HF signatures of powerful lightning recorded on DEMETER

Abstract

Emissions in the HF range (1.7 – 3 MHz) are observed at the times of powerful lightning on the low‐altitude satellite DEMETER. At ∼700 km, wave activity observed on the E field spectrograms recorded on DEMETER during nighttime is mainly dominated by upgoing 0+ whistlers and higher‐order dispersed whistlers. Over a period of 30 months, 130 events with HF emissions at frequency ∼2 MHz have been observed at the time of intense 0+ whistlers on VLF spectrograms. A global map of the distribution of events indicates that they do not occur above regions of most thunderstorm activity such as the upper part of South America or the middle of Africa. This lack of occurrence above these two near‐equatorial regions is consistent with the high value of the critical frequency of the F layer which prevents the transionospheric propagation of the HF component of the lightning pulses up to 700 km. The time, location, intensity, and polarity of the lightning discharges related to a subset of these HF events are determined above the North American region with the National Lightning Detection Network. As most of the events are recorded in Survey Mode when full resolution of the data is not available, the neural network on board DEMETER is used to determine the times of the 0+ whistlers recorded by the satellite with a time accuracy of ∼0.1 s. The impulsive HF events correspond to intense lightning discharges occurring in regions immediately below the satellite. It is assumed that the propagation of these HF pulses up to the altitude of the satellite is favored by local ionospheric heating due to high thunderstorm activities.