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

The current density carried by photoelectrons emitted from the GEOTAIL spac ecraft 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 Octobe r 31, 1998, by assuming balance of the currents carried by photoelectrons a nd ambient thermal electrons. Behaviour of the photoelectron current as a f unction of spacecraft potential is consistent with the current profile pred icted by Grard (1973), and the emitted photoelectrons consist of several co mponents with different temperatures. The saturation density of the low ene rgy component of the photoelectron current is 85+/-33 x 10(-6) [Am-2]. Numb er density of the photoelectrons is estimated to be 2.9 +/- 1.4 x 10(9) [m( -3)] at the surface of the spacecraft, and the average energy of the photoe lectrons is 2.1 +/- 0.5 [eV]. These values are higher than the prediction b y Grard but consistent with previous in-flight measurements from GEOS-1, IS EE-1 or Viking.