A Simple Way to Estimate the Soft X-ray Class of Far-Side Solar Flares Observed with STEREO/EUVI
aa r X i v : . [ a s t r o - ph . S R ] J un Solar PhysicsDOI: 10.1007/ ••••• - ••• - ••• - •••• - • A Simple Way to Estimate the Soft X-ray Class ofFar-Side Solar Flares Observed with STEREO/EUVI
I.M. Chertok · A.V. Belov · V.V. Grechnev Received ; acceptedc (cid:13)
Springer ••••
Abstract
Around the peaks of substantial flares, bright artifact nearly horizon-tal saturation streaks (B-streaks) corresponding to the brightest parts of the flaresources appear in the STEREO/EUVI 195 ˚A images. We show that the lengthof such B-streaks can be used for the solution of an actual problem of evaluatingthe soft X-ray flux and class of far-side flares registered with double STEREOspacecraft but invisible from Earth. For this purpose from data on about 350flares observed from January 2007 to July 2014 (mainly exceeding the GOESM1.0 level) both with GOES and STEREO, an empirical relation is establishedcorrelating the GOES 1–8 ˚A peak flux and the B-streak length. This allowed usfor the same years to estimate the soft X-ray classes for approximately 65 strongfar-side flares observed by STEREO. The results of this simple and promptmethod 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 EUVIimages can be used to reconstruct a probable time history of strong far-sideflares.
Keywords:
STEREO; Extreme Ultraviolet; Solar Flares; Saturation Streaks;GOES; Soft X-Rays
1. Introduction
In October 2006, the double
Solar Terrestrial Relations Observatory (STEREO;Kaiser et al. , 2008) was launched, and at the end of January 2007 the twospacecraft separated and entered into heliocentric orbits near 1 AU in oppositedirections. The Ahead (A) probe leads the Earth, while the Behind (B) probe Pushkov Institute of Terrestrial Magnetism, Ionosphere andRadio Wave Propagation (IZMIRAN), Troitsk, Moscow,142190 Russia email: [email protected] ; [email protected] Institute of Solar-Terrestrial Physics SB RAS, LermontovSt. 126A, Irkutsk 664033, Russia email: [email protected]
SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 1 hertok et al. lags behind the Earth, drifting about 22 ◦ per year from the Sun–Earth line.In February 2011, the two STEREO spacecraft were already in the quadraturewith the Earth, providing the first ever complete 360 ◦ view of the Sun. At theend of July 2014, the two spacecraft were located on the opposite sides of theEarth’s orbit; STEREO-A was ahead of the Earth by 164 ◦ , and STEREO-B was162 ◦ behind the Earth. Each STEREO spacecraft is equipped with an almostidentical set of extreme ultraviolet, optical, radio, and in situ instruments. Inparticular, the Sun Earth Connection Coronal and Heliospheric Investigationsuit (SECCHI; Howard et al. , 2008) includes the
Extreme Ultraviolet Imager (EUVI; Wuelser et al. , 2004) providing solar images in four channels of 171,195, 284, and 304 ˚A.With an increase of the longitudinal separation between the two STEREOspacecraft and each of them with the Sun–Earth line, the number of flaresregistered with the A and/or B probes but invisible from Earth has been growing.Among the problems in studies of the backside flares is classifying their impor-tance, which would allow one to compare them with flares recorded by near-Earthsatellites. STEREO observations and information about the importance of far-side flares are significant for investigations of such solar activity phenomenaas flares themselves, eruptions, large-scale coronal waves, coronal mass ejec-tions (CMEs), solar energetic particles (SEPs). These observational studies arepromising to achieve further progress in understanding and modeling these phe-nomena as well as space weather forecasting (e.g., Hudson, 2011; Lugaz et al. ,2012; Webb and Howard, 2012; Nitta et al. et al. ,2014; Li et al. , 2014; Richardson