Radio Science Techniques for Deep Space Exploration

4 Solar and Fundamental Physics

In Collaboration with J. W. Armstrong

4.1 Principles of Heliospheric Observations

Radio science (RS) observations can probe the inner heliosphere and the solar wind on spatial scales inaccessible to in-situ observations. The first use of the deep space network (DSN) for heliospheric research was a spectral broadening experiment as the line-of-sight (LOS) of the Mariner 4 spacecraft passed within 0.6 degrees (deg) of the Sun. An open-loop receiver (see Chapter 5 ) was used to record the pre-detection electric field data. In 1968, Goldstein (1969) duplicated the experiment during the superior conjunction of Pioneer 6. Table 4.1 summarizes observable quantities and their relevance to heliospheric investigation goals. Appendix 4A describes solar corona observation methodology using Mars Express data for illustration.

Table 4.1 Radio science observables relevant to heliospheric studies.

Observation	Relevance	Comment
Phase scintillation Spectrum Allan variance	Directly related to electron density spectrum over wide range of spatial scales Sense changes with solar distance Sense changes in/out of ecliptic	Sensitive and w/large dynamic range (does not “saturate” like intensity scintillation) S-X or X-K isolates downlink plasma w/excellent control of systematics ~10−4–10 Hz Useful over whole sky: Sun–Earth–spacecraft angles 0–180 deg
Intensity fluctuations Spectrum Statistical moments (variance, third central moment) Bi-spectrum