Lightning-Induced D region Disturbances Detected by Broadband VLF Sferics

Broadband very low frequency (VLF) sensors located near Duke University have being operating with the aim of studying the variability of the midlatitude ionospheric D region. Significant temporal variability of the D region is well known, and we aim to study the time scales and possible sources of that variability in this work. We probe the D region ionosphere by measuring the high power, broadband signals launched by lightning (sferics) and that propagate in the Earth-Ionosphere waveguide. We analyzed nighttime sferics data of July and August, 2005 recorded by our sensors through comparing measured sferic spectra to model results and extracted the two-parameter exponential electron density profile for each measurement. The derived nighttime D region electron density profiles showed large variations on some nights and relatively stable behavior on others. Significant changes were observed on time scales of approximately 1 hour and longer. We explore the nature of this variability to try to understand what fraction originates in direct lightning-ionsosphere coupling and what fraction is driven through other mechanisms.