[P-EM16] Dynamics of Earth's Inner Magnetosphere and Initial Results from Arase

[PEM16-25] On the field line length dependence of variations in anisotropies for pitch angle distributions of keV electrons at dipolarization sites in the tailside

★Invited Papers

*Kaiti Wang¹、Zhuo-Ren Lang²、Ching-Huei Lin³、Tohru Hada⁴、Alfred B. C. Chen²、Toshi Nishimura⁵ (1.Tamkang Univ.、2.NCKU、3.Chien-Hsin Univ.、4.Kyushu Univ.、5.Boston Univ.)

キーワード：pitch angle distributions, dipolarization, THEMIS mission

Previous studies had reported observations from various satellite missions on changes in pitch angle distributions of electrons during dipolarization periods at Earth’s tailside associated with geomagnetic activities. In this study, we quantitatively analyzed the variations in anisotropies for pitch angle distributions of electrons based on observations from 2008 to 2011, by particle instruments ESA and SST onboard the probes of THEMIS mission, at dipolarization sites in the inner plasma sheet. The ratio of this anisotropy during dipolarization to that before dipolarization is found to overall exhibit an increasing trend on its upper limit as the length of field line the probe located increases, specifically for electrons with energies on the order of keV. The effectiveness of Fermi acceleration at different field line length may account for this feature. We will present these results on the aspects of observations as well as theories.