Determining the size of lightning‐induced electron precipitation patches

Abstract

We analyze Trimpi signatures during 23 and 24 April 1994 at four sites on or near the Antarctic Peninsula (Palmer, Faraday, Rothera, and Halley) on subionospheric VLF signals received from four U.S. naval transmitters (NAA, NSS, NLK, and NPM). Electron precipitation patches are found to be large, i.e., ∼1500 km × 600 km, with the longer axis orientated east‐west. Calculations using a three‐dimensional Born scattering model, where patch densities are 1.5 electrons cm−3 above ambient at the center at ∼84 km altitude, provides results that are consistent with this picture. A high proportion (38%) of the Trimpi events were associated with strong lightning flashes in eastern United States. When lightning discharges had currents >65 kA (positive or negative), there was a >80% chance of seeing an associated Trimpi event. The chance of seeing any Trimpi events fell to near zero for discharges of <45 kA. The largest Trimpi perturbations occur when the center of the precipitation patch is 700–800 km from the receivers. This result is consistent with the modeling calculations for large patches. The equatorward edge of the precipitation patch was estimated to be at ∼60°S, close to the magnetic conjugate of the lightning. The close association of the equatorward edge of the precipitation patch with the conjugate location of the causative lightning is consistent with a quasi‐ducted whistler‐induced precipitation mechanism. Nonducted whistler‐induced precipitation mechanisms would predict a 5°–10° latitudinal gap between the lightning and the equatorward edge of the patch. However, the lack of observed whistlers at the time of the Trimpi events is consistent with the nonducted whistler mechanism and is not consistent with the quasi‐ducted mechanism, although the distances from duct exit point to receiver may have been too large (∼700–1000 km) for the signals to be detectable. Using the significantly larger patch dimensions determined in this study, it is estimated that lightning may well be 10–100 times more effective at depleting the radiation belts than hiss.

Investigating radiation belt losses though numerical modelling of precipitating fluxes

“…4a and 5a resemble those observed in experimental data (e.g. Dowden et al, 2001;Clilverd et al, 2002). For example, if one contrasts the WEP Trimpi presented in the upper panels of Figs.…”

“…In this section we define the process through which we attempt to understand the nature of precipitation fluxes by modelling a specific set of precipitation conditions (transmitter-receiver great circle path) described by Clilverd et al (2002), combing flux spectrum observations, a model of the atmosphere, and a VLF scattering model.…”

“…The WEP-modified ionospheric modification is termed a Lightning Induced Enhancement (LIE) "patch". This position and ionospheric modification size are taken from Clilverd et al (2002), and are determined by observations at multiple Antarctic Peninsula stations. Inside the WEP modified ionospheric region the perturbation to the collision frequency profile is assumed to be zero due to rapid plasma cooling (∼0.1 s at 85 km, and faster for lower altitudes; Rodger et al, 1998), and thus the modification is described solely in terms of perturbations to the electron number density profile dN e (x,y,z), determined in Sect.…”

“…A number of theoretical studies have made use of electron density perturbations with a vertical Gaussian profile to represent the WEP produced ionospheric electron density modification (Nunn and Strangeways, 2000;Clilverd et al, 2002), while others have studied modifications derived from satellite observed WEP fluxes (e.g. Pasko and Inan, 1994;Rodger et al, 2002).…”

Modeling polar ionospheric effects during the October–November 2003 solar proton events

“…However, the drift of the frequency standard crystal at this location makes long-timescale phase comparisons essentially impossible. While Omnipals have been used to investigate short-timescale ionization changes ($100 s) using received amplitude and phase [e.g., Clilverd et al, 2002], long-timescale comparisons require locking levels currently unavailable for this instrument at this location. Upgraded versions of the OmniPAL have been locked to GPS, creating the AbsPAL, which has proved successful for studies undertaken over hours to days [e.g., Thomson et al, 2004].…”