# 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 '20 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 '20 at 9:29
• @uhoh this might be spot on viviss.si/download/viviss/ZBORNIK%20MGB/… somebody might make an answer out of it. Feb 25 '20 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 '20 at 0:06

You can definitely see a large geomagnetic storm with a compass, if you have the timing to catch one and the patience to sit and stare for a few minutes.

If you look at these minutely measurements from Lerwick observatory for one of the more recent large storms (the 2003 Halloween storm), you can see a roughly 4.5 degree swing in declination (labelled "D", the horizontal magnetic field angle to geographic North) over about 15 minutes at about 07:00. "I" is inclination, vertical field angle to the horizontal plane, "F" is total intensity of the field.

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.

It will depend on where you are as to how big this change in the magnetic field is, and how big the storm is. For this storm, the UK geomagnetic observatory network gives a good idea of how the effect seen depends on where you are.

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.

• This is excellent, exactly what I was hoping for. Thanks! I hope you don't mind that I added the plot and a short description of the deflection. Feel free to edit further or roll back.
– uhoh
Feb 29 '20 at 1:41
• Thanks for the good edit, I've also added an image I couldn't find at first, so you can see how the compass variation can change with location.
– WJB
Mar 2 '20 at 11:21