Transmitter-Induced Precipitation of Radiation Belt Electrons


The Threat of
Killer Electrons

Energetic charged particles can become trapped in the Earth’s magnetic field
much like they can be trapped in the magnetic bottle of a plasma confinement chamber.

The Threat of
Killer Electrons

Energetic charged particles can become trapped in the Earth’s magnetic field
much like they can be trapped in the magnetic bottle of a plasma confinement chamber. These trapped particles comprise the Van Allen radiation belts
and are a threat to space vehicles, potentially causing single event upsets and deep dielectric charging. As electronic instruments grow smaller in size, the potential for these electronic upsets grows higher. These damaging effects of such high energy electrons have earned them the name ”killer electrons.”

Overview of TIPER

Very Low Frequency (VLF) electromagnetic waves propagating in the magnetosphere are capable of interacting with trapped energetic electrons to remove them from the radiation belts. VLF waves propagating in the whistler mode can efficiently change the bouncing motion of energetic electrons if a certain gyroresonance condition is met between the wave and the particle’s motion. A particle’s motion can be modified so that instead of remaining trapped in the magnetic bottle of the magnetosphere, it precipitates upon the upper atmosphere where it will harmlessly collide with the neutral particle population and lose its energy.

Some VLF waves are generated by natural phenomena, such as lightning, and these waves play a major role in controlling the radiation belt populations, but the waves can also be generated by man-made transmitters. A large antenna is required to efficiently radiate in the VLF range because the wavelengths are 10-100 km. Since longer wavelengths also penetrate greater distances into sea water, a number of powerful VLF transmitters have already been constructed for the purpose of Naval communication. These transmitters are located throughout the world and are almost always transmitting. Some of the VLF waves from these transmitters will leak through the ionosphere, couple into the magnetosphere, and gyroresonate with trapped particles leading to particle precipitation. This phenomenon is termed Transmitter-Induced Precipitation of Electron Radiation (TIPER). Experiments with the Naval VLF transmitters help us better understand the phenomenon of induced particle precipitation.

Figure 1:
Transmitter-Induced Precipitation of Electron Radiation.

Image Tiper

Detection Methods

A major challenge in the study of TIPER is reliably measuring your effect upon the trapped particle population of the radiation belts. The effects of a transmitter keying experiment upon the entire trapped population are not detectable, so efforts focus on measuring the induced precipitation (i.e. measuring the flux of formerly trapped energetic particles that are now colliding with the upper atmosphere and exiting the radiation belts). There are two primary methods for performing this measurement: satellite-based detection, and subionospheric detection.

Satellite-based detection can provide the most direct measurement of induced precipitation. The satellite DEMETER
used in our experiments has both an electric field instrument and an instrument for particle detection. This facilitates simultaneous measurement of the transmitted VLF waves and the energetic particles that have been influenced. Since DEMETER resides in a near-polar, lower earth orbit at just below 700 km altitude, it is capable of passing through a region of precipitation once each night and recording the necessary measurements. Transmission experiments are often coordinated with DEMETER passes to facilitate these measurements.

Figure 2:
DEMETER Satellite.


Subionospheric detection provides a less direct form of measurement, but is available at any time and can still provide insight into the quantities of induced precipitation. In this detection method, a probe signal from a secondary VLF transmitter propagates through the precipitation region and is detected by a receiver on the opposite side. The precipitating energetic electrons results in secondary ionization which perturbs the VLF probe signal. The detected probe signal is analyzed for such perturbations in alignment with the transmission schedules of our experiments, and modeling efforts can provide an estimate of the induced precipitation based upon these observations.

Figure 3:
Subionospheric Detection.

Image SubionosphericDetection

Previous Work and Ongoing Experiments

Much work has been performed on the topic of particle precipitation leading up to this interest in TIPER. Successful subionospheric detection and modeling of whistler and Lightning-induced Electron Precipitation (LEP) was performed (Inan and Carpenter, 1987; Lauben et al., 2001; Johnson et al., 1999; Bortnik et al., 2006; Inan, 1987). Modeling was extended to focus on transmitters due to an interest in radiation belt dynamics and controlled precipitation (Kulkarni et al., 2008; Abel and Thorne, 1998). The most recent rounds of controlled experiments began with Naval VLF transmitters in 2005 and continue to today (Graf et al., 2009; Inan et al., 2007). Experiments are ongoing with transmitters NAA in Cutler, ME and NWC in North West Cape, Australia.

Figure 4:
Subionospheric detection network for NAA transmitter induced-precipitation experiments.

Image NAAdetection


This work is supported by a William R. and Sara Hart Kimball Stanford Graduate Fellowship and by the Defense Advanced Research Projects Agency (DARPA) and the High Frequency Active Auroral Research Projects Agency (HAARP). DEMETER satellite data is analyzed with the cooperation and assistance of the Laboratoire de Physique et Chimie de l’Environnement, Centre National de la Recherche Scientifique, Orléans, France, and the Centre d’Etude Spatiale des Rayonnements, Centre National de la Recherche Scientifique, Toulouse, France.


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