In-flight measurements of energetic radiation from lightning and thunderclouds

In the certification procedure for new aircraft, manufacturers carry out so-called icing test flights, where the altitude at which the temperature reaches zero degrees Celsius is deliberately sought and crossed in or under thunderstorms. Airbus also used these flights to test ILDAS, a system aimed at determining the severity and attachment points of lightning during flight from high-speed data on the electric and magnetic fields at the aircraft’s surface. We used this unique opportunity to enhance the ILDAS systems with two x-ray detectors coupled to high-speed data recorders in an attempt to determine the x-rays produced by lightning in situ, with synchronous determination of the lightning current distribution and electric field at the aircraft. Such data are of interest in a study of lightning physics. In addition, the data may provide clues to the x-ray dose for personnel and equipment during flights. The icing campaign ran in April 2014; in six flights we collected data from 61 lightning strikes on an Airbus test aircraft. In this communication we briefly describe ILDAS and present selected results on three strikes, two aircraft-initiated and one intercepted. Most of the x-rays have been observed synchronously with initiating negative leader steps, and as bursts immediately preceding the current of the recoil process. Those processes include the return stroke. The bursts last one to four microseconds and attain x-ray energies up to 10 MeV. The intensity and spectral distribution of the x-rays and their association with the current distribution are discussed. ILDAS also continuously records x-rays at low resolution in time and amplitude.