Geomagnetic Storm Tonight But Timing Couldn't Be Worse

Our last G2 moderate storm occurred on March 19-20, 2021 when bright rays and arcs played about the northern sky all night. Bob King

UPDATE7:30 p.m. CDT: The storm's arrival time has been revised. A G1 minor storm should be underway from nightfall until around 10 p.m. Central Time, then develop into a G2 moderate storm from 10 p.m. to 1 a.m. 

I sat up in my chair when I saw the NOAA space weather forecast yesterday. Scanning the April 25th column, the number "6" jumped out. The numbers indicate the Kp index, a measure of how rattled Earth's magnetic field becomes when it's hit by a wave of particles from the sun. The index ranges from no disturbance (0) to severe geomagnetic storm (9). The higher the number the greater the chances of seeing the aurora borealis.

For the northern regions of the northern states, a Kp of 5 often bodes a minor geomagnetic storm and a modest auroral display low in the northern sky. A Kp of 6 takes us to the moderate or G2 geomagnetic storm level, where auroras typically spread across the upper Midwest, Maine, New York state and the upper Plains. 

The latest forecast indicates a G2 storm arriving early Sunday morning, April 25th. If memory serves, it's only the second time this year that forecasters have predicted a G2 storm. While the hour isn't exactly sleep-friendly, ardent aurora watchers will want to set their phone alarms. 

That the good news. Now the bad. The moon will be less than a day from full. Bright moonlight is often an aurora-killer. Minor storms are only visible in the camera, when a time exposure of the northern sky will reveal the aurora's telltale green glow otherwise masked by moonlight.

If a more intense display appears tomorrow morning, it should be visible without a camera though diluted by the moon. Auroras are expected to start dancing around 4 a.m. Central Daylight Time (5 a.m. EDT; 3 a.m. MDT and 2 a.m. PDT) and continue through dawn. The moon will shine off to the southwest in Virgo and set during morning twilight about a half-hour before sunrise. 

A modest flare explodes in the small sunspot group AR 2816 on April 22. NASA's Solar Dynamics Observatory captured the event in these images taken in far ultraviolet (UV) light. NASA / SDO

What sets this aurora apart from nearly all others this year is its cause — a solar flare. Sunspot groups have been mostly small potatoes in recent months, unable to produce the kind of explosions that eject billows of subatomic particles called CMEs (coronal mass ejections). The source of most northern lights displays have instead been coronal holes, gaps in the sun's atmosphere that allow particles from the surface to passively spray into space like water from an unattended hose. 

The flare occurred early April 22nd, with its package of goodies aimed squarely at the Earth. It should arrive Sunday morning. Aurora forecasting is an inexact science. Sometimes these particle blasts arrive earlier or later or have the wrong magnetic configuration and blow right past Earth without setting off auroras. Since clear skies are forecast for my region, I'll be monitoring the storm's progress and update you with the latest.

I'll also be setting my alarm for 3 or 4 a.m. for a peek. You never know.