Experimental evidence of D region heating by lightning‐induced electromagnetic pulses on MF radio links

Abstract

Broadband medium frequency (MF) measurements are used to study perturbations of the lower ionosphere related to lightning activity. A station measuring the vertical electric field in the MF band (500–1500 kHz) was set up in France in 2004. Just after the lightning return stroke, a strong attenuation is very frequently observed in the MF waves of radio transmissions. This perturbation effect is studied in each case of about 4000 return strokes and at nine radio frequencies. The main characteristics of the perturbation are a mean peak attenuation of 12 dB, an onset time of less than 1 ms, and a duration varying from 3 to 10 ms. Peak attenuations and durations are roughly proportional to the lightning‐induced peak current. Perturbations are characterized by a circular structure with a radius reaching 250 km, which is centered on the return stroke location. The recovery time of the electromagnetic pulse (EMP) induced electron density is too long compared with the duration of the observed perturbation. However, we show that the origin of these MF wave perturbations, on timescales involved in the heating produced, can be explained by a lightning‐induced electric field at 90 km altitude. The observed attenuation is compatible with a heating of 4–14 eV of the lower ionospheric electrons produced by the lightning electric field. These perturbations are then the MF radio signature of an EMP effect on the lower ionosphere, in the same way as elves correspond to their optical signature.