在类太阳活动事件中检验磁螺旋度守恒
Testing magnetic helicity conservation in a solar-like active event
摘要：
磁螺度是理想磁流体力学（MHD）的一个守恒量。因此，它可以作为磁化等离子体磁场演化的有效示踪剂。理论估计表明，磁螺旋度也基本上是保守的非理想MHD过程，如磁重联。然而，这个猜想几乎没有得到实验或数值的检验。由于磁螺旋度估计方法的最新进展，现在可以在一般的三维数据集中对其耗散水平进行数值测试。我们首先重新审视的相对磁螺旋度的时间变化的一般公式在一个完全有界的体积时，没有假设的规范。我们介绍了一种方法来精确地估计其耗散独立的非理想MHD过程发生的类型。在一个类太阳爆发事件的模拟中，我们使用不同的量规，比较其在有限体积中的估计与其通过边界的时间积分通量，从而测试螺旋度的守恒和耗散。我们给出了磁螺旋度真实耗散的一个上界：在准理想MHD阶段，它是准零的。即使当磁重联起作用时，磁螺旋度的相对耗散也非常小（<2.2%），特别是与磁能的相对耗散（大于30倍）相比。最后，我们说明了如何螺旋度通量项涉及速度分量的规范依赖，从而限制了它们的物理意义。
英文摘要：
Magnetic helicity has the remarkable property of being a conserved quantity of ideal magnetohydrodynamics (MHD). Therefore, it could be used as an effective tracer of the magnetic field evolution of magnetized plasmas. Theoretical estimations indicate that magnetic helicity is also essentially conserved with non-ideal MHD processes, e.g. magnetic reconnection. This conjecture has however been barely tested, either experimentally or numerically. Thanks to recent advances in magnetic helicity estimation methods, it is now possible to test numerically its dissipation level in general three-dimensional datasets. We first revisit the general formulation of the temporal variation of relative magnetic helicity on a fully bounded volume when no hypothesis on the gauge are made. We introduce a method to precisely estimate its dissipation independently of the type of non-ideal MHD processes occurring. In a solar-like eruptive event simulation, using different gauges, we compare its estimation in a finite volume with its time-integrated flux through the boundaries, hence testing the conservation and dissipation of helicity. We provide an upper bound of the real dissipation of magnetic helicity: It is quasi-null during the quasi-ideal MHD phase. Even when magnetic reconnection is acting the relative dissipation of magnetic helicity is also very small (<2.2%), in particular compared to the relative dissipation of magnetic energy (>30 times larger). We finally illustrate how the helicity-flux terms involving velocity components are gauge dependent, hence limiting their physical meaning.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
