The remarkable thing is that it took an hour for it to come back from this disturbance. It's a very unusual event and was three orders of magnitude more intense than the 1998 one, which we thought was very intense.
We observed the ionosphere respond to that. The ionosphere was in fact pulsating at night.
At higher altitudes, there isn't enough air for ionized molecules to combine and become neutral again, so the region stays ionized. That's what the ionosphere is.
This region has been called the 'ignore-o-sphere' because it isn't an easy region to measure. The VLF technique that we have developed is particularly suitable for looking at this altitude range, which is not otherwise measurable.