Energetic electron acceleration during dipolarization events in Mercury's magnetotail

Energetic particle bursts in association with dipolarization events within Mercury's magnetotail have been a source of curiosity and controversy since Mariner 10. Mariner 10 observed particle events analogous to injection and dipolarization events at Earth, but instrumental effects prevented an unambiguous determination of species, flux, and energy spectrum. At Earth, such energetic particle events closely correlate with sudden increases in the northward component of the near-tail magnetic field, termed dipolarization events, typically near substorm onset. Results from the Energetic Particle Spectrometer aboard MESSENGER have shown that energetic particle bursts at Mercury are composed entirely of electrons with energies from 30 - 300 keV. Here we use the Gamma-Ray Spectrometer (GRS) high time resolution (10 ms) energetic electron (>50 keV) measurements and the Magnetometer data to discover the relationship between energetic electron bursts and dipolarizations of the magnetic field in Mercury's magnetotail. From March 2013 to April 2015, we identified 472 electron burst events within the magnetotail, of which 150 were closely associated with dipolarization. The dipolarizations were detected on the basis of their rapid ( 1 s) increase in the northward tail field (ΔBz 30 nT), which persist for 10 s. The GRS energetic electron bursts are typically coincident with the leading edge of the dipolarization event and last for 5 s. Surprisingly, these events display a strong dawn-dusk asymmetry with more events on the dawn side of the magnetotail.