Techniques for Remote Sensing of Ionospheric Electron Density from a Spacecraft.

Abstract : This paper examines a variety of techniques for the remote sensing of ionospheric electron density from a satellite platform. Most of the current satellite techniques are limited to determining ionospheric characteristics along the satellite track or directly beneath the satellite. This study considers techniques by which a more general areal survey can be made so that the electron density in three dimensions can be determined. Existing techniques are reexamined for additional potential, and the feasibility of new techniques is considered. Topside sounders currently offer the best areal survey capability with the ISIS satellites. A spacecraft version of the 'Digisonde' sounder offers enhanced measurements. Laser radars or lidars have substantial potential for ionospheric measurements but not yet been developed for spacecraft use or for operation in the vacuum ultraviolet region of the spectrum where ionic species such as 0+ must be detected. It is anticipated that such developments are likely. Thomson scatter radars are useful in ground-based installations but would be cumbersome on a satellite. They are huge and require high power to operate effectively. Various Faraday and phase methods have been used over the years but yield only the column electron density and generally require a co-operating ground station. They are useful in conjunction with other measurements but probably not alone. Numerous optical or chemiluminescent methods are possible. They often use relatively simple equipment and, hence, would be easy to include with other detectors on a satellite system.