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Title: Local time variations of high-energy plasmaspheric ion pitch angle distributions

Abstract

Recent observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument revealed a persistent depletion in the 1–10 eV ion population in the postmidnight sector during quiet times in the 2 < L < 3 region. This study explores the source of this ion depletion by developing an algorithm to classify 26 months of pitch angle distributions measured by the HOPE instrument. We correct the HOPE low energy fluxes for spacecraft potential using measurements from the Electric Field and Waves (EFW) instrument. A high percentage of low count pitch angle distributions is found in the postmidnight sector coupled with a low percentage of ion distributions peaked perpendicular to the field line. A peak in loss cone distributions in the dusk sector is also observed. Here, these results characterize the nature of the dearth of the near 90° pitch angle 1–10 eV ion population in the near-Earth postmidnight sector. This study also shows, for the first time, low-energy HOPE differential number fluxes corrected for spacecraft potential and 1–10 eV H + fluxes at different levels of geomagnetic activity.

Authors:
 [1];  [1];  [2];  [2];  [1];  [3];  [4]
  1. Univ. of Michigan, Ann Arbor, MI (United States)
  2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  3. West Virginia Univ., Morgantown, WV (United States)
  4. Univ. of Minnesota, Minneapolis, MN (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
National Aeronautic and Space Administration (NASA); USDOE
OSTI Identifier:
1291224
Report Number(s):
LA-UR-15-29276
Journal ID: ISSN 2169-9380
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Space Physics
Additional Journal Information:
Journal Name: Journal of Geophysical Research. Space Physics; Journal ID: ISSN 2169-9380
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Heliospheric and Magnetospheric Physics

Citation Formats

Sarno-Smith, Lois K., Liemohn, Michael W., Skoug, Ruth M., Larsen, Brian Arthur, Moldwin, Mark B., Katus, Roxanne M., and Wygant, John R. Local time variations of high-energy plasmaspheric ion pitch angle distributions. United States: N. p., 2016. Web. doi:10.1002/2015JA022301.
Sarno-Smith, Lois K., Liemohn, Michael W., Skoug, Ruth M., Larsen, Brian Arthur, Moldwin, Mark B., Katus, Roxanne M., & Wygant, John R. Local time variations of high-energy plasmaspheric ion pitch angle distributions. United States. doi:10.1002/2015JA022301.
Sarno-Smith, Lois K., Liemohn, Michael W., Skoug, Ruth M., Larsen, Brian Arthur, Moldwin, Mark B., Katus, Roxanne M., and Wygant, John R. 2016. "Local time variations of high-energy plasmaspheric ion pitch angle distributions". United States. doi:10.1002/2015JA022301. https://www.osti.gov/servlets/purl/1291224.
@article{osti_1291224,
title = {Local time variations of high-energy plasmaspheric ion pitch angle distributions},
author = {Sarno-Smith, Lois K. and Liemohn, Michael W. and Skoug, Ruth M. and Larsen, Brian Arthur and Moldwin, Mark B. and Katus, Roxanne M. and Wygant, John R.},
abstractNote = {Recent observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument revealed a persistent depletion in the 1–10 eV ion population in the postmidnight sector during quiet times in the 2 < L < 3 region. This study explores the source of this ion depletion by developing an algorithm to classify 26 months of pitch angle distributions measured by the HOPE instrument. We correct the HOPE low energy fluxes for spacecraft potential using measurements from the Electric Field and Waves (EFW) instrument. A high percentage of low count pitch angle distributions is found in the postmidnight sector coupled with a low percentage of ion distributions peaked perpendicular to the field line. A peak in loss cone distributions in the dusk sector is also observed. Here, these results characterize the nature of the dearth of the near 90° pitch angle 1–10 eV ion population in the near-Earth postmidnight sector. This study also shows, for the first time, low-energy HOPE differential number fluxes corrected for spacecraft potential and 1–10 eV H+ fluxes at different levels of geomagnetic activity.},
doi = {10.1002/2015JA022301},
journal = {Journal of Geophysical Research. Space Physics},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 7
}

