain intact.

Califf, S.; Li, X.; Blum, L.; Jaynes, A.; Schiller, Q.; Zhao, H.; Malaspina, D.; Hartinger, M.; Wolf, R.; Rowland, D.; Wygant, J.; Bonnell, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2014

YEAR: 2014     DOI: 10.1002/2014JA020360

convection; double probe; electric field; inner magnetosphere

Peculiar pitch angle distribution of relativistic electrons in the inner radiation belt and slot region

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

Van Allen Probes


First Results from CSSWE CubeSat: Characteristics of Relativistic Electrons in the Near-Earth Environment During the October 2012 Magnetic Storms

Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65\textdegree) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10\textdegree) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km \texttimes 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context.

Li, X.; Schiller, Q.; Blum, L.; Califf, S.; Zhao, H.; Tu, W.; Turner, D.; Gerhardt, D.; Palo, S.; Kanekal, S.; Baker, D.; Fennell, J.; Blake, J.; Looper, M.; Reeves, G.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2013

YEAR: 2013     DOI: 10.1002/2013JA019342

RBSP; Van Allen Probes

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