What you should know about the giant sunspot that is once again facing Earth

The culprit? A geomagnetic storm caused by high-energy particles and magnetism emanating from a sunspot – a blue spot-like discoloration on the surface of the sun. The same sunspot cluster that was hiding on the far side of the sun for several weeks is now rotating back toward Earth.

There is a chance that more geomagnetic storms of varying strength are in store over the next two weeks as the sunspot moves from left to right across the solar disk. (The sun takes about 27 days to complete one rotation.) It is unlikely that another extreme geomagnetic storm will occur in the near future, but moderate to severe geomagnetic storms are not out of the question if a solar flare erupts from the favorably positioned sunspot cluster.

Technically, this is the third time this sunspot cluster has faced Earth. It was pointed at us in the first half of May, then returned at the end of the month into early June. Now Earth is in the firing line again.

Each time the sunspot cluster returns to the side of the sun facing Earth, it is assigned a new number. In May it was Active Region 3664. Then 3697. Now it is AR3723 – and it crackles with magnetism.

On Sunday, for example, AR3723 spewed out a Class M solar flare, the second-highest level on the scale (it goes above A, B, C, M, and X, with the Class X flare being the strongest). The pulse of radiation helped ionize Earth’s upper atmosphere, causing a shortwave radio blackout over the Atlantic that lasted several hours. And AR3723 is expected to spew out even more magnetic disturbances in the coming days and weeks.

Solar flares are violent explosions of high-energy particles and electrons that race through space at nearly the speed of light. They appear as intense flashes of light that flicker on the Sun. Sometimes they are followed by slower magnetic shock waves called coronal mass ejections (CMEs); they race through space like interstellar tsunamis. When a CME hits Earth, its chaotic magnetism can interact with Earth’s magnetic field, creating episodes of the Northern (and Southern) Lights.

It’s too early to know exactly what AR3723 has in store.

It will be about a day before the planet comes into view of observation satellites that will help scientists at the Space Weather Prediction Center in Boulder, Colorado, determine the planet’s magnetic structure more precisely. From there, scientists can make probabilistic predictions — for example, projecting the likelihood of a Class M or X solar flare within a certain time frame. These flares send high-energy particles toward Earth that can cause shortwave radio blackouts on the sunlit side of the planet.

The sunspot cluster is much smaller than it used to be. But its magnetic structure is apparently still sufficient to emit powerful solar flares. The Space Weather Prediction Center noted that AR3723 “remained the most magnetically complex sunspot group,” but has not evolved much in recent days.