Title: Exploring the Hermean magnetosphere
Author(s): I. A. Daglis, S. Orsini, S. Livi, A. Milillo, and P. Wurz (Poster)
Contact: I. A. Daglis, National Observatory of Athens

ABSTRACT
  In the two decades since the Mariner 10 mission there has been no other spacecraft mission to Mercury. Now, the innermost planet of our solar system is the target of both European and USA missions: ESA is preparing the Cornerstone mission BepiColombo (to be launched in 2009), while NASA recently (June 2001) gave approval for the MESSENGER mission to move into full-scale spacecraft development (launch in 2004). The authors of this paper are members of an international consortium, led by S. Orsini as Principal Investigator, proposing a Neutral Particle Analyzer (NPA) to be flown on board the BepiColombo MPO (Mercury Planetary Orbiter). The proposed instrument consists of three spectrometers, able to detect and characterize the thermal, suprathermal and energetic neutral particles respectively, populating the hermean space environment. The processes that will be investigated via neutral atom detection will contribute to address the following major scientific objectives:
-        Solar-Hermean coupling, i.e. interaction of the Hermean magnetic field with the solar wind.
-        Volatiles in the Hermean exosphere.
-        Chemical composition of the Hermean surface
  This presentation will focus on the first objective. The interaction of Mercury’s magnetosphere with the solar wind is poorly understood, mainly due to the extreme drought of observational data. Actually, prior to the Mariner 10 flybys it was believed that the interaction of the solar wind with Mercury would be lunar-like, because Mercury was not expected to have an intrinsic magnetic field. Mariner 10 had 3 close encounters with the planet in 1974 and 1975, and obtained plasma, energetic charged particles and magnetic field measurements providing evidence of a “miniature” magnetosphere approximately 5% of the size of the terrestrial magnetosphere, although the planetary radii differ by less than a factor of 3. There are more striking differences between Mercury’s and Earth’s magnetospheres. Because of the weakness of the Hermean magnetic field, the magnetospheric standoff distance is rather small and ranges from the planetary surface to approx. one planetary radius above the surface (in comparison, the terrestrial standoff distance is at 10 Earth radii). As a consequence, the surface of Mercury is directly exposed to the solar wind whenever the interplanetary magnetic field points southward, permitting magnetic merging on the dayside magnetopause. The data of Mariner 10 determined that the Hermean magnetosphere does not contain a plasmasphere or permanent radiation belts. During the third flyby, Mariner 10 observed substorm-like activity in the Hermean magnetosphere, which would imply that electric currents flow in the near-Hermean space. However, it is not clear how such currents could close in the absence of an ionosphere, which has important consequences not only for global electric currents, but also for plasma circulation patterns at Mercury.
  We are presenting a schematic outline of important features of the Hermean magnetosphere, comparing them with the corresponding features of our own magnetosphere.