Ion acoustic waves and related plasma observations in the solar wind

Abstract

This paper presents an investigation of solar wind ion acoustic waves and their relationship to the macroscopic and microscopic characteristics of the solar wind plasma. Comparisons with the overall solar wind corotational structure show that the most intense ion acoustic waves usually occur in the low‐velocity regions ahead of high‐speed solar wind streams. Of the detailed plasma parameters investigated, the ion acoustic wave intensities are found to be most closely correlated with the electron to proton temperature ratio Te/Tp and with the electron heat flux. Investigations of the detailed electron and proton distribution functions also show that the ion acoustic waves usually occur in regions with highly non‐Maxwellian distributions characteristic of double‐proton streams. The distribution functions for the double‐proton streams are usually not resolved into two clearly defined peaks, but rather they appear as a broad shoulder on the main proton distribution. Two main mechanisms, an electron heat flux instability and a double‐ion beam instability, are considered for generating the ion‐acoustic‐like waves observed in the solar wind. Both mechanisms have favorable and unfavorable features. The electron heat flux mechanism can account for the observed waves at moderate to large ratios of Te/Tp but has problems when Te/Tp is small, as sometimes occurs. The ion beam instability appears to provide more flexibility in the Te/Tp ratio; however, detailed comparisons using observed distribution functions indicate that the ion beam mode is usually stable. Possible resolutions of these difficulties are discussed.