太阳射电尖峰的第一次频率-时间分辨成像光谱观测
First Frequency-Time-Resolved Imaging Spectroscopy Observations of Solar Radio Spikes
摘要：
太阳射电尖峰是在GHz到几十MHz范围内观测到的短持续时间、窄带宽的动态频谱精细结构。它们的持续时间非常短，频率带宽很窄，表明太阳日冕中的亚秒级小规模能量释放，但它们的起源尚不清楚。利用低频阵列射线（LOFAR），我们提出了空间，频率和时间分辨的观测与日冕物质抛射（CME）的个别无线电尖峰。个别的射电尖峰成像表明，观察到的面积随时间的推移而增加，个别尖峰的质心位置平行于太阳边缘超光移动。与在同一事件中观察到的单个IIIb型条纹的尖峰特征的比较表明，在持续时间，带宽，漂移率，偏振和观察面积，以及尖峰和条纹在图像平面中的运动的相似性，这表明基本等离子体发射与尖峰发射区域的数量级为${\sim}\:10^8$ cm，亮度温度高达$10^{13}$ K。所观察到的空间，光谱和时间特性的个人尖峰爆发也表明辐射负责尖峰逃逸通过各向异性密度湍流在闭环结构中的散射优先沿引导磁场定向平行于肢体的散射区域。在观测到的时间分布上，散射占主导地位，这表明能量释放时间可能比通常假设的要短。观测结果还表明，沿闭合磁场线的密度湍流各向异性高于沿开放磁场线的密度湍流各向异性。
英文摘要：
Solar radio spikes are short duration and narrow bandwidth fine structures in dynamic spectra observed from GHz to tens of MHz range. Their very short duration and narrow frequency bandwidth are indicative of sub-second small-scale energy release in the solar corona, yet their origin is not understood. Using the LOw Frequency ARray (LOFAR), we present spatially, frequency and time resolved observations of individual radio spikes associated with a coronal mass ejection (CME). Individual radio spike imaging demonstrates that the observed area is increasing in time and the centroid positions of the individual spikes move superluminally parallel to the solar limb. Comparison of spike characteristics with that of individual Type IIIb striae observed in the same event show similarities in duration, bandwidth, drift rate, polarization and observed area, as well the spike and striae motion in the image plane suggesting fundamental plasma emission with the spike emission region on the order of ${\sim}\:10^8$ cm, with brightness temperature as high as $10^{13}$ K. The observed spatial, spectral, and temporal properties of the individual spike bursts are also suggesting the radiation responsible for spikes escapes through anisotropic density turbulence in closed loop structures with scattering preferentially along the guiding magnetic field oriented parallel to the limb in the scattering region. The dominance of scattering on the observed time profile suggests the energy release time is likely to be shorter than what is often assumed. The observations also imply that the density turbulence anisotropy along closed magnetic field lines is higher than along open field lines.
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