Lightning Processes And Dynamics Of Large Scale Optical Emissions In Long Delayed Sprites

Simultaneous measurements of high altitude optical emissions and the magnetic field produced by sprite-associated lightning discharges enable a close examination of the link between low altitude lightning process and high altitude sprite process. In this work, we report results of the coordinated analysis of high speed (1000--10000 frames per second) sprite video and wideband (0.1 Hz to 30 kHz) magnetic field measurements made simultaneously at the Yucca Ridge Field Station and Duke University during the June through August 2005 campaign period. We investigate the relationship of lightning charge transfer characteristics and long delayed (>30 ms) sprites after the lightning return stroke. These long delayed sprites initiated after a total vertical charge moment change from a few thousand C km to more than ten thousand C km. Continuing currents provide about 50% to 90% of this total charge transfer depending on the sprite delayed time and amplitude of continuing current. Our data also show that intense continuing current bigger than a few kA plays an important role in sprites whose primary optical emissions last unusually long (>30 ms). On one observation night (4 July 2005), a large mesoscale convective system produced many sprites that were part of complex transient luminous event (TLE) sequences that included optical emission elements that appear well after any return stroke and initiate at apparently relatively low altitudes (~ 50 km). These low initiation altitude sprite events are typically associated with intense continuing currents and total charge moment changes of 4000 C km or more. With the estimated lightning source current moment waveform, we also employ a 2-D FDTD model to numerically simulate the electric field at different altitudes and compare it with the breakdown field. This reveals the initiation altitude of those long delayed sprites and the effect of electric field dependence of the electron mobility.