Masakazu Kojima

Solar wind velocity and normalized scintillation index from single-station IPS observations

The recently refurbished Ooty Radio Telescope in southern India was used in a two-month campaign of interplanetary scintillation (IPS) observations in collaboration with the Cambridge IPS array in England during April–May 1992. The unique feature of this campaign was that, for the first time, scintillation enhancements were predicted in real time by observing solar events on 7–8 May, 1992 and then detected at both Ooty and Cambridge. Also, for the first time, high spatial resolution (∼ 100 sources sr−1) solar wind all-sky velocity maps were obtained at Ooty. Good consistency is found between the IPS observations from both observatories andin-situ shocks detected at Earth by IMP-8.Yohkoh soft X-ray images were used to infer the generation of a coronal mass ejection on 7 May, 1992.

The spectrum of electron density fluctuations in the solar wind and its variations with solar wind speed

We describe simultaneous interplanetary scintillation (IPS) measurements for the solar distance range 70-185 R⊙ made using the Ooty Radio Telescope and the Solar Terrestrial Environment Laboratory three-antenna system during the first half of the current solar cycle 22. These measurements have been used to establish the spectral characteristics of the electron density fluctuations in the solar wind, and we study the relation of these to the plasma flow speed. At times of low activity, for high-speed streams (Vsw ≈ 600 km s−1) the spectra (sensitive to the IPS temporal frequency range 1-10 Hz) are steep, with a power law exponent α ≈ 3.8. During high levels of activity for high-speed streams, the spectra are less steep with α ≈ 3.4. In the low-speed wind (Vsw ≈ 400 km s−1) the spectra are clearly flatter, α ≈ 2.9, and nearly invariant with solar activity. The average shape of the spectrum obtained from this study is consistent with results obtained via other techniques. By combining the present estimates with earlier spacecraft data, it is possible to consider the shape of the density fluctuation spectrum in the frequency range 10−5-10¹ Hz. It is shown that the spectrum has different slopes in different spectral domains and is well described by a three-component model as proposed by Coles and Harmon [1989]. Moreover, the high-frequency components of the spectra in the low- and high-speed streams involve different spatial frequency ranges.