The proposed effort during 1996-1997 is to 1) study the locations and size distribution of magnetospheric ducts, 2) greatly improve current estimates of loss rates of radiation belt electrons due to VLF waves from lightning, chorus, and other coherent waves, 3) clarify the relative roles of ducted and non-ducted whistler mode waves in the diffusion and precipitation of radiation belt particles, and 4) study the mechanisms responsible for high latitude precipitation bursts.
The study will be based on precipitating electron records acquired in 1982 by the SEEP (Stimulated Emission of Energetic Particles) satellite and by VLF wave receivers on the ground and on satellites. Previous work with the S81-1 satellite data has shown that ducted whistlers interact with electrons and deflect some of these electrons into the atmosphere below the ducts connected to the interaction region. However these data have not been exploited to determine duct dimensions or global electron loss rates. By observing the precipitating electrons with a low altitude satellite, information on the location and the size distribution of magnetospheric ducts can be obtained.
Electron precipitation by ducted whistlers and by other VLF waves is thought to be the major loss mechanism for radiation belt electrons. However, the relative importance of ducted whistlers, unducted whistlers, chorus, and hiss is controversial. The SEEP satellite with its low altitude orbit, large geometric factor detectors, and high data collection rates has acquired an unique data set which can be used to address these issues. In addition, VLF wave data from ground stations and satellites were obtained during the time interval SEEP was operating and will be used to associate wave characteristics with the particle precipitation.
This work will improve knowledge of magnetospheric duct dimensions, will provide quantitative information on LEP fine structure and on the number of electrons precipitation by ducted whistlers, should give information on the precipitation rates caused by non-ducted whistlers and other VLF emissions. Analysis of the high resolution electron data should also constrain the possible mechanisms responsible for the high latitude electron precipitation bursts.
In this proposal, the Taylor-Stanford-Lockheed team describes a research program to extract scientific results from existing satellite and ground based data of energetic electron precipitation caused by lightning and by other VLF waves. A new approach will be used to study magnetospheric ducts and wave particle interaction regions by measuring the ionospheric footprints of electrons precipitated by the waves. These electrons follow magnetic field lines and thus project the wave-particle interaction region onto the ionosphere. Six months of energetic electron data obtained by the SEEP satellite is available for this study, along with extensive VLF wave data from both northern and southern hemisphere ground stations and from the ISIS 1, 2 and DE 1 satellites. An analysis of these data should give new, independent estimate of sizes of whistler ducts. Additional information to be obtained from this study will include the trapped electron loss rates from lightning induced electron precipitation and an assessment of the global importance of these processes.
In 1982 a team from Stanford University and Lockheed Missiles and Space Company conducted a successful satellite and ground based experiment to test whether Navy VLF transmitter signals could precipitate electrons from the radiation belts. This experiment, called the SEEP project (Stimulated Emission of Energetic Particles), observed transmitter stimulated precipitation and also found numerous cases of lighting-induced electron precipitation (LEP), a phenomenon which has received further study by the Stanford/Lockheed group [Voss et al. 1984], [Imhof et al. 1986, 1989], [Inan et al. 1989] and by others [Goldberg et al. 1987]. The data obtained from the SEEP instruments are unique in that the satellite was at very low altitude (200 km) and had detectors with large geometric factors (0.2 cm²str) and fine energy (2 kev) and time resolution (64 ms). LEP events as recorded by SEEP show unprecedented clarity and detail, and these data have not been exploited for their full scientific value.
Analysis of the VLF transmitter experiments and initial identification of the LEP phenomena was funded by the Office of Naval Research, and the results of that work have been published. Although the objectives of the SEEP experiment were achieved, the Navy is unable to support further work relating to this experiment. Therefore, in this proposal we seek NSF funding to explore the broader scientific implications of LEP, the character and sizes of ionospheric and magnetospheric ducts, and the influence of coherent whistler-mode radiation on trapped electron lifetimes.