Malara F, Primavera L, Veltri P
Dipartimento di Fisica, Università della Calabria, 87030 Arcavacata di Rende (CS), Italy.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 1999 May;59(5 Pt B):6023-31. doi: 10.1103/physreve.59.6023.
Compressive fluctuations in solar wind slow speed streams are studied by means of a magnetohydrodynamics (MHD) model, which represents the plasma in the vicinity of the heliospheric current sheet. The model contains a current sheet, as well as density and temperature variations, corresponding to a large scale modulation of the specific entropy. Alfvénic fluctuations are initially superimposed on the background equilibrium and compressive fluctuations are consequently generated during the time evolution. The resulting correlation between density and temperature fluctuations at various spatial scales is interpreted in terms of both generation of magnetosonic fluctuations and of an "entropy cascade." The latter phenomenon arises as a consequence of the interaction between the MHD turbulence and the underlying large scale entropy structure. In particular, it is responsible for anticorrelated density and temperature fluctuations detected at various scales. The results of the model are compared with the proton density-temperature correlation calculated during several crossings of solar wind slow speed streams by the Helios spacecraft. The model reproduces to a good extent the main observed features, in particular the dependence of the correlation coefficient on location (close to or far from the current sheet) and on the fluctuation scale. The results show that large scale inhomogeneities, in particular, that of specific entropy, are important ingredients in the dynamics of the MHD turbulence in slow speed streams.
利用磁流体动力学（MHD）模型研究了太阳风低速流中的压缩波动，该模型代表了日球电流片附近的等离子体。该模型包含一个电流片，以及与比熵的大规模调制相对应的密度和温度变化。阿尔文波动最初叠加在背景平衡上，因此在时间演化过程中会产生压缩波动。在各种空间尺度上，密度和温度波动之间产生的相关性可以通过磁声波波动的产生和“熵级联”来解释。后一种现象是MHD湍流与潜在的大规模熵结构相互作用的结果。特别是，它导致了在各种尺度上检测到的密度和温度波动的反相关。将该模型的结果与太阳神号航天器在几次穿越太阳风低速流期间计算出的质子密度-温度相关性进行了比较。该模型在很大程度上再现了主要的观测特征，特别是相关系数对位置（靠近或远离电流片）和波动尺度的依赖性。结果表明，大规模不均匀性，特别是比熵的不均匀性，是低速流中MHD湍流动力学的重要组成部分。
