MEASUREMENTS OF AC MAGNETIC-FIELDS AND CURRENTS IN THE HERMEAN MAGNETOSPHERE

Due to the lack of a dense resistive ionosphere and to the slow rate o f planetary rotation, the plasma trapped inside the Hermean magnetosph ere is unlikely to corotate. Therefore, unlike other magnetized planet s, a large convection electric field should prevail close to Mercury. In the absence of collisions, even at low altitudes, kinetic processes such as wave-particle interactions are likely to play a very importan t role. Some of these processes are discussed in a companion paper by Blomberg (Planet. Space Sci. 45, 143-148, 1997). Here, the ULF part of the spectrum is focused on. The magnetosphere of Mercury evolves over very short times and small spatial scales. Therefore small-scale fluc tuating currents involving Alfven waves are expected to develop and to exert a control on the magnetospheric dynamics. In view of this, it i s important to be able to measure a.c. magnetic fields and possibly a. c. currents. Technical solutions to carry out these measurements will also be briefly described. (C) 1997 Published by Elsevier Science Ltd.