Radial dependence of the level of amplitude fluctuations of spacecraft radio signals probing circumsolar plasma

The statistical characteristics of amplitude fluctuations of radio signals propagating through circumsolar plasma at various distances from the Sun are investigated during various phases of solar activity. Primary data obtained in large-scale series of radio-probing sessions are systemized. During these sessions, circumsolar plasma was probed with signals from spacecraft launched by Russia and other countries. It is found that the dependences of the relative level of amplitude fluctuations (scintillation index) on the solar-offset distance of a radio beam can be approximated with power functions with exponents depending on the heliocentric distance. When the distance between a probed region and the photosphere is large, this exponent is close to the value 1.5. As the Sun is approached, the exponent grows to a value of 1.8–2.05 reached at a heliocentric distance of about four solar radii. The scintillation index is calculated theoretically with allowance for the radial dependence of turbulence characteristics, such as the spectral index and outer scale.