Mercury’s Solar Wind Interaction as Characterized by Magnetospheric Plasma Mantle Observations With MESSENGER

Abstract

We analyze 94 traversals of Mercury’s southern magnetospheric plasma mantle using data from the MESSENGER spacecraft. The mean and median proton number densities in the mantle are 1.5 and 1.3 cm−3, respectively. For sodium number density these values are 0.004 and 0.002 cm−3. Moderately higher densities are observed on the magnetospheric dusk side. The mantle supplies up to 1.5 × 108 cm−2 s−1 and 0.8 × 108 cm−2 s−1 of proton and sodium flux to the plasma sheet, respectively. We estimate the cross‐electric magnetospheric potential from each observation and find a mean of ~19 kV (standard deviation of 16 kV) and a median of ~13 kV. This is an important result as it is lower than previous estimations and shows that Mercury’s magnetosphere is at times not as highly driven by the solar wind as previously thought. Our values are comparable to the estimations for the ice giant planets, Uranus and Neptune, but lower than Earth. The estimated potentials do have a very large range of values (1–74 kV), showing that Mercury’s magnetosphere is highly dynamic. A correlation of the potential is found to the interplanetary magnetic field (IMF) magnitude, supporting evidence that dayside magnetic reconnection can occur at all shear angles at Mercury. But we also see that Mercury has an Earth‐like magnetospheric response, favoring −BZ IMF orientation. We find evidence that −BX orientations in the IMF favor the southern cusp and southern mantle. This is in agreement with telescopic observations of exospheric emission, but in disagreement with modeling.Key PointsProton and sodium ions in Mercury’s southern plasma mantle have mean number densities of ~1.5 and 0.004 cm−3, respectivelyThe highest estimates of mantle proton and sodium flux supply to the plasma sheet are 1.5 × 108 cm−2 s−1 and 0.8 × 108 cm−2 s−1, respectivelyAn average cross‐electric magnetospheric potential of ~19 kV is determined, which is enhanced for increased IMF strength and −BZ