Friday, April 16, 2010

Wait! Mercury Has a Tail (and an astreoid occultation)

A few days ago I posted some images of Mercury with a tail as seen in the STEREO spacecraft H1b imager. But I realised that the image of the Mercury tail in the H1a imager I referred too was pretty rubbish. So I've now posted one of my more recent images. The image on the right is Mercury as seen in the H1a imager on January 16 of 2010.

For contrast I've put the image from the H1b imager of 8 April on the right. The tail in the H1a imager is better on the 17th, but I've shown the 16th because of a rather unusual circumstance. On the 16th an asteroid passes behind Mercury, so you get a tail and an asteroidal occultation in the same picture. The tail (it points to the left in the lefthand image, and right in the righthand image, the vertical bar is an imager artefact) is easier to see in the animations (see below) which show the asteroid shooting behind Mercury. I haven't identified the asteroid yet, the horizons ephemeris for Mercury from H1a seems off, but I'll sort it out eventually. Update: The asteroid is 88 Thisbe.

Image Credit: Center for Space Physics, Boston University Mercury as seen from Earth in narrow band sodium light.

As I noted in the last article, we are pretty convinced the tail we're seeing is the sodium tail of Mercury, where the solar wind blasts sodium ions off Mercury's surface. From the STEREO imagers, you can only see the tail when Mercury is at inferior conjunction, when most of Mercury's surface is dark, and it's light doesn't overwhelm the image. However, inferior conjunction as seen from Earth is not the same for the STEREO ahead and behind craft. Mercury was at inferior conjunction on 5 January, but it was not until 16-17 January the Mercury was close to that position as seen from STEREO ahead. Similarly, inferior conjunction is not due to be seen from Earth until April 29, but was seen at this position on the 8th from the STEREO behind space craft. Of course, being stupid I didn't realise this apsect of the geometry, and my surveys to find the tail in H1b failed. Once Jiangao Ruan spotted the H1b tail, I finally realise what had been wrong.

Mercury as seen from STEREO A (ahead) on January 16 and STEREO B (behind) on April 8.

As well as the right geometry from the point of view of the spacecraft being needed to see the tail, the tail is most intense when Mercury is at Perihelion (ie closest to the Sun), Mercury was about a month from aphelion on the 16th of January, and was 8 days past Perihelion on the 8th of April. You can see the tail is more distinct in the image from the 8th than the 16th. The stereoimagers let only 5% of the sodium spectrum through, which is enough to see the tails, but not enough for them to be prominent.

You can download an avi of Mercury showing the tail on 16 January 2010 with the astreiod shooting behind Mercury (caution 2.4 Mb) here and a smaller version (0.2 Mb) here. Or you can play the YouTube video below, it's a bit jerky and hard to see the asteroid though.


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