Plasma and magnetic field around Mercury

International audienceMeasurements during flybys of Mercury by the MESSENGER spacecraft in 2008-2009 confirm, 34 years after Mariner-10, that this planet possesses an intrinsic magnetic field. This planetary field likely is strong enough to stand off the solar wind and form a magnetosphere with characteristic spatial and temporal scales much smaller than those at Earth. So far, unlike the puzzling measurements of Mariner-10, MESSENGER data do not show evidences of bursty energetic particle injections from the magnetotail but they do display signatures of processes at work in the terrestrial magnetosphere such as flux transfer events, traveling compression regions, or Kelvin-Helmholtz instabilities. Mass resolved measurements from MESSENGER also reveal that a variety of ion species of planetary origin such as O , Na -Mg , K -Ca are present in the inner magnetosphere. Moreover, because of the weak intrinsic magnetic field and proximity to the Sun, the magnetosphere of Mercury is expected to be very dynamical, likely leading to specific nonadiabatic features of the magnetospheric ion dynamics. On the modeling side, a number of studies using distinct approaches (single-particle, MHD, hybrid) have been carried out to investigate the large-scale plasma circulation at Mercury. I will review some of these results, both for characteristics of their own and to put them into perspective with the terrestrial case