A stethoscope for the Sun
posted: May 06, 2015

The STEREO spacecraft are still in safe mode as they go through their superior conjunction, passing behind the Sun as seen from Earth. This means that for an extended period of time, we cannot receive any imagery from the far-side of the Sun due to radio interference from the Sun. See the STEREO-website for up-to-date information.

For sure, nothing can replace the backside imagery that STEREO has provided. Worse, this situation will persist till at least July this year, with normal operations expected to resume only in January 2016. In order to get a small idea on what is happening on the Sun's backside, space weather forecasters fall back on tools that existed before the start of the STEREO era. For example, there exist maps with the location of known active regions that are currently doing their far-side transit (see e.g. here). From the far-side example of 02 May underneath to the left, one can see that old group NOAA 2322 (moving to the "right") will round the "east" limb within about 3-4 days and thus become visible from Earth. NOAA 2322 was an M-class flare producing sunspot group when it rounded the west limb on 20 April. On the right is the white light picture of the earth-facing solar hemisphere for that day.

Unfortunately, the above far-side maps do not provide any information on the further evolution of existing sunspot groups or the development of any new active regions. Enter helioseismology. This branch of solar research is very similar to seismology on Earth, where by studying the waves induced by earthquakes, scientists get an idea on the earth's interior. Indeed, by carefully listening to the up and down movements all over the solar surface, helioseismologists get a pretty good idea on the specifics of the solar interior. Moreover, by studying the related acoustic waves that are travelling through the solar interior, forecasters can virtually "see" any rather big, active region that may be present on the far-side. This branch of solar research is called "helioseismic holography" and was developed by Charles Lindsey and Doug Braun back in the late 1990's (see NSO/NISP for more info and recent images).

The maps produced by this technique show sound wave travel time variations, with locations of shorter travel times appearing darker. These darker regions indicate locations where there is an accumulation of magnetic field on the surface. Data necessary for the production of these maps are obtained through the Global Oscillation Network Group or "GONG". The image above shows a map dated 02 May with the Earth side and the far-side of the solar surface indicated. The "P98" marks a potential active region on the far-side, possibly corresponding to old region NOAA 2322. For reference, also NOAA 2335 on the Earth facing side has been indicated.

There's of course a bit of noise in these helioseismic data, and it occasionally happens that apparently active regions on the far-side turn out to be just large facular fields with no or only very small spots. So, prediction of the appearance of a new active group can sometimes be somewhat disappointing. In this case though, it seems that region "P98" really had some punch. Indeed, from or at least close to the area indicated on the helioseismic far-side map, several strong coronal mass ejections (CME) have billowed away on 1, 3 and 4 May. These have been indicated with white arrows on the map underneath. They probably originate from the same active region. Note how, as this active region is transiting the far-side, the main direction of the CME gradually co-moves to the Sun's east limb as seen from Earth.

Interestingly, an SDO-image of the earth-facing solar hemisphere on 5 May shows that an active sunspot region has indeed rounded the northeast limb on 5 May. This region, numbered as NOAA 2339, immediately produced an M1.9 class flare followed later on 5 May at 22:11UT by a strong X2 flare (see this press release). Recent imagery indicates this is a complex and rather large sunspot region.