Whenever there is a geomagnetic storm that causes changes in the magnetic field of the Earth, or conditions in the so-called magnetotail region behind the Earth. This is often correlated with solar storms, or changes in the density and magnetic fields in the solar wind as it passes the Earth. They rise and fall in frequency with sunspot numbers, and there are maxima in March and September when the Earth in its orbit is in a better location with respect to the Sun to get 'blasted' by solar storms in the mid-latitude zones of the Sun.