Investigating the possible association between thunderclouds and plasmaspheric ducts

It has been suggested that quasi-electrostatic electric fields from thunderclouds might produce whistler ducts by driving plasma interchange of geomagnetic flux tubes. Using lightning activity as a proxy for the presence of electrified clouds, the relation between thunderclouds and duct number was investigated through the study of two periods of 10 days, that is from June 13–22, 1993, and August 6–15, 1993. Typical duct numbers observed using NAA transmissions (24.0 kHz, Cutler, Maine) received at Faraday during the nights of June and August 1993 were ∼ 7, while the maximum duct numbers were 9–12. Duct numbers dropped down to zero or near zero during the day. Times of increased duct number could not be clearly associated with increased thunderstorm activity in the duct footprint region. Duct numbers observed on the path from NLK (24.8 kHz, Seattle, Washington) to Dunedin, New Zealand, were also studied, where very low thunderstorm activity levels imply that thundercloud-driven duct formation should not be significant. The typical nighttime peak was ∼ 5 at about 1000 UT (23 LT). The difference between duct numbers in areas with high thunderstorm activity and quiet areas might be thought to be due to thundercloud duct formation. However, the F region absorption on the NLK footprints at 1000UT (23 LT) in June was found to be equivalent to 0000 UT (20 LT) in June on the NAA duct footprints. The typical number of ducts observable on the NAA path at 0000 UT in June is ≈ 5, very similar to the number observed on NLK at 0930 UT. The results found in this study are consistent with the idea that ionospheric absorption is a significant influence on the number of ducts observed. It does not appear that quasi-electronstatic fields from thunderclouds play a significant role in the formation of plasmaspheric whistler ducts. The similarity in quiet time midlatitude duct nuumbers at these two different locations suggest a globally consistent duct formation mechanism, such as the interchange of magnetospheric tubes of plasma induced by spatially varying electric fields in the E region.

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