Energetics of the Kelvin-Helmholtz instability induced by transverse waves in twisted coronal loops
Energetics of the Kelvin-Helmholtz instability induced by transverse waves in twisted coronal loops
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DOI：
10.1051/0004-6361/201731178
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发表时间：
2017-08
影响因子：
6.5
通讯作者：
T. Howson;I. Moortel;P. Antolin
中科院分区：
文献类型：
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作者：
T. Howson;I. Moortel;P. Antolin
目标。我们量化了扭曲磁场对横向振荡日冕环中磁开尔文-亥姆霍兹不稳定性（KHI）发展的影响。方法。我们使用磁流体力学代码Lare3d在一个直的、密度增强的磁通量管中建立了一个基本的直立扭结模式。为了评估磁场方位分量的影响，在磁通管的磁场中加入了不同程度的扭转。结果。共振吸收过程仅受扭曲磁场存在的微弱影响。然而，KHI的后续演变对场的方位分量的强度很敏感。增加的捻度值抑制了线圈密度分布的变形，这与不稳定性的增长有关。尽管如此，当存在非零方位分量时，磁场中产生的尺度要小得多。因此，在（甚至是弱）扭曲场的情况下，不稳定性更强。一旦不稳定性形成，环形顶点处的场向流就会以扭曲的形式建立起来。此外，在直线场的情况下，在整个环流积分时，不存在涡度的净垂直分量。然而，方位角磁场的加入使涡旋度在扭结模式下产生了一个优先的振荡方向。结论。由于产生了小长度尺度和湍流状态，KHI可能对太阳大气中的波浪加热有影响。虽然磁扭确实抑制了与不稳定性相关的漩涡的发展，但由于产生的电流更大，即使只有弱扭，在扭曲状态下形成的KHI也会伴随着更大的欧姆耗散。磁扭曲的存在可能会使日冕的不稳定性更难被探测到，但会增强它对加热太阳大气的贡献。此外，沿着环路速度的发展可能有观测应用，用于推断日冕结构中磁扭曲的存在。
Aims. We quantify the effects of twisted magnetic fields on the development of the magnetic Kelvin-Helmholtz instability (KHI) in transversely oscillating coronal loops. Methods. We modelled a fundamental standing kink mode in a straight, density-enhanced magnetic flux tube using the magnetohydrodynamics code, Lare3d. In order to evaluate the impact of an azimuthal component of the magnetic field, various degrees of twist were included within the flux tube’s magnetic field. Results. The process of resonant absorption is only weakly affected by the presence of a twisted magnetic field. However, the subsequent evolution of the KHI is sensitive to the strength of the azimuthal component of the field. Increased twist values inhibit the deformation of the loop’s density profile, which is associated with the growth of the instability. Despite this, much smaller scales in the magnetic field are generated when there is a non-zero azimuthal component present. Hence, the instability is more energetic in cases with (even weakly) twisted fields. Field aligned flows at the loop apex are established in a twisted regime once the instability has formed. Further, in the straight field case, there is no net vertical component of vorticity when integrated across the loop. However, the inclusion of azimuthal magnetic field generates a preferred direction for the vorticity which oscillates during the kink mode. Conclusions. The KHI may have implications for wave heating in the solar atmosphere due to the creation of small length scales and the generation of a turbulent regime. Whilst magnetic twist does suppress the development of the vortices associated with the instability, the formation of the KHI in a twisted regime will be accompanied by greater Ohmic dissipation due to the larger currents that are produced, even if only weak twist is present. The presence of magnetic twist will likely make the instability more difficult to detect in the corona, but will enhance its contribution to heating the solar atmosphere. Further, the development of velocities along the loop may have observational applications for inferring the presence of magnetic twist within coronal structures.