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Found 7 entries in the Bibliography.

Showing entries from 1 through 7


Evolution of Pitch Angle-Distributed Megaelectron Volt Electrons During Each Phase of the Geomagnetic Storm

Using Relativistic Electron Proton Telescope measurements onboard Van Allen Probes, the evolution of electron pitch angle distributions (PADs) during the different phases of magnetic storms is studied. Electron fluxes are sorted in terms of storm phase, urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0001 value, energy, and magnetic local time (MLT) sectors for 55 magnetic storms from October 2012 through May 2017. To understand the potential mechanisms for the evolution of electron PADs, we fit PADs to a sinusoidal function urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0002, where urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0003 is the equatorial pitch angle and n is a real number. The major inferences from our study are (i) at L urn:x-wiley:jgra:media:jgra55457:jgra55457-math-00045, the prestorm electron PADs are nearly isotropic (n urn:x-wiley:jgra:media:jgra55457:jgra55457-math-00050), which evolves differently in different MLT sectors during the main phase subsequently recovering back to nearly isotropic distribution type during the storm recovery phase; (ii) for urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0006 urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0007 3.4 MeV, the main phase electron PADs become more pancake like on the dayside with high n values (>3), while it becomes more flattop to butterfly like on the nightside, (iii) at L = 5, magnetic field strength during the storm main phase enhances during the daytime and decreases during the nighttime. (iv) Conversely, at L urn:x-wiley:jgra:media:jgra55457:jgra55457-math-00083, the electron PADs neither respond significantly to the different phase of the magnetic storm nor reflect any MLT dependence. (v) Main phase, electron fluxes with urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0009 <4.2 MeV shows a persistent 90\textdegree maximum PAD with n ranging between 0 and 2, while for urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0010 urn:x-wiley:jgra:media:jgra55457:jgra55457-math-0011 4.2 MeV the distribution appears flattop and butterfly like. Our study shows that the relativistic electron PADs depend upon the geomagnetic storm phase and possible underlying mechanisms are discussed in this paper.

Pandya, Megha; Bhaskara, Veenadhari; Ebihara, Yusuke; Kanekal, Shrikanth; Baker, Daniel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2019

YEAR: 2019     DOI: 10.1029/2019JA027086

electron flux; inner magnetosphere; Pitch angle distribution; Radiation belts; Van Allen Probes

Variation of Radiation belt electron flux during CME and CIR driven geomagnetic storms: Van Allen Probes observations

Relativistic electron flux responses in the inner magnetosphere are investigated for 28 magnetic storms driven by Corotating Interaction Region (CIR) and 27 magnetic storms driven by Coronal Mass Ejection (CME), using data from the Relativistic Electron-Proton Telescope (REPT) instrument on board Van-Allen Probes from Oct-2012 to May-2017. In this present study we analyze the role of CIRs and CMEs in electron dynamics by sorting the electron fluxes in terms of averaged solar wind parameters, L-values, and energies. The major outcomes from our study are: (i) At L = 3 and E = 3.4 MeV, for >70\% cases the electron flux remains stable, while at L = 5, for ~82\% cases it changes with the geomagnetic conditions. (ii) At L = 5, ~53\% of the CIR storms and 30\% of the CME storms show electron flux increase. (iii) At a given L-value, the tendency for the electron flux variation diminishes with the increasing energies for both categories of storms. (iv) In case of CIR driven storms, the electron flux changes are associated with changes in Vsw and Sym-H. (v) At L ~ 3, CME storms show increased electron flux, while at L ~ 5, CIR storms are responsible for the electron flux enhancements. (vi) During CME and CIR driven storms, distinct electron flux variations are observed at L = 3 and L = 5.

Pandya, Megha; Veenadhari, B.; Ebihara, Y.; Kanekal, S.G.; Baker, D.N.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 07/2019

YEAR: 2019     DOI: 10.1029/2019JA026771

electron flux; innermagnetosphere; Magnetic Storms; Radiation belt; solar wind driver; Van Allen Probes


Void structure of O + ions in the inner magnetosphere observed by the Van Allen Probes

