Pitch angle distributions of electrons at dipolarization sites during geomagnetic activity: THEMIS observations

Kaiti Wang, Ching Huei Lin, Lu Yin Wang, Tohru Hada, Yukitoshi Nishimura, Drew L. Turner, Vassilis Angelopoulos

    Research output: Contribution to journalArticlepeer-review

    8 Citations (Scopus)

    Abstract

    Changes in pitch angle distributions of electrons with energies from a few eV to 1 MeV at dipolarization sites in Earth's magnetotail are investigated statistically to determine the extent to which adiabatic acceleration may contribute to these changes. Forty-two dipolarization events from 2008 and 2009 observed by Time History of Events and Macroscale Interactions during Substorms probes covering the inner plasma sheet from 8 RE to 12 RE during geomagnetic activity identified by the AL index are analyzed. The number of observed events with cigar-type distributions (peaks at 0° and 180°) decreases sharply below 1 keV after dipolarization because in many of these events, electron distributions became more isotropized. From above 1 keV to a few tens of keV, however, the observed number of cigar-type events increases after dipolarization and the number of isotropic events decreases. These changes can be related to the ineffectiveness of Fermi acceleration below 1 keV (at those energies, dipolarization time becomes comparable to electron bounce time). Model-calculated pitch angle distributions after dipolarization with the effect of betatron and Fermi acceleration tested indicate that these adiabatic acceleration mechanisms can explain the observed patterns of event number changes over a large range of energies for cigar events and isotropic events. Other factors still need to be considered to assess the observed increase in cigar events around 2 keV. Indeed, preferential directional increase/loss of electron fluxes, which may contribute to the formation of cigar events, was observed. Nonadiabatic processes to accelerate electrons in a parallel direction may also be important for future study. Key Points Pitch angle distributions of electrons up to MeV at tailside are analyzed Changes in the numbers of observed cigar and isotropic events are connected Adiabatic accelerations can explain these changes over a large range of energy

    Original languageEnglish
    Pages (from-to)9747-9760
    Number of pages14
    JournalJournal of Geophysical Research: Space Physics
    Volume119
    Issue number12
    DOIs
    Publication statusPublished - Dec 2014

    All Science Journal Classification (ASJC) codes

    • Space and Planetary Science
    • Geophysics

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