While last week was quite a bummer with respect to sunspot activity, the chromosphere offered a much more appealing view. Indeed, during a few days, various filaments of medium length were visible all over the solar surface, with one even attaining 400.000 km. These filaments can be seen in the Kanzelhöhe image underneath, taken on 12 June.
Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk using special filters like H-alpha. However, cool does not mean cold. Using the same filters, one can see the glowing profile of these filaments as they round the solar limb. They are then called prominences. See this 8 December 2011 image (Kanzelhöhe) for a prominence rounding the northeast limb (upper left) and of which a part is already visible on the solar disk as a filament. Notice in this image also the large number of filaments on the northern hemisphere.
A quick comparison of last week's activity with other days during this and previous solar cycles, reveals that there have been instances with a comparable number of filaments or with filaments that were much longer than last week's. The image underneath shows a very long filament that was visible near the end of the previous solar cycle. Imaged by USET on 19 February 2005, and by Kanzelhöhe 3 days later, the filament reached a length of 850.000 km, or more than twice the Earth-Moon distance!
Another example of long filaments is shown in the image underneath, with Kanzelhöhe images taken on 11 August 1980, 28 January 1999 and 6 August 2012. The lengths here vary between nearly two to more than three times the Earth-Moon distance! Just as in the other images, the difference in "darkness" of the filaments is a consequence of various parameters, such as the imaging system (filter, camera,...) and the image processing.
Space weather forecasters keep an eye on these long filaments. Indeed, as the magnetic areas may become unstable, the filament can be ejected into space and -if well positioned- directed towards Earth. Kanzelhöhe images underneath show a filament eruption with flare signature (Hyder flare) that occurred on 12 September 2000 (resp. 07:00UT, 12:12UT and 16:00UT). The eruption was accompanied by a moderate proton event, and the ejected plasma cloud caused minor geomagnetic storming 3 days later.
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