Magnetic Helicity Reversal in the Corona at Small Plasma Beta
Magnetic Helicity Reversal in the Corona at Small Plasma Beta
复制标题
小等离子体 Beta 下日冕中的磁螺旋度反转
DOI：
10.3847/1538-4357/aae97a
复制
发表时间：
2018
期刊：
影响因子：
--
通讯作者：
A. Brandenburg
中科院分区：
文献类型：
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作者：
P. Bourdin;N. Singh;A. Brandenburg
太阳和恒星发电机散发出相反符号的小尺度和大尺度的磁螺旋度。然而，太阳风观测和模拟表明，在发电机以上的一些距离上，小尺度和大尺度的磁螺旋度都有反转的迹象。现在有了对活动区域上方太阳日冕的真实模拟，我们就可以获得沿日冕环的磁场和电流密度。我们表明，当等离子体β的局部最大值降至1以下，磁场几乎完全不受力时，磁螺旋度的水平平均会发生符号反转。因此，这种逆转预计会发生在日冕内部，帕克太阳探测器或太阳轨道器无法直接进行现场测量。我们还表明，这种反转与具有正、负磁螺旋度的结构相对优势的微妙变化有关。
Solar and stellar dynamos shed small-scale and large-scale magnetic helicity of opposite signs. However, solar wind observations and simulations have shown that some distance above the dynamo both the small-scale and large-scale magnetic helicities have reversed signs. With realistic simulations of the solar corona above an active region now being available, we have access to the magnetic field and current density along coronal loops. We show that a sign reversal in the horizontal averages of the magnetic helicity occurs when the local maximum of the plasma beta drops below unity and the field becomes nearly fully force free. Hence, this reversal is expected to occur well within the solar corona and would not directly be accessible to in situ measurements with the Parker Solar Probe or SolarOrbiter. We also show that the reversal is associated with subtle changes in the relative dominance of structures with positive and negative magnetic helicity.