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太阳射电Ⅲ型爆是一种常见的快速射电爆发形式,在太阳耀斑、CME等剧烈太阳活动过程中常常成群出现。每一群一般包含几个到几十个单个Ⅲ型爆。在太阳活动中等强度的年份,每年可观测到成千上万次射电Ⅲ型爆。它们有时出现在耀斑开始时刻,有时出现在耀斑的峰值期间,有时也出现在耀斑后相中。同太阳耀斑有关的射电Ⅲ型爆有时还会周期性地出现,呈准周期性脉动(Quasi-Periodic Pulsation,QPP)特征,显示非热高能粒子的产生,或者粒子加速过程可能也具有准周期性发生的特点[21]。
太阳射电Ⅲ型爆的显著特征是快速的频率漂移。在低频段,太阳射电Ⅲ型爆的频漂率均为负,即爆发从高频迅速向低频漂移,频漂率也随频率的减小而降低。人们利用大量观测进行统计分析,发现在550 MHz~74 kHz频段,频漂率与频率之间近似满足下列拟合关系[22]
$$ \frac{\mathrm{d}f}{\mathrm{d}t}\approx -0.01{f}^{1.85} $$ (1) 其中:频漂率
$ \dfrac{\mathrm{d}f}{\mathrm{d}t} $ 的单位为$\mathrm{M}\mathrm{H}\mathrm{z}/{{\rm{s}}}$ ,频率$ f $ 的单位为MHz。上述拟合关系在低频和甚低频段基本适用,但是在厘米−分米波段的射电Ⅲ型爆的频漂率则常常显著偏离上述结果[23]。科学家甚至可以观测向低频和高频方向漂移的Ⅲ型爆发生,即频漂率符号相反的Ⅲ型爆对(type III pair),符号反转对应的频率称为反转频率($ {f}_{\mathrm{r}} $ )。统计表明,太阳射电Ⅲ型爆的反转频率在不同的耀斑事件中差别较大,大约介于300 MHz~3.4 GHz之间[24-25]。反转频率对应于耀斑爆发的初始能量释放源区,也即粒子加速区附近的射电辐射频率,也间接指示了源区的空间高度。如果知道不同频率随高度的变化函数关系,也就可以给出粒子加速源区的空间位置。在不同爆发事件中反转频率在米波段到分米波段变化这一特征表明,粒子加速源区的高度在不同爆发事件中是有显著差别的,大致介于从1万~10万 km之间。从反转频率往高频方向漂移的射电Ⅲ型爆,频漂率为正,称为反向Ⅲ型爆,反映了高能粒子离开加速源区后向太阳表面的飞行过程;与此相反,从反转频率向低频漂移的射电Ⅲ型爆的频漂率为负,通常称为正常Ⅲ型爆,反映了高能粒子离开加速源区后向远离太阳的行星际空间飞行的过程。式(1)仅给出了正常Ⅲ型爆频漂率的拟合规律。空间甚低频段的射电Ⅲ型爆通常也都属于正常Ⅲ型爆。
利用WIND/WAVES、SWAVES等空间低频−甚低频射电探测器,人们在该频段的射电Ⅲ型爆的研究方面已经取得了大量非常重要的研究[26]。图1是一例由SWAVES于2009年7月18日观测到的太阳射电Ⅲ型爆事件[27]。在该事件中,射电Ⅲ型爆从频率16 MHz一直向低频段延续到大约26 kHz附近,总持续时间超过2 h。在16 MHz附近,爆发的持续时间不足1 min,在1 MHz附近,持续时间为2~3 min,在100 kHz附近,爆发的持续时间则接近半0.5 h之久。与此相对比,在米波段的射电Ⅲ型爆的持续时间通常只有几s,而在厘米–分米波段的射电Ⅲ型爆,有时其持续时间甚至不到100 ms[28]。
利用地面太阳射电望远镜和空间太阳射电探测器的观测数据进行组合,可以得到从分米波直到千米波以下射电Ⅲ型爆的超宽带频谱图。图2是利用瑞士Phoenix 4射电频谱仪(200~900 MHz)、法国Nancay 10米波射电频谱仪(14~80 MHz)和空间WIND/WAVES射电探测器(0.02~14.0 MHz)等包括地基和空间望远镜在内共同获得的2014年2月16日一个超宽带射电Ⅲ型爆事件[29]。该事件中覆盖了从分米波一直延伸到波长为10 km的超长波段。
辐射频率在几百MHz米波段到1 GHz左右的分米波段的射电Ⅲ型爆辐射源区非常靠近粒子加速源区,即低日冕的太阳耀斑源区附近,距离太阳光球表面大约只有0.01~0.1倍太阳半径左右。但是,波长为几千米到十几千米的超长波(频率 < 0.1 MHz)射电Ⅲ型爆,它们的辐射源区则非常靠近地球轨道附近,与太阳耀斑的爆发源区大约距离200倍太阳半径以上。图2中的频谱结构表明,在不同频段的Ⅲ型爆在如此巨大的空间跨度上实际上是同一个连续的过程,即它们很可能直接反映了同一束非热高能粒子流在日冕和行星际空间高速飞行的轨迹。
在关于太阳射电Ⅲ型爆的源区研究方面,人们利用靠近SVLF频段的地面低频射电成像望远镜观测发现,太阳射电Ⅲ型爆的源区距离太阳表面的高度随频率的降低而逐渐增加,源区的尺寸也随频率的减小而逐渐增加。例如,同一个Ⅲ型爆的从169 MHz变化到43 MHz时,其源的大小则从大约5'而逐渐增加到20'。图3给出了人们利用欧洲LOFAR低频射电阵观测得到的频率为30~90 MHz的Ⅲ型爆群的频谱结果和其中一个III爆的源区图像,可以看出,Ⅲ型爆源从50~55 MHz演变到30~35 MHz时,其源区位置逐渐从太阳表面附近增加到大约3倍太阳半径处,源区尺寸也从不到0.5倍太阳半径(16')逐渐增加到1.0倍太阳半径以上[30]。
根据上述低频射电的成像观测结果,似乎可以外推,在SVLF频段的射电Ⅲ型爆的源区尺寸很可能会超过一个太阳直径以上。在行星际空间,比如辐射频率在1 MHz附近的射电Ⅲ型爆,其源区尺寸甚至可能达到数倍太阳直径大小,其角径可能达到1°甚至几度的数量级。
