将ICME截面模拟为静态MHD柱的研究
On modeling ICME cross-sections as static MHD columns
摘要：
众所周知，太阳日冕物质抛射在穿过日光层时会膨胀。尽管如此，它们的截面通常被建模为静态等离子体柱内的磁流体力学（MHD）的框架。我们测试这种方法的有效性，使用现场等离子体数据从151磁云（MC）观测的风航天器和45观测的太阳神航天器。我们发现WIND事件最可能的横截面膨胀速度仅为MC内部Alfvén速度的$\approx 0.06$倍，而Helios事件最可能的横截面膨胀速度为$\approx 0.03$。因此，MC截面在阿尔文穿越时间尺度上几乎是静态的。使用库仑碰撞产生的电导率的估计，我们发现，在MC的Lundquist数是高的（$\approx 10^{13}$），这表明MHD的描述是合理的。焦耳加热率使用我们的电导率估计是几个数量级低于要求的等离子体加热内MC靠近地球。虽然我们计算的（低）加热速率与MHD描述一致，但与加热要求的差异表明可能偏离MHD，需要更好地了解MC中的等离子体加热。
英文摘要：
Solar coronal mass ejections are well known to expand as they propagate through the heliosphere. Despite this, their cross-sections are usually modeled as static plasma columns within the magnetohydrodynamics (MHD) framework. We test the validity of this approach using in-situ plasma data from 151 magnetic clouds (MCs) observed by the WIND spacecraft and 45 observed by the Helios spacecrafts. We find that the most probable cross-section expansion speeds for the WIND events are only $\approx 0.06$ times the Alfvén speed inside the MCs while the most probable cross-section expansion speeds for the Helios events is $\approx 0.03$. MC cross-sections can thus be considered to be nearly static over an Alfvén crossing timescale. Using estimates of electrical conductivity arising from Coulomb collisions, we find that the Lundquist number inside MCs is high ($\approx 10^{13}$), suggesting that the MHD description is well justified. The Joule heating rates using our conductivity estimates are several orders of magnitude lower than the requirement for plasma heating inside MCs near the Earth. While the (low) heating rates we compute are consistent with the MHD description, the discrepancy with the heating requirement points to possible departures from MHD and the need for a better understanding of plasma heating in MCs.
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