A Simple Way to Estimate the Soft X-ray Class of Far-Side

Solar Flares Observed with STEREO/EUVI

I.M. Chertok (1), A.V. Belov (1), V.V. Grechnev (2)

(1) Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), Troitsk, Moscow, Russia
(2)  Institute of Solar-Terrestrial Physics SB RAS, Irkutsk, Russia

Abstract.  Around the peaks of substantial flares, bright artifact nearly horizontal saturation streaks (B-streaks) corresponding to the brightest parts of the flare sources appear in the STEREO/EUVI 195 Å images. We show that the length of such B-streaks can be used for the solution of an actual problem of evaluating the soft X-ray flux and class of far-side flares registered with double STEREO spacecraft but invisible from Earth. For this purpose from data on about 350 flares observed from January 2007 to July 2014 (mainly exceeding the GOES M1.0 level) both with GOES and STEREO, an empirical relation is established correlating the GOES 18 Å peak flux and the B-streak length. This allowed us for the same years to estimate the soft X-ray classes for approximately 65 strong far-side flares observed by STEREO. The results of this simple and prompt method are consistent with the estimations of Nitta et al. (Solar Phys., 288, 241, 2013) based on the calculations of the EUVI full-disk digital number output. In addition, we studied some features of the B-streaks in impulsive and long-duration flares and demonstrated that B-streaks in several consecutive EUVI images can be used to reconstruct a probable time history of strong far-side flares.

Figure 1. The STEREO/EUVI 195 A B-streaks typical of C-, M-, and X-class flares.


Table 1: A list of flares registered both with GOES and STEREO/EUVI 


Figure 2. Scatter plot of the relative lengths of the STEREO/EUVI B-streaks versus the
GOES 18 A fluxes.


Figure 3. Impulsive GOES M-class flares and their thin, single, long B-streaks.


Figure 4. Long-duration GOES X-class flares and their thick, two or multi-element,
relatively short B-streaks. 

Figure 5. The distribution of the time difference between the observations of the longest
STEREO/EUVI B-streak and the peak of the SXR flux measured by GOES.

Figure 6. Full-Sun STEREO/EUVI 195 A images of six strongest far-side flares detected
from B-streaks (see Table 2).

Table 2. A list of major far-side flares detected from B-streaks in STEREO/EUVI 195 A images. 

Figure 7. Probable time history of the 18 A flux of two famous far-side flares estimated
from STEREO/EUVI B-streaks. 

 PDF file  of the paper

Solar Phys. Volume 290, Issue 7, pp. 1947-1961, 2015. DOI:  10.1007/s11207-015-0738-4.


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