An assessment of the length and variability of Mercuryʹs magnetotail

We employ Mariner 10 measurements of the interplanetary magnetic field in the vicinity of Mercury to estimate the rate of magnetic reconnection between the interplanetary magnetic field and the Hermean magnetosphere. We derive a time-series of the open magnetic flux in Mercuryʹs magnetosphere, from which we can deduce the length of the magnetotail. The length of the magnetotail is shown to be highly variable, with open field lines stretching between 15RH and 850RH downstream of the planet (median 150RH). Scaling laws allow the tail length at perihelion to be deduced from the aphelion Mariner 10 observations.