Mariner 10 observations of field-aligned currents at Mercury

The Mariner 10 observations firmly established the presence of a modest intrinsic magnetic field (M ∼ 300 nT RM3) at Mereury and a magnetosphere with much the same structure as that found at the Earth. Due to the limited duration of the two passes through this small magnetosphere, about 1 h in total, the opportunities to learn about its dynamics were very limited. However, clear evidence of substorm activity was obtained in the form of intense energetic particle injections similar to those observed in the near-tail region at Earth. In this study the Mariner 10 magnetic field measurements taken during the two nightside fly-bys are re-examined for evidence of field-aligned currents (FACs). Given the very tenuous nature of the Hermean atmosphere and the high resistivity anticipated for its regolith, the FACs coupling the magnetosphere netosphere to the weak ionosphere and/or planetary surface might be expected to be low in intensity and short-lived. Surprisingly, compelling evidence is found for intense FACs following a substorm-type energetic particle injection observed during the first fly-by in 1974. The interplanetary magnetic field orientation observed several minutes later when Mariner 10 exited into the magnetosheath was southward and thus, favorable for dayside reconnection which could power substorm processes. The implications of these FAC observations for the height integrated electrical conductivity at low altitudes, for the electrodynamics of this magnetosphere, and for the planning of a future Mercury mission are discussed.