Does a dark hole in the sun’s atmosphere give northern skywatchers an opportunity to spot auroras this week? It appears so, as solar forecasters have spotted a large hole in the sun’s atmosphere approaching Earth and sending powerful solar winds toward the planet. For people interested in auroras, this information matters because a coronal hole in the sun is not an explosion similar to a coronal mass ejection. On the contrary, it is a persistent region that can cause geomagnetic unrest for days as the sun rotates.
According to NOAA, coronal holes look dark in extreme ultraviolet images because they are cooler and less dense than the surrounding corona. Moreover, their magnetic field lines are open instead of closed, allowing solar winds to escape into space. When a coronal hole is located in equatorial regions of the sun and approaches Earth, it causes a co-rotating interaction region, which triggers a powerful solar wind stream, causing geomagnetic unrest at level G1 or G2.
Aurora forecasters cannot guarantee an impressive show, but they can explain how geomagnetic unrest works. According to NASA Jet Propulsion Laboratory, auroras happen because of solar particles entering Earth’s magnetosphere, interacting with oxygen and nitrogen, and making their emission. The result is an impressive glow of various colors, ranging from green arcs to red and purple hues. This phenomenon happens continuously in the form of a so-called “auroral oval” that expands during geomagnetic storms, affecting regions in and out of polar areas.
Thus, during a minor geomagnetic storm, people in Alaska, Canada, Scandinavia, and Scotland can see auroras; but stronger events can make auroras visible even to those living in the northern United States. Coronal holes do not cause as sharp geomagnetic storms as coronal mass ejections because solar wind streams produced by coronal holes can persist for several days instead of a couple of hours. However, the general rule remains unchanged solar winds from coronal holes make aurora forecasts switch from quiet conditions to active ones and back within days, depending on how much energy flows into the geomagnetic system.
The underlying physics of auroras and their influence on Earth are known to science. As mentioned above, the main thing about auroras is solar winds carrying energy from the sun to Earth. However, scientists still fail to predict each solar wind burst far in advance because it heavily depends on solar wind’s magnetic properties. Therefore, it is difficult to make accurate predictions until solar winds arrive at the Earth’s vicinity and interact with the magnetosphere.
On top of that, it should be noted that people are limited by terrestrial factors when it comes to observing auroras. Geomagnetic conditions can be favorable but darkness, absence of clouds, and the observer’s latitude are crucial factors in determining aurora visibility. While NOAA provides forecasts that can hint where aurora can appear in North America, people can use storm classifications to estimate how far south auroras can appear.