Quantifying Weak Nonthermal Solar Radio Emission at Low Radio Frequencies
Quantifying Weak Nonthermal Solar Radio Emission at Low Radio Frequencies
复制标题
量化低射频弱非热太阳能无线电发射
DOI：
10.3847/1538-4357/aa9d96
复制
发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Mihir Arjunwadkar
中科院分区：
文献类型：
--
作者：
Rohit Sharma;D. Oberoi;Mihir Arjunwadkar
最近，从默奇森宽场阵列（MWA）等新一代低射频仪器获得的细粒度高灵敏度数据为使用新技术表征这些频率下太阳发射的性质提供了机会。在这里，我们利用这个机会寻找在100-240 MHz频段存在弱非热发射的证据，其水平比迄今为止所探测的要弱。这些特征的存在被认为是基于纳米耀斑的日冕和色球加热理论的必然结果。我们将校准的MWA太阳动态光谱分为缓慢变化的和脉冲的，因此是非热的，成分。我们证明了高斯混合模型可以用来鲁棒地模拟后者，我们估计了磁通密度分布以及脉冲非热发射在频率-时间平面上的流行。据报道，在低至~ 0.2 SFU （1 SFU = 104 Jy）的水平存在非热排放的证据，使其成为该性质报道的最弱的排放。我们的研究表明，在中等太阳活动期间，非热脉冲辐射的占比在17%-45%之间。我们还发现，脉冲非热发射中辐射的磁通密度与慢变分量的强度非常相似，而慢变分量以热轫致辐射为主。弱脉冲非热发射的如此显著的普遍性和强度以前从未被认识到。
The recent availability of fine-grained high-sensitivity data from the new generation of low radio frequency instruments such as the Murchison Widefield Array (MWA) has opened up opportunities for using novel techniques for characterizing the nature of solar emission at these frequencies. Here we use this opportunity to look for evidence for the presence of weak nonthermal emissions in the 100–240 MHz band, at levels weaker than have been probed so far. The presence of such features is believed to be a necessary consequence of nanoflare-based coronal and chromospheric heating theories. We separate the calibrated MWA solar dynamic spectra into a slowly varying and an impulsive, and hence nonthermal, component. We demonstrate that Gaussian mixture modeling can be used to robustly model the latter, and we estimate the flux density distribution as well as the prevalence of impulsive nonthermal emission in the frequency-time plane. Evidence for the presence of nonthermal emission at levels down to ∼0.2 SFU (1 SFU = 104 Jy) is reported, making them the weakest reported emissions of this nature. Our work shows the fractional occupancy of the nonthermal impulsive emission to lie in the 17%–45% range during a period of medium solar activity. We also find that the flux density radiated in the impulsive nonthermal emission is very similar in strength to that of the slowly varying component, which is dominated by thermal bremsstrahlung. Such significant prevalence and strength of the weak impulsive nonthermal emission has not been appreciated before.
影响因子：
6.5
作者：
van Haarlem, M. P.;Wise, M. W.;van Zwieten, J.
通讯作者：
van Zwieten, J.