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  1. ; ; ; ; ; ; ( , Applied Sciences)
    Leaders of subsequent strokes in negative cloud-to-ground lightning are known to produce X-ray/gamma-ray emissions detectable at distances of a few kilometers or less from the lightning channel. These leaders usually develop in decayed but still warm channels of preceding strokes. We computed electric field waveforms at different points along the path of subsequent leader as those points are traversed by the leader tip. For a typical subsequent leader, the electric field peak is a few MV/m, which is sufficient for production of energetic radiation in a warm (reduced air density) channel. We examined the dependence of electric field peak on the leader model input parameters, including the prospective return-stroke peak current (a proxy for the leader tip potential) and leader propagation speed, and compared model predictions with measurements. 
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  2. ; ; ; ; ; ( , Atmosphere)
    We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 µs) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity. 
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  3. ; ; ; ; ; ; ; ( , Journal of Geophysical Research: Atmospheres)