Characterizing Turbulent Magnetic Field, Density and Velocity Fluctuations in Earth-Directed ICMEs

Abstract

Earth-directed solar Coronal Mass Ejections (CMEs) often interact with the magnetosphere, causing geomagnetic storms that disrupt a range of technologies we routinely rely on. A good understanding of the CME-magnetosphere interactions are key to mitigating the consequences of such storms. The extent of CME-magnetosphere interaction is governed substantially by conditions in the CME sheath (which is the leading part of Earth-directed CMEs). It is possible that turbulence in the sheath and magnetic obstacle (MO) of CMEs could show different characteristics compared to background/ambient solar wind. We have attempted to characterize the CME sheath based on a comparison with power present inside the turbulent high-frequency fluctuations in CME sheaths, MO and background solar wind by the help of Power Spectral Density (PSD) calculation where frequencies were around the inertial subrange. This comparison was based of a quantity (modulation index) is a measure of power inside these high-frequency fluctuations to that of low-frequency fluctuation. Previous studies have shown that there is a difference in the geomagnetic impact of sheath and MO based on Dst Index values, we have tried to check for such differences through correlations with the modulation index. The comparison was carried out with data from NASAsWINDspacecraft for proton density (Np), magnetic field magnitude (B) and velocity magnitude (V). This data was separated into two different set of events one consisted of 130 events with ICME shock and another with 271 events that have mix of events with and without shock along with Dst index values. We found that the modulation index effectively characterizes the sheath as a highly turbulent structure, MO is comparatively less turbulent, and the chosen background solar wind is the least turbulent. Spectral indexes furthermore show that there is a similarity in MO and sheath spectra however, background solar wind spectra is different.