Hiroaki Misawa

IPS observations at stelab for the fruitful coming decade

We have been carrying out solar wind measurements using the interplanetary scintillation (IPS) method. Our IPS observation system is operated at a frequency of 327MHz and consists of four stations located at Toyokawa, Fuji, Sugadaira and Kiso. The present system, however, has insufficient sensitivity to measure enough IPS sources for observing the solar wind with adequate spatial and temporal resolution. Therefore we have been excuting the upgrade project since 1994 in order to observe a larger number of compact radio sources. The Fuji system has been improved successfully and has achieved sensitivity by a factor over five compared with the previous system. The upgrade project is now in progress for the Toyokawa and Sugadaira station.

Velocity observations at high latitude and the acceleration phenomena

Abstract Interplanetary scintillation (IPS) observations of polar solar wind streams are discussed. The IPS observed several streams which show low speeds (≤400 km/s) inside of 0.3 AU and high speeds (≥700 km/s) outside of 0.3 AU. The foot points of these streams were located in the HeI coronal holes or at their boundaries. Since the IPS measurements are indirect measurements of the solar wind speed, the derived speeds might be biased by one or several mechanisms. We investigate whether the discrepancy between the observations inside and outside of 0.3 AU can be explained by the geometry of low and high speed regions along the line of sight. We also investigate the effect of changing the turbulence level of the electron density, and finally, by comparing observations from different antenna systems, we discuss the effect of the transition from the weak to strong scattering region.

Dependence of velocity fluctuations on solar wind speeds from an IPS analysis method

Interplanetary scintillation (IPS) observations were made with radio telescopes of the STE Lab. (327 MHz) and the EISCAT facilities (932 MHz) in 1993 and 1994 to investigate the dependence of the parallel component of velocity fluctuation ({delta}V{sub parallel}) on flow speeds. Preliminary analyses show the following results; 1) {delta}V{sub parallel} in high speed winds (V{sub IPS}> or approx. 500 km/s at distances beyond 0.3 AU) increases with approaching the sun, 2) Low speed winds (V{sub IPS} < or approx. 400 km/s at distances beyond 0.3AU) have a small {delta}V{sub parallel} at all distances. Here, V{sub IPS} is the solar wind velocity observed with the IPS method. Since the parallel fluctuation component is expected to be related to magnetoacoustic waves near the sun, the results infer that magnetoacoustic waves have some relation to the acceleration of high speed solar winds.

Acceleration Phenomena Observed by Interplanetary Scintillation at 0.1–0.3 AU

Interplanetary scintillation has observed high-latitude streams which show low speeds (≤400 km/s) within 0.3 AU and high speeds (≥700 km/s) beyond 0.3 AU. The foot points of these streams were located in the Hel coronal holes or at their boundaries. Since the IPS measurement is biased by one or several mechanisms, we investigated whether this acceleration phenomena could be caused by bias effects or not. Speed increase of more than 100 km/s is left without being explained by the bias effects.