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"dalek" writes:

NOAA's Space Weather Prediction Center (SWPC) has issued a rare G4 watch for incoming coronal mass ejections (CMEs) that are expected to [noaa.gov]arrive as early as 18-21 UTC on Friday [noaa.gov]. NOAA's scale for geomagnetic storms [noaa.gov] is based on the planetary K-index [noaa.gov], and goes as high as G5. The K-index is a measure of horizontal disturbances in Earth's magnetic field caused by the interaction of the CME with Earth's magnetosphere, and is estimated from observations collected by many ground-based magnetometers [wikipedia.org].

Although G4 conditions occurred as recently as March 23 of this year, the SWPC has not forecasted G4 conditions since January 2005. The most recent time G5 conditions were reached was during the 2003 Halloween solar storms [wikipedia.org]. In a G4 geomagnetic storm, auroras may be visible at geomagnetic latitudes as low as 45°.

A large cluster of sunspots ejected several CMEs which have merged during their approach to Earth. The incoming geomagnetic storm is currently forecasted to be most severe from 03-12 UTC on Saturday, with the highest planetary K-index expected to be 8.33. Another CME is expected to start impacting Earth around 15 UTC on Saturday, with the geomagnetic storm peaking at G2 conditions between 03-06 UTC on Sunday.

If you'd like to monitor geomagnetic disturbances, there are guides for DIY projects (1 [hackaday.com], 2 [hackaday.com], and 3 [hackaday.io]) where you can construct your own magnetometer capable of measuring nanoTesla-scale variations in the magnetic field to monitor for auroras.

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