A blast wave swept across the face of the Sun on Wednesday, rippling outward from the site of a large solar flare. Blast waves that spread all the way across the Sun like this one did are rare, especially when the Sun is in the quiet phase of its 11-year cycle, as it is right now.

The wave was imaged by the Optical Solar Patrol Network telescope at the National Solar Observatory (NSO) in Sunspot, New Mexico, US. Watch a video of the expanding blast wave. The wave compresses and heats the solar plasma as it passes, causing it to brighten.

The wave raced outwards from the site of the flare at 400 kilometres per second, says K S Balasubramaniam of NSO. It occurred near one edge of the Sun's face and traveled to the other edge in about 30 minutes, he says.

This was the second major flare from the same group of sunspots. Another flare spewed out from the group on Tuesday. The fact that this region of the Sun spewed out two major flares just a day apart "implies that there must have been some kind of tremendous energy buildup", Balasubramaniam told New Scientist.

The buildup of energy is thought to be related to the twisting of the Sun's magnetic field. Such a large buildup and release of energy on the Sun is a rare occurrence, and especially unusual around solar minimum, when the Sun is normally at its quietest.Low point

The Sun appears to be just past the lowest point in its cycle, Balasubramaniam says. The last maximum was in 2000, but the exact time between maximum and minimum varies from cycle to cycle.

The last time a similar blast wave was seen by the NSO telescope was in November 2003, though it might have missed others if they occurred when the Sun was not visible from the site, he says.

The expanding blast wave also caused two dark filaments to disappear as it passed by, one at upper left and the other at lower middle. The filaments are arches of relatively cool material clinging to magnetic field lines above the Sun.

The passage of the wave may temporarily disperse this material, Balasubramaniam says. See a video showing the brief disappearance of the filaments.

Exactly how the release of magnetic energy leads to solar outbursts is not known. Whether flares provide the trigger for the waves or the relationship is more complicated is also not clear. "If we see a lot more of these, it will tell us a lot more about why these things happen at certain times," he says.