Investigating Sub-visible Lightning with the Long Wavelength Array

Today, one of the more reliable and accurate total lightning detection networks is the Lightning Mapping Array (LMA). However, in a recent study about one-third of lightning was not detected by the LMA during the DC-3 aircraft field campaign (Brune 2021). The missing lightning was termed “sub-visible lightning”, and was identified by chemical constituents sampled by the campaign that act as lightning indicators. This study investigates whether sub-visible flashes are present in thunderstorms. To get more sensitive measurements than the LMA, we used the Long Wavelength Array (LWA), which is a VHF radio telescope located in New Mexico. The LWA is capable of observing signals as low as -53 dBW/MHz, which is 20 dB more sensitive than the most sensitive LMA deployments. It has 256 VHF antennas with 40 MHz of observing bandwidth at two locations, LWA-SV and LWA-1. All 256 LWA antennas were combined into a single beam to measure radio waves in 10 ms increments continuously for 12-hour periods. The beam is steered to observe directly overhead, and the large number of antennas enables us to detect signals below the galactic background noise level. We will present a detailed comparison between LMA and LWA observations to identify how much lightning the LMA is seeing. Most LMA flashes aligned well with LWA signals, but there are emissions the LMA missed. In addition, the LWA observed up to minute-long VHF glows with no corresponding LMA sources, many which correspond to solar radio bursts identified by LWA Orville imagery.