Short‐term variations of Mercury's Na exosphere observed with very high spectral resolution

Abstract

Short time variations of Mercury's exosphere cannot be tracked easily from ground based observatories because of the difficulty of distinguishing them from Earth atmospheric effects. On July 13th 2008, using THEMIS solar telescope, we were able to simultaneously measure brightness, Doppler shift and width of the exospheric sodium D2 emission line during half a day with a resolving power of ˜370,000. Mercury's exosphere displayed an emission brightness peak in the Northern hemisphere which vanished in few hours and a more persistent Southern Hemispheric peak. The bulk Doppler shift of the exosphere suggests a period of strong escape from Mercury. The global changes of the Doppler shift and of the Doppler width suggest that a cloud of sodium atoms ejected before or at the beginning of our sequence of observations passed through THEMIS field of view moving anti-sunward. A preferentially southern ejection of sodium atoms leading to the observed persistent southern emission peak is consistent with the orientation of the Interplanetary Magnetic Field during that period

Spatial variations of the sodium/potassium ratio in Mercury’s exosphere uncovered by high-resolution spectroscopy

“…Some easily vaporized elements (in particular Na and K) have been detected and studied from such ground observations Morgan, 1985, 1986). Subsequent observing campaigns from Earth-based telescopes Morgan, 1987, 1990;Sprague, 1990;Sprague et al, 1997Sprague et al, , 1998 have found the abundance of Na and K undergoes strong changes from 10 6 to 10 12 atoms cm À2 for Na and 10 4 to 10 9 atoms cm À2 for K. Such observations revealed in particular the close relations with Mercury's magnetosphere through solar wind penetration along open magnetospheric field lines, subsequent sputtering of Mercury's surface leading to remarkable features in Mercury's exosphere (Potter and Morgan, 1990;Potter et al, 1999;Killen et al, 2001;Leblanc et al, 2008Leblanc et al, , 2009. Part of these observations have also been interpreted through exospheric volatile particles recycling from hot to cold surfaces (Leblanc and Johnson, 2003), the later being high latitude and nightside regions Sprague et al, 1997;Barbieri et al, 2004;Schleicher et al, 2004), Caloris basin and observed radar bright spots at high latitude (Harmon and Slade, 1992), most probably corresponding to small craters partly permanently shadowed .…”

“…Therefore ground-based observations have shown to be an unexpected and useful source of information on Mercury's exosphere but also that Mercury's exosphere can be used as a marker of Mercury's magnetosphere and upper surface composition. In particular, long and short time variabilities of Mercury's sodium exosphere have shown to be an unprecedented guide to better understand what drives the dynamic of Mercury's exosphere Potter et al, 1999;Killen et al, 2001Killen et al, , 2004Potter et al, 2006Potter et al, , 2007Leblanc et al, 2008Leblanc et al, , 2009). …”

Mercury exosphere I. Global circulation model of its sodium component

“…Brightness calibration and calculation of the seeing value for THEMIS data are explained in Leblanc et al (2008Leblanc et al ( , 2009). The measured averaged seeing during this observation corresponds to panel e conditions with a dispersion due to variable atmospheric conditions covering a range of cases from panels d to f. Comparing e and c, it is seen that the simulated brightness emission spatial distribution and intensity are in good agreement with the observed emission brightness when taking into account atmospheric seeing.…”

“…We used one camera to measure the D 2 at 5890 Å Na emission line. Details on the calibration and the extraction of the data and their uncertainty are described in Leblanc et al (2008Leblanc et al ( , 2009. During the first two years of the THEMIS campaign, we obtained complete scans of Mercury's Na exosphere for 43 days of observations.…”

Resonance scattering polarization in the magnetosphere of Mercury

“…The solar wind may penetrate along open magnetospheric field lines and entail the sputtering of the surface, in turn leading to the observed variations and distributions of neutral species in the exosphere (Killen et al, 2001;Potter et al, 1999Potter et al, , 2007Potter et al, , 2006Potter et al, , 2009Potter and Morgan, 1990;Potter and Killen, 2008;Leblanc et al, 2006Leblanc et al, , 2008Leblanc et al, , 2009Mangano et al, 2009). Part of these observations have also been interpreted in terms of exospheric volatile particles being released from the surface and migrating from hot to cold areas, i.e., high-latitude and nightside regions (Barbieri et al, 2004;Hunten and Sprague, 1997;Sprague et al, 1997) or large topographic features such as the Caloris basin (Sprague et al, 1990).…”