The big wave!...
posted: March 13, 2014

EIT-waves are named after the Extreme-ultraviolet Imaging Telescope (EIT) onboard SOHO, with which this phenomenon was discovered in 1996-1997. They are large-scale bright fronts observed in extreme ultraviolet (EUV) and propagating over a significant portion of the solar surface. An example is given underneath, from the 15 February 2011 event (X2-flare in the active region NOAA 1158). As these fronts are not so easily visible, scientists often use difference images, i.e. the difference between two subsequent images is shown. The image to the right shows the difference between the two PROBA2/SWAP images to the left.

17 years later, the true nature of these waves remains under debate, though there is a gradual convergence towards it being primarily a fast magnetosonic wave (directly related to the presence of a coronal mass ejection, CME, rather than a flare), but often also with a contribution from the CME expansion (see Note 1). Other typical characteristics are its relatively low average speed of 200-600km/s, and that these fronts can be stopped at the boundary of coronal holes or near active regions. An example is given underneath, with the EIT-wave propagating from an X1.4 flare (22 September 2011, NOAA 1302) at the east limb, part of which was stopped by a coronal hole in the southern solar hemisphere.

Last week on 5 March around 14:00UT, scientists of the PROBA2 Science Centre noticed something strange near the Sun's north pole. There was some brightening wave quickly fading as it was progressing further onto the solar disk, as a sea wave extinguishing when it reaches a beach. As there were no notable events on the solar disk facing Earth, this wave must have come from the Sun's backside. EUV imagery from STEREO-B quickly revealed that a strong eruptive flare had taken place at around 13:26UT, and that a bright front had been propagating away from the eruption site and to the north pole, where it arrived just over 30 minutes later.

The estimated average speed corresponds to about 500 km/s. No obvious front signature was found towards the south, as a number of active regions and filaments prevented its propagation. This may also explain the outlook of the associated (backside) halo CME, which was clearly more pronounced towards the north than to the south.

This movie has clips from all of the three events discussed above. More movies on EIT-waves as observed by SDO/AIA can be found here.

Note 1 - A fast magnetosonic wave is a longitudinal wave of charged particles in a magnetized plasma propagating in all directions, including perpendicularly and parallel to the magnetic field. See image underneath (Source: Wikipedia).

Credits - Data and imagery were taken from PROBA2/SWAP, SDO, STEREO, SOHO/LASCO, and SOHO/Gallery.