Mercuryʹs magnetosphere: another look

The measurements made of Mercuryʹs magnetic field during the Mercury I flyby and the Mereury III flyby have been incorporated into models of the Hermean magnetosphere-magnetotail system. When the magnetic field data for the first half of the Mereury I flyby and all of the Mercury III flyby were incorporated into a single fit of a scaled version of the Beard (J. Geophys. Res. 84, 2118–2122, 1979) Earth magnetosphere-magnetotail system, a r.m.s. deviation of 9.3 nT for the magnetic field vector was obtained (Bergan and Engle, J. Geophys. Res. 86, 1617–1620, 1981). This paper presents results of a study that employs an adaptation of that Beard model but also adopts the assumption that the incident solar wind pressure was different at the times of the two Mercury magnetosphere encounters. Resulting different stand-off distances and scaling factors for the data of the two respective flybys result directly from that single assumption. The study yields a comparable fit of reduced r.m.s. deviation of 7.1 nT and a strength of the Mercury planetary dipole moment D (before any displacement effects are incorporated) between 154 nT RM3 (Merc 1) and 182 nT RM3 (Merc 3). The corresponding standoff distances are 1.31 RM for the Merc 3 encounter and 1.08 RM for the Merc 1 encounter.