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arXiv 2009.10604astro-ph.SR
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静止太阳观测中的传播效应

Propagation Effects in Quiet Sun Observations at Meter Wavelengths

Rohit Sharma, Divya Oberoi

摘要:

安静的太阳米波发射产生于MK日冕的热韧致辐射,并可能成为日冕诊断的丰富来源。在到达观测者的途中,由于传播效应(主要是折射和散射),它通过磁化和湍流的日冕介质得到了很大的改变,导致图像平面中强度的重新分布。通过比较在一个安静的太阳周期和模拟的热韧致辐射辐射的全磁盘米波太阳地图,我们验证了这些传播效应。100至240 MHz之间的太阳射电图来自Murchison Widefield Array。在球冠模型外,采用自洽磁流体动力学算法,利用FORWARD软件包模拟热韧致辐射图像。FORWARD模型不包括传播效应。观测到的和模拟的地图之间的差异被解释为由于散射和折射而产生的。有一个很好的预测和观察到的亮度分布之间的一般对应关系,虽然也观察到显着的差异。我们发现明显的传播效应,包括各向异性散射的存在下,有明确的证据。观测到的太阳的射电尺寸在面积上要大25- 30%。对应于唯一可见活性区的发射峰位移了8 ′ ~ 11 ′,大小增加了35--40%.我们的简单模型表明,散射通量密度的分数总是大于百分之几十,并在不同地区之间变化显着。我们估计密度不均匀性在1- 10%的范围内。

英文摘要:

Quiet sun meterwave emission arises from thermal bremsstrahlung in the MK corona, and can potentially be a rich source of coronal diagnostics. On its way to the observer, it gets modified substantially due to the propagation effects - primarily refraction and scattering - through the magnetized and turbulent coronal medium, leading to the redistribution of the intensity in the image plane. By comparing the full-disk meterwave solar maps during a quiet solar period and the modelled thermal bremsstrahlung emission, we characterise these propagation effects. The solar radio maps between 100 and 240 MHz come from the Murchison Widefield Array. FORWARD package is used to simulate thermal bremsstrahlung images using the self-consistent Magnetohydrodynamic Algorithm outside a Sphere coronal model. The FORWARD model does not include propagation effects. The differences between the observed and modelled maps are interpreted to arise due to scattering and refraction. There is a good general correspondence between the predicted and observed brightness distributions, though significant differences are also observed. We find clear evidence for the presence of significant propagation effects, including anisotropic scattering. The observed radio size of the Sun is 25--30\% larger in area. The emission peak corresponding to the only visible active region shifts by 8'--11' and its size increases by 35--40\%. Our simple models suggest that the fraction of scattered flux density is always larger than a few tens of percent, and varies significantly between different regions. We estimate density inhomogeneities to be in the range 1--10\%.

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