MESSENGER X-ray observations of magnetosphere–surface interaction on the nightside of Mercury

S.T Lindsay,M.K James,E.J Bunce,S.M Imber,H Korth,A Martindale,T.K Yeoman

doi:10.1016/j.pss.2016.03.005

Abstract

The recently completed MESSENGER mission to Mercury has detected X-ray fluorescence events on the nightside surface of the planet, induced by the precipitation of electrons. We expand upon previously reported catalogues of such events, using a filter based on elemental fluorescence lines to construct a catalogue covering the full five years of the MESSENGER mission. We find that the locations of the majority of these events are ordered in two clear latitudinal bands on the dawn side of the planet centred at ~50°N and ~20°S. Electron precipitation is implied to be either stable or occurring repeatedly on timescales of up to several minutes, long in relation to characteristic times of the Mercury magnetospheric environment. Conversely, X-ray fluorescence events are observed on only ~40% of MESSENGER orbits, although we note that some events are inevitably lost during the filtering process. We suggest that the regions of most intense precipitation are determined by the location of the relevant magnetic field line footprints on the surface. We are able to place speculative limits on the energies of electrons precipitating in this manner based on fluorescence lines in the observed X-ray spectra. The poleward boundaries of the regions of most intense precipitation are found to be collocated with the open-closed field line boundary. We use a magnetic field model to trace field lines from these fluorescence sites to implied locations of origin in the magnetotail.

Highlights

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft was launched in August 2004 and achieved Mercury orbit on 18 March 2011 (Solomon et al, 2007)
Results from MESSENGER orbital operations in 2011 included the detection of X-ray fluorescence taking place on the nightside surface of Mercury (Starr et al, 2012), which was attributed to the impact of energetic particles on the planetary surface
When extended to cover the primary and extended MESSENGER mission from March 2011 to April 2015, our catalogue consists of 2048 fluorescence events, each consisting of one or more consecutive X-Ray Spectrometer (XRS) records

Summary

Introduction

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft was launched in August 2004 and achieved Mercury orbit on 18 March 2011 (Solomon et al, 2007). We have developed an automated filter to isolate candidate events over the full MESSENGER mission from March 2011 to April 2015, based on the detection of Si and/or Ca-Kα fluorescence from the unlit planetary surface This filter applies a series of criteria to each XRS calibrated data record (CDR) as follows: 1. The KT14 model was used to trace the implied magnetic field line associated with each fluorescence event back from its magnetic footprint (assumed to be collocated with the intersection of the XRS boresight with the planetary surface at the time of the event) to the magnetic equator.

Results and discussion

Identification of the fluorescing agent

Surface mapping

Frequency and duration of events

Magnetic field modelling

Conclusions