RF radiation observations of positive cloud-to-ground flashes

During the summers of 1995 and 1996 we conducted broadband HF-UHF and narro wband VHF radio frequency (RF) observations of positive cloud-to-ground (+C G) flashes at Langmuir and Los Alamos laboratories, New Mexico. These obser vations indicate that positive leaders to ground produce no or very weak ra diation from KF to UHF. The broadband system was able to record 2 ms data e ach time it was triggered. For a I-CG the system was usually triggered by t he return stroke, and a 1 ms pretrigger period was coincident with the posi tive leader process. It was commonly observed that no or little radiation w as associated with the leader process in the 1 ms pretrigger period. The na rrowband VHF system employed a logarithmic power amplifier and recorded one 1 ms data each time it was triggered. The narrowband observations show tha t strong and often continuous radiation occurs at the beginning of the +CGs , but the radiation usually becomes intermittent and impulsive during the l ast few tens of milliseconds preceding the return strokes. The observations for most of the +CGs also show complete lack of radiation a few ms before the beginning of the return strokes, suggesting that the ongoing downward p ositive leaders were quiet at VHF, at least during the final few ms. The re sults of this study for natural positive leaders are in agreement with the results obtained from laboratory gap discharges and rocket-triggered lightn ing.