Evidence for space weather at Mercury

Abstract

Abstract. Mercury's sodium atmosphere is known to be highly variable both temporally and spatially. During a week-long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind-magnetosphere interactions. We assume that photon-stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercury's magnetic moments are required to confirm the results.

Mercury exosphere I. Global circulation model of its sodium component

“…If confirmed by further observation within this range of TAA, this would rule out any significant enhancement due to coronal mass ejection (Potter et al 1999), due to a significant change of both solar wind and solar flux conditions (Killen et al 2001), due to an enhanced source in the Caloris basin (Sprague et al 1990;Yan et al 2006), or due to a shift of the bombarded surface by solar wind sputtering . Figure 3 suggests that the conjunction of variable IMF conditions with the increase in the scattering efficiency due to the increase of Mercury's heliocentric velocity, and the variation of the solar radiation pressure may be able to produce the size of the observed increase in brightness.…”

“…The yield is a measurement of the efficiency by which a solar wind ion ejects surface particles. In LJ2003, we set Y SWS = 0.06 surface atoms/solar wind particles smaller than the value of 0.15 suggested by Killen et al (2001) because we took into account the effect of the surface porosity. Cassidy and Johnson (2005) calculated that porosity effect should reduce the yield measured in laboratory on flat surface by a factor close to the factor 0.4 as suggested earlier by Johnson (1989).…”

“…Q PSD was chosen equal to 10 -20 cm 2 in LJ2003 following Yakshinskiy and Madey (1999) laboratory measurements, but the effective value may vary with respect to the surface porosity (Cassidy and Johnson 2005). On the other hand, the solar flux F ph may vary strongly following the solar activity (Killen et al 2001). …”

“…In LJ2003, this ejection rate was set equal to 5×10 23 sodium atoms / s at perihelion (derived from Killen et al 2001). …”

MESSENGER: Exploring Mercury’s Magnetosphere

“…Extensive analysis and modeling have been devoted to the investigation of how ion impact sputtering, aided by solar radiation, may result in the injection and acceleration of newly created ions followed by further re-circulation, as diagrammed in Figure 8 Ip, 1987;Killen et al, 2001;Delcourt et al, 2003).…”