SOLAR-CYCLE CHANGES IN THE LEVEL OF COMPRESSIVE MICROTURBULENCE NEAR THE SUN

Observations of the coherence bandwidth of interplanetary scintillations (IPS) made at the Nancay Radio Observatory between 1982 and 1992 have been used to estimate the variance of electron density delta N-e(2) at solar distances of 5 to 15 R(s). These IPS observations are sensitive to scales of the order of 10 km; i.e., they measure the ''microscale variance,'' which is a small fraction of the total density variance. Measurements in the equatorial region are invariant over the solar activity cycle and show the same behavior with distance as the mean density, i.e., delta N-e(2) proportional to N-e(2). They do not show a statistically significant change in the ''transonic region'' as reported by other observers (Lotova et al., 1985). Measurements over the polar region show a clear variation with the solar cycle. At solar maximum, delta N-e(2) is invariant with latitude, as is N-e (Withbroe; 1988). However, the apparent delta N-e(2) measured by IFS is about 10 times lower in the polar streams which form during the declining and minimum phases of solar activity. Compensation for the line of sight integration inherent in the IPS observations indicates that the local value of SN,2 is actually about 15 times lower in the polar streams. We cannot estimate the latitude variation of the ''development'' of the compressive turbulence delta N-e/N-e between 5 and 15 R(s) because measurements of N-e in the polar hole are lacking.