Effects of M-Components on Needle Activity in Positive Cloud-To-Ground Lightning Flashes

High-speed video and electric field records of two positive cloud-to-ground (+CG) flashes were used to examine the effect of M-components on needle activity after the return stroke onset. We observed enhancements of needle activity that were associated with the occurrence of M-components identified by channel luminosity enhancements both at cloud altitudes and near the ground. Full-fledged M-components enhance needle activity via injection of negative charge into the bottom of grounded channel and reversing the direction of the radial electric field at the channel core, similar to +CG return strokes. Attempted M-components, identified by channel luminosity enhancements at the cloud but not near the ground, did not enhance needle activity because of the absence of significant reflection from the ground, which causes electric field reversal at the core.