Should you be graced with clear night skies over the next couple days, you might want to look up: there's a (solar) storm coming.
The National Oceanic and Atmospheric Administration (NOAA) has issued a storm watch for a moderate G2 geomagnetic storm for Sept. 11, with auroral activity possible as far much of the northern United States.
But what about that hole in the sun?
This latest storm comes courtesy of a massive, canyon-shaped hole in the sun's corona. These dark regions, sometimes the size of 50 Earths or larger, are cooler and contain less dense plasma than the rest of the sun's outer atmosphere. They're also responsible for much of the sun's fast solar wind, charged particles that escape at speeds of up to 1 to 2 million miles per hour. As of Tuesday morning, the solar wind hitting Earth was clocked traveling at up to 370 miles per second!
When these coronal holes are lined up towards Earth, the solar wind they send our way interacts with Earth's magnetosphere to create brilliant auroras over both the North and South Poles. If the wind is strong enough, such as the one the sun launched our way last week, it can push auroras down to lower latitudes.
Unlike sunspots, coronal holes are only visible in certain types of extreme ultraviolet light. Since 2010, NASA has been monitoring their appearance from space using the Solar Dynamics Observatory (SDO) satellite. You can see a time-lapse SDO captured in ultraviolet of one such coronal hole in the video below.
According to NOAA, coronal holes can occur at any moment on the sun, but are most persistent during the years around solar minimum, the period of least solar activity in the 11-year solar cycle of the sun. During this dip in the cycle, which will occur from 2019-2020, sunspots fade away and a period of relative calm forms over the sun's surface.