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NOAO Newsletter - National Solar Observatory - March 2000 - Number 61


Rapid Changes in Mercury's Sodium Exosphere

Drew Potter (NASA/JSC)

Sodium in the atmosphere of Mercury can be detected by sunlight scattered in the D1 and D2 resonance lines. Images of the sodium emission show that the sodium density changes from day to day and is often concentrated in regions at high or mid latitudes. Drew Potter (NASA/JSC) and Tom Morgan (SWRI) suggested that sputtering by magnetospheric particles was the origin of the sodium. A problem with this is that the magnetic field of Mercury is strong enough that it is believed to shield the surface from solar particles much of the time, although particle precipitation at the magnetospheric cusps could deposit particles to the surface at high latitudes. Ann Sprague (UA/LPL) noted that the "spots" of sodium emission tended to coincide with major geologic features, such as the Caloris Basin. She proposed that the sodium is released from sodium-rich surface rocks that are associated with these features; however, some spots have appeared where there are no obvious geologic features.

Some of the difficulty in ascribing a source for the sodium arises from the effect of terrestrial atmospheric blurring of the image. It is hard to tell exactly where the sodium emission originates after the atmosphere has blurred the image. Potter, Killen (SWRI), and Morgan recently developed a technique for correcting sodium images for atmospheric blurring, using images made with a large-area image slicer. They applied this technique to a series of Mercury sodium observations made in November, 1997 at the McMath-Pierce Solar Telescope (Planet. and Space Sci. 47, 1441, 1999). Their technique for producing images from the spectroscopic data provides images of both the sodium emission and of the sunlight reflected from the surface. These images are exactly simultaneous with one another, so that the atmospheric blurring is exactly the same for each. They computed a good guess for the actual seeing function by comparing the observed surface reflection image with a theoretical model of the surface reflection image. They then used this function to correct the sodium images for atmospheric blurring. The corrected images show large daily changes in the distribution of sodium over the planet. The sodium emission was brightest at longitudes near the sub-solar longitude in the range 130 - 150, with excess sodium at northern latitudes on some days, and excess sodium at southern latitudes on other days. There are no obviously outstanding geologic features in this range of longitudes. The rapid changes observed during this period suggest a connection with solar activity, since the planet itself is apparently geologically inactive. The total planetary sodium increased by a whopping factor of about 3 during these few days, while the F10.7-cm solar flux during this period varied only slightly, with an increase of about 15%, which is probably insufficient to account for the observed changes. However, there were a number of coronal mass ejection (CME) events, some of which were directed towards the general area of Mercury. The figure below shows the total sodium plotted as a function of time, with the three major CMEs. It is suggested that the changes in the visible neutral sodium atmosphere might be a direct result of solar weather, in particular, the effects of CMEs on Mercury.


Caption: Total visible sodium on Mercury, November 13-20, 1997


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