Abstract
A summary is presented of experimental optical observations at 4278 Å from close to a powerful (~150 kW) VLF transmitter (call sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 s after transmitter turn-on, a sudden increase in optical emissions at 4278 Å was detected using a dedicated camera/charge-coupled device (CCD) monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 s, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 Å reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.
Highlights
The Earth’s outer radiation belt consists of trapped high-energy electrons whose motion results from a combination of their bounce, gyration, and drift, along, around, and perpendicular to, the magnetic field, B, respectively
The discovery of the radiation belts [Van Allen et al, 1958] and subsequent descriptions of the large-scale physical processes operating in the region [e.g., Hess, 1968, and references therein] have been followed by attempts to understand, model, and predict the natural variation of electrons which make up the outer belt
The system comprises an Andor DU-888 camera with Electron Multiplying Charge Coupled Device (EMCCD) providing a field-of-view of ~30° of sky, centered on the zenith, with recordings taken at 10 frames per second with a digital resolution of 256 × 256
Summary
The Earth’s outer radiation belt consists of trapped high-energy electrons whose motion results from a combination of their bounce, gyration, and drift, along, around, and perpendicular to, the magnetic field, B, respectively. One natural loss mechanisms for radiation belt electrons is related to the occurrence of whistler-mode chorus waves [Dungey, 1963; Helliwell et al, 1973]. Whistler-mode chorus waves cause pitch angle scattering of electrons. This results in the temporal decay of the number of electrons—scattered electrons penetrate deeper into the atmosphere where losses are more likely. The first suggestion that electrons could be lost from the radiation belts via cyclotron-resonant interactions was (remarkably) accompanied by a prediction that the same population could be artificially removed with radio wave transmitters; If...the failure of whistlers to remove the trapped radiation...is due to lack of power...it may be asked whether man-made transmitters can do better [Dungey, 1963]. Numerous studies have since suggested mechanisms to carry out this process—namely the artificial reduction of damaging fluxes of energetic electrons [e.g., Bell et al, 1985; Inan et al, 1985; Abel and Thorne, 1998; Rodger et al, 2006]
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