Diagnosing Mercury's Magnetotail Asymmetries: Detecting Seasonal Effects of Mercury's Orbit Using MESSENGER Observations
Abstract
The MESSENGER spacecraft has observed substantial asymmetries across Mercury's magnetotail. In the plasma sheet, ions of solar wind origin (H+, He2+) appear denser on the dawn side of the magnetotail while ions of planetary origin (Na+, O+) appear denser on the dusk side. The current sheet appears thinner and with weaker Bz on the dusk side, while products from reconnection (dipolarization fronts, flux ropes) are substantially more frequent on the dawn side. The Earth's magnetotail, by comparison, exhibits the same trend in cross-tail current sheet thickness but the opposite trend with reconnection products more common on its dusk side. Recent studies have proposed that the dusk side abundance of heavy planetary ions and/or the Hall electric field in conjunction with the small ion scales at Mercury may modify the cross-tail reconnection rate and explain the differences between Earth's and Mercury's magnetotail asymmetries. We investigate the extent of Mercury's magnetotail asymmetries by examining both plasma and magnetic field parameters synoptically in the magnetotail throughout the lifetime of the MESSENGER orbital mission. We confirm, statistically, the same asymmetries as reported by previous studies. However, by quantifying the cross-tail gradients in thermal ion and magnetic pressures, we observe a magnetotail not in MHD stress balance: both pressures are substantially greater on the dawn side of the magnetotail than the dusk side. We find these pressure gradients contain artifacts of Mercury's orbit about the Sun, a result of both Mercury's highly eccentric orbit and MESSENGER's fixed orbital plane with respect to the Sun. Utilizing the spacecraft orbit's apsidal precession, we devise a statistical scheme to examine effects of heliocentric distance on plasma and magnetic field parameters within the plasma sheet and magnetotail lobes. Correcting for these effects from Mercury's orbit, the cross-tail trend in magnetic pressure reduces in magnitude while the thermal ion pressure asymmetry disappears to within uncertainty. The possible implications of these seasonal trends on Mercury's magnetosphere and exosphere will be discussed, and an updated MHD stress-balance of Mercury's magnetotail will be presented.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSM51A..07D
- Keywords:
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- 2740 Magnetospheric configuration and dynamics;
- MAGNETOSPHERIC PHYSICS;
- 2756 Planetary magnetospheres;
- MAGNETOSPHERIC PHYSICS;
- 5435 Ionospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 5443 Magnetospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS