[论文解读] Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure

Simon Good, O. K. Rantala+4 more|arXiv (Cornell University)|Jul 19, 2023

Solar and Space Plasma Dynamics参考文献 53被引用 7

一句话总结

本研究分析了太阳轨道器和帕克太阳探测器在0.25至0.95 au范围内观测到的28个星际激波前太阳风爆发（ICMEs）中的磁场波动。结果表明，通量绳结构主导了大尺度谱，减去背景后谱指数从−5/3降至−3/2，而惯性尺度范围的谱指数保持−5/3，且与径向距离或质子β无关，表明湍流标度具有系统无关性。

ABSTRACT

Interplanetary coronal mass ejections (ICMEs) have low proton beta across a broad range of heliocentric distances and a magnetic flux rope structure at large scales, making them a unique environment for studying solar wind fluctuations. Power spectra of magnetic field fluctuations in 28 ICMEs observed between 0.25 and 0.95 au by Solar Orbiter and Parker Solar Probe have been examined. At large scales, the spectra were dominated by power contained in the flux ropes. Subtraction of the background flux rope fields reduced the mean spectral index from $-5/3$ to $-3/2$ at $kd_i \leq 10^{-3}$. Rope subtraction also revealed shorter correlation lengths in the magnetic field. The spectral index was typically near $-5/3$ in the inertial range at all radial distances regardless of rope subtraction, and steepened to values consistently below $-3$ with transition to kinetic scales. The high-frequency break point terminating the inertial range evolved approximately linearly with radial distance and was closer in scale to the proton inertial length than the proton gyroscale, as expected for plasma at low proton beta. Magnetic compressibility at inertial scales did not show any significant correlation with radial distance, in contrast to the solar wind generally. In ICMEs, the distinctive spectral properties at injection scales appear mostly determined by the global flux rope structure while transition-kinetic properties are more influenced by the low proton beta; the intervening inertial range appears independent of both ICME features, indicative of a system-independent scaling of the turbulence.

研究动机与目标

• 研究ICME中通量绳结构和低质子β如何影响内日球层中磁场波动谱。
• 确定ICME中湍流标度是否因独特的磁性和等离子体特性而不同于太阳风。
• 研究ICME湍流中谱指数、转折尺度和相关长度的径向演化。
• 评估质子β和通量绳几何结构在塑造压缩性和谱转折点中的作用。

提出的方法

• 对0.25至0.95 au范围内由太阳轨道器和帕克太阳探测器观测到的28个ICME中磁场波动进行谱分析。
• 使用Gold-Hoyle模型减去背景通量绳场，以分离湍流波动。
• 在惯性和动能尺度范围内计算幂律谱指数，重点关注−5/3和−3/2区间。
• 从功率谱估算谱转折点和相关长度，并比较其径向趋势。
• 利用质子β（βi）和磁压缩性评估等离子体条件和湍流特性。
• 将谱特征与径向距离、通量绳参数（如扭转度、轴向取向）及βi进行相关性分析。

Figure 1: Smoothed, trace PSD of magnetic field fluctuations for a selection of the ICMEs analyzed. Red points mark spectral break frequencies.

实验结果

研究问题

• RQ1 通量绳结构的存在如何影响ICME中磁场波动的功率谱？
• RQ2 ICME湍流惯性尺度范围内的谱指数是否随径向距离或质子β变化？
• RQ3 ICME中谱转折点和相关长度如何随日心距离演化？
• RQ4 低质子β在塑造ICME中动能尺度湍流和压缩性方面起什么作用？
• RQ5 ICME中惯性尺度湍流在多大程度上独立于整体ICME结构和等离子体参数？

主要发现

• 减去通量绳背景后，kd_i ≤ 10⁻³范围内的平均谱指数从−5/3降至−3/2，表明大尺度能量主要由通量绳结构主导。
• 所有径向距离（0.25–0.95 au）下，惯性尺度范围的谱指数均保持在−5/3附近，无论是否减去通量绳或质子β如何，表明湍流标度具有系统无关性。
• 高频谱转折点（代表惯性尺度范围结束）随径向距离近似线性演化，且更接近质子惯性长度而非质子回旋半径，与低βi条件一致。
• 惯性尺度上的磁压缩性在径向上无显著相关性，与一般太阳风中压缩性随距离增加的趋势相反。
• 向动能尺度过渡时，谱指数在−3以下出现显著陡化，表明出现动能尺度湍流效应。
• ICME中的惯性尺度湍流独立于通量绳结构和低质子β，支持一种普遍且系统无关的能量级联机制。

Figure 2: Example spectra from two ICMEs. From top to bottom, the panels show the trace PSD with power-law fits in the inertial and transition ranges, magnetic compressibility spectrum, and spectral index of the trace PSD. Vertical lines mark spacecraft-frame frequencies associated with the spectral