magnetic storm

In most cases when such a "magnetic storm" occurs, there is an unusually large or active spot on the Sun.

Thus in February 1892, when a very large spot was on the Sun, a violent magnetic storm broke out.

There was a magnetic storm in the photosphere of a sun which was not supposed to have such things.

At first a favorite theory was, that communication had been interrupted by a magnetic storm.

This magnetic storm affected every telegraphic station in the kingdom.