[P-EM04_28PM2] New Perspectives on Earth's Inner Magnetosphere

[PEM04-P02_PG] Solar-cycle variation of the plasmasphere observed from the Akebono PWS data

3-min talk in an oral session

*Shuhei HASEGAWA¹, Yoshizumi MIYOSHI¹, Naritoshi KITAMURA¹, Kunihiro KEIKA¹, Masafumi SHOJI¹, Atsushi KUMAMOTO², Shinobu MACHIDA¹ (1.Solar-Terrestrial Environmment Laboratory, Nagoya University, 2.Department of Geophysics, Graduate School of Science, Tohoku University)

Keywords:plasmasphere, electron density, akebono satellite, solar-cycle

Plasmaspheric density structures have been studied for a long time. Although it has been clarified that the density is roughly constant along field lines in the outer plasmasphere, field-aligned density distributions of the inner plasmasphere has not been studied intensively. Moreover, continuous observations longer than one-solar cycle have not been reported. Consequently, long-term variations of the plasmaspheric density over a solar cycle remain unknown. In this study, using electron density data based on plasma wave observations from the PWS experiments on board the Akebono satellite from 1989 to 2008, we conduct statistical analyses on variations of structures of the plasmasphere and plasmatrough. In order to investigate the latitudinal distribution of the electron density, we assumed that electron density distribution along field lines are described by a power law form N_e = N_e0(LR_E/R)^α, where N_e0 is the equatorial electron density. Using the dataset during geomagnetically quiet periods and altitude higher than 4000 km, we derived solar cycle variations of the equatorial density N_e0 and field-aligned density distributions α. N_e0 and α are almost constant for the solar cycle (N_e0 ≈ 2000 cm^-3 and α = 0–1) in the inner plasmasphere at L = 2.1–2.3, which distribution is close to diffusive equilibrium. In contrast, N_e0〜200 cm^-3 and α=0–1 at solar minimum which distribution is close to diffusive equilibrium and N_e0 〜30 cm^-3 and α= 2–3 at the solar maximum which distribution is close to collisionless in the outer plasmasphere at L = 4.2–4.7.