Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations. I. Observations and evaluation of methods
Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations. I. Observations and evaluation of methods
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DOI：
10.1051/0004-6361/201219354
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发表时间：
2013-04
影响因子：
6.5
通讯作者：
A. Warmuth;G. Mann
中科院分区：
文献类型：
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作者：
A. Warmuth;G. Mann
上下文。在太阳耀斑中，大量的热能和非热能以加热的等离子体和加速粒子的形式被脉冲释放。这些过程可以通过硬X射线(HXR)诊断进行研究。除了光谱观测之外，彻底了解热和非热粒子群还需要知道HXR源的大小。目标。给出了太阳耀斑中热源和非热源的几何源参数。我们比较和评估了四种不同的获取震源大小的方法，然后得出了最可靠的震源大小以及系统的不确定性。方法：研究方法。我们使用RHESSI仪器获得了从GOES C3.4级到X17.2级的24个耀斑的HXR图像时间序列。使用的四种成像技术是CLEAN、像素化、可见性前向拟合和MEM_NJIT。从这个数据集中，我们得到了热源和非热源的几何参数。使用不同的成像技术使我们能够量化系统测量的不确定性。结果。我们发现，不同的方法对HXR源的大小给出了一致的结果。这种关联性对于热源来说是非常好的，而对于脚点的关联性则略低。MEM_NJIT算法系统地提供了比其他方法更小的大小，这可能是过度分辨率的结果。热源体积为2×1025−1.2×1028 cm~3(中位数相对不确定度为30%)，非热足点面积为2×1016−6×1017 cm~2(中位数相对不确定度为40%)。我们的样品的热体积与微耀斑的热体积在相同的范围内，这意味着源大小不是耀斑能量学的重要参数。结论。使用不同的成像算法来确定HXR源大小的优点是，可以更好地量化不确定度，从而使导出的参数更可靠。结合在大量耀斑中作为时间序列得出的几何参数，这将使我们能够研究热和非热HXR源的标度关系和时间演化。
Context. In solar flares, a large amount of thermal and nonthermal energy is released impulsively in the form of heated plasma and accelerated particles. These processes can be studied via hard X-ray (HXR) diagnostics. In addition to spectroscopic observations, a thorough understanding of the thermal and nonthermal particle populations requires the knowledge of the HXR source sizes. Aims. We derive the geometric source parameters of both thermal coronal sources and the nonthermal HXR footpoints in solar flares. We compare and evaluate four different methods for obtaining source sizes, and then derive the most reliable source sizes, as well as the systematic uncertainties. Methods. We obtained time series of HXR images for 24 flares from GOES class C3.4 to X17.2 using the RHESSI instrument. The four imaging techniques employed are CLEAN, Pixon, visibility forward fit, and MEM_NJIT. From this data set, we derived the geometric parameters of the thermal HXR sources and the nonthermal footpoints. Using the different imaging techniques allowed us to quantify systematic measurement uncertainties. Results. We find that the different methods give consistent results on HXR source sizes. The correlations are very good for the thermal sources, and somewhat lower for the footpoints. The MEM_NJIT algorithm gives systematically smaller sizes than the other methods, possibly a result of over-resolution. Thermal source volumes are in the range of 2 × 1025−1.2 × 1028 cm3 (with a median relative uncertainty of 30%), and nonthermal footpoint areas in the range of 2×1016−6×1017 cm2 (median relative uncertainty: 40%). The thermal volumes of our sample are in the same range as those derived for microflares, which would imply that source size is not an important parameter for flare energetics. Conclusions. Using different imaging algorithms for determining HXR source sizes offers the advantage that uncertainties can be better quantified thus making the derived parameters more reliable. Combined with geometric parameters that were derived as time series in a larger number of flares, this will allow the study of the scaling relations and the temporal evolution of thermal and nonthermal HXR sources.