28 February 2017 - Mission Day: 7760 - DOY: 059

CME, then a Comet

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LASCO C3 image of comet LASCO C3 image of comet
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LASCO C2 image of comet LASCO C2 image of comet
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Caption: A very bright Sun-grazing comet was discovered by SOHO while a pair of coronal mass ejections were heading for Earth, following two explosions above active region 9672 on Monday, 22 October. The expanding cloud(s) will likely strike our planet's magnetosphere on 24-25 October, triggering more geomagnetic activity.

The last one of the CMEs can be seen in both the C2 and C3 movies, and the comet's tail can be seen growing as it approaches the Sun. Solar radiation heats the comet, causing an outgassing of water molecules and dust. The dust scatters sunlight at visible wavelengths, making the comet bright in LASCO images. The water molecules are broken down by ultraviolet light into oxygen and hydrogen atoms, and the hydrogen atoms interact with the coronal plasma.

In these images, the shaded disk is a mask in the instrument that blots out direct sunlight. The white circle added within the disk shows the size and position of the visible Sun.

In just 6 years, SOHO has discovered more than 365 comets, making it the most prolific comet finder in the history of astronomy. With the SOHO/LASCO images available in near real time on the web, the majority of the discoveries are made not by professional astronomers, but by amateur stargazers firing off reports through the Sungrazer web site.

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Instruments: LASCO (Large Angle and Spectrometric Coronagraph);
Taken: 22-24 October 2001
Picture credits: SOHO/LASCO (ESA & NASA)

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