Detection of Solar Flares from the Analysis of Signal-To-Noise Ratio Recorded by Digisonde at Ebro Observatory

This work proposes a new indirect method to detect solar flares based on the effects that produce in the signal of the radio waves reflected from the ionosphere as observed by a digisonde. Analysis of the signal-to-noise ratio (SNR) measured from ionograms for Ebro Observatory ionospheric station made possible to estimate a daily pattern of the SNR under no flare conditions and to detect absorption effects in the SNR caused under flare conditions. The method allows to characterize the ionospheric absorption of High Frequency radio waves as a product of these energetic events, and provides observational data to the international Service of Rapid Magnetic Variations (SRMV) to confirm Sfe (Solar Flare Effects). To set up the method, we have analyzed a data set of solar flares occurred during 2011—2014 at daylight hours at EB040 (262 flares, 17 X-class, 124 M-class, and 121 C-class). This led to impose a minimum threshold of -20dB in the SNR for at least four consecutive frequencies to confirm that a given solar flare happened. The method is particularly sensitive in the X-class solar flares detection, performs quite well with M-class flares and even detects some C-class flares. Furthermore, we deeply studied the observational and energetic constraints that affect the detection of the solar flares from the analysis of GOES hard X-Ray; e.g. for each event, we computed the solar altitude angle at the time of the ionospheric sounding to get an estimation of the geoeffective irradiance which has an effect on the local ionosphere. According to these constraints, we can confirm that the method is more effective for detection of flares that occur when the solar elevation angle is higher than 18.94º and have a geoeffective hard X-Ray irradiance above of 3.30·10^-6 W/m².