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.

We observed the ionosphere respond to that. The ionosphere was in fact pulsating at night.

Our response continued because the Earth of course didn't saturate. The Earth is much too large a detector to saturate.