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.

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

“…After MESSENGER enters the high-latitude nightside magnetosphere the Na + density decreases and the suggestion of an energy dispersion is present in Figure 7a with the higherenergy H + becoming depleted as the spacecraft moves to lower latitudes deeper in the northern tail lobe. These observations are all consistent with MESSENGER entering the plasma mantle that makes up the outer high-latitude layer of the magnetotail Jasinski et al, 2017).…”

“…At this point the spacecraft entered the high-latitude magnetosphere at an altitude of 410 km and a local time of 13:00. These observations are all consistent with MESSENGER entering the plasma mantle that makes up the outer high-latitude layer of the magnetotail Jasinski et al, 2017). It is expected that the He ++ density tracks with that of the H + since they are both of solar wind origin.…”

“…Solar wind particles generally have a significant component of their velocity parallel to the interplanetary magnetic field. The direct impact of solar wind protons not only sputters neutral atoms into Mercury's exosphere (e.g., Pfleger et al, 2015) but also sputters ions into the high-latitude magnetotail where they mix with solar wind that mirrored at low altitudes to form the plasma mantle Jasinski et al, 2017). Cusp magnetic flux tubes within which the magnetic field is depressed due to the diamagnetism of the plasma injected by reconnection at the magnetopause are known as cusp plasma filaments .…”

A Comparative Study of the Proton Properties of Magnetospheric Substorms at Earth and Mercury in the Near Magnetotail

“…Lyons and Speiser () showed that this cross‐tail acceleration is sensitive to the ratio of the dawn‐to‐dusk electric field ( E y ) to the vertical B z . The high E y due to relatively high cross‐tail potential (DiBraccio et al, ; Jasinski et al, ; Slavin et al, , ) would result in strong cross‐tail acceleration for protons at Mercury. The other possible candidate is wave‐particle interactions (e.g., Catapano et al, ; Hasegawa et al, ; Shizgal, ).…”

Saturn's Open‐Closed Field Line Boundary: A Cassini Electron Survey at Saturn's Magnetosphere

“…This causes the solar wind to increase in Alfvenic Mach number with distance from the Sun, and therefore, the shocks formed at the outer planetary magnetospheres produce a much higher plasma‐β (ratio of plasma to magnetic pressure) in the magnetosheath. Such conditions have been shown to not be conducive for reconnection onset (Masters et al, ; Swisdak et al, , ), in comparison to Earth and Mercury (e.g., Zhong et al, ; Slavin et al, ; Slavin et al, ; Jasinski, Slavin, et al, ). Regardless, reconnection under such conditions at Saturn has been observed to occur at multiple x‐lines to form flux transfer events (Jasinski, Slavin, et al, ) similar to observations at the inner planets.…”