Solar cycle evolution of ICME sheath regions at 1 AU

TL;DR

This paper investigates the solar-cycle evolution of ICME sheath regions at 1 AU by analyzing 900+ sheath events across solar cycles 23–25, using interquartile range ($IQR$) and Turbulence Index ($TI$) to quantify variability and turbulence. It contrasts full-cycle (FCS) and rising-phase (RPS) samples and relates sheath properties to upstream solar-wind conditions and the open solar flux ($OSF$), finding that SC24 experienced substantial reductions in sheath pressure ($p_{T}$) and magnetic field strength ($B$) and lower turbulence, while SC25 showed increased magnetic complexity and a shift toward normal-dominated non-radial flows. A key result is the lack of significant correlation between OSF and sheath properties, indicating that local solar-wind conditions and ICME-specific factors primarily drive sheath evolution. The work highlights the importance of treating sheath regions as distinct, structurally evolving components in ICME modeling, with implications for space weather forecasting and heliospheric modeling, and notes that multi-spacecraft missions will be essential to fully resolve 3D sheath dynamics.

Abstract

We investigate the evolution of interplanetary coronal mass ejection (ICME) sheath regions at 1 AU across solar cycles 23, 24, and the rising phase of 25, focusing on their variability and turbulence in relation to upstream solar wind conditions and the global heliospheric state. Using a dataset of over 900 ICME sheath events, we apply statistical metrics such as the interquartile range (IQR) and the Turbulence Index (TI) to quantify variability and turbulence. The analysis compares full and rising phases of solar cycles and examines both local ICME parameters (e.g., sheath total pressure, non-radial flows) and global interplanetary indicators such as open solar flux (OSF). From SC23 to SC24, sheath total pressure and magnetic field strength decreased by over 40% and 25%, respectively, accompanied by reduced turbulence and variability. In contrast, the rising phase of SC25 shows increased magnetic complexity, particularly in non-radial field components, despite stable bulk parameters. Non-radial flow patterns also shift from tangentially dominated in SC23-SC24 to normal-dominated in SC25, suggesting changes in ICME orientation and sheath formation mechanisms. No significant correlation is found between OSF and sheath properties, indicating that local solar wind and ICME specific factors are the primary drivers of sheath evolution. The study reinforces the importance of upstream solar wind dynamics in relation to variations in plasma and magnetic field measured components of ICME sheaths. The derived trends in turbulence, magnetic orientation, and flow geometry suggest that sheath regions are sensitive indicators of solar cycle phase and should be considered as distinct, structured components in ICME modeling.

Figures (8)

• Figure 1: OSF solar cycle boxplot. The FCS box is colored blue with the median line in red and for the RPS the box is colored black with the median line orange.
• Figure 2: $p_{T}$ solar cycle boxplot. The FCS box is colored blue with the median line in red and for the RPS the box is colored black with the median line orange.
• Figure 3: Speed $NRF$ solar cycle boxplot. The FCS box is colored blue with the median line in red and for the RPS the box is colored black with the median line orange.
• Figure 4: $NRB_{TN}$ solar cycle boxplot. The FCS box is colored blue with the median line in red and for the RPS the box is colored black with the median line orange.
• Figure 5: $NRB_{TR}$ solar cycle boxplot. The FCS box is colored blue with the median line in red and for the RPS the box is colored black with the median line orange.