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An interplanetary shock wave hit Earth's magnetic field on April 19th around 23:50 UT. When the shock arrived, the density of the solar wind flowing around our planet abruptly quadrupled. Moreover, magnetic fields in the solar wind near Earth have since intensified 10-fold. These developments set the stage for possible geomagnetic storms in the hours ahead.

Auroras are now being reported in northern Michigan. S. Evans sends this picture from Big Bay, MI:

"The auroras are brightening after midnight on April 20th with some rays and beams, and good color," says Evans.
What is an interplanetary shock wave? It is a supersonic disturbance in the gaseous material of the solar wind. These waves are frequently delivered by coronal mass ejections (CMEs). Indeed, this one might have been a minor CME that left the sun unrecognized earlier this week. Or it might be an unusually sharp co-rotating interaction region (CIR). CIRs are transition zones between slow- and fast-moving streams of solar wind. They contain plasma density gradients that often do a good job of sparking auroras.



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