The application of artificial electron beams to magnetospheric research

Abstract

In the last decade, more than 25 large research rockets have injected electron beams into the ionosphere and magnetosphere to probe the magnetosphere or to investigate various basic problems in space plasma physics. The problem of vehicle neutralization has been studied in the altitude region 100–300 km, and it is now known that the beam injection produces a hot plasma extending possibly as much as 100 m from the vehicle. Vehicle neutralization is enhanced by a discharge associated with the beam or with the return current. The neutralization current may be locally enhanced by neutral gas. Beams have been projected for long distances and appear to be stable in many cases. However, certain beam‐plasma discharges similar to those observed in laboratory experiments may have also occurred in space, tending to thermalize high‐energy beams. Several types of experiments search for beam echoes from field‐aligned potential differences thought to cause auroras. Others have made observations at conjugate regions in the opposite hemisphere for magnetic geometry studies. In the Minnesota ‘Echo’ technique, conjugate hemisphere beam echoes are detected on board the source rocket. From the Echo experiments it was found that ionospheric electric fields map along auroral zone field lines as equipotentials through the equatorial plane. Echo bounce times can be fitted in some cases to model fields, but in other cases the field may be distorted or extended in the night region. Examples from the Echo program at Minnesota are used to illustrate electron beams as magnetospheric ‘probes.’ The injection of such beams creates rich sources of plasma and electromagnetic waves, which have been detected by ejected subsystems near the mother rocket or at ground level. The frequency range extends from a few kilohertz to at least 50 MHz and includes lower hybrid, whistler mode, electron cyclotron harmonics, upper hybrid, and plasma frequency emission. Electron beams have been used to produce artificial auroras that were observed by sensitive television techniques, by other optical methods, and by radar reflections. The scattering, energy loss, and magnetic reflection of beams in the 100‐km atmosphere region have also received detailed study. Future programs including Space Shuttle are discussed. Summaries of the current experimental programs and complete literature references are included.