Solar corona effects on angle of arrival fluctuations for microwave telecommunication links during superior solar conjunction

摘要

During the superior solar conjunction of spacecraft, the transmitted signals from a spacecraft to the Earth ground station graze the region near the Sun's photospheric surface, passing through dense and turbulent regions of solar-charged particles. Phase changes due to solar coronal irregularities cause raypath wandering, wavefront tilting, and fluctuations in the apparent angle of arrival when observed from the Earth. This study presents a first theoretical investigation of solar wind and solar coronal effects on the angle of arrival fluctuations for RF signals. On the basis of the Chandrasekhar relationship between phase and angular fluctuations, an analytical integrating solution for angle of arrival fluctuations is derived by applying solar corona and plasma irregularity spectrum models. It is found that angular fluctuations rapidly decrease with increasing heliocentric distance at a rate of ～r~(-55) and also decrease with increasing frequency at a rate of ～1/f~2. It is found that when using K_a band at α = 0.4° (r = 1.6 solar radii), there is a 19 millidegrees (mdeg) angular scattering, corresponding to a 9 dB gain degradation. In comparison, lower-frequency X and S band signals undergo much worse degradation effects. Beyond α = 2° (r > 8 solar radii), angular fluctuations at microwave frequency bands can almost be neglected (θ_(RMS) < 1 nideg). A solution to minimize this degradation is to use K_a- or higher-frequency bands for the telecommunication link during periods of solar conjunction. This study not only quantifies the angular fluctuations caused by solar corona irregularities but also provides an effective method for diagnosing the plasma density fluctuations in a region very close to the solar photospheric surface.