Abstract
The response of the global ionosphere to solar flares is an important topic in the field of space weather. The global ionospheric response to solar flares is comprehensively analyzed from the perspectives of total electron content (TEC) and very low frequency (VLF) signals by using solar flare data on the eruption days of X-class flares from 2006 to 2019, including flare level, flare duration, geographical location, and local time. In addition, the relationship between X-ray flux and VLF phase variation is studied through correlation analysis. The concepts of disturbance intensity (DI) and disturbance angle are defined, and a DI evaluation model is established to help detect the difference in sensitivity to solar flares between TEC and VLF signals. Results show the following. (1) The higher the flare level and the longer the duration, the greater the disturbance to the ionosphere. Simultaneously, a phenomenon exists in which the disturbance caused by low-level and long-lasting flares is greater than that caused by high-level flares. (2) VLF phase variation and flare level exhibit a good correlation, and they are also closely related to geographical location and local time. The disturbance degree of a station facing the sun is more evident than that of a station facing away from the sun. The TEC disturbance of stations in the morning (local time) is more obvious than that in the afternoon, and disturbance increases along the direction of Earth’s rotation. (3) When DI is at the same level, the lowest flare level that causes TEC response is higher than the lowest flare level that causes VLF signal response. The disturbance angle of TEC is unevenly distributed within the interval [0°, 90°], and that of VLF signals is more than 85°. The sensitivity of VLF signals to flare response is considerably higher than that of TEC, and the difference between the two even stride across DI level.
Highlights
A solar flare is a type of high-energy radiation burst phenomenon that is frequently emitted by the sun; it is considered one of the most important types of space weather [1], [2]
(3) When disturbance intensity (DI) is at the same level, the lowest flare level that causes total electron content (TEC) response is higher than the lowest flare level that causes very low frequency (VLF) signal response
The disturbance angle of TEC is unevenly distributed within the interval [0◦, 90◦], and that of VLF signals is more than 85◦
Summary
A solar flare ( known as a chromospheric burst) is a type of high-energy radiation burst phenomenon that is frequently emitted by the sun; it is considered one of the most important types of space weather [1], [2]. The phenomenon of sudden phase anomaly (SPA) in the VLF phase causes abnormal changes; that is, when electron density in the D-layer increases sharply, the equivalent ionospheric reflection height decreases and the VLF signal propagation phase suddenly moves towards the advance phase; this phenomenon is called SPA [16]. The sensitivity of VLF signals to solar flares is reflected in the change in effective reflection height when electron density in the D-layer increases. Abnormal disturbances of TEC and VLF signals caused by solar flare eruptions under different factors are studied from a global perspective on the basis of the C- to X-class flare data among all the X-class flares from 2006 to 2019. The sensitivity difference of ionospheric TEC and VLF signals to solar flares is detected, and the response characteristics of the ionosphere to solar flares are identified
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