Evidence of energy conversion in weakly collisional plasma during an interplanetary coronal mass ejection

TL;DR

The paper investigates energy conversion and wave activity in weakly collisional plasma inside a magnetic cloud of an interplanetary coronal mass ejection (ICME) using Wind data from 8–9 June 2000. By applying spectral analysis, ion-scale magnetic helicity, and wave polarization (along with wavelet coherence), the authors distinguish between Alfvén ion-cyclotron (AIC) and fast magnetosonic/whistler (FM/W) wave populations in two intervals exhibiting enhanced fluctuations. They find interval 1 dominated by left-handed quasi-parallel AIC waves, and interval 2 showing coexisting AIC-like and FM/W-like fluctuations with strong compressive signatures and associated electron heating, implying multiple ion-scale dissipation channels near marginal stability. The results highlight species-dependent heating (notably alpha-particle perpendicularly heated) and electron–magnetic-field coupling as key factors shaping energy partition and wave activity in CME magnetic clouds, with implications for turbulence and heating in space plasmas.

Abstract

Intervals of enhanced turbulent fluctuations are typically less frequent within the magnetic cloud region of an interplanetary coronal mass ejection (ICME). We investigate two such intervals inside an ICME observed by the \textit{Wind} spacecraft on 8--9 June 2000 and characterize their associated wave populations. We focus on spectral analysis and plasma instability analysis, using ion-scale normalized magnetic helicity and polarization properties with respect to the background magnetic field $B_0$. In the first interval, the ion-scale normalized magnetic helicity shows a left-handed circularly polarized signature. In the second interval, the left-handed signature persists and an additional high-frequency right-handed population appears. The propagation is approximately parallel to $B_0$. The left-handed fluctuations are compatible with Alfvén ion-cyclotron (AIC) waves, while the right-handed fluctuations are consistent with fast magnetosonic/whistler (FM/W) waves. The ICME plasma accesses resonance conditions that support multiple ion-scale wave modes. Evolving anisotropies in the plasma and the approach to marginal stability allow the coexistence of AIC-like and fast-magnetosonic/whistler-like fluctuations, with enhanced electron heating favoring the growth of the FM/W contribution and strengthening the density--magnetic-field magnitude correlation.

Figures (7)

• Figure 1: Interplanetary parameters associated with the ICME observed on 08–09 June 2000. From top to bottom, the panels show: the total magnetic-field magnitude $B_{\mathrm{mag}}$ (in nT, GSE coordinates); the normalized magnetic-field fluctuation amplitude $\delta B/B_{0}$, computed as the centered-difference variation of $\vec{B}$ and normalized by the local background field; the magnetic-field vector components ($B_{\mathrm{comp}}$ in nT)); the proton bulk speed $V_p$ (km s$^{-1}$); proton density $N_p$ (cm$^{-3}$); proton temperature anisotropy $T_{\perp}/T_{\parallel}$; proton temperature $T_{p}$; and plasma beta $\beta$. The ICME sheath is shaded in cyan, the magnetic cloud in pink, and the two yellow intervals mark periods of enhanced magnetic variability within the magnetic cloud. The dashed line at the end part indicates the time at which the spacecraft entered the bow shock.
• Figure 2: Power spectral density (PSD) of the magnetic field trace for the two intervals shown in Figure 1. The red solid line indicates a reference $-5/3$ slope associated with Kolmogorov-like inertial-range scaling, while the vertical dashed line marks the local proton cyclotron frequency $f_{ci}$.
• Figure 3: The spectral analysis of (a) $\sigma_c$, (b) $\sigma_r$, (c) magnetic compressibility ($C_B$) along with (d) the distribution of $\sigma_m$ as a function of flow angle $\theta_{VB}$, is shown for the fluctuating interval 1 (top) and 2 (bottom). For $C_B$ and $\sigma_m$, high-resolution data at 11 Hz are utilized, whereas $\sigma_c$ and $\sigma_r$ were computed using 3-second data. The solid dashed lines represent the ion gyrofrequency ($f_{ci}$) and skin depth ($f_{di}$).
• Figure 4: The upper panel of the figure shows the temperature anisotropy of alpha particle, and the middle panel depicts the ratio of perpendicular and parallel components of Alpha and proton, respectively. The bottom panel depicts ratio of differential velocity ($V_d=V_{\alpha}-V_p$) to the Alfvén speed ($V_A$).
• Figure 5: Wavelet transform coherence (WTC) between magnetic field magnitude fluctuations ($\delta |B|$) and proton density fluctuations ($\delta n$) for two studied intervals. The color scale indicates the coherence magnitude, ranging from 0 (no coherence) to 1 (perfect coherence). Black arrows denote the relative phase between $\delta |B|$ and $\delta n$: right (left) arrows correspond to in-phase (anti-phase) behavior, while upward (downward) arrows indicate that $\delta |B|$ leads (lags) $\delta n$ by $90^\circ$. The white dashed horizontal line marks the local ion cyclotron frequency ($f_{\mathrm{ci}}$). Cross-hatched regions indicate the cone of influence (COI), where edge effects may affect the reliability of the coherence estimates.