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  • Some of the intial observations made with the solid-state proton detector on Explorer 45 are discussed. Descriptions are given of (l) the energy spectra and pitch angle distributions of ring current protons observed during the- main and fast recovery phases of the Dec. 17, 1971 storm; and (2) appearances of characteristic changes in the pitch angle distributions of ~100keV protons, which are interpreted as pitch angle dispersion of rapidly injected protons induced by their azimuthal drift at L values above 5.
  • We investigate the propagation of {approx}0.3-300 keV electrons in five solar impulsive electron events, observed by the WIND three-dimensional Plasma and Energetic Particle instrument, that have rapid-rise and rapid-decay temporal profiles. In two events, the temporal profiles above 25 keV show a second peak of inward-traveling electrons tens of minutes after the first peak, followed by a third peak due to outward-traveling electrons minutes later-likely due to reflection/scattering first at {approx}0.7-1.7 AU past the Earth, and then in the inner heliosphere inside 1 AU. In the five events, below a transition energy E{sub 0} ({approx}10-40 keV), the pitch-angle distributions aremore » highly anisotropic with a pitch-angle width at half-maximum (PAHM) of <15{sup 0} (unresolved) through the time of the peak; the ratio {Lambda} of the peak flux of scattered (22.{sup 0}5-90{sup 0} relative to the outward direction) to field-aligned scatter-free (0{sup 0}-22.{sup 0}5) electrons is {approx}<0.1. Above E{sub 0}, the PAHM at the flux peak increases with energy up to 85{sup 0} at 300 keV, and {Lambda} also increases with energy up to {approx}0.8 at 300 keV. Thus, low-energy electrons propagated essentially scatter-free through the interplanetary medium, while high-energy electrons experienced pitch-angle scattering, with scattering strength increasing with energy. The transition energy E{sub 0} between the two populations is always such that the electron gyroradius ({rho}{sub e}) is approximately equal to the local thermal proton gyroradius ({rho}{sub Tp}), suggesting that the higher energy electrons were scattered by resonance with turbulent fluctuations at scale {approx}>{rho}{sub Tp} in the solar wind.« less
  • A diffusive approximation of the Fokker-Planck equation is presented which contains adiabatic focusing and pitch angle scattering as elementary processes. The normalized anisotropic part of the distribution function, presented in closed analytical form, is uniquely determined by observable quantities and is independent of time throughout a solar event. It depends only on the local values of the ratio of the mean free path to the focusing length and on the shape of the pitch angle diffusion coefficient. The solution is not limited to weak focusing as long as the solar injection process lasts sufficiently long. Some numerical examples are given,more » and the application of the model to observations represented by a Legendre expansion up to the fourth order are discussed. Deviations from quasi-linear theory and the influence of helicity on the angular distributions are discussed. The method is also applied to solar particle events aboard the space probes Helios 1 and 2. 40 references.« less
  • Ion pitch angle distributions, measured in a near-synchronous orbit, are predominantly field aligned at low energies and predominantly peaked perpendicular to the magnetic field at higher energies. The transition from field-aligned fluxes to fluxes peaked predominantly perpendicular to the magnetic field occurs over a narrow energy range. These ion distributions have been observed at all local times between 5.3 and 7.8 R/sub E/. This transition energy correlates with the deep minimum observed in the ion spectra. There is no apparent correlation between the ion transition energy and magnetic local time, L, Kp, or Dst. However, the transition energy does respondmore » to observed particle injections. The transition energy decrease prior to injection, increases abruptly at injection by as much as 10--20 keV, and then decreases slowly after injection, increases abruptly at injection by as much as 10--20 keV, and then over several hours to instrument threshold level. Ion drift trajectory calculations indicate that the low-energy component below the transition energy drifts in from the nightside plasma sheet via local morning to the dayside. The high-energy component, above the transition energy, arrives on the dayside via local evening.« less