In situ observations of magnetic reconnection within the magnetotails of Saturn and Mercury

Abstract

This thesis concerns magnetotail reconnection at the planets Saturn and Mercury. Magnetotail reconnection generates large magnetic structures that may pass over orbiting spacecraft, creating identifiable signatures in in situ magnetic field and plasma data. At Saturn, a comprehensive survey of Cassini magnetometer data was performed; the distribution, recurrence and properties of the identified reconnection related events were investigated. The identifications were most frequently observed post-midnight, while the reconnection site was inferred to lie between ~ 20 - 30 R_S from the planet. Mass loss through magnetotail reconnection was found to be insufficient to balance the addition from sources such as the moon Enceladus. Dipolarization fronts were further investigated with regard to their plasma properties. The energized populations behind the fronts were found to be ~ 4 - 12 times hotter and ~ 3 - 10 times less dense than the local ambient plasma sheet. The plasma properties of the dipolarizations were found to be more variable in the post-midnight events, perhaps suggesting a more variable x-line location. Two case studies were presented, the first evidenced short duration flows (~10 minutes), indicating bursty or azimuthally limited reconnection. The second displayed a series of dipolarizations carrying increasingly hot and tenuous plasma, suggesting that the reconnection site was retreating tailward.

For Mercury's magnetotail, a method was presented by which encounters with quasiforce-free flux ropes can be automatically identified in magnetic field data. The method combined new techniques such as wavelet transforms, while automating the analysis of properties that are commonly used to identify flux ropes. The technique was applied to 319 intervals in which MESSENGER was within the Hermean plasma sheet, locating a total of 248 flux ropes. The identifications exhibited a small dawnward preference. Identifications were more likely if the preceding lobe magnetic field was enhanced. Two Monte Carlo based techniques were presented to investigate the effects of spacecraft sampling and selection criteria. The Hermean magnetotail reconnection site was found to be most consistent with an offset ~0.4 R_M dawnward of midnight, with a reconnection width of ~ half the model magnetotail. Dissipation mechanisms planetward of the reconnection site were required to explain the observed distributions. Two sets of selection criteria were compared and found to preferentially select different subsets of the intrinsic population. The method allowed the quantification of these effects, firstly allowing distributions of properties to be corrected (e.g. flux rope radii), and secondly allowing the rate of identifications to be corrected for the fraction of the population that is undetected.

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Identifiers

ORCID for Andrew Smith: orcid.org/0000-0001-7321-4331

ORCID for Caitriona Jackman: orcid.org/0000-0003-0635-7361

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