BepiColombo measurements of Mercury's internal field

In situ measurements of Mercury's internal magnetic field are complicated by the short distance of the magnetopause from the planetary surface. Magnetospheric currents therefore strongly contribute to the measured field, even at small orbital radii. We suggest exploiting the fact that the orbital and rotation periods of Mercury are non-synchronous, in order to separate the internal field from external contributions. Here, we simulate the magnetic field measurement onboard both BepiColombo orbiters (MMO and MPO). We make use of a new empirical model of the Hermean magnetosphere [Scuffham, J.W., Balogh, A. A new model of Mercury's magnetospheric magnetic field, in press, 2006] to realistically represent the external contribution. Using standard inversion techniques, and working in a planetocentric reference frame, we show how to disentangle the magnetospheric field, periodically modulated by the orbital motion around the Sun, from the static internal field. We conclude that collecting data from both orbiters over one or more Mercury years allows an accurate measurement of the internal field multipoles, including higher-order terms.