CHARACTERISTIC BOUNDARIES OF THE HERMEAN MAGNETOSPHERE AND ENERGETIC PARTICLES CLOSE TO THE PLANET

Characteristic Hermean magnetospheric boundaries identified in magneti c field and plasma data recorded on board the Mariner 10 spacecraft du ring fly-bys of Mercury in March 1974 and March 1975 are described. En ergetic particle measurements contemporaneously made on board, and amb iguities inherent in their interpretation, are discussed and investiga tions to detect an atmosphere and ionosphere at Mercury described. Sci entific questions concerning the magnetosphere that could be addressed using a future Mercury Orbiter are outlined and reasons advanced as t o why it is important to include an Energetic Particle Detector in the payload of such a mission. The relative locations of a Bow Shock and Magnetopause identified at Mercury in magnetic and plasma electron dat a recorded during the two encounters considered, correlate well with t he calculated positions of the same boundaries in the Earth's magnetos phere when equivalent interplanetary conditions are assumed, when aber ration in the solar wind flow is taken into account and when observati ons made in the Hermean magnetosphere at a distance of xR(M) are compa red with corresponding observations in the Terrestrial Magnetosphere a t a distance of 8XR(E). The Bow Shock and Magnetopause signatures reco rded at Mercury are consistent with the geometry which is expected for interaction between a planet centred magnetic dipole and the solar wi nd. Although the origin of the magnetic field at Mercury is not yet un iquely established, its dipole moment is estimated to be 5.1 +/- 0.3 x 10(22) G cm(3) (or about 0.04-0.09% that of the Earth). The geometric ally determined distance to the Magnetopause stagnation point of solar wind flow is 1.45 +/- 0.15 R(M). On average, this distance can, howev er, be expected to be somewhat larger, 1.8 +/- 0.2 R(M), to solar wind variability and to the highly eccentric orbit of Mercury. Right-hand polarized whistler modes (less than or equal to 10 Hz) upstream of the Bow Shock, a Polar Cusp region about twice the size of that pertainin g at the Earth, a Current Sheet and an extended Magnetic Tail were eac h recorded at Mercury. The impulsive acceleration of particles in this Tail to energies of several hundred keV suggests similarities with te rrestrial substorm events. It is a matter of debate if electrons alone , or electrons and protons, participate,in the acceleration process. T he thin atmosphere makes no significant contribution to the plasma of the Magnetosphere and there is no appreciable ionosphere. The restrict ed nature of the magnetic measurements made aboard Mariner 10, and the effect of technical problems suffered by the instruments recording pl asma and energetic particle data in limiting the deductions that can p resently be made using these records, require-further ''particles and fields'' studies to be carried out in the Hermean Magnetosphere. A can didate mission to enable such investigations is the ''Mercury Orbiter' ' (MO) currently under study by the European Space Agency. Since the s ource of the magnetic field at Mercury is presently in doubt, observat ions made over an extended period from this spacecraft, while flying i n a low orbit, are required to determine the higher order multiple mom ents of the planetary magnetic field. and to deduce its topology withi n the Magnetosphere. Such data would also provide insights into the pr esent nature of the interior of Mercury and elucidate its thermal hist ory. It is recommended to include in the MO pay-load an Energetic Part icle Detector to investigate transient intense bursts of relativistic electrons (possibly also protons) in the Magnetic Tail at energies of several ''hundred keV. This would allow the basic process of charged p article acceleration in a Magnetic Tail-Neutral Sheet configuration to be investigated. Interdisciplinary investigations of the link between magnetospheric and exospheric dynamics would provide an insight into mechanisms whereby, in the absence of a significant ionosphere, solar wind interaction leads to the development of a magnetospheric current system which connects with the planetary surface. Energetic particle m easurements would also support Cruise Phase studies made on board the planned Mercury Orbiter. It is expected in this connection that outsid e, but close to, the Hermean Magnetosphere, the dynamics of the relati vely unevolved solar wind flow could be studied from a unique vantage point using the proposed Energetic Particle Detector. (C) 1997 Elsevie r Science Ltd.