Thunderstorms as Possible HF Radiation Sources of Propagation Teepee Signatures

Propagation teepee is a type of HF spectral feature often recorded at 15-30 MHz by a group of citizen scientists whose main interest is in observing radio emissions from Jupiter. The feature is characterized as spectral enhancements with the frequency of enhancement first increasing and then decreasing with time, resulting in a “triangular spectral feature.” Its shape is reminiscent of teepee tents (or TPs for short), the moveable dwellings of some groups of native-Americans.  TPs usually have sharp or well-defined upper frequency limits for both the leading and trailing edges (see figure). While some TPs are observed in isolation, they are often seen in groups, distributed either in time or in apex frequency as a nested group at a particular time. As reported by Fung et al. [2020], most TPs appear to be diffuse even at high time resolution, but a few TPs seen at high time resolution reveal that those TPs consist actually of discrete bursts, strongly suggestive that the band noise could be produced by lightning storms. TP signatures are thus believed to be HF signals produced by remote lightning storms and reflected by the bottom-side ionosphere. By analyzing a few events with TP signatures detected simultaneously by multiple spectrograph stations, we will use a relationship between the TP apex frequency and the distance to its radiation source to identify the lightning storms responsible for the observed TP signatures.