et al. , 2014; Kwon, Zhang, and Vourlidas, 2015).Currently, in addition to the classification of flares based on the intensity andemission area in the H α line, the soft X-ray flare (SXR) classification is generallyaccepted and widely used. The C, M, and X SXR classes of flares are determinedaccording to the peak fluxes measured by the Geostationary Operational Envi-ronmental Satellite (GOES; Garcia, 1994) 1–8 ˚A detectors in the ranges of (1–10) × − , 10 − , and 10 − W m − , respectively.Nitta et al. (2013a) addressed the problem of classification of the STEREObackside flares calculating the EUVI 195 ˚A full-disk emission fluxes as a to-tal output of the charge-coupled device (CCD) camera in units of digital datanumber (DN) per second. Using data from June 2010 to September 2012, theauthors selected the flares that were recorded by GOES and simultaneouslywere observed with one or both STEREO spacecraft and found an empiricalrelation between the observed GOES 1–8 ˚A peak fluxes and the calculatedEUVI 195 ˚A full-disk DN output. Using this relation, one can estimate theGOES peak X-ray flux of sufficiently intense backside flares observed only bySTEREO. Nitta et al. (2013a) presented a list of 16 such major far-side flaresdetected with this procedure.In the present paper, we propose a somewhat simpler technique for esti-mations of the X-ray class of STEREO flares. The technique is based on themeasurements of the length of a bright artifact saturation streak, which appearsin the EUVI 195 ˚A images nearly along its East–West axis around the peaksof sufficiently strong flares (Figure 1). Such a streak is a consequence of the SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 2 oft X-ray Class of Far-Side Flares
Figure 1.
The STEREO/EUVI 195 ˚A B-streaks typical of C, M, and X class flares. The spatialscale shown in panel (c) is the same for all of the three images. The 2011-03-09, 2011-12-26,and 2013-05-13 flares are labeled 77, 157, and 295 in Figure 2 (see also Table 1). so-called blooming, i.e. , saturation of CCD cells, corresponding to the bright-est part of a flare source, and spilling of excessive electrons from these cellsalong CCD columns (see Wuelser et al. , 2004). Similar overexposure effects occuralso in the
Extreme ultraviolet Imaging Telescope (EIT; Delaboudini`ere et al. ,1995) images gathered with SOHO ( e.g. , Andrews, 2001) and in the
Atmo-spheric Imaging Assembly (AIA; Lemen et al. , 2012) images obtained with SDO(Schwartz, Torre, and Piana, 2014).Henceforth we will refer to the blooming streaks as B-streaks. The B-streaksaffect the images of flare cores around the peak time. These sources and momentsare most interesting physically. The B-streaks are considered usually as a seriousinterference for the image processing and studies. Our analysis shows howeverthat B-streaks contain useful information about the flare importance and thebrightest EUV sources in general. This idea is based on the fact that the largeremission flux produces the stronger blooming in CCD cells, and therefore alonger B-streak is formed (see Figure 1). In the next section 2, we describeour approach and data selected for the analysis. Section 3 is devoted to strongflares observed with both GOES and one or two STEREO spacecraft. Theseconcurrent observations allowed us to obtain an empirical relation between therelative lengths of B-streaks and the SXR fluxes ( i.e. , the GOES classes) ofthe STEREO flares. In Section 4, we present illustrations and a list of majorbackside flares (mainly above the M3.0 class), registered by STEREO spacecraftin 2007–2014. We also compare the SXR classes of far-side flares evaluated byNitta et al. (2013a), using the calculated DN fluxes in the EUVI 195 ˚A channel,and those estimated with our technique. The paper ends in Section 5 with asummary and concluding remarks.
