A reverse-polarity sunspot has released a solar flare expected to trigger a moderate to strong geomagnetic storm on Earth either tonight or tomorrow. People in the right locations could get another treat similar to the auroras of late February and March 23. For those suitably minded, there would also be the intellectual thrill of knowing that what they are seeing comes from a rare class of sunspots.
On Sunday evening UTC, an M1.5 solar flare was reported by Spaceweather.com. By Monday morning, American time, Space Weather Watch was announcing an accompanying coronal mass ejection (CME). Both flares and CMEs, which often occur together, can trigger geomagnetic storms when high-energy photons or charged particles encounter the Earth’s upper atmosphere.
Nevertheless, at first sight, this event might not seem like one that would cause much excitement. M-class is only the second largest class of flares, after X, and with a scale that goes up to M10, 1.5 is pretty modest. Three years ago, in the depths of the solar quiet, this would have been big news, but the last few months have seen flares more than 10 times as large.
However, while it never got that big, the flare was unusually long, lasting six hours. More importantly, the CME appears to be headed straight for Earth. A direct hit from a small CME can cause more auroral activity than a glancing blow from something larger.
Knowing that a CME is likely to hit Earth’s magnetosphere, and knowing when it will occur are different things, however. This aurora is estimated to be streaming toward Earth at a speed somewhere between 700 and 1,100 kilometers per second (1.6 million - 2.5 million miles per hour). With a distance of 150 million kilometers (93 million miles) to cover, the difference between being top and bottom of that range makes for a gap in arrival time of more than a day. If the resulting auroras only last a few hours, that will determine which parts of the planet have auroras during daylight when no one can see them.
The National Oceanic and Atmospheric Administration aurora dashboard currently predicts mild storm activity from 11 pm-3 am Wednesday night/Thursday morning UTC. "With a chance of isolated G2 (moderate) storms and a slight chance of G3 (strong)", this would make for optimum viewing times in North America, but predictions of events like these still carry plenty of uncertainty.
If an aurora does occur, observers will have something special to add to the memory. Sunspots all have bipolar magnetic fields. As far back as 1909, George Hale noticed that the sunspots in one hemisphere usually have the same polarity – at the time all those in the Northern Hemisphere had their north pole leading and the south pole trailing, while the opposite was true in the Southern Hemisphere. At the end of an 11-year sunspot cycle, the polarities between the hemispheres swapped, and have done so between every cycle since.
A small minority (around 3 percent) of sunspots show reverse polarity, with their fields in the opposite direction to others in their hemisphere. Some observers claim these Hale’s law violators are usually small and weak, but others dispute this. Certainly, the sunspot region known as AR3296 is large and powerful. Its negative polarity is on the right in standard orientation images from Earth, in contrast to all those around it. Reverse polarity sunspots more often develop tangled magnetic fields, leading to an increased chance of explosions such as the one heading our way.