Radiation belt electron precipitation fluxes associated with lightning

Abstract

[1] In this study we consider the dependence of precipitation fluxes arising from whistler-induced radiation belt losses on the strength of the associated lightning's return stroke current. As a result of this work, it will be possible to use lightning activity data sets to estimate globally induced precipitation flux rates and thus determine the lightning-induced effect on the radiation belts more accurately. Four study days were selected, during which a high proportion of the lightning activity occurring near the east coast of North America produced observable Trimpi effects on VLF transmitter signals propagating in the region of the Antarctic Peninsula ($L = 2-2.5). The lack of lightning in Antarctica gives this location a unique advantage for this study. The functional dependence of the relative scattered field amplitude with the return stroke peak current of the lightning discharge suggests that during these events diffusion conditions are occurring near the precipitating radiation belt particles loss cone due to strong whistler wave fields, probably caused by ducted signals. The range of observed Trimpi scatter amplitude of À10 to À35 dB was produced by precipitation bursts with energy fluxes estimated to range over 6.5-0.4 Â 10 À3 ergs cm À2 s À1 . The largest fluxes were found to be driven by lightning currents of about 250 kA, while the smallest detectable fluxes relate to lightning currents of 70 kA. Although 3 of the 4 study days showed a high degree of consistency between the levels of lightning return stroke peak current required to produce any given perturbation scatter amplitude value, conditions were significantly different on 23 April 1994. On this day, observed Trimpi signatures were 6-7 dB greater for any given lightning intensity than on the other study days. These events are consistent with a significantly harder radiation belt precipitation spectra, probably caused by geomagnetic storm-time acceleration processes of radiation belt electrons.

Lightning-driven inner radiation belt energy deposition into the atmosphere: implications for ionisation-levels and neutral chemistry

“…Intense geomagnetic disturbances can inject energetic electrons into the slot and inner radiation belt, leading to a harder energy spectrum for the trapped population. In these cases the WEP energy spectra will also harden, as has been experimentally observed (Inan et al, 1989;Clilverd et al, 2004). As injections into the inner radiation belt are quite rare, we are able to assume that the "quiet" WEP energy spectra are more indicative of typical conditions.…”

“…Building on this modelling, the dependence of WEP precipitation fluxes on the strength of the associated lightning's return stroke peak current has been identified (Clilverd et al, 2004). In previous studies it has been shown that lightning return stroke current strengths have an approximately linear relationship with the received whistler field strength, and whistler strengths with the size of Trimpi perturbations (Carpenter and LaBelle, 1982;Carpenter and Orville, 1989), from which we would expect a relation between Trimpi scatter amplitude and lightning currents.…”

“…1 are combined with WEP energy spectra, in both cases calculated following the approach outlined in Rodger et al (2003Rodger et al ( , 2004b. The parameters of the WEP burst were selected by comparison with the calculations of Faraday Trimpi produced by WEP with a mean precipitation energy flux at L=2.23 of 2×10 −3 ergs cm −2 s −1 (Rodger et al, 2004b), specifically the "normal" Faraday Trimpi spectral conditions (Clilverd et al, 2004). Following the approach outlined in the Rodger et al (2003Rodger et al ( , 2004bRodger et al ( , 2005 studies, this starting point is used to derive global variations.…”

“…Satellite observations indicate that the global annual average total lightning flash rate is 44±5 flashes per second (Christian et al, 2003), while Clilverd et al (2004) found that the most significant WEP bursts were due to lightning with return stroke peak currents of I ≥70 kA, which includes ∼14% of all cloud to ground lightning. As such the global annual average rate for significant WEP bursts should be ∼1.5 s −1 , with all remaining lightning flashes producing a much higher rate of WEP bursts with smaller intensities.…”

“…In previous studies it has been shown that lightning return stroke current strengths have an approximately linear relationship with the received whistler field strength, and whistler strengths with the size of Trimpi perturbations (Carpenter and LaBelle, 1982;Carpenter and Orville, 1989), from which we would expect a relation between Trimpi scatter amplitude and lightning currents. Clilverd et al (2004) considered four study days during which a high proportion of the lightning activity occurring near the east coast of N. America produced observable Trimpi effects on VLF transmitter signals propagating in the region of the Antarctic Peninsula (∼L=2-2.5). The dependence of the scattered field amplitude on the return stroke peak current of the lightning discharge suggests that during these events diffusion conditions are occurring near the loss cone of the precipitating radiation belt particles due to strong whistler wave fields, probably caused by ducted signals.…”