# Does a geomagnetic storm visibly deflect a compass?

Reading Solar Storms Could Confuse Whales and Cause Mass Strandings and the linked open access paper in Current Biology Gray whales strand more often on days with increased levels of atmospheric radio-frequency noise I see that there is a significant correlation between the strandings and certain kinds of electromagnetic disturbances, but not a significant correlation with perturbations of the Earth's "DC" or static geomagnetic field alone.

That led me to wonder if the direction of the magnetic field that I would see on a compass or some instrument that averages over seconds or longer would show any significant deflection during a Geomagnetic storm.

Can there be deflections of say 1 degree or more? Or would I have to use a microscope or put a small mirror on the needle and bounce a laser pointer off of it and on to a wall to see the deflection?

• This is probably going to depend on where in the world you are. – gerrit Feb 25 at 8:52
• @gerrit excellent, I'll take anyplace on earth and the worst known storm, and even a special compass that handles fields near the poles that have little horizontal component. outdoors.stackexchange.com/q/21360/12619 – uhoh Feb 25 at 9:29
• @uhoh this might be spot on viviss.si/download/viviss/ZBORNIK%20MGB/… somebody might make an answer out of it. – trond hansen Feb 25 at 19:06
• @trondhansen Thank you! The article is quite enticing but I think it will be necessary to drill down into the references before we can know if there can "...be deflections of say 1 degree or more?" If I'd asked for 0.1 degree then obviously yes, and if I'd asked for 0.5 degree then "probably", but for 1 degree (0.017 rad) we'll have to get some actual $\mathbf{H}$ and $\Delta \mathbf{H}$ vectors and calculate a deflection of some kind. – uhoh Feb 26 at 0:06

Lerwick (LER) based on 1-minute definitive data 2003-10-29 shows plots for D and I in degrees and F in nT. D shows a dip to -2 degrees in the hour before 07:00 UT and then a sudden rise to about +1.5 degrees at about 07:00 with spikes to +2 degrees.
Compass angles (positive West, negative East) measured at BGS Lerwick, Eskdalemuir and Hartland observatories during 2003-10-29 and 2003-10-30. The the biggest change is seen at Lerwick, a smaller change is seen at Eskdalemuir further to the south (further away from the auroral zone where the field changes are occurring), and a smaller change still at Hartland.