The Viking Solar Corona Experiment

Abstract

The 1976 Mars solar conjunction resulted in complete occultations of the Viking spacecraft by the sun at solar minimum. During the conjunction period, coherent 3.5‐ and 13‐cm wavelength radio waves from the orbiters passed through the solar corona and were received with the 64‐m antennas of the NASA Deep Space Network. Data were obtained within at least 0.3 and 0.8 Rs of the photosphere at the 3.5‐ and 13‐ cm wavelengths, respectively. The data can be used to determine the plasma density integrated along the radio path, the velocity of density irregularities in the coronal plasma, and the spectrum of the density fluctuations in the plasma. Observations of integrated plasma density near the south pole of the sun generally agree with a model of the corona which has an 8:1 decrease in plasma density from the equator to the pole. Power spectra of the 3.5‐ and 13‐cm signals at a heliocentric radial distance of about 2 Rs have a ½‐power width of several hundred hertz and vary sharply with proximate geometric miss distance. Spectral broadening indicates a marked progressive increase in plasma irregularities with decreasing ray altitude at scales between about 1 and 100 km.

Solar wind velocity measurements near the sun using Ulysses radio amplitude correlations at two frequencies

“…20-29 R x ). sured electron content during C1 (Bird et al, 1994) to the solar minimum electron content used by Tyler et al (1977). These ratios, as well as the actual values for DL, and N T , are also presented in Table 1.…”

“…Clearly, the correlation function K(s) peaks at and is symmetric about s = 0 for the solar wind regions beyond 20 R x . There are two obvious reasons for this: first, the separation of the S-and X-band ray paths from differential refraction is considerably less than a kilometer (Tyler et al, 1977) and second, the solar wind velocity in this region is relatively high. The correlation coefficient at the maximum is about 0.3, i.e., lower than the theoretical value 0.7 predicted by Woo (1975).…”

“…No adjustments are made here for a possible heliographic latitude dependence during the solar maximum epoch applicable to these observations. Tyler et al (1977). b Ulysses measurements (Bird et al, 1994).…”

“…6). The values of DL must be determined from numerical ray-tracing calculations, for example those presented by Tyler et al (1977), valid for a solar minimum corona (reproduced here as Fig. 7).…”

“…Values of the cross correlation velocity have been determined according to the relation V cc = DL/s cc (Tyler et al, 1977;Bird, 1982), where DL is the radial separation of the S-and X-band radio ray paths from differential refraction in a spherically symmetric corona (see sketch, Fig. 6).…”