射电Ⅲ型爆源区的空间尺度随频率的减小而逐渐增加的主要原因是射电波辐射受行星际空间介质(InterPlanetary Medium,IPM)的散射效应,对于一个遥远的点源发射的射电信号,在经过行星际介质的散射后,其角径(
$ {\theta }_{\mathrm{I}\mathrm{P}\mathrm{M}} $ )将被扩大,成为一个有一定展宽的弥散面源,面源的大小近似为$$ {\theta }_{\mathrm{I}\mathrm{P}\mathrm{M}}\approx \frac{100'}{{(f \rm /MHz)}^{2}} $$ (2) 例如,在频率为100 MHz时,一个点源将被散射成一个角径大约为0.6″大小的面源;而在30 MHz时,该点源则变成一个大约为7″的面源,在1 MHz处则成了一个角径为100′(即1.6°)大小的弥散面源。这对望远镜的空间分辨率提出了一个限制。例如,在观测频率为1 MHz处望远镜是无法区分小于1.6°的结构细节的。
Research Advances of Solar Radio Type Ⅲ Bursts at Space Very Low Frequencies
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摘要:太阳非热高能粒子流是产生灾害性空间天气事件最主要的驱动源之一,其主要观测特征是具有快速频率漂移特征的射电Ⅲ型爆。主要介绍了国际上在空间甚低频波段(<30 MHz)太阳射电Ⅲ型爆的主要观测设备和研究进展,包括具备高时间–频率分辨能力的空间和月基甚低频频谱仪等,对存在的主要问题进行了详细讨论,系统分析了空间甚低频射电探测器观测数据在太阳射电Ⅲ型爆研究方面的主要科学目标和应用前景。Abstract:Solar non-thermal high-energy particle flows are one of the main triggering sources of the disastrous space weather events. Their main observational manifestation is radio Type Ⅲ bursts with fast frequency drifting rate. This work introduces the main advances of observational instruments and scientific researches on the solar radio Type Ⅲ bursts at the space very low frequency (< 30 MHz SVLF) in the world, including the space-based or lunar-based solar SVLF spectral polarimeters with high temporal and spectral resolutions. We also discuss carefully the existing problems on the solar radio observations and related researches, also including the main scientific aims and prospective of the space-based or lunar-based solar SVLF observations.
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Key words:
- solar activity/
- solar radio/
- radio observations/
- space weather
Highlights● The importance and significance of observations at solar radio type Ⅲ bursts at space very low frequency are given. ● A comprehensive introduction of the main international space SVLF detection equipment and working condition is presented. ● The main research results of solar radio Type Ⅲ bursts at SVLF in the world are summarized. ● The existing problems on the solar radio observations and related researches, also including the main scientific aims and prospective of the space-based or lunar-based solar SVLF observations are discussed. -
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