Implications of Flat Optically Thick Microwave Spectra in Solar Flares for Source Size and Morphology
Implications of Flat Optically Thick Microwave Spectra in Solar Flares for Source Size and Morphology
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DOI：
10.3847/1538-4357/ac0fdb
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发表时间：
2021-07
期刊：
影响因子：
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通讯作者：
Shaheda Begum Shaik;D. Gary
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文献类型：
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作者：
Shaheda Begum Shaik;D. Gary
这项研究的目的是研究太阳耀斑中低频、光学厚度的回旋同步微波辐射的光谱动力学，以确定发射源的特征。我们提出了一组微波爆发的高分辨率光谱观测扩展欧文斯谷太阳能电池阵列（EOVSA）在其调试阶段在2.5-18 GHz的频率范围内，1秒的时间分辨率。在本研究分析的12个事件中，9个爆发的光学厚光谱指数α l随时间直接下降，α l是源形态的指标。特别是，与同质/均匀源的预期光谱（α l ≈ 2.9）相比，五个爆发显示出“平坦”光谱（α l ≤ 1.0）。低频（<10 GHz）下的这些平坦光谱可以定义为来自具有大面积和/或具有多个发射分量的空间不均匀源的发射。在具有部分互相关数据的六个事件的子集中，具有平坦谱的两个事件在2.6- 3GHz处显示出120″的源大小。基于非均匀性的建模支持多个离散源只能再现平坦频谱的结论。我们报告说，这些平坦的光谱主要出现在衰变阶段，通常在大多数爆发的持续时间内增长平坦，这表明随着耀斑的发展，发射量的不均匀性和复杂性越来越大。这种充满被捕获的高能粒子的大量耀斑发射在其他波长下通常是不可见的，如硬X射线，这可能是由于这些低环境密度和磁场强度的区域中的无碰撞条件。
The study aims to examine the spectral dynamics of the low-frequency, optically thick gyrosynchrotron microwave emission in solar flares to determine the characteristics of the emitting source. We present the high-resolution spectra of a set of microwave bursts observed by the Expanded Owens Valley Solar Array (EOVSA) during its commissioning phase in the 2.5–18 GHz frequency range with 1 second time resolution. Out of the 12 events analyzed in this study, nine bursts exhibit a direct decrease with time in the optically thick spectral index α l , an indicator of source morphology. Particularly, five bursts display a “flat” spectrum (α l ≤ 1.0) compared to that expected for a homogeneous/uniform source (α l ≈ 2.9). These flat spectra at low frequencies (<10 GHz) can be defined as the emission from a spatially inhomogeneous source with a large area and/or with multiple emission components. In a subset of six events with partial cross-correlation data, both the events with flat spectra show a source size of ∼120″ at 2.6–3 GHz. Modeling based on inhomogeneity supports the conclusion that multiple discrete sources can only reproduce a flat spectrum. We report that these flat spectra appear predominantly in the decay phase and typically grow flatter over the duration in most of the bursts, which indicates an increasing inhomogeneity and complexity of the emitting volume as the flare progresses. This large volume of flare emission filled with the trapped energetic particles is often invisible in other wavelengths, like hard X-rays, presumably due to the collisionless conditions in these regions of low ambient density and magnetic field strength.