II型射电暴中谱峰突起的成像光谱观测
Imaging spectroscopy of spectral bump in a type II radio burst
AI总结：
本文利用LOFAR首次对具有谱峰突起的II型射电暴进行射电成像观测，通过分析源位置与频率漂移发现谱峰突起对应局部等离子体密度下降25%，并推断激波可能遭遇日冕洞上部区域。
摘要：
研究背景。太阳II型射电暴的观测为分析日冕激波加速的非热电子以及诊断日冕中的等离子体密度分布提供了独特的机会。然而，能够追踪这些电子的、具有高频率分辨率的干涉观测在II型射电暴中非常罕见。研究目标。最近，利用低频阵列（LOFAR）记录到了更多具有空间分辨能力的II型射电暴高分辨率观测。利用这些观测，我们旨在追踪一个经历突然谱峰突起的II型射电暴的位置。研究方法。在此，我们首次展示了对一个具有谱峰突起的II型射电暴进行的射电成像观测。我们测量了该II型射电暴源位置和频率漂移的变化，并推导出其传播方向上的密度分布。研究结果。我们识别出一个在其频率-时间剖面中经历突然谱峰突起的II型射电暴。其整体频率漂移率为0.06 MHz/s，对应估计速度为295 km/s。由成像得到的射电源投影速度为380 km/s，方向朝东。在谱峰突起处，观测到II型分裂频带源位置发生偏离。频带间距在南北方向上显著增大。结论。该谱峰突起在60 MHz处显示出8 MHz的偏离，对应等离子体密度下降25%。在II型射电暴谱峰突起期间估计的穿越距离为29 Mm，表明这一密度变化发生在一个受限区域内。这表明激波很可能遇到了一个日冕洞的上部延伸区域。
英文摘要：
Context. Observations of solar type II radio bursts provide a unique opportunity to analyze the non-thermal electrons accelerated by coronal shocks and also to diagnose the plasma density distribution in the corona. However, there are very rare high-frequency resolution interferometric observations for type II radio bursts that are capable of tracking these electrons. Aims. Recently, more spatially resolved high-resolution observations of type II radio bursts have been recorded with the Low-Frequency Array (LOFAR). Using these observations, we aim to track the location of a type II radio burst that experiences a sudden spectral bump. Methods. Here, we present the first radio imaging observations for a type II burst with a spectral bump. We measure the variation in source location and frequency drift of the type II burst, and deduct the density distribution along its propagation direction. Results. We identified a type II burst that experiences a sudden spectral bump in its frequency-time profile. The overall frequency drift rate is 0.06 MHz/s and it corresponds to an estimated speed of 295 km/s. The projected speed of the radio source obtained from imaging is 380 km/s towards the east direction. At the spectral bump, a deviation in the source locations of the type II split bands is observed. The band separation increases significantly in the north-south direction. Conclusions. The spectral bump shows an 8 MHz deviation at 60 MHz which corresponds to a 25% decrease in the plasma density. The estimated crossing distance during the spectrum bump in type II is 29 Mm suggesting that this density variation occurs in a confined area. This indicates that the shock most likely encounters the upper extent of a coronal hole.
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