The force-free parameter $α$, also known as helicity parameter or twist parameter, bears the same sign as the magnetic helicity under some restrictive conditions. The single global value of $α$ for a whole active region gives the degree of twist per unit axial length. We investigate the effect of polarimetric noise on the calculation of global $α$ value and magnetic energy of an analytical bipole. The analytical bipole has been generated using the force-free field approximation with a known value of constant $α$ and magnetic energy. The magnetic parameters obtained from the analytical bipole are used to generate Stokes profiles from the Unno-Rachkovsky solutions for polarized radiative transfer equations. Then we add random noise of the order of 10$^{-3}$ of the continuum intensity (I$_{c}$) in these profiles to simulate the real profiles obtained by modern spectropolarimeters like Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, SOLIS etc. These noisy profiles are then inverted using a Milne-Eddington inversion code to retrieve the magnetic parameters. Hundred realizations of this process of adding random noise and polarimetric inversion is repeated to study the distribution of error in global $α$ and magnetic energy values. The results show that : (1). the sign of $α$ is not influenced by polarimetric noise and very accurate values of global twist can be calculated, and (2). accurate estimation of magnetic energy with uncertainty as low as 0.5% is possible under the force-free condition.
Spectro-Polarimetric Properties of Small-Scale Plasma Eruptions Driven by Magnetic Vortex Tubes
太阳对流的高度湍流性质导致了次表层与光球层和色球层之间强烈的多尺度相互作用。根据真实的三维辐射MHD数值模拟无处不在的小尺度涡管产生的湍流下可见的表面和集中在颗粒间的车道。涡管可以捕获和放大磁场，穿透色球层，并引发准周期性的流动爆发，产生Alfvénic波，将质量和能量输送到太阳大气中。模拟揭示了高速流动模式，复杂的热力学和磁结构的爆发涡流管。近地层强烈的压力梯度是自发喷发的启动和驱动力，而低色球层的洛伦兹力则加速了自发喷发。在本文中，模拟数据被用来进一步调查的喷发，其光谱偏振特性的Fe I 6301.5和6302.5 A谱线的动力学，并展示预期的签名的喷发日出SP数据。我们发现，复杂的动力学结构的涡管（在涡核和周边的上升流下降）可以捕捉到的斯托克斯I型。在喷发过程中，下降流和上升流的比例会突然改变，这种效应可以在斯托克斯V剖面中观察到。此外，在喷发期间，线偏振信号增加，这也可以用Hinode SP检测到。
Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images
