Time Variations of the Nonpotential and Volume-threading Magnetic Helicities
Time Variations of the Nonpotential and Volume-threading Magnetic Helicities
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DOI：
10.3847/1538-4357/aadae7
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
L. Linan;'Etienne Pariat;K. Moraitis;G. Valori;James E. Leake
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文献类型：
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作者：
L. Linan;'Etienne Pariat;K. Moraitis;G. Valori;James E. Leake
相对磁螺旋度是一个规范不变的量，适用于研究日球等离子体的磁螺旋度含量。相对磁螺旋度可以唯一地分解为两个规范不变量，磁场的非势分量的磁螺旋度和一个互补的体积穿透螺旋度。最近对模拟太阳爆发产生的数值实验的分析表明，非势螺旋度与总相对螺旋度之比是磁系统爆发性的一个明显标志，该数值的高值可能是产生爆发的不稳定性开始的充分条件。本研究介绍了第一次分析检查的时间变化，这些nonpotential和体积线程螺旋。通过对日冕动力学的三维磁流体动力学（MHD）模拟分析，证实了所推导的解析公式的正确性。分析研究和数值计算结果表明，与磁螺旋度不同，非势螺旋度和体穿螺旋度即使在理想磁流体状态下也不是守恒量.对应于非势能螺旋度和体积螺旋度之间的转换的一个项经常支配它们的动力学。这一发现有一个重要的后果，他们在太阳日冕的估计：与相对螺旋度不同，它们的体积日冕演化不能确定这些量的流量通过体积的边界。只有外推三维日冕场的技术才能正确研究非势螺旋度和体积螺旋度，并对基于螺旋度的太阳喷发代理进行观测分析。
Relative magnetic helicity is a gauge-invariant quantity suitable for the study of the magnetic helicity content of heliospheric plasmas. Relative magnetic helicity can be decomposed uniquely into two gauge-invariant quantities, the magnetic helicity of the nonpotential component of the field and a complementary volume-threading helicity. Recent analysis of numerical experiments simulating the generation of solar eruptions have shown that the ratio of the nonpotential helicity to the total relative helicity is a clear marker of the eruptivity of the magnetic system, and that the high value of that quantity could be a sufficient condition for the onset of the instability generating the eruptions. The present study introduces the first analytical examination of the time variations of these nonpotential and volume-threading helicities. The validity of the analytical formulae derived are confirmed with analysis of 3D magnetohydrodynamics (MHD) simulations of solar coronal dynamics. Both the analytical investigation and the numerical application show that, unlike magnetic helicity, the nonpotential and the volume-threading helicities are not conserved quantities, even in the ideal MHD regime. A term corresponding to the transformation between the nonpotential and volume-threading helicities frequently dominates their dynamics. This finding has an important consequence for their estimation in the solar corona: unlike with relative helicity, their volume coronal evolution cannot be ascertained by the flux of these quantities through the volume’s boundaries. Only techniques extrapolating the 3D coronal field will enable both the proper study of the nonpotential and volume-threading helicities and the observational analysis of helicity-based solar-eruptivity proxies.