The Van Allen Probes Helium Oxygen Proton Electron instrument observed a new type of enhancement of O+ ions in the inner magnetosphere during substorms. As the satellite moved outward in the premidnight sector, the flux of the O+ ions with energy ~10 keV appeared first in the energy-time spectrograms. Then, the enhancement of the flux spread toward high and low energies. The enhanced flux of the O+ ions with the highest energy remained, whereas the flux of the ions with lower energy vanished near apogee, forming what we call the void structure. The structure cannot be found in the H+ spectrogram. We studied the generation mechanism of this structure by using numerical simulation. We traced the trajectories of O+ ions in the electric and magnetic fields from the global magnetohydrodynamics simulation and calculated the flux of O+ ions in the inner magnetosphere in accordance with the Liouville theorem. The simulated spectrograms are well consistent with the ones observed by Van Allen Probes. We suggest the following processes. (1) When magnetic reconnection starts, an intensive equatorward and tailward plasma flow appears in the plasma lobe. (2) The flow transports plasma from the lobe to the plasma sheet where the radius of curvature of the magnetic field line is small. (3) The intensive dawn-dusk electric field transports the O+ ions earthward and accelerates them nonadiabatically to an energy threshold; (4) the void structure appears at energies below the threshold.

Nakayama, Y.; Ebihara, Y.; Ohtani, S.; Gkioulidou, M.; Takahashi, K.; Kistler, L.; Tanaka, T.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 11/2016

YEAR: 2016     DOI: 10.1002/2016JA023013

injections; nonadiabatic acceleration; substorms; Van Allen Probes

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

Ozaki, M.; Shiokawa, K.; Miyoshi, Y.; Kataoka, R.; Yagitani, S.; Inoue, T.; Ebihara, Y.; Jun, C.-W; Nomura, R.; Sakaguchi, K.; Otsuka, Y.; Shoji, M.; Schofield, I.; Connors, M.; Jordanova, V.;

Published by: Geophysical Research Letters      Published on: 08/2016

YEAR: 2016     DOI: 10.1002/2016GL070008

fast modulation; Pc1 geomagnetic pulsations; pulsating proton aurora; subpacket structure; Van Allen Probes; wave-particle interactions

Pulsating proton aurora caused by rising tone Pc1 waves

We found rising tone emissions with a dispersion of \~1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over \~30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7\textdegreeN, 246.7\textdegreeE, magnetic latitude 61.7\textdegreeN). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, \~10\% variations in intensity, and fine structures of 3\textdegree in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of \~1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2016

YEAR: 2016     DOI: 10.1002/2015JA021681

EMIC-triggered waves; Pc1 waves; proton aurora


Formation process of relativistic electron flux through interaction with chorus emissions in the Earth\textquoterights inner magnetosphere

We perform test particle simulations of energetic electrons interacting with whistler mode chorus emissions. We compute trajectories of a large number of electrons forming a delta function with the same energy and equatorial pitch angle. The electrons are launched at different locations along the magnetic field line and different timings with respect to a pair of chorus emissions generated at the magnetic equator. We follow the evolution of the delta function and obtain a distribution function in energy and equatorial pitch angle, which is a numerical Green\textquoterights function for one cycle of chorus wave-particle interaction. We obtain the Green\textquoterights functions for the energy range 10 keV\textendash6 MeV and all pitch angles greater than the loss cone angle. By taking the convolution integral of the Green\textquoterights functions with the distribution function of the injected electrons repeatedly, we follow a long-time evolution of the distribution function. We find that the energetic electrons are accelerated effectively by relativistic turning acceleration and ultrarelativistic acceleration through nonlinear trapping by chorus emissions. Further, these processes result in the rapid formation of a dumbbell distribution of highly relativistic electrons within a few minutes after the onset of the continuous injection of 10\textendash30 keV electrons.

Omura, Yoshiharu; Miyashita, Yu; Yoshikawa, Masato; Summers, Danny; Hikishima, Mitsuru; Ebihara, Yusuke; Kubota, Yuko;

Published by: Journal of Geophysical Research: Space Physics      Published on: 11/2015

YEAR: 2015     DOI: 10.1002/2015JA021563

Chorus; nonlinear wave-particle interaction; Particle acceleration; Radiation belts; relativistic electrons; simulation

Energetic electron precipitation associated with pulsating aurora: EISCAT and Van Allen Probe observations

Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also sub-relativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler-mode wave\textendashparticle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Troms\o VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from ~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to Troms\o and the satellite observed rising tone emissions of the lower-band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave\textendashparticle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of sub-relativistic electrons and the pulsating aurora.

Miyoshi, Y.; Oyama, S.; Saito, S.; Kurita, S.; Fujiwara, H.; Kataoka, R.; Ebihara, Y.; Kletzing, C.; Reeves, G.; Santolik, O.; Clilverd, M.; Rodger, C.; Turunen, E.; Tsuchiya, F.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 03/2015

YEAR: 2015     DOI: 10.1002/2014JA020690

EISCAT; pitch angle scattering; pulsating aurora; Van Allen Probes