A stealth solar storm hit Earth on Nov. 20. It arrived without warning and may have contributed to auroras visible at mid latitudes.
This event didn't trigger a geomagnetic storm; however, such solar storms, or coronal mass ejections (CMEs), are still exciting because they erupt almost undetected and disrupt the solar wind conditions around Earth. They are generally more common when the sun enters the declining phase of its 11-year solar cycle, the periodic rise and fall in the sun's magnetic activity.
