Observation and Analysis of Solar Flares that Cause Large-area Short-wave Communication Interruption

When a solar ﬂare erupts, the sun emits a ﬂood of X-rays and high-energy particles that reach Earth at the speed of light, causing a sudden ionospheric disturbance event (SID event). The D layer of the ionosphere absorbs high-frequency radio signals. With the increase of ﬂare intensity, the D layer’s absorption capacity becomes stronger, which leads to the decline of shortwave communication quality and even the interruption of shortwave communication. In this paper, solar ﬂares, which caused large area short-wave communication interruption in recent years, are observed and analyzed by very low frequency (VLF) method, and the inﬂuence of solar ﬂares on shortwave communication is summarized. Finally, several methods to deal with the short-wave communication interruption caused by solar ﬂares are proposed.

Schematic diagram of shortwave communication factors, including the equivalent height of the ionosphere, the electron density of the ionosphere, and the influence of day and night, season, climate, etc. In particular, the eruption of solar flares is more likely to cause a large area of short wave communication interruption. VLF signal mainly propagates in the waveguide formed between the ground and the lower ionosphere, and shortwave radio signal mainly propagates in a long distance through the reflection of the ground and the ionosphere, and its propagation path is shown in Figure 1. When a flare suddenly erupts, the sun radiates a lot of X-rays and high-energy particles. When X-rays arrive near the earth at the speed of light, the ionization degree of D layer in the low ionosphere increases, the electron concentration increases, and the equivalent reflection height of the low ionosphere decreases, resulting in the phase advance of VLF signal propagation. When the flare level reaches m or above, the D layer of the ionosphere absorbs the short wave signal strongly, and the short wave communication may be interrupted. Therefore, by monitoring the VLF phase change, we can alarm the short wave communication interruption events by observing the advance of VLF signal phase, and understand the exact reason of short wave interruption in time, which is very beneficial and necessary for taking necessary measures in advance to ensure the smooth shortwave communication line as far as possible.
2 Analysis of the response of very low frequency (VLF) propagation characteristics to solar flares Very low frequency (VLF) signals propagate very stably in the ground low ionospheric waveguide. When solar flares erupt, intense X-rays arrive near the earth, resulting in the increase of ionospheric ionization, the increase of electron concentration, the decrease of equivalent reflection height of low ionosphere, and the sudden advance of VLF signal phase. The phase of VLF signal is very sensitive to Sid events, so VLF method can be used as a very effective means to monitor solar activity. At the same time, the variation characteristics of low ionosphere can be studied by inversion.
When solar flares erupt, the phase advance of VLF signal ∆ϕ is affected by many factors, including the equivalent reflection height of the low ionosphere h 0 the frequency of the signal radiated by the VLF transmitting antenna f the direction of propagation path, the long distance of the path, and the ground conductivity.VLF radio waves propagate in the spherical shell space between the earth's surface and the lower ionosphere for a long distance. Generally, the "waveguide mode" theory is used to analyze the propagation. With the increase of distance, the higher-order mode decays rapidly, while the lower order mode decays slowly. In the place far away from the VLF transmitter,the existence of the higher-order mode can be ignored, and only the first-order mode can be calculated.
According to the waveguide mode theory, the first-order mode phase ve- Where,v c is the speed of light propagation in free space,a is the average radius of the earth,taking a = 6371km. The phase offset of VLF ∆ϕ can be expressed as: Where, d is the great circle distance of the earth between the transmitting station and the receiving point,v p is the phase velocity of the first-order mode of VLF signal without solar flare, and v ′ p is the phase velocity of the first-order mode of VLF signal with solar flare.
The relationship between the offset of equivalent reflection height ∆h 0 the offset of phase (radian) ∆ϕ is: Where,λ is the wavelength of VLF signal. When a solar flare erupts,the peak Finally, the level of solar flare can be judged according to the size of F 0 . When the flare level reaches M level, the D layer of ionosphere absorbs the short wave signal strongly, which may cause the interruption of short wave communication. Therefore, by monitoring the VLF phase change, we can give an alarm to the short wave communication interruption events, understand the exact reason of the short wave interruption in time, and take corresponding measures.
3 Observation and analysis of solar flares causing interruption of large area shortwave communication In Haikou, we received three signals from the Russian alpha VLF navigation system The amplitude and phase curves of the VLF signal with the radio station frequency of 11.9khz are plotted as shown in Fig. 2 to Fig. 4 (the solid line is the amplitude and phase curve on October 29, 2003, and the dotted line is the amplitude and phase curve on November 3, 2003).It can be seen from the figure that at 13:06BT on October 29, 2003, the solar flare eruption reached its peak, and the maximum phase lead of 11.9khz VLF signals from west sub station, main station, East sub station to Haikou are 25cec, 27cec and 52cec respectively; according to the earth's long distance, the distances from west sub station, main station and East sub station to Haikou receiving point of alpha system can be calculated to be about 25cec, 27cec and 52cec respectively It is 7022.5km, 4546.7km and 3875.7km.Through calculation, we get that the solar flare level is M3.8,the results observed by the US goes satellite are M3.5, which is basically consistent with our observation results;At 09:30 (BT) on November 3, the solar flare eruption reached its peak. The maximum phase advance of 11.9khz VLF signals from west sub station, main station and East sub station to Haikou are 53cec, 48cec and 72cec respectively. Through calculation, we get that the solar flare level is X2.4, and the result observed by goes satellite is X2.7, which is consistent with our calculation results within the allowable error range It is in accordance with this law;The specific data of solar flares in these two days are shown in Table 1. Similarly, we observed the solar flare activity on November 5, 2003, but when the solar flare erupted at X28 on November 5, 2003, it was at 03:50 am in China, so we failed to receive the corresponding VLF signal phase change. The flare has a wide range of influence. The short wave communication at home and abroad has been greatly affected. The global communication has been interfered, the maritime emergency call system has been paralyzed, the communication of the Everest expedition has been interrupted, and the accuracy of the global positioning system has been  reduced. Among them, the situation of short wave communication in northern China was seriously damaged, and the short wave signal of radio observation points in Beijing and Manzhouli was once interrupted,in foreign countries, a power system in the south of Malmo city in Sweden was damaged, and the power demand of more than 50000 people was affected, which had a great impact on people's lives. Therefore, the second solar flare is during the western countries' Halloween and is known as the "Halloween solar storm". Based on the analysis of the solar activity in the event of short wave communication interruption, we can draw a conclusion: if the solar flare burst level is low (below M level) or the duration of the event is very short, the change of low ionospheric electron concentration is not obvious, the change of low ionospheric height is not obvious, and the VLF signal advance is not obvious, it may only cause the instability of short time short wave communication signal; but Under the influence of some special space weather, the intensity and duration of the flare is large, the phase of VLF signal is seriously advanced,

CONCLUSION
The eruption of solar flares will not only affect the earth's ionosphere, but also affect the earth's climate and magnetic field. The magnetic storms caused by solar flares will also have a serious impact on the reception of short wave communication signals. In the period of frequent solar activity, the received signal is often weak or even interrupted. We can use VLF observation methods to monitor the electron concentration in the D-Layer ionosphere or to predict the critical frequency in the F-layer, and take corresponding preventive measures before solar flare, so as to ensure the stability of short wave communication.