The plasma environment around Mercury

Our understanding of the plasma environment of planet Mercury is mostly derived from the limited amount of data which has been collected by the NASA flyby mission, Mariner 10, in 1974-1975. The prospect for new missions, the NASA Messenger orbiter and the ESA BepiColombo cornerstone have, however, stimulated a renewed interest in this planet. Like the Moon, Mercury has a vestigial atmosphere and no ionosphere, but - against all expectations - it possesses a small, but definite, intrinsic magnetic field. Due to the weakness of this field, of the order of a few 100 nT at the equator, and the relatively high solar wind pressure which prevails at the orbit of Mercury, the size of the magnetospheric cavity is much smaller than that of the Earth. The two environments have somewhat similar topologies but their dynamics differ widely. The solar wind may even reach the surface of Mercury when its pressure is sufficiently large. The formation of radiation belts is probably impeded for lack of a sufficiently strong magnetic confinement. The environment is permeated by a current system which is basically unknown and the plasma is rarefied, like in the Earth magnetosphere. Due to the proximity of the Sun, the emission rate of photoelectrons is up to 10 times larger than at the-Earth orbit and leads to unique electrostatic phenomena off-orbiting spacecraft and at the surface of the planet.