SOLAR-WIND MOTION NEAR THE SUN DERIVED FROM SIMULTANEOUS INTERPLANETARY SCINTILLATION OBSERVATIONS AT 2-GHZ AND 8-GHZ

Single-station observations of interplanetary scintillation (IFS) at two frequencies; 2 GHz and 8 GHz, have been made simultaneously between 1990 and 1992 at the Kashima Space Research Center. The solar wind velocity near the sun is derived from these IFS observations by means of a co-spectrum technique. Derived velocities clearly reveal that solar wind acceleration occurs at radial distances between 12 and 80 Rs (solar radii), although the velocity structure on the solar surface occasionally influences the radial variation of our IFS data. A close association between high-speed winds and the coronal hole is demonstrated by comparing our IFS data with He (1083 nm) observations. Our velocity estimates are then compared with speeds obtained from three-station IFS measurements at 327 MHz. Consequently, a good agreement is found with IFS data taken in 1992, whereas the correlation is lost with IFS data from 1991 owing to the systematic bias between two measurements. It is shown that our velocity estimates are slightly higher than three-station estimates at large radial distances, and that this relation is reversed within 30 Rs. While the cause of this radial dependence is not fully understood, some possible effects are discussed here.