Highly structured slow solar wind emerging from an equatorial coronal hole
Highly structured slow solar wind emerging from an equatorial coronal hole
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DOI：
10.1038/s41586-019-1818-7
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发表时间：
2019-12-12
期刊：
影响因子：
64.8
通讯作者：
Wygant, J. R.
中科院分区：
文献类型：
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作者：
Bale, S. D.;Badman, S. T.;Wygant, J. R.
在太阳活动最少的太阳极小期，太阳风(1，2)在高纬度地区被观测到，主要是一种速度很快(每秒超过500公里)的高度阿尔文稀薄的等离子体流，起源于日冕洞深处。在更靠近黄道面的地方，太阳风夹杂着一种变化更大的每秒不到500公里的慢风。慢风流的确切来源不太确定(4)；理论和观测表明，它们可能起源于头盔流光的尖端(5，6)，来自日冕洞边界附近的互换重联(7，8)，或位于磁场高度分散的日冕洞内(9，10)。驱动太阳风所需的加热机制也尚未解决，尽管候选机制包括阿尔芬波湍流(11，12)、纳米耀斑中的重新连接加热(13)、离子回旋波加热(14)和热梯度加速1。在一个天文单位的距离，风是混合和演变的，因此这些源和过程的大部分诊断结构已经丢失。在这里，我们介绍了帕克太阳探测器(15)在36到54个太阳半径范围内的观测结果，这些观测结果表明，缓慢的阿尔文太阳风从一个小的赤道冕洞中涌出。测量到的磁场显示出与等离子体喷流和增强的坡印廷磁通有关的大的间歇性反转，并散布在更平滑和更不湍动的流动中，并带有近径向磁场。此外，等离子体波测量表明，存在与等离子体加热和热化过程有关的电子和离子速度空间微观不稳定性(10，16)。我们的测量表明，存在与太阳风能化有关的脉冲机制，微观不稳定性在加热中起到了一定作用，我们提供了证据，表明低纬冕洞是缓慢太阳风的关键来源。
During the solar minimum, when the Sun is at its least active, the solar wind(1,2) is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvenic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind(3) of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain(4); theories and observations suggest that they may originate at the tips of helmet streamers(5,6), from interchange reconnection near coronal hole boundaries(7,8), or within coronal holes with highly diverging magnetic fields(9,10). The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfven-wave turbulence(11,12), heating by reconnection in nanoflares(13), ion cyclotron wave heating(14) and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe(15) at 36 to 54 solar radii that show evidence of slow Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities(10,16) that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.