Earth Planets Space, Vol. 52 (No. 4), pp. 283-292, 2000

Net current density of photoelectrons emitted from the surface of the GEOTAIL spacecraft

Tomoko Nakagawa¹, Takuma Ishii¹, Koichiro Tsuruda², Hajime Hayakawa², and Toshifumi Mukai²

¹Tohoku Institute of Technology, 35-1 Yagiyama Kasumi-cho, Taihaku-ku, Sendai, Miyagi 982-8577, Japan
²The Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

(Received September 22, 1999; Revised February 14, 2000; Accepted February 15, 2000)

Abstract: The current density carried by photoelectrons emitted from the GEOTAIL spacecraft is estimated from the electric potential of the spacecraft measured in the single probe mode of GEOTAIL/EFD and plasma density and temperature obtained by GEOTAIL/LEP during the period from September 14, 1993 to October 31, 1998, by assuming balance of the currents carried by photoelectrons and ambient thermal electrons. Behaviour of the photoelectron current as a function of spacecraft potential is consistent with the current profile predicted by Grard (1973), and the emitted photoelectrons consist of several components with different temperatures. The saturation density of the low energy component of the photoelectron current is 85 ± 33times 10^-6[Am^-2]. Number density of the photoelectrons is estimated to be 2.9 ± 1.4times 109 [m^-3] at the surface of the spacecraft, and the average energy of the photoelectrons is 2.1 ± 0.5 [eV]. These values are higher than the prediction by Grard but consistent with previous in-flight measurements from GEOS-1, ISEE-1 or Viking.