The solar wind, blowing at 2 million miles per hour from the hole, just caused a geomagnetic storm here on Earth
A coronal hole on the Sun, as seen by a NASA spacecraft between Dec. 2 and 9, 2016. (Source: NASA SDO/Helioviewer.org) Over the past week, NASA's Solar Dynamics Observatory spacecraft watched a massive coronal hole rotate into view as the Sun spun on its axis. Click on the screenshot above to bring up a video I posted to my Youtube channel showing all the action as seen by SDO between December 2nd and 9th. Such holes occur in areas of the solar atmosphere, called the corona, where the Sun's magnetic field is open to space, rather than closed in on itself. This allows charged particles to stream out at high speed, lowering the density and temperature of material in the parts of the corona where this occurs. The result: When the Sun is viewed in x-ray wavelengths, as it is above, we see a dark region, or "hole," in the corona.
Charged particles spew from the corona all the time, creating the solar wind. That's because material in the corona is so hot, and therefore moving so energetically, that the Sun's gravity cannot hold on to some of it. On average, the solar wind blows at a speed of about 1 million miles per hour (400 kilometers per second). But the wind emanating from coronal holes can blow at twice that velocity. The coronal hole seen in the animation above directed a stream of solar wind particles toward Earth, triggering a minor geomagnetic storm here on Friday, Dec. 9, 2016, according to the Space Weather Prediction Center. A geomagnetic storm occurs when a period of high-speed solar wind transfers significant energy into Earth's enclosing magnetic bubble, called the magnetosphere. This can trigger major changes in the currents, plasmas, and fields in the magnetosphere, resulting in beautiful auroras but also disruption to satellites and risks to power grids and pipelines on the ground.