The excitation of active whistler mode signal paths in the magnetosphere by lightning: Two case studies

In two ∼ 1 hr case study periods, the properties of whistlers propagating along multiple geomagnetic-field aligned paths from points of origin in the northern hemisphere were compared to data on the location and intensity of lightning. In the two studies, which represented times near 0700-0800 LT and relatively quiet magnetospheric conditions, between one quarter and one half of the two-hop whistlers observed at Lake Mistissini were found to have originated in ground flashes detected by the network. The uncorrelated whistlers are believed to have originated in lightning outside the network viewing area or in undetected ground flashes within the network. The authors confirm evidence from other workers that lightning can excite ducted whistler paths whose ionospheric endpoints are at ranges up to 2,500 km or more from the lightning location. Once in the magnetosphere, the ducted waves were found to spread in L value by an interpath coupling process that was operative over the entire L range of detected whistler paths. An approximately linear relation was found between the normalized first stroke peak magnetic field and whistler amplitude observed during a 10-min period. The detection of whistler waves on a particular magnetospheric path and the relative intensity of the waves in successive whistlers appear to be strongly dependent upon the field strength of the impulsive radio signal from lightning at the point of ionospheric wave injection