Speaker
Description
How lightning starts is still an open question in the field of lightning physics. The in-situ electric fields observed within thunderstorms from decades of balloon measurements are far too low for both the breakdown of virgin air or what is known as the the streamer-to-leader transition. Past observations have implicated fast breakdown (FB) as a method by which lightning does initiate, however this is only witnessed in a subset of the particularity powerful thunderstorms. LOFAR has uncovered two new initiation mechanisms, one that shares similar features with FB and a second dubbed ultra-slow-propagation that is totally new and unexpected. The events that share features with FB have compatible e-folding lengths to previous observations, but very wildly in length, duration, and altitude at which they take place. Additionally, these discharges have a exponential growth in intensity. This is suggestive of a similar mechanism to FB, however, what is especially surprising is that in spite of their varying properties they all have a speed of roughly of 5.0 * 10^6 m/s. This suggests a steady state process, which is not typically seen as compatible with exponential growth. Ultra slowly propagating discharges contain bursts which individually propagate at speeds on the order of 10^6 m/s for up to a millisecond, but collectively these bursts follow a trajectory which only travels speeds on the order of 1-10 km/s. Furthermore, these bursts tend to repeat on timescales of 4 us, but also have periods of inactivity of tens to hundreds of milliseconds while still happening within the same region of space when activity is again visible.
Within this work we will present updated results on observations of lightning initiation, which is in part due to the recent refinement of the LOFAR lightning beamforming into the Adaptive Time Resolved Interferometric 3D (ATRI-D) imager. These techniques, developed by the LOFAR lightning group, leverage the dual polarity of the Low-Band Antennas (LBA) to produce observations with enhanced detail of initiation events as seen by the LOFAR array, and as such provide further clues on precisely how lightning starts. These refinements provide a means to test previously developed hypotheses of lightning initiation. LOFAR, due to its long baselines, high timing precision, and large number of antennas, has immense capability and potential as an instrument for studying lightning initiation, and as such is currently the best instrument in the world for these reasons.
