Artificial electron beams as probes of the magnetosphere

During the past decade a series of large suborbital vehicles has carried high-powered electron guns into the ionosphere over the eastern United States, Canada, and Alaska to inject electron beams as probes of the distant plasma medium in the magnetosphere. The electron beams were reflected from the conjugate hemisphere and were detected and analyzed to study the origins of the Van Alien radiation belts and the aurora by measuring plasma perturbations in space during magnetic storms. This paper summarizes the technical details of the ac- celerators, the method of injecting and analyzing the beams using television techniques and particle counters in space, and how the beams in their behavior simulate the natural trapped electrons and give clues as to the ac- celeration processes. NE of the earliest and most important results of the space era was the discovery of intense zones of trapped particles circulating in the outer regions of the Earth's magnetic field above the ionosphere.1 The electrons and protons were found to have energies in the million electron volt range and to vary as a result of magnetic storm activity detected by magnetometer stations on the ground. Early experiments with identical counters measuring simultaneously in the radiation belts and in interplanetary space far from the Earth showed that the trapped particle intensity increased remarkably, even in the million electron volt range, without the presence of similar particles arriving with the solar plasma from the sun.2'3 Thus the acceleration of particles to high energy was known to occur near the Earth in what is now called the Earth's magnetosphere. These particles radiate radio waves of great intensity and it was by means of radio waves from the planet Jupiter that the existence of intense radiation belts around Jupiter was predicted. The prediction was verified by the Pioneer and Voyager spacecrafts, which in penetrating Jupiter's magnetosphere found energies and intensities far exceeding those present near the Earth. Again it was found that these particles were locally accelerated. Many observations of the plasmas, magnetic fields, and high-energy particles of the planetary magnetospheres have now been made. The production of energetic particles has been frequently observed near the solar surface in solar flares and detected by indirect means elsewhere in the cosmos as well. Besides the basic scientific interest in these naturally accelerated particles, many practical applications of space both manned and unmanned must contend with the high radiation dosage encountered in various parts of the radiation belts. The deterioration of solar cells and other solid-state devices and disturbances in the operation of sensitive detectors are examples of radiation damage effects. There was great concern that the Pioneer and Voyager spacecrafts would suffer damage by Jupiter's radiation belts and in fact some effects were recorded.4 Many near-Earth missions are subject to deterioration by the intense inner radiation belt protons and electrons, or to spacecraft charging or radiation damage in the important synchronous orbit region during magnetic storms. Beyond the Earth's magnetosphere there are solar flare accelerated low-energy cosmic rays which are par- ticularly hazardous for extra-vehicular activities or ex- ploration of the surface of the planets. Yet there is a sur- prising lack of direct measurements which identify exactly the accelerating processes. More than a decade ago the author began a series of controlled experiments in which electron beams were used to probe the outer reaches of the Earth's magnetic field to establish the cause and effect relationship between the presence of electrodynami c processes during magnetic storms and the injection and changes in energy and other properties of the trapped electrons. A technique known as the "electron echo" has been developed for this purpose. The procedure is shown schematically in Fig. 1. The figure is drawn in per- spective but more or less to scale and illustrates a very useful location on the Earth for carrying out the experiment, namely in central Alaska in the auroral zone. A large sounding rocket is launched into the ionosphere flying for periods up to 10 min above the atmosphere carrying a powerful electron ac- celerator. Pulses of electrons are injected and spiral outward in the Earth magnetic field in exactly the manner of the natural electrons which are present in the same region of space as part of the radiation belts. The electron pulses pass through the equatorial plane and reflect from the converging fields in the Southern Hemisphere, and like the natural particles, return near their point of origin over Alaska. These magnetic lines of force pass through the equatorial plane in a region where plasma clouds are often injected from the magnetospheric tail plasma sheet. Instabilities cause fluc- tuating fields which accelerate the trapped particles to higher energies, making this region of space particularly important. By various techniques the returning beams are detected and analyzed to give information about the changes which they