Pitch-Angle Distribution for Electrons at Dipolarization Sites: Field Aligned Anisotropy and Isotropization

Friday, 18 December 2015
Poster Hall (Moscone South)
Kaiti Wang1, Ching-Huei Lin2, Tohru Hada3, Toshi Nishimura4, Vassilis Angelopoulos5 and Wei-Jung Lee1, (1)NCKU National Cheng Kung University, Tainan, Taiwan, (2)Chien Hsin University of Science and Technology, Zhongli, Taiwan, (3)Kyushu University, Fukuoka, Japan, (4)University of California Los Angeles, Los Angeles, CA, United States, (5)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, CA, United States
Investigation of Earth’s radiation environment is important not only because of its geophysical significance but also because it can inform the design of future satellites. The observed dipolarization effects on pitch-angle distributions (PAD) of electrons at the tailside in the inner plasmasheet during geomagnetic activity identified by AL index has been studied via analyzing data from THEMIS mission. We have shown that cigar distributions below about 1keV tend to become isotropized at the fronts at the dipolarization sites whereas isotropic distributions above 1keV tend to become more cigar-shaped (i.e., fluxes peak at pitch-angle of 0o and 180o). We have previously suggested that the ineffectiveness of Fermi acceleration below 1keV could be the factor causing this difference. We examine the dependence of this effect on radial distance from Earth taking place at or near dipolarization sites during times of geomagnetic activity. Because both the field line length and the properties of dipolarizations vary with radial distance. We anticipate significant dependence of this effect on radial distance. Our study contributes to our understanding of the electron environment during dipolarizations in Earth's magnetosphere.