Some quantitative aspects of electron precipitation in and near the auroral zone
Authors: Bailey D. K. 
Published: February 2008

Observations made with the International Quiet Sun Years (IQSY) forward‐scatter paths, supplemented with a special program of ground‐based observations at the midpoint of one of the two forward‐scatter paths in Alaska, have been used as a basic body of experience against which to discuss some of the quantitative aspects of electron precipitation into the ionosphere and mesosphere. Because most of the electron precipitation events from which we can extract quantitative spectral information occur in or near the auroral zone, the results obtained in Alaska are the main subject of our study and review.

The limitations of the forward‐scatter technique as now in use are discussed in some detail. They stem from uncertainties about the actual height of scattering at any moment and from uncertainties in estimating the radio‐wave absorption during an electron precipitation event, that result from the competing effects of simultaneous enhancement of the intensity of the scattered signals. In spite of these inherent handicaps progress can be made. Unmistakable daytime decreases in the intensity of the scattered signals can be identified. They result from intense and abnormal ionization below the scattering stratum. We can identify and eliminate from consideration SID and PCA effects, which were rare during the observing program. The remaining effects have been shown to be due to the precipitation of electrons and to result from electrons in the high‐energy, tail of the spectrum describing the precipitation. We also show that the spectra describing daytime precipitation are usually considerably harder than the spectra describing the nighttime precipitation associated with the aurora and with auroral absorption observed by riometers, a result confirmed by a number of other investigations. The Es propagation that occurs from time to time, mostly at night, masks the scatter signals but provides information about the comparatively soft nighttime electron precipitation.

We conclude that exponential energy spectra for electron energies greater than about 10 kev will always describe adequately the electron precipitation responsible for the riometer absorption, the behavior of the scattered signal, and the r‐type Es (or night E) that is found to be responsible for the Es propagation observed with the scatter‐path. The results from the forward‐scatter program are related quantitatively to other observations concerned with electron precipitation, such as riometer absorption, auroral luminosity, fluxes of precipitating electrons observed by satellite and rocket, and auroral bremsstrahlung X rays observed by balloon; and in a more limited way the results are related to magnetometer behavior. The electron precipitation results are also related, but only qualitatively, to such recently developed concepts as the auroral oval, the auroral substorm, and the magnetosphere.

Suggestions are made for improving the ground‐based observing techniques, and for investigations that are required to allow a more exact interpretation of the ground‐based observations.