Measuring ionospheric variations near the Hunga Tonga volcanic explosion using VLF/LF lightning waveforms

In this study, we use VLF/LF lightning waveforms observed by Earth Networks Total
Lightning Network (ENTLN) to examine the ionosphere near the Hunga Tonga volcanic
explosion on 15 January 2022. The lightning waveforms used in this study (negative cloud-to-ground
events) consist of a ground wave (which travels to the station along the ground) and an
ionospheric reflection wave (which travels to the station by reflecting off the ionosphere). These
events can be used to probe the ionosphere halfway between the lightning event and the
detecting station, as both the time difference and peak ratio between these two waves depend on
the electron density at the probing location. So far, we have focused on stations in eastern
Australia, particularly AUK06, AUK89, and AUK98, which are in the northeast. We calculated
the time differences and peak ratios for lightning events 400-900 km away from each station.
These parameters can also depend on the distance of the lightning event from the station and
normal daily ionosphere variations, so these factors were accounted for by fitting best fit lines to
distance and local time and subtracting out the trend. We show the ionospheric variation, in
terms of the time differences and peak ratios against both local time and the probing locations’
distances from Hunga Tonga.