Mv. Space Res. Vol. 13, No. 10, pp. (10)331—(10)335, 1993 0273—1l77~3 $24.00 Printed in Great Britain. All rights reserved. Copyright © 1993 COSPAR THE VELOCITY DISTRIBUTION FUNCTIONS OF OXYGEN AND SULPHUR IONS IN THE 10 PLASMA TORUS M. Banaszkiewicz and W.-H. Ip Max-Planck-Institutflir Aeronomie, D-3411 Katlenburg-Lindau, Germany ABSTRACT The Smith-Strobe! mode! for the energy transfer and ion velocity distributions of the Jo torus plasma is updated to take into account the heating effect of the secondary hot electrons and hot ions. The new radiative cooling rates - which led to the current energy crisis in the maintenance of the ultraviolet emission of the Jo plasma torus - are also used in the compu- tation. It is found that reasonable agreement with the Voyager observations can be achieved with such external contributions. It is expected that comparison of the theoretical models with the Galileo observations would provide very useful guidance in our understanding of the complicated processes involved in the energy coupling among the different ion and electron populations in the inner Jovian magnetosphere. INTRODUCTION Because of active volcanic outgassing, the Galilean satellite, Io, maintains an atmosphere of mainly SO 2 in composition [1]. From surface and atmospheric sputtering and ionospheric pickup process, a large quantity of neutral atoms and ions are injected into the Jovian mag- netosphere. The formation of a Jo plasma torus was first detected by Kupo et al. [2] from the optical emission of the Sil ions at 6716 and 6731 A. This plasma cloud located well inside the orbit of Jo was found to be very dense (ne 3000 cm— 3) and relatively cold with the elec- tron temperature being on the order of 2.2 eV [3]. A much warmer and extended ion torus outside Jo’s orbit was subsequently discovered by Voyager 1 during its flyby of the Jovian system. The plasma (PLS) instrument detected oxygen and sulfur ions at a characterictic temperature of 60-90 eV and electrons at temperature of 5-7 eV [4,5,6]. No less dramatic, the ultraviolet spectrometer (UVS) experiment detected very intense emissions of the Oil, 0111, SlI, Sill and SIV ions from the Jo torus [7]. The corresponding UV radiative energy loss was estimated to be about 3 —6 x i0’~ W [8]. The interpretation, immediately after the Voyager encounters, was to invoke the energy input associated with the production of new ions in a distributed neutral cloud as the main source for the observed UV emission . The basic idea is that after creation, a new ion will be accelerated by the corotational electric field to move in a cycloidal orbit; the gyration energy will be 250 eV for 011 and 500 eV for SIl ions at the Jo orbit. The new ions will then interact with the thermal populations of ions and electrons via Coulomb collision and hence transfer energy to the bulk plasma. The ultimate sink for this kinetic energy input is electron impact excitation of the UV emissions. Assuming local thermodynamic equilibrium and a homogeneous structure of the Jo plasma torus, it could be found that a mass loading rate of 1.1 x 1029 AMU/s would be sufficient to maintain the electron temperature and consequently the EUV emissions as observed [7]. The supplier of the new ions was generally considered to be the atomic neutral cloud emitted from Jo. [Note that an average number density of 30 cm3 for the oxygen atoms was deduced by Brown [9] from ground-based measurements and similar results were obtained by Ballester et a!. [10] from their IUE observations.] The energy transfer process in the framework of the Neutral Cloud Theory (NCT) has been studied in detail by several authors [11,12] and reasonable agreements were found until Shemansky [13] reported that the cooling rate for the S+ ions used before was too low by a factor of 5-9 in the temperature range 4-6 eV. Of smaller magnitude, but still significant corrections, upward by factors of 1.3-2, were found for other species [13]. Because of the large abundance of the S+ ions their higher efficiency (10)331