Effects of a series of large flares from a sunspot group eruption on VLF propagation

Abstract

Sunspot groups are constantly evolving, causing flare eruption under the action of the solar atmospheric magnetic field. After the flare, the ejected X-rays cause the electron concentration of the Earth’s ionosphere to increase and the equivalent reflection height of the ionosphere to decrease. We studied the correlation between a series of flares erupted by sunspot groups and VLF signal propagation. Using NOAA sunspot activity area data and solar flare data released by the GOES satellite, we counted a series of flares erupted by sunspot groups in 2000. It is found that the AR9077 sunspot group continuously erupted 15 M-class and above flares and 3 X-class flares. Alpha VLF navigation station data were received in Haikou, and the phase anomalies of the VLF signal caused by a series of large flares erupted by AR9077 were observed. According to the phase anomaly, the flare is analyzed, and the relationship between X-ray flux density and ionospheric equivalent reflection height is fitted. We incorporated the change of solar zenith angle at the observation point in our calculation to correct the equivalent reflection height of the ionosphere and thus improve the accuracy of flare prediction. The calculated results are consistent with the data released by the GOES satellite. The observation results show that VLF phase anomaly correlates well with solar flares and sunspots. VLF signal can be used as a reliable scheme to predict the space environment, avoiding the impact of sunspot flares.