We present low-frequency (80 – 240 MHz) radio observations of circular polarization in 16 isolated type III solar radio bursts using the Murchison Widefield Array (MWA) between August 2014 and November 2015. For most of the bursts, near burst onset, we find on average 9 % $9\%$ circular polarization at 80 MHz and 22 % $22\%$ at 240 MHz whereas these percentages are 5 % $5\%$ and 20 % $20\%$ near burst maximum. The polarization fractions are neither constant in time nor uniform over the spatial extents of the bursts. We measure polarization fractions as a function of burst source’s position. On average, near both burst onset and maximum, we find higher polarization near the disk center and lower polarization when the burst source is near the limb. We study total intensity (Stokes I $I$ ), circularly polarized intensity (Stokes V $V$ ), and polarization fraction ( | V | / I $| V| /I$ ) profiles for type III bursts with and without source motion as a function of position at times when the intensity of bursts is maximum. For the burst event with no source motion, we find symmetric profiles for Stokes I $I$ , V $V$ , and | V | / I $| V| /I$ . We find symmetric I $I$ and V $V$ but asymmetric | V | / I $| V| /I$ profiles for burst events which have source motion. We argue that this is due to the fundamental emission at the front of a type III electron beam and motion of the burst source. We then perform spectropolarimetric imaging studies of moving burst sources and analyze their motion. At burst onset, we obtain relatively higher polarization fractions, which is considered to be due to a large contribution from fundamental plasma emission at the front of the beam. At burst maximum, the polarization fraction is lower due to the combination of fundamental and harmonic components. After peak intensity, the emission is dominated again by the fundamental component that decays until the end of a burst with lesser polarization fraction than earlier. We argue that the fundamental radiation that decays over time after peak burst intensity is strongly scattered. This pattern of fundamental, fundamental and harmonic, and then fundamental emission with time at each frequency is consistent with the interpretations of Dulk, Suzuki, and Sheridan ( Astron. Astrophys. 130 , 39, 1984 ), Robinson, Cairns, and Willes ( Astrophys. J. 422 , 870, 1994 ), and Robinson and Cairns ( Solar Phys. 181 , 363, 1998 ). We propose that scattering effects can be a viable reason for low polarization fractions in type III events. Finally, we investigate the variations of the decay time ( t d $t_{d}$ ) for three events with frequency ( f $f$ ), finding that t d ∝ f − 2.0 ± 0.1 $t_{d} \propto f^{-2.0\pm 0.1}$ and decreases more rapidly with increasing f $f$ compared with previous lower-frequency observations ( t d ∝ f − 1.1 ± 0.1 $t_{d} \propto f^{-1.1\pm 0.1}$ ). This is interpreted in terms of radial variations of the turbulence properties.

中文翻译：

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公制III型太阳射电暴的光谱偏振成像

我们使用默奇森宽场阵列 (MWA) 在 2014 年 8 月至 2015 年 11 月期间对 16 个孤立的 III 型太阳射电暴中的圆极化进行了低频（80 – 240 MHz）无线电观测。对于大多数暴发，接近暴发开始，我们平均 9 % $9\%$ 圆极化在 80 MHz 和 22 % $22\%$ 在 240 MHz 而这些百分比是 5 % $5\%$ 和 20 % $20\%$ 接近突发最大值。在突发的空间范围内，极化分数在时间上既不恒定也不均匀。我们测量极化分数作为脉冲源位置的函数。平均而言，在爆发开始和最大值附近，当爆发源靠近边缘时，我们发现磁盘中心附近的极化较高，而爆发源的极化较低。我们研究总强度 (Stokes I $I$ )，圆偏振强度 (Stokes V $V$ )，和极化分数 (|V|/I$|V|/I$) 的剖面图，在爆发强度最大的时候，有和没有源运动的 III 型爆发作为位置的函数。对于没有源运动的突发事件，我们找到了斯托克斯 I $I$ 、V $V$ 和 | 的对称分布。V | /我$| V| /I$ 。我们发现对称的 I $I$ 和 V $V$ 但不对称 | V | /我$| V| 具有源运动的突发事件的 /I$ 配置文件。我们认为这是由于 III 型电子束前端的基本发射和爆发源的运动造成的。然后我们对移动的突发源进行光谱偏振成像研究并分析它们的运动。在爆发开始时，我们获得了相对较高的极化分数，这被认为是由于光束前部基本等离子体发射的巨大贡献。在最大爆发时，由于基波和谐波分量的组合，极化率较低。在峰值强度之后，发射再次由基本分量主导，该分量衰减直到具有比之前更小的偏振分数的爆发结束。我们认为，在峰值爆发强度之后随时间衰减的基本辐射被强烈散射。这种基波、基波和谐波，然后是每个频率的基波发射模式与 Dulk、Suzuki 和 Sheridan (Astron. Astrophys. 130 , 39, 1984)、Robinson、Cairns 和 Willes ( Astrophys . J. 422, 870, 1994)，以及 Robinson 和 Cairns (Solar Phys. 181, 363, 1998)。我们提出散射效应可能是 III 型事件中低极化率的一个可行原因。最后，我们研究了具有频率 ( f $f$ ) 的三个事件的衰减时间 ( td $t_{d}$ ) 的变化，发现 td ∝ f − 2.0 ± 0.1 $t_{d} \propto f^{-2.0 \pm 0.1}$ 并且随着 f $f$ 的增加与之前的低频观察相比下降得更快（ td ∝ f − 1.1 ± 0.1 $t_{d} \propto f^{-1.1\pm 0.1}$ ）。这是根据湍流特性的径向变化来解释的。