Propagation Teepee: A High Frequency (HF) Radio Spectral Feature Identified by Citizen Scientists

We report on the observations of a high frequency (HF) spectral feature that appears often in ground‐based spectral data at 15‐30 MHz.The feature, likely of terrestrial origin, is often recorded by a group of amateur radio astronomers, the Spectrograph User Group (SUG), whose main interest is in observing radio emissions from Jupiter. The feature appears 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. While some TPs are observed in isolation, they are often seen in groups, distributed either in time or in frequency as a nested group at a particular time. 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 produced from lightning as possible radiation sources of the TPs. In this paper, we investigate the possible generation of TPs as a result of ionospheric reflection of band noise produced by remote lightning storms.