Earth Planets Space, Vol. 57 (No. 9), pp. 885-894, 2005

Pitch angle diffusion of electrons at the boundary of the lunar wake

Tomoko Nakagawa¹ and Masahide Iizima²

¹Tohoku Institute of Technology, 35-1 Yagiyama Kasumi-cho, Taihaku-ku, Sendai, Miyagi 982-8577, Japan
²Tohoku University, Aramaki Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan

(Received December 15, 2004; Revised July 6, 2005; Accepted July 6, 2005)

Abstract: Velocity distribution of the solar wind electrons that penetrate through the lunar wake boundary is investigated by calculating orbits of the electrons injected into model structures of layers of electric fields. Only the electrons with sufficient energy to overcome the potential difference penetrate through the wake boundary. The electrons injected along the magnetic field lines which intersect the model structure undergo pitch angle scattering due to electric field component perpendicular to the magnetic field. After the passage through the electric field, the electrons have significant perpendicular component of velocity as well as the parallel component larger than a lower limit, which is dependent on the electric potential of the wake boundary. The velocity distribution can account for the cyclotron resonance with sunward-propagating whistler mode waves that were detected by GEOTAIL at 27 lunar radii upstream of the moon on October 25, 1994.
Key words: Lunar wake, pitch angle diffusion, electric field, wake potential structure, electron distribution function.