The sunspot AR2975 has fired off 17 flares since Monday and it may not be done yet. Some of the charged particles blasted out in the process look likely to hit the Earth's atmosphere, including a “Cannibal Coronal Mass Ejection”, which isn't quite as scary as it sounds. If these do indeed come our way observers at high latitudes can look forward to spectacular auroral displays over the next three days.
Sunspots are cooler areas of the Sun caused by magnetic fields that weaken the processes by which heat rises to the Sun's surface. Although the spots themselves are emitting less energy than surrounding areas, they are associated with flares and coronal mass ejections (CMEs) where plasma bursts into space. When these occur charged particles stream off the Sun. Those that head in our direction can be funneled by Earth's magnetic field towards the geomagnetic poles. When they strike the upper atmosphere they can produce auroras that range from tantalizing to breathtaking.
So far, Spaceweather.com reports, AR2975 has released 11 C-class and six of the more powerful M-class flares, as well as some of the A and B classes too minor to keep track of. Nevertheless, we have yet to see an X-class flare, the most powerful category.
Flare strength matters little if the particles are directed away from Earth. In this case, however, the prediction is for geomagnetic storms starting a few hours after midnight UTC tonight. Predictions of such storms are getting better as satellites like the Solar Dynamics Observatory give us more information about our local star, but remain far from reliable. Nevertheless, the current prediction is for this round to reach G3 class, which the NOAA says can trigger false alarms on protection devices, require voltage corrections and cause intermittent interference with satellite navigation. We haven't experienced many of these for several years. However, since a typical solar cycle produces 200 G3 class storms, we probably should get used to this.
The storm will be larger because the second CME is traveling faster than the first (1,700 km/s to 1,259 km/s). Thus the second CME is expected to catch and engulf its predecessor, creating a “cannibal CME”, making for one strong storm rather than two smaller ones.
North Americans should be ideally positioned to witness any resulting auroras without having to wait up too late. Europeans may have better prospects getting up before dawn. If the predictions are correct, auroras may be visible as far as 40 degrees from the geomagnetic poles, which are currently located in the Arctic Ocean north of Canada and near the Antarctic coast south of Australia respectively.
Those living closer to the equator, or with no opportunity to get away from city lights, will have to look at the images with envy.
Solar activity takes place on an 11-year cycle. The last low came in 2019, when three-quarters of the time the Sun had no sunspots at all. Activity has been building up since then – this year has had no days without a sunspot. Activity is not expected to peak until 2025, but these cycles are hard to predict precisely. Particularly during an active cycle, there can be plenty going on several years away from the peak.
AR2975 is far from the only sunspot present at the moment – a host of others are close by on the Sun, along with several others half a hemisphere away, making a level of activity similar to the last peak.