For my PhD I researched the source, propagation, and effects of lightning in the Earth-ionosphere system. Most of my research is done with the World Wide Lightning Location Network (WWLLN) along with upgrading and expanding the network. While I am writing my thesis the draft is available on GitHub.
My work on WWLLN has been to expand the network through station design, construction, maintenance, and deployment. Part of the work has been a redesign of the heart of the sytem by embedding an ARM computer (Gumstix) to eliminate the need for an external desktop computer. I have also created several new data products for WWLLN, the most widely used is the stroke energy with the latest product, thunderstorm clusters, just beginning distribution to researchers.
Lightning radiates broadband electromagnetic radiation into the Earth-ionosphere waveguide, at large distances (>1000 km) the peak of the radiated energy is in the very low frequency (VLF) band of 6-18 kHz. WWLLN is able to estimate the total VLF energy radiated into the waveguide by using the U.S. Navy Long Wave Propagation Capability code to model the necessary stroke energy required to produce the electric fields seen at the locating WWLLN stations. The details of this process are described in a JTECH paper (code available on GitHub) and it is shown that the energies are found with a global median of near 700 J with an average 22% uncertainty.
The global electric circuit is the system of charging and discharging of the ionosphere through thunderstorms (charging) and fair weather return currents (discharging). Utilizing clustering methods, WWLLN derived thunderstorms give a measure of the thunderstorm current contribution to the global electric circuit, thunderstorms. With the WWLLN archive and high time resolution, both long term activity (months to years) and short term activity (minutes) can be monitored. Future work on the global circuit with WWLLN requires an accurate measurement of the fair weather return current to enable comparisons between source models and the measured current.
I used WWLLN to study the propagation of VLF waves in the Earth-ionosphere waveguide. The measured electric field from lightning at a given stations is normalized by the source energy to measure the attenuation of the signal along the stroke-receiver path. Using various subsets of WWLLN stations show how the attenuation changes with the magnetic azimuth of propagation.
As part of the support work for using WWLLN I performed several analysis of the detection efficiency of the network. One of the first analysis was the creation of a relative detection efficiency model that is described in a Radio Science article, with the code available as well. The model creates hourly, global maps of the network performance under several assumptions about the distribution of lightning energies.
Along with the model I have made direct comparisons of WWLLN to the Earth Networks Total Lightning Network (ENTLN) that were presented at EGU. ENTLN itself needs to be validated before it can be compared to WWLLN, and this is done with the Lightning Imaging Sensor (LIS) onboard the TRMM satellite. The paper discussing the ENTLN-LIS comparison is currently under review.
I developed a linear regression method to examine the difference between oceanic and land lightning. The method was developed to account for the detection efficiency bias present in both WWLLN and ENTLN (different biases). The contrast was observed to occur on spatial scales smaller than the transition between oceanic and continental thunderstorms; implying the chang more local factors.