A study of density fluctuations in the solar wind acceleration region

Abstract

Interplanetary scintillation (IPS) measurements of the density fluctuations in the fast and slow solar wind were made at 2 GHz and 8 GHz in 1994 at the Kashima Space Research Center, Communications Research Laboratory. Using the observations which cover the distance range from 5 to 76 solar radii (Rs), we investigate the radial evolution of the dissipation scale length of the density fluctuations, the so‐called “inner scale.” Our IPS observations reveal that the inner scale shows different radial profiles between the inside and outside of the acceleration region. The size of the inner scale outside 25 Rs increases linearly with radial distance, showing good agreement with previous observations made at r ≥ 50 Rs. The inner scale inside 25 Rs, on the other hand, deviates from the linear relation. We simulate the radial variation of the inner scale using a solar wind acceleration model and then compare the results to that of the observed inner scale. We find that, in the low‐speed solar wind, the radial profile of the computed inner scale is in good agreement with that of the observed inner scale and that the solar wind acceleration causes the deviation of the inner scale from a linear relation. However, in the high‐speed wind, we cannot reproduce the radial profile of the observed inner scale with the acceleration model, even though the line‐of‐sight integration effect on IPS measurements is taken into account. We propose that this disagreement is due to the density and magnetic field fluctuations not being correlated in the high‐speed solar wind, as shown at greater heliocentric distances by Helios measurements.