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arXiv 2203.04385astro-ph.SR
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冕环和扩口环流体动力学模拟中的几何假设

Geometric Assumptions in Hydrodynamic Modeling of Coronal and Flaring Loops

Jeffrey W. Reep, Ignacio Ugarte-Urra, Harry P. Warren, Will T. Barnes

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Accepted to ApJ
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摘要：

在冠状环建模中，通常假设环是具有均匀横截面积的半圆形。然而，观察到的环很少是半圆形的，并且磁场的外推表明，磁场强度随着高度而减小，这意味着横截面积随着高度而扩大。我们直接研究这两个假设，以了解它们如何影响短，热回路强，脉冲电子束加热事件的流体动力学和辐射响应。面积膨胀的幅度和速率都直接影响动力学，并且膨胀的横截面显著延长了环路冷却和排水的时间，增加了上流持续时间，并抑制了声波。辐射冷却的标准$T \sim n^{2}$关系式不适用于膨胀回路，膨胀回路在相对较少的排水下冷却。另一方面，环的偏心率的增加仅增加排水时间尺度，并且通常是较小的影响。谱线强度也受到横截面积变化的强烈影响，因为它们取决于发射区域的体积以及密度和电离状态。随着更大的膨胀，密度降低，因此所有高度的线强度相对降低，并且由于冷却时间的增加，最热的线保持明亮的时间明显更长。区域扩张对流体动力学和辐射的精确建模至关重要，需要观测来约束扩张的幅度、速率和位置。

英文摘要：

In coronal loop modeling, it is commonly assumed that the loops are semi-circular with a uniform cross-sectional area. However, observed loops are rarely semi-circular, and extrapolations of the magnetic field show that the field strength decreases with height, implying that the cross-sectional area expands with height. We examine these two assumptions directly to understand how they affect the hydrodynamic and radiative response of short, hot loops to strong, impulsive electron beam heating events. Both the magnitude and rate of area expansion impact the dynamics directly, and an expanding cross-section significantly lengthens the time for a loop to cool and drain, increases upflow durations, and suppresses sound waves. The standard $T \sim n^{2}$ relation for radiative cooling does not hold with expanding loops, which cool with relatively little draining. An increase in the eccentricity of loops, on the other hand, only increases the draining timescale, and is a minor effect in general. Spectral line intensities are also strongly impacted by the variation in the cross-sectional area since they depend on both the volume of the emitting region as well as the density and ionization state. With a larger expansion, the density is reduced, so the lines at all heights are relatively reduced in intensity and, because of the increase of cooling times, the hottest lines remain bright for significantly longer. Area expansion is critical to accurate modeling of the hydrodynamics and radiation, and observations are needed to constrain the magnitude, rate, and location of the expansion or lack thereof.

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