2. Approach and Data
In order to judge to what extent B-streaks are suitable for an assessment of theSXR class of backside flares, it is necessary to analyze the relationship between
SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 3 hertok et al. these parameters for coincident flares observed simultaneously with GOES andone or both STEREO spacecraft. Proceeding from the STEREO trajectories, itis clear that initially the number of such coincident flares was large, but thengradually decreased. After February 2011, when the two STEREO spacecraftwere in the near-quadrature configuration relative to the Sun–Earth line, onlythose flares were observed both with GOES and one of the STEREO probes thatwere sufficiently removed from the central meridian visible from Earth. In 2013and 2014, only near-the-limb flares, from the Earth view, could be observed withGOES and one of the STEREO spacecraft; the west- and east-limb GOES flareswere observed with A and B stations, respectively.We have analyzed the NOAA GOES flare list up to July 2014, i.e. , during theascent and maximum phases of Solar Cycle 24, ( ftp://ftp.ngdc.noaa.gov/STP/space-weather/solar-data/solar-features/solar-flares/x-rays/goes/ )and selected for further analysis practically all coincident flares above the M1.0class. The exceptions were a small number of flares overlapping in time, butoccurring in different active regions; flares with obvious partial limb occultationfor either STEREO or GOES; and some M1–M2 flares, in which a relativelyshort B-steak was not discernible against extended pre-existing structures. Inaddition to strong flares, a certain amount of C-class flares with a visible B-streak was randomly chosen to be included in our analysis. We considered theseevents to reveal more clearly the tendency of the increasing length of the EUVB-streak with an increasing SXR flare importance.For the considerations of B-streaks we used EUVI images in the 195 ˚A channel,which encompasses the Fe xii line with a peak temperature of ≈ . xxiv line at 192 ˚A that makes the 195 ˚A channel the closest analog of the SXRGOES monitors (see Nitta et al. , 2013a). As the main quantitative parametercharacterizing B-streaks, we took its maximum length, L , measured in fractionsof the solar radius, R S , in the same EUVI image, i.e. , the L/R S ratio. As willbe shown later, the length of a B-streak can considerably exceed 1 R S in majorflares.We endeavored to develop a method, which would be as simple as possible.For this reason, we did not analyze the source FITS files, and used instead theEUVI 195 ˚A images in JPEG and movies in MPEG, which are available at http://stereo-ssc.nascom.nasa.gov/browse/ . They are already processed with thestandard SolarSoft routine secchi prep.pro, and rotated so that solar north is up.In spite of their non-linear brightness scale, most B-streaks are clearly discerniblein the 512 ×
512 movies. We also use the 2048 × τ exp . For each image, information on τ exp canbe found at http://sharpp.nrl.navy.mil/cgi-bin/swdbi/secchi flight/img short/form ,using the full-page output. It turned out that more often the EUVI telescopesoperated with τ exp = 8 s. The 16 s exposure time was almost regularly used inearly observations until January 2010. Later on, τ exp = 16 s had only the imagesrecorded every even hour at the 16th minute, i.e. , at 00:16, 02:16, 04:16, and soon ( all times hereafter refer to UT ). We corrected all the considered images to SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 4 oft X-ray Class of Far-Side Flares
Table 1.
Extraction from the list of flares registered both with GOES and STEREO (seesection 2). Asterisks mark the events in which the correction for an exposure time of 8 s wasapplied. GOES SXR STEREONo. Date Time Class Flux Location AR A/B Time
L/R S UT F G UT(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)17 2010-02-06 18:59 M2.9 29 N21 E17 11045 A 19:03 0.3427 2010-02-12 11:26 M8.3 83 N26 E11 11046 B 11:26 0.5477 ∗ ∗ ∗ ∗ ∗ ∗ τ exp = 8 s. Namely, if the longest B-streak occurred in an image with τ exp = 16 s,we halved its length and compared it with B-streaks in adjacent images. We thenexcluded flares (mainly of C-class) in which after such a correction the maximumrelative length of B-streaks, L/R S , was less than 0.1. The reason is that shortB-streaks in such events could become visible against the background of otherflare structures just because τ exp = 16 s.The list of events selected for our analysis contains about 275 flares, whoseGOES importance was M1 or higher, and 75 C-class flares. The complete list of SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 5 hertok et al. flares observed with both GOES and STEREO/EUVI is accessible at ∼ ichertok/STEREO/ .Table 1 presents some flares extracted from this list and contains, in particular,those events that will be mentioned below, as well as a number of major flares.Column 1 presents the serial number of a flare in the complete list. Columns2–7 contain information on GOES SXR observations of a flare in 1–8 ˚A, in-cluding date; peak time; COES class; corresponding SXR flux, F G , in units of10 − W m − ; coordinates; and the NOAA number of an active region. Columns8–10 specify STEREO A or B spacecraft; the observation time of the B-streak(rounded to 1 min); and its maximum relative length, L/R S .
