Turbulent Properties of Interplanetary Coronal Mass Ejections Observed by Solar Orbiter in the Inner Heliosphere

TL;DR

The paper tackles how interplanetary coronal mass ejections (ICMEs) differ in turbulence from the ambient solar wind in the inner heliosphere. Using 12 ICMEs observed by Solar Orbiter between 0.29 and 1.01 AU, the authors analyze magnetic power spectral densities, break scales, and cross-field (v–b) correlations across ICME substructures (sheath and ME) and surrounding wind, applying Taylor’s hypothesis and a KS-based framework to detect inertial-range breaks. They find that the sheath is consistently the most turbulent region, while ME and solar wind show variable inertial-range scalings—often near $f^{-5/3}$ in ICMEs but with solar-wind regions displaying $f^{-3/2}$ or a coexistence of both, tied to Alfvénicity; spectral breaks in ICMEs occur at higher frequencies and align near the ion inertial length, especially in low-$\beta$ ME. These turbulence signatures can serve as effective, physics-based indicators of ICME boundaries and potentially enable near-real-time ICME detection, provided high-cadence magnetic-field data (around 10 Hz) are available.

Abstract

We investigate the turbulent properties of 12 interplanetary coronal mass ejections (ICMEs) observed by Solar Orbiter between 0.29 and 1.0 AU. We analyze fluctuation power, spectral indices, break scales, and correlations between magnetic and velocity fluctuations (v-b) to quantify differences between ICME substructures (sheath and magnetic ejecta (ME)) and the surrounding solar wind. The ICME sheath is consistently the most turbulent region at all distances. In the solar wind, Alfvénicity influences inertial-range scaling, resulting in either single power laws near f^-3/2 or f^-5/3, or a coexistence of both, whereas ICME substructures consistently exhibit Kolmogorov-like f^-5/3 spectra. Alfvénicity is reduced within ICMEs, particularly in the ejecta, indicating more balanced Alfvénic fluctuations than in the solar wind. Spectral breaks shift to higher frequencies in ICME regions, with average break frequencies of 0.53 +/- 0.35 Hz (solar wind), 1.87 +/- 1.46 Hz (sheath), and 1.46 +/- 1.28 Hz (ME), reflecting differences in underlying microphysical scales. Our findings highlight distinct turbulence regimes in ICMEs compared to the solar wind and support the use of fluctuation power, spectral breaks, and v-b correlations as effective diagnostics for identifying ICME boundaries.

Figures (8)

• Figure 1: The magnetic field and plasma data for an ICME observed by SolO spacecraft on October 17, 2023. Panel (a): Magnetic field components and magnitude, Panel (b): Proton velocity components and magnitude, Panel (c): Proton density, Panel (d): Proton temperature ($T_p$, blue points) and expected proton temperature ($T_{\text{exp}}$) derived from the $V_{\text{sw}}$ relation (red points), Panel (e): Ratio of $T_p/T_{\text{exp}}$ ( dashed red line corresponds to $T_p/T_{\text{exp}} = 0.5$), Panel (f): Proton plasma beta ($\beta_p$)( dashed red line corresponds to $\beta_p = 1$). Colored areas represent four selected regions: light salmon pink – SW1, light green – sheath, light blue – ME, and light peach – SW2.
• Figure 2: Magnetic PSD traces for the selected regions of the ICME observed on 2023 October 17, smoothed using a running mean window. The insets show the fitted PSDs within the inertial range ($10^{-2}$– $5 \times 10^{-1}$ Hz) for each region.
• Figure 3: Absolute cross-correlation coefficients between magnetic and velocity field fluctuations in the R, T, and N components, plotted as a function of frequency ($1/\tau$), for the SW1, sheath, ME, and SW2 regions of the ICME event on 2023 October 17. Here, $\tau$ is the fluctuation timescale.
• Figure 4: Inertial range spectral slopes for the SW1, sheath, ME, and SW2 regions plotted against event numbers listed in Table \ref{['tab:icme_events']}. While the SW1 and SW2 regions occasionally exhibit double power-law behavior with coexisting $-3/2$ and $-5/3$ slopes, the sheath and ME regions consistently display a single spectral slope close to $-5/3$ across the inertial range.
• Figure 5: $f_{b}$, $f_{d_i}$, and $f_{\rho_i}$ plotted against event numbers from Table \ref{['tab:icme_events']}, which are ordered by increasing heliocentric distance, for the SW1, sheath, ME, and SW2 regions across all analyzed events.