Around 8 p.m. Eastern Time on Thursday night, an interplanetary shock wave slammed into Earth at supersonic speed. The density of the solar wind quadrupled and cracked open Earth’s magnetic field. Planetary K-Index skyrocketed. Bright, colorful aurora, visible to the naked eye, lit up the night sky in both hemispheres. It took until late morning Friday for the solar wind density to return to normal.
It’s not as scary as it sounds, but it is fascinating.
An interplanetary shock wave is a disturbance in the solar wind that is always flowing outward from the sun through our solar system. Imagine that the solar wind is like a fluid flowing from the sun to Earth — sometimes it flows slowly, and sometimes it is fast. There can also be waves of high-density fluid within the flow that break on our planet just as ocean waves would break on a sea wall.
Earth’s magnetic field protects us from solar wind radiation, but sudden solar events — solar flares, coronal mass ejections and interplanetary shock waves — can strain the magnetic field to the point that it ruptures, allowing some of the energy into our atmosphere. Fortunately, once the stress of the solar event is over, the magnetic field readjusts to its normal, protective state.
It’s possible Thursday night’s shock wave came from a coronal mass ejection that went undetected by satellites. Satellites are good at catching changes in the solar wind, but sometimes things are missed: The space between Earth and our sun is pretty big, and minor events can go undetected.
Another explanation may have been that it was just a very sharp transition between slow- and fast-moving solar winds, like turning a hose up from a trickle to a hard spray. If it happens quickly enough, the change can have the same impact as a coronal mass ejection.
Whatever the cause, the result here on Earth was a minor geomagnetic storm — not strong enough to disrupt GPS, communications or satellites in any meaningful way. It did, however, create a colorful aurora that was seen as far south as Illinois. At the higher latitudes in Canada and New Zealand, the aurora was visible to the naked eye.