GEOMAGNETIC STORM PREDICTED: NOAA forecasters say there is a 65% chance of minor G1-class geomagnetic storms on July 24th when a high-speed stream of solar wind is expected to hit Earth's magnetic field. The gaseous material is flowing from a large hole in the sun's atmosphere, shown here in an extreme ultraviolet image from NASA's Solar Dynamics Observatory:

This is a "coronal hole," a vast region in the sun's atmosphere where magnetic fields open up and allow solar wind to escape. They look dark in ultraviolet images because the hot glowing plasma normally contained there is missing. In this case, the plasma is making a beeline for Earth.

Some readers have asked, how can we have a geomagnetic storm during solar minimum? It happens all the time. Sunspots, whose counts define the solar cycle, are not the only source of storms. When sunspots vanish, coronal holes replace them as a primary source of solar activity. Studies show that coronal holes not only open more frequently, but also last longer when sunspots are absent. During the last solar minimum in 2007-2009, one coronal hole stayed open for 27 consecutive solar rotations. As the sun slowly turned on its axis, that hole fire-hosed Earth with a stream of solar wind almost once a month for nearly two years. Explosive sunspots make stronger storms than the relatively gentle breezes that emerge from coronal holes, but geomagnetic storms never go away, not even during solar minimum.

High-latitude sky watchers should be alert for auroras on July 24th when the solar wind arrives. G1-class storms can produce Northern Lights as far south as US states ranging from Maine to Washington.


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