Electron temperature of the solar wind
Electron temperature of the solar wind
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DOI：
10.1073/pnas.1917905117
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发表时间：
2020-04-28
影响因子：
11.1
通讯作者：
Egedal, Jan
中科院分区：
文献类型：
--
作者：
Boldyrev, Stanislav;Forest, Cary;Egedal, Jan
太阳风提供了一个例子，弱碰撞等离子体膨胀从热源在空间发散磁场线的存在。观测结果表明，在日球内层，电子温度随距离的增加而降低，近似为T_e（r），与r（-0.3）近似。r（-0.7），这明显慢于类似于r（-4/3）的绝热膨胀定律。出于这样的观察，我们提出了一个动力学理论，解决了非绝热演化的一个几乎无碰撞的等离子体从一个中心热源膨胀。我们专注于动态的高能电子传播沿着径向发散磁通管。由于它们的磁矩守恒，电子形成一束沿磁场线沿着准直的电子束。由于与背景等离子体的弱能量交换，束群缓慢地失去其能量并加热背景等离子体。我们提出，无论碰撞有多弱，在离源足够大的距离处，都建立了一个普遍的膨胀机制，其中电子温度以与r（-2/5）成比例的方式下降。这接近于观测到的日光层内部电子温度的比例。因此，我们的第一原理动力学推导可能提供一个解释慢于绝热温度下降的太阳风。更广泛地说，它可能有助于描述来自G型恒星的磁化无碰撞风。
Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic-field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as T-e(r) similar to r(-0.3) ... r(-0.7), which is significantly slower than the adiabatic expansion law similar to r(-4/3). Motivated by such observations, we propose a kinetic theory that addresses the nonadiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic-flux tube. Due to conservation of their magnetic moments, the electrons form a beam collimated along the magnetic-field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as T-e(r) proportional to r(-2/5). This is close to the observed scaling of the electron temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind. More broadly, it may be useful for describing magnetized collisionless winds from G-type stars.