Ground-based photometric imaging of lightning EMP-induced transient luminous events [electronic resource]
Keywords: Electrical Engineering

With around 2000 thunderstorms active on the planet's surface at any given time, lightning is one of Earth's more prevalent natural phenomena. Each lightning return stroke radiates a wideband electromagnetic pulse (EMP), and it has long been known that fields radiated by lightning return strokes have far-reaching effects. In the Earth-ionosphere waveguide, lightning-radiated fields can propagate efficiently to great distances, being detectable at ranges in excess of several thousands of kilometers. Additionally, they can propagate through the magnetized plasma of the ionosphere and enter the magnetosphere as whistler-mode waves where they can interact with geomagnetically trapped charged particles in the Earth's radiation belts. In 1989, an entirely new class of lightning return stroke field effects was discovered in the form of large and brilliant but brief lightning-associated optical flashes in the upper atmosphere, collectively known as transient luminous events (TLEs). Elves, the most abundant kind of TLE, are rapidly expanding rings of light produced by lightning EMP-heating of the lower ionosphere. Centered above their parent lightning return strokes at 85--90 km altitudes, elves can expand to diameters of several hundreds of kilometers on sub-millisecond timescales. However, their very short lifetimes make elves difficult to observe, and most in-the-field studies of elves have featured instruments requiring manual triggering that allow for detailed study of captured events but necessarily involve high rates of missed detections. In this work, we present three years of elve observations made by a new free-running (non-triggered), ground-based, high-speed photometric imaging instrument called PIPER. This instrument is unique among ground-based instruments in that it does not require triggering and can observe nearly all elve activity within its field of view as it tracks a storm across the horizon over its several-hour lifetime. PIPER is a multi-wavelength, 64-anode photometer array composed of two horizontally-oriented and two vertically-oriented 16-anode photometer arrays. With a sampling rate of 25 kHz, the array provides ample time-resolution for resolving elves and adequate spatial resolution for discriminating elves from other transient optical phenomena (sprites, cloud flashes, meteors, etc.). We develop an algebraic technique for reconstructing the geometry of a particular elve from its photometric array observation. We then present aggregate observations of elves from four different multi-week summer observation campaigns and investigate many features of bulk elve activity that have not hitherto been possible to investigate with previous data sets. These features include peak storm-time elve production rates, the storm-to-storm and within-storm variability in elve production rates, elve production probability dependence on lightning return stroke parameters and local time of night, and distributions of elve geometric parameters. We also present observations of an unusual (and very infrequently reported) category of elves we call "elve doublets" and interpret their causative mechanism in terms of the EMP radiated from compact intracloud discharges.