Phenomenology of transionospheric pulse pairs: Further observations

We report on further observations of transionospheric pulse pairs (TIPPs), which are the most powerful transient radio signals observed by the Blackbe ard broadband digital radio receiver on the ALEXIS satellite. The source of these signals is unknown but appears to be associated with thunderstorm ac tivity. The signals do not resemble those reported for known lightning proc esses. We have previously reported observations of these events in the freq uency band 28-95 MHz. In this paper we report observations of TIPPs in the 117- to 166-MHz band, with the subsatellite point situated over the contigu ous United States. The main results are that the measured pulse parameter s tatistics are nearly the same as reported for the low-frequency events, wit h the exception that the pulse separation distribution is biased toward sma ller values in the high-frequency observations. The radiated power does not drop off appreciably even at 166 MHz, which further constrains the possibl e size and timescale of the source(s). We also report results of experiment s designed to measure the apparent reflectivity of dry, flat ground at freq uencies around 100 MHz. We find that the apparent reflectivity can exceed 9 0%. This result helps to explain how the second pulse in a TIPP can have so much energy relative to the first.