Seasonal variations of Mercury's magnesium dayside exosphere from MESSENGER observations

The Ultraviolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer instrument aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft made near-daily observations of solar-scattered resonant emission from magnesium in Mercury's exosphere during the mission's orbital phase (March 2011-April 2015, similar to 17 Mercury years). In this paper, a subset of these data (March 2013-April 2015) is described and analyzed to illustrate Mg's spatial and temporal variations. Dayside altitude profiles of emission are used to make estimates of the Mg density and temperature. The main characteristics of the Mg exosphere are (a) a predominant enhancement of emission in the morning (6 am-10 am) near perihelion, (b) a bulk temperature of 6000 K, consistent with impact vaporization as the predominant ejection process, (c) a near-surface density that varies from 5 cm(-3) to 50 cm(-3) and (d) a production rate that is strongest in the morning on the inbound leg of Mercury's orbit with rates ranging from 1 x 10(5) cm(-2) s(-1) to 8 x 10(5) cm(-2) s(-1). (C) 2016 Elsevier Inc. All rights reserved.