LF and MF observations of the lightning electromagnetic pulse at ionospheric altitudes



For the first time the full ionospheric signature of the lightning electromagnetic pulse (EMP) was measured up to a frequency of 2 MHz. At altitudes below 225 km, the upward-going whistler wave is found to have a nose-whistler wave shape with the fastest propagating frequency (nose frequency) near 80 kHz. The bulk of the EMP energy is at the nose frequency, and there is a sharp upper limit near 175 kHz where the group delay is very long. We believe the group delay is due to a propagation resonance for the whistler mode (slightly non-longitudinal propagation) associated with the low plasma frequencies in the F-layer valley. MF emissions were seen below the F peak, but not above. These results verify earlier speculation that the leading intense edge of the lightning EMP was carried by 50–125 kHz waves. In addition, we present tantalizing evidence for detection of a pulse pair prior to the stroke that is similar to transionospheric pulse pairs (TIPPs) detected in satellite data.