A prominence is a large, bright feature extending outward from the Sun’s surface, often in a loop shape. Prominences are anchored to the Sun’s surface in the photosphere, and extend outwards into the Sun’s corona. While the corona consists of extremely hot ionized gases, known as plasma, which do not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. A prominence forms over timescales of about a day, and stable prominences may persist in the corona for several months. Some prominences break apart and give rise to coronal mass ejections.
The Solar Dynamics Observatory never fails to deliver absolutely stunning images from the Sun: as of 18:49 UT , the above picture is what the Sun looked like in the ultraviolet spectrum. The prominence that you are seeing looping off the Sun is estimated at over 700,000 km across, which is about the radius of the entire Sun.
These features are known as prominences, and they form in the sun’s photosphere. Cooler plasma pushes out into the hotter, ionized gases of the Sun’s corona, creating massive loops that take about a day to form but can sometimes persist for months. This particular prominence, one of the biggest we’ve ever seen, isn’t expected to last much longer than a few hours, after which it should harmlessly break up.
But right about now, there’s a loop of plasma running around the Sun that could encircle the Earth twenty times over. That’s absolutely amazing, and here’s the particularly crazy bit. That picture above was taken in ultraviolet light.
The prominence is incredibly long, but it’s also very thin, making it completely invisible next to the light coming off of the surface of the Sun.
This video shows the buildup up this most recent spectacular solar show, as this portion of the Sun comes into view from a 48-hour period between December 4th and 6th:
Here is a video that shows the prominence eruption as it expanded:
Spaceweather.com also has some other fantastic images that are linked to on their front page. Prominences like this can come crashing down quickly when they become unstable, so head over to the SDO site to watch the action as it develops.