INTERACTION WITH THE LOWER IONOSPHERE OF ELECTROMAGNETIC PULSES FROM LIGHTNING - HEATING, ATTACHMENT, AND IONIZATION

A Boltzmann formulation of the electron distribution function and Maxw ell's equations for the electromagnetic (EM) fields are used to simula te the interaction of lightning radiated EM pulses with the lower iono sphere. Ionization and dissociative attachment induced by the heated e lectrons cause significant changes in the local electron density (N(e) ). Due to 'slow' field changes of typical lightning EM pulses over tim e scales of tens of mus, the distribution function follows the quasi-e quilibrium solution of the Boltzmann equation in the altitude range of interest (70 to 100 km). The EM pulse is simulated as a planar 100 mu s long single period oscillation of a 10 kHz wave injected at 70 km. U nder nighttime conditions, individual pulses of intensity 10-20 V/m (n ormalized to 100 km horizontal distance) produce changes in N(e) of 1- 30% while a sequence of pulses leads to strong modification of N(e) at altitudes <95 km. The N(e) changes produce a 'sharpening' of the lowe r ionospheric boundary by causing a reduction in electron density at 7 5-85 km (due to attachment) and a substantial increase at 85-95 km (du e to ionization) (e.g., the scale height decreases by a factor of appr oximately 2 at approximately 85 km for a single 20 V/m EM pulse). No s ubstantial N(e) changes occur during daytime.