Intermittent turbulent fluctuations in solar coronal mass ejections

Technical Summary: Intermittent Turbulent Fluctuations in Solar Coronal Mass Ejections

Problem Statement
Earth-directed solar coronal mass ejections (CMEs) are primary drivers of near-Earth space weather disturbances, causing significant risks to satellite electronics, GPS systems, and ground-based infrastructure. While the large-scale characteristics of CMEs (such as bulk velocity and magnetic field orientation) are well-studied, the nature of intermittent, localized, broadband fluctuations within CMEs remains less understood. Although intermittency is a recognized feature of solar wind and space plasma turbulence, specific studies regarding turbulent fluctuations within CMEs are limited. This paper addresses the need to characterize these "turbulent spots"—localized regions of high-intensity fluctuations resembling those found in fluid mechanics—to better understand collisionless plasma turbulence and improve space weather prediction.

Methodology
The study analyzes in-situ spacecraft observations from 125 well-observed, non-interacting, Earth-directed CMEs. The dataset includes measurements of plasma velocity ($V$), proton density ($n$), proton temperature ($T$), and magnetic field magnitude ($B$) obtained by spacecraft at the L1 Lagrange point (e.g., Wind).

The analysis employs continuous wavelet transforms (using the Morlet mother wavelet) to decompose the time-series data of $B(t)$, $V(t)$, and $n(t)$. This technique allows for the identification of coherent structures and intermittent fluctuations in the high-wavenumber tails of the fluctuation spectrum. The authors compute the wavelet power spectral density (PSD) and integrate it over a frequency range bounded by the "cone of influence" (to avoid edge effects) and the maximum data frequency. This integration yields the Integrated Power Spectral Density (IPSD) for each timestamp, denoted as $IPSD_X(t)$ where $X$ represents $B$, $V$, or $n$. Peaks in the IPSD curves are identified as "turbulent spots."

The CME events are segmented into three distinct regions for comparison:

1. Solar Wind Background: A 24-hour period immediately preceding the event.
2. Sheath: The region between the shock front and the CME start.
3. CME Interior: The region following the sheath.

Key Contributions and Results
The paper presents statistical results derived from the 125-event sample, offering novel discriminators for identifying CME regions and understanding plasma heating:

1. Characterization of Turbulent Spots: The study confirms that turbulent spots are localized regions of intense, broadband fluctuations, typically occupying 1/30 to 1/50 of the CME's spatial extent. There is no strict one-to-one correspondence between turbulent spots in magnetic field, velocity, and density; a spot in one quantity does not necessarily imply a spot in the others.
2. Discrimination of CME Regions: The intensity of the strongest turbulent spot serves as a reliable indicator for distinguishing the sheath from the ambient solar wind background.
   • The ratio of the peak IPSD in the sheath to that in the background is significantly higher for magnetic field ($B$) and proton temperature ($T$).
   • On average, the logarithm of the ratio for the strongest $B$ peak is 4.2, and for temperature, it is 4.3. In contrast, the ratio for density ($n$) is only 1.2.
   • This suggests that the intensity of the strongest turbulent spot, particularly in $B$, is a robust statistical discriminator for the onset of the sheath.
3. Occurrence Rates: The rate of turbulent spot occurrence is substantially higher in the sheath compared to the background solar wind. For the 10 largest peaks, the mean occurrence rate of magnetic field turbulent spots ($P_{DB}$) in the sheath is approximately 20.03 spots per day, compared to 1.25 spots per day in the background.
4. Proton Heating Correlation: The study finds a temporal correlation between turbulent spots and proton heating. In the sheath, the largest proton temperature peak occurs approximately 13 minutes after the largest magnetic field turbulent spot peak. This time lag is interpreted as consistent with a scenario where turbulent spots act as sites of enhanced dissipation and subsequent proton heating, manifesting as a temperature peak after a typical conduction timescale.

Significance and Claims
The authors claim that these findings enhance the understanding of intermittency in collisionless plasma turbulence. Specifically, the study suggests that:

• Space Weather Prediction: The intensity of the strongest turbulent spot and the occurrence rate of these spots can serve as reliable indicators for the onset of the sheath, which often triggers geomagnetic storms. This could improve current space weather prediction models, which currently rely primarily on bulk parameters like CME speed and the southward $B_z$ component.
• Physical Mechanisms: The similarity between CME turbulent spots and coherent structures in wall-bounded fluid turbulence (where the CME sheath acts analogously to a boundary layer) provides a new context for studying turbulence-mediated reconnection and CME-magnetosphere coupling.
• Methodological Scope: By analyzing $B$, $V$, and $n$ simultaneously across a large sample (125 events), the study provides a more comprehensive statistical picture than previous works focusing solely on magnetic field turbulence.

The authors note limitations, specifically that the relatively low time cadence of the data prevents meaningful analysis of spectral slopes, suggesting that future studies utilizing high-cadence data from missions like Parker Solar Probe and Solar Orbiter would be beneficial.