Characteristic boundaries of the Hermean magnetosphere and energetic particles close to the planet

Abstract

Characteristic Hermean magnetospheric boundaries identified in magnetic field and plasma data recorded on board the Mariner 10 spacecraft during fly-bys of Mercury in March 1974 and March 1975 are described. Energetic particle measurements contemporaneously made on board, and ambiguities inherent in their interpretation, are discussed and investigations to detect an atmosphere and ionosphere at Mercury described. Scientific questions concerning the magnetosphere that could be addressed using a future Mercury Orbiter are outlined and reasons advanced as to 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 data recorded during the two encounters considered, correlate well with the calculated positions of the same boundaries in the Earth's magnetosphere when equivalent interplanetary conditions are assumed, when aberration in the solar wind flow is taken into account and when observations made in the Hermean magnetosphere at a distance of xR M are compared with corresponding observations in the Terrestrial Magnetosphere at a distance of 8 XR E. The Bow Shock and Magnetopause signatures recorded at Mercury are consistent with the geometry which is expected for interaction between a planet centred magnetic dipole and the solar wind. Although the origin of the magnetic field at Mercury is not yet uniquely established, its dipole moment is estimated to be 5.1 ± 0.3 × 10 22 G cm 3 (or about 0.04–0.09% that of the Earth). The geometrically determined distance to the Magnetopause stagnation point of solar wind flow is 1.45 ± 0.15 R M. On average, this distance can, however, be expected to be somewhat larger, 1.8 ± 0.2 R M, due to solar wind variability and to the highly eccentric orbit of Mercury. Right-hand polarized whistler modes (≤ 10 Hz) upstream of the Bow Shock, a Polar Cusp region about twice the size of that pertaining at the Earth, a Current Sheet and an extended Magnetic Tail were each recorded at Mercury. The impulsive acceleration of particles in this Tail to energies of several hundred keV suggests similarities with terrestrial substorm events. It is a matter of debate if electrons alone, or electrons and protons, participate in the acceleration process. The thin atmosphere makes no significant contribution to the plasma of the Magnetosphere and there is no appreciable ionosphere. The restricted nature of the magnetic measurements made aboard Mariner 10, and the effect of technical problems suffered by the instruments recording plasma and energetic particle data in limiting the deductions that can presently be made using these records, require further “particles and fields” studies to be carried out in the Hermean Magnetosphere. A candidate mission to enable such investigations is the “Mercury Orbiter” (MO) currently under study by the European Space Agency. Since the source of the magnetic field at Mercury is presently in doubt, observations made over an extended period from this spacecraft, while flying in a low orbit, are required to determine the higher order multiple moments of the planetary magnetic field, and to deduce its topology within the Magnetosphere. Such data would also provide insights into the present nature of the interior of Mercury and elucidate its thermal history. It is recommended to include in the MO payload an Energetic Particle 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 particle 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 measurements would also support Cruise Phase studies made on board the planned Mercury Orbiter. It is expected in this connection that outside, but close to, the Hermean Magnetosphere, the dynamics of the relatively unevolved solar wind flow could be studied from a unique vantage point using the proposed. Energetic Particle Detector.