JJI甚低频台站信号对太阳耀斑事件的响应特性

太阳耀斑发生时，日地空间的X射线通量会随之增大，进而影响到地球电离层的电子密度分布，导致地球-电离层波导状态发生改变，因此接收到的甚低频（VLF，Very Low Frequency）信号会表现出对应的扰动现象.2017年9月8日，位于湖北省内武汉和随州两站点的VLF接收机分别监测到与X射线太阳耀斑相关的来自日本宫崎县（130°49'E，32°04'N）的JJI甚低频台站信号（22.2 kHz）的振幅异常事件.分析当日的数据发现JJI信号的振幅对不同的太阳耀斑出现不同的响应类型，而且对于同一个耀斑，两地的信号响应类型不尽相同.通过统计2017—2019年间与太阳耀斑相关的JJI信号振幅扰动事件，发现两接收站点的JJI信号响应类型都与耀斑强度及其发生时间存在一定的关系，且呈现出四种响应类型，即两次上升下降型、先上升后下降型、下降型和上升型，但是四种响应类型的事件占比不同.拟合结果表明信号的扰动幅度与X射线通量的积分成正相关，但是两站点的线性拟合斜率存在差异.JJI信号到武汉和随州均属于近似沿纬度方向的短距离传播，且两接收站点相距较近，因此两传播路径大致相似.研究两路径上JJI信号对太阳耀斑响应的差异性有助于理解VLF信号的传播以及探索其在太阳活动监测方面的应用.

Response characteristics of very low frequency signals from JJI transmitter to solar flare events

The increase in X-ray flux during solar flares affects the electron density in the Earth′s ionosphere and thus changes the waveguide state,leading to the disturbance in signals observed by the VLF receiver.During the solar flares that occurred on 8 September 2017,both the VLF receiver at Wuhan and Suizhou in Hubei Province detected abnormal signal perturbations at 22.2 kHz from the JJI transmitter(130°49′E,32°04′N)in Japan.The observations show that the amplitude of the received JJI signal has different responses to different solar flares.Interestingly,we also found that the response types of the two receivers at Wuhan and Suizhou are significantly different for the same solar flare.Based on the observations of Wuhan and Suizhou receivers during 2017—2019,the statistical analysis of signal amplitude disturbance events from JJI station related to solar flares is performed.The results reveal that the response types in both paths are related to flare intensity and the occurrence time.Four different types of response are investigated,namely,twice rising and falling,first rising and then falling,falling and rising.However,the proportion of the four response types are different.The fitting results also show that the disturbed amplitude of the signal is positively correlated with the integration of X-ray flux,while the linear fitting slope of the two paths is different.The JJI signals received at Wuhan and Suizhou both belong to the short-distance transmission along the latitude direction,and the two receiving stations are close to each other.Thus,the two propagation paths are quite close.These results for the responses of the JJI signal to solar flares in the two paths are helpful to understand the propagation of VLF signals and shed light on monitoring solar activities by VLF signals.