Heliospheric tomography using interplanetary scintillation observations: 3. Correlation between speed and electron density fluctuations in the solar wind

Abstract

We have examined the relationship between solar wind speed and electron density fluctuations on scale sizes around 100 km in the heliocentric distance range of 0.3 to 0.8 AU using interplanetary scintillation (IPS) data obtained at the Solar‐Terrestrial Environment Laboratory. The solar wind properties derived from the IPS data are biased by line of sight integration through a three‐dimensional structured solar wind. Therefore we have applied a computer‐assisted tomography (CAT) method to deconvolve the line of sight integration and reconstruct the solar wind structure. The analysis was made for the solar wind speed V and electron density fluctuations δ N e in the solar activity minimum phase when high‐speed regions are separated from an equatorial low‐speed region by a sharp velocity gradient. From results of the CAT analysis we derived the best fit power law relation of δ N e ∝ V −γ with γ = 0.5 ± 0.15, indicating that fractional density fluctuations δ N e / N e in the high‐speed wind are larger than those in the low‐speed wind. Combining this relation with results of previous workers [ Coles et al. , 1995; Manoharan , 1993; Celnikier et al. , 1987; Jackson et al. , this issue], we suggest that the fractional density fluctuation level of the high‐speed wind evolves with heliocentric distance.