Interplanetary scintillation at large elongation angles: Response to solar wind density structure

Synoptic interplanetary scintillation (IPS) index measurements were taken at 34.3 MHz during May–December 1974 using the University of Iowa Cocoa Cross radiotelescope on a ‘grid’ of 150 selected radio sources covering solar elongation angles up to 180°. Over 80 of these sources displayed definite IPS. The solar elongation dependence of the 34.3‐MHz IPS index is consistent with the elongation angle dependence measured at higher frequencies. Large enhancements (factors of ≳2) of the IPS index are found to coincide with the solar wind (proton) density increases greater than 10 cm−3 as measured by Imp 7 and 8 for nearly all observed IPS sources throughout the sky. These ‘all‐sky’ IPS enhancements appear to be caused by increased contributions to the scintillation power by turbulent plasma in regions close to the earth (≲0.3 AU) in all directions. Correlation analysis confirms the IPS response to solar wind density and indicates that the events are due primarily to the corotating solar wind turbulent plasma structures which dominated the interplanetary medium during 1974. The expected IPS space‐time signature for a simple model of an approaching corotating turbulent structure is not apparent in our observations. In some cases, the enhancement variations can be attributed to structural differences in the solar wind density turbulence in and out of the ecliptic.