East–West Asymmetry in Interplanetary-Scintillation-Level Variation Associated with Solar-Wind Disturbances

Interplanetary-scintillation (IPS) observations provide useful information on large-scale solar-wind disturbances, such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs), which impact the Earth and drive space weather. In the present study, we derived the G_ave -index, which represents daily variations in the density-fluctuation level of the inner heliosphere, based on IPS observations at the Institute for Space-Earth Environmental Research of Nagoya University between 1997 and 2019, and investigated the response of G_ave to ICME and SIR events. A clear difference was observed in the temporal profile of G_ave obtained from the superposed-epoch analysis between ICME and SIR events. The G_ave -values for the east and west sides of the sky plane for ICME events increased simultaneously and peaked at the ICME start time, which is consistent with the analysis of ICMEs directed toward the Earth. In contrast, the analysis of SIR events showed an asymmetric response between eastern and western G_ave , with a distinct increase in G_ave observed on the west side after the SIR start time and higher G_ave -values observed on the east side before the start time. These findings were explained by the effect of the spiral-shaped structure of the SIR. Significant positive correlations were found between G_ave and solar-wind density and speed, which also showed east–west asymmetry. These phenomena were ascribed to the effect of SIR events, while the occurrence of peak correlations between G_ave and density at zero delay time for Cycle 23 was ascribed to the effect of ICMEs. The difference in correlations between Cycles 23 and 24 was ascribed to the weakening of activity in Cycle 24. The occurrence of a correlation peak for a positive delay time suggests that eastern and western G_ave data are useful for predicting the arrival of the solar wind with increased density and speed, respectively, although the correlation magnitudes were weak.