Abstract
AbstractThe first extensive study of interplanetary magnetic field (IMF) characteristics and stability at Mercury is undertaken using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetometer data. Variations in IMF and solar wind conditions have a direct and rapid effect upon Mercury's highly dynamic magnetosphere; hence, understanding of the time scales over which these variations occur is crucial because they determine the duration of magnetospheric states. We characterize typical distributions of IMF field strength, clock angle, and cone angle throughout the duration of MESSENGER's mission. Clock and cone angle distributions collected during the first Earth year of the mission indicate that there was a significant north‐south asymmetry in the location of the heliospheric current sheet during this period. The stability of IMF magnitude, clock angle, cone angle, and IMF Bz polarity is quantified for the entire mission. Changes in IMF Bz polarity and magnitude are found to be less likely for higher initial field magnitudes. Stability in IMF conditions is also found to be higher at aphelion (heliocentric distance r ∼ 0.31 AU) than at perihelion (r ∼ 0.47 AU).
Highlights
18 The Hermean magnetosphere is often compared to that of the Earth because the dipole mo19 ments of both planets share the same sense of orientation [Ness et al, 1975]
(this being an average for the parts of the sheath sampled by MESSENGER). 190 Figure 1b depicts a bimodal distribution of clock angles, with peaks at 90 and -90◦, where 191 there is some level of bias toward a clock angle of 90◦ present in this distribution
The analysis undertaken by this study shows that, after 40 minutes since the last measurement of the interplanetary magnetic field (IMF), there is a 70% chance of the sign of Bz remaining the same for large initial field magnitudes, but only ∼ 13% chance of Bz keeping its polarity for small initial field values
Summary
18 The Hermean magnetosphere is often compared to that of the Earth because the dipole mo ments of both planets share the same sense of orientation [Ness et al, 1975]. While expected solar wind velocities of ~200–800 km s−1 at Mercury [Russell et al, 1988; Burlaga, 2001] are similar to those experienced at 1 AU, the number density is typically up to ten times higher at ~30–70 cm−3 [Burlaga, 2001; Blomberg et al, 2007; Fujimoto et al, 2007]. This means that the dynamic pressure, Pdyn, is significantly higher at Mercury (~11.0– 26.5 nPa [Fujimoto et al, 2007]), which, when combined with Mercury’s relatively weak dipole
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