Determining the source of strong LF/VLF TIPP events: Implications for association with NPBPs and NNBPs

Transionospheric pulse pairs (TIPPs) as detected by the Blackbeard VHF radi o instrument onboard the ALEXIS satellite have been shown to correlate with pulses detected by stations in the National Lightning Detection Network (N LDN). The short peak-to-zero time of these NLDN-detected pulses (<10 mu s) are indicative of interior cloud processes, as opposed to the longer pulses associated with cloud-to-ground discharges. TIPPs are most probably genera ted by the same discharge responsible for narrow bipolar pulses (NBPs), whi ch have been detected on the ground but are also believed to be generated e ntirely inside the cloud. Here we report on five TIPPs detected by Blackbea rd that are correlated with cloud-generated pulses detected by multiple sta tions in the NLDN during powerful storms. Previously, a maximum of two stat ions at a time showed such a correlation. Given the greater area of detecti on, these pulses radiate more powerfully in the LF/VLF frequencies than tho se previously detected as TIPPs by Blackbeard. We are able to place the sou rces of the TIPPs in Hurricane Fausto off the coast of Mexico, and in a rem nant of the same storm eight days later over Texas. The heights of the sour ces are higher than those previously determined for TIPPs. The higher altit udes and the greater power of the TIPP-correlating signals may be related t o the intensity of the storms. Two of these five TIPPs were also correlated with the ground detection of HF signals, and the TIPPs and HF signals were determined to originate from the same source region. These HF signals are similar to those previously recorded with the field change signature of nar row positive bipolar pulses (NPBPs), but we find that both polarities are d etected, four out of the five being negative. This indicates that TIPPs cor relate with narrow negative bipolar pulses (NNBPs) as well as NPBPs. The de tection of NNBPs, which has previously been much rarer than NPBPs, may also be related to the intensity of the storms and the resultant higher-altitud e sources.