Satellite Observations of Lightning-Induced Electron Precipitation


Useful Links

PIPER - A photometer designed and built by students to image LEP and Transient Luminous Events (TLEs).

TIPER - Transmitter-induced precipitation of electron radiation.

LEP - A look at experiments to detect LEPs from the ground.

HAIL - Holographic Array for Ionospheric Lightning.

Introduction

Lightning discharges are well known sources of electromagnetic radiation in the frequency range of a few kHz to many MHz, with the most intense radiation typically being in the range of $5-10$ kHz (Rakov and Uman, 2003, p. 6). Electromagnetic waves originating in lightning discharges often propagate through the ionized highest regions of the Earths atmosphere into the radiation belts. High energy electrons in this region constitute a hazard to the increasing number of scientific and commercial spacecraft that orbit the Earth, and quantitative understanding of this radiation and its sources and loss are accordingly important. Observations have indicated that electromagnetic whistler waves injected by lightning discharges can scatter the energetic electrons and cause them to precipitate out of the radiation belts. Individual cases (localized in space/time) of lightning-induced electron precipitation (LEP) have been observed, but the degree to which waves from lightning induce loss of trapped radiation on a global scale is not known (Abel and Thorne, 1998).

Figure 1: Cartoon illustrating propagation of electromagnetic waves (red) away from a lightning flash through the Earth-ionosphere waveguide. A portion of this energy leaks through the ionosphere and couples into the magnetosphere. (Peter, 2007)
Image ionosphere

The mechanism of LEP events is as follows: individual lightning discharges launch whistler-mode wave packets that undergo cyclotron-resonant interaction with trapped electrons. These interactions cause pitch-angle scattering of the electrons, sometimes by as much as 1 degree (Inan et al., 1989), and can move electrons just above the loss-cone edge into the loss cone. Subionospheric VLF measurements of LEP indicate large regions affected in individual bursts, with tens to hundreds of events observed from isolated storms (Peter and Inan, 2004). Subsequent interactions with lightning-generated whistler waves can result in scattering by multiple individual discharges within a storm or series of storms.

Figure 2: Cartoon illustrating wave-particle interactions.
Image wpi

Observations of LEP

Transient bursts of lightning-induced electron precipitation (LEP) are observed directly on satellites (Voss et al., 1984; Voss et al., 1998; Inan et al., 2007) and via scattering of sub-ionospheric very low frequency (VLF) signals from secondary ionization enhancements they produce in the D-region ionosphere (Inan et al., 1988). Such electrons are in the so-called bounce loss cone, i.e., destined to precipitate within $< 1$ s. Energetic electrons scattered by whistlers at longitudes west of the spacecraft but destined to precipitate at the South Atlantic Anomaly (SAA) in the course of their eastward drift are in the so-called drift loss cone, and enhanced fluxes of such electrons have also been observed in association with lightning (Blake et al., 2001).

The DEMETER satellite has an instrument for particle detection that can be used to track changes in electron precipitation at low energies. Traversing the Earth in a sun-synchronous orbit at $  700$ km altitude, DEMETER sees drift loss cone particles through a majority of its orbit. With DEMETER measurements of energetic electrons and data from the National Lightning Detection Network (NLDN) describing lightning over the United States, it is possible to do a statistical assessment of the role of lightning in the precipitation of energetic electrons.

Figure 3: Seasonal variation of drift loss cone fluxes and lightning.
Image seasonal

Recent work (Gemelos et al., 2009) has looked at the seasonal dependence of energetic particle precipitation in relation to lightning. Figure 3a shows the logarithm of the median monthly nighttime flux of 126 keV electrons in bins of 1$^{\circ}$ latitude by 1$^{\circ}$ longitude, for August and December of 2006, 2007 and 2008. Only nighttime data are considered since trans-ionospheric absorption of VLF is significantly higher during the day (Helliwell, 1965, p. 71). The energy of 126 keV is within the observed range of $\approx{100}-250$ keV for LEP events (Voss et al., 1984; Voss et al., 1998; Inan et al., 2007); data for other energies in this range ($100-300$keV) exhibit similar behavior. Figure 3b shows the median nighttime fluxes in the geomagnetic conjugate region. Figure 3c shows the number of nighttime lightning strokes per month, weighted by peak current, in a given 1$^{\circ}$ by 1$^{\circ}$ latitude/longitude bin, as detected by the National Lightning Detection Network (NLDN) (Cummins et al., 1998) for the same months. Note that VLF whistler intensity from lightning is proportional to peak current (Reising et al., 1996), and LEP flux is proportional to VLF amplitude (Inan and Carpenter, 1986). Substantially higher lightning activity in August is accompanied by notably higher fluxes of drift loss cone electrons in both hemispheres, while lower lightning activity in December is accompanied by lower fluxes of precipitating electrons. The largest difference is in the central United States, where the lightning activity is most intense and widespread. The sharp termination of electron flux near the eastern boundary of the continental U.S. is due to the SAA; even though lightning activity does extend beyond the coast, DEMETER observes only bounce loss cone fluxes in this region. Increased fluxes are, however, apparent in the geomagnetic conjugate region.

Further statistical studies are underway to explore the correlation between lightning and energetic particle precipitation over the U.S. as a function of energy and location (L-shell). Future students may also be able to study particle precipitation data from TARANIS. Together wish ground-based observations, satellite observations provide a key link in determining the global extent to which LEP plays a role in shaping the radiation belts.

Acknowledgements

This work was supported by the Office of Naval Research under a Multi-University Research Initiative (MURI) program through subcontract to Stanford University from University of Maryland.

Bibliography

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