Triggered Negative Lightning-Leaders That Propagated Into Thunderstorm Lower Positive Charge

When the electric field below a thunderstorm or other electrified cloud is around 10 kV/m, it is sometimes possible to initiate ("trigger") an upward-propagating lightning-leader by launching a rocket that uncoils a wire from the ground. The triggered leader propagates upward from the tip of the wire lifted by the rocket. When the channel is hot enough, a flash is visible. Triggering is common when the leader carries positive charge, but not when it carries negative charge. This article is about four flashes consisting of triggered negative leaders that branched into low-altitude regions of positive cloud charge over Langmuir Laboratory in central New Mexico. Measurements of current and the locations of leader channels are available for three of the four flashes. Some current pulses at the ground for Flash 2 originated at negative leader steps more than 3 km away, which is a greater distance than has been reported from video measurements. Flashes 3 and 4 propagated only into thunderstorm lower positive charge, and the average lightning-charge densities inside the volumes occupied by these two flashes are remarkably close. Our best estimate of density for Flashes 3 and 4 lies between -4.2 and -1.8 C/km(3), which is compatible with the large spread in cloud-charge densities derived from instruments carried on airplanes or balloons into low positive regions in thunderstorms.