Professor Umran S. Inan

Winter 1998-99

Date: Wednesday, February 24, 1999
Time: 4:15-5:30 PM; Refreshments at 4:00 PM
Location: GESB 124

Coordinated FORTE satellite VHF and National Lightning Detection Network study of lightning

Dr. Abram R. Jacobson
Los Alamos National Laboratory


This work compares contemporaneous observations of lightning from two highly complementary systems. FORTE is a low-Earth-orbit satellite carrying radio-wave and optical instruments for the study of lightning. The radio receivers aboard FORTE observe very-high-frequency (VHF) emissions from from the air-breakdown preceding large-scale current flow. Only VHF (and higher) emissions from lightning can reliably penetrate the ionosphere to a satellite, especially along grazing-incidence paths. The National Lightning Detection Network (NLDN) is a ground-based array of sensors in the continental United States (CONUS) observing the low-frequency (LF) and very-low-frequency (VLF) radiation from the large-scale currents. Prior to the launch of FORTE in 1997, essentially no work had been done on the statistical correlations between (a) ground-based LF/VLF and (b) spaced-based VHF remote sensing of lightning. During April - September 1998, FORTE was tasked with taking maximum triggered VHF data over and near the CONUS, and NLDN data was specially post-processed in a loosened-criterion mode that provided enhanced detection range beyond the borders of the United States. The time history of reported events from the two systems was compared, and event pairs (one VHF event from FORTE, the other LF/VLF event from NLDN) that were candidate correlations (closer than 200 millisec from each other) were scrutinized to determine if there was a statistically meaningful timing relationship. We have found that there is a statistically significant correlation , consisting of a prompt coincidence between a subset of NLDN events and a subset of FORTE events. This coincidence is most likely to occur for intracloud, and less likely to occur for cloud-to-ground discharges. The prompt coincidences mostly are within 50 microsec, after correction for the propagation of the VHF signal to FORTE from the NLDN-geolocated discharge. The NLDN-furnished geolocation of the promptly-coincident FORTE-observed VHF pulses allows the pulses to be better interpreted. In particular, we can deduce, from the lag of the VHF ground-reflection echo, the height of the VHF emission region in the storm.