MESSENGER observations of dipolarizations in Mercury's magnetotail

Dipolarizations are not unique to Earth's magnetosphere. Mercury, whose magnetosphere closely resembles Earth's topologically, experiences rapid ( 2 s) increases in the northward component of the tail field (ΔB_z 30 nT) analogous to dipolarization fronts at Earth. However, differences between the two magnetospheres - namely Mercury's lack of an ionosphere, shorter temporal/spatial scales, and stronger upstream solar wind forcing - result in differences in the dynamics of dipolarizations in these magnetospheres. At Mercury, for example, dipolarizations occur substantially more frequently in the post-midnight plasma sheet, opposite to that at Earth. We use thermal ion, energetic electron, and magnetic field measurements from the MESSENGER spacecraft to describe dipolarizations at Mercury statistically. We compare the dipolarization fronts observed at Mercury by MESSENGER with those measured at Earth by missions such as THEMIS. The implications of our results for our understanding of the substorm process at terrestrial planets will be discussed.