3. Relation between the GOES Flux and EUVI B-streak
In the preceding section, was mentioned how to correct for the EUVI exposuretime. Some other factors can affect the relations between the lengths of the EUVIB-streaks and the GOES SXR fluxes. These are:– Different temperature responses of the SXR GOES detectors and the EUVI195 ˚A channel, as mentioned in section 2.– Possible time difference between the flare peak in SXR and EUV emissions.– Limited imaging rate of the EUVI (section 2) and possible related omissionsof flare peaks.– Significant differences between impulsive (compact) flares and long-durationevents (LDEs, which are usually associated with big CMEs; see below).– A difference (though small) between the distances of the STEREO-A andB spacecraft from the Sun.– Particularities of the B-streak formation in the blooming process.– We measure the longest B-streak only, although several B-streaks occur insome flares (mainly LDEs; see below).We disregard these factors for simplicity of the method. Nevertheless, asFigure 2 shows, a clear relationship does indeed exist between the maximumrelative length of the B-streak,
L/R S , and the peak GOES 1–8 ˚A flux, F G . Onaverage, when L/R S increases from 0.03 to 1.5, F G rises from 3 to 600 (hereafter F G is expressed in the unit of 10 − W m − ) that corresponds to the SXR flareclass from C3 to X6. The C-class flares confirm the trend which M- and X-classflares show. We remind that only a limited number of C flares is presented herewhich were selected randomly and had a conspicuous B-streak. The visibility ofshort B-streaks is obviously hampered, because pre-flare and flaring structureshave some longitudinal extent, too. Therefore, the points of C-class flares inFigure 2 are biased to relatively longer B-streaks due to their visual selectionand the limited number of events. Further we consider only the most powerfulflares above the M1.0 level. Because we do not control any of the variables in the F G − L/R S relationship, the geometric mean regression method was chosen toobtain an empirical dependence between these quantities. Using this method, weobtained a power-law regression equation to fit the relation between the lengthof the B-streak and the SXR flux, F G = 392 × ( L/R S ) . (1) SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 6 oft X-ray Class of Far-Side Flares
77 17 2799157 171 189 295337
Figure 2.
Scatter plot of the relative lengths of the STEREO/EUVI B-streaks versus theGOES 1–8 ˚A fluxes. The gray open circles denote C-class flares, and the black filled circlesdenote ≥ M1-class flares. The line corresponds to the regression equation (1). The numbersspecify the flares shown in Figures 1, 3, and 4. (the line in Figure 2). The correlation coefficient between
L/R S and F G is r ≈ .
81. For the majority of the ∼ > M1.0 flares, deviations of the SXR flux from theregression line down or up do not exceed a factor of 2. Equation (1) can be usedfor estimations of the SXR fluxes and GOES classes of far-side flares observedby STEREO but invisible from Earth. We will return to this issue in section 4.Our analysis shows that many impulsive and LDE flares are characterizedby different B-streaks. As Figure 3 demonstrates, in the standard 3-day GOESplots, impulsive flares look like spikes almost without any distinct decay phase.In such flares, the whole duration and decay times measured when the fluxlevel decays to a point halfway between the maximum flux and the pre-flarebackground level, are 10–20 min and 3–10 min, respectively. Such a B-streak isvisible usually only in 2–3 EUVI frames of the 5-min cadence. Figure 3 illustratesthat impulsive flares produce a single, thin and relatively long B-streak (seeFigure 2 and Table 1). This is due to the fact that almost the entire flux fromsuch flares is emitted by a single compact core corresponding to a few pixels.Accordingly, such events reside under the regression line in Figure 2.In contrast, LDE flares produce relatively short, but thick B-streaks with twoor more blooming elements (Figure 4). Apparently, a number of bright sourcekernels corresponding to these blooming elements contribute to their peak fluxes.In its most developed form, such a multi-element blooming structure does not
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Figure 3.
Impulsive GOES M-class flares (upper row) and their thin, single, long B-streaks(bottom). The 2010-02-06, 2010-02-12, and 2011-07-30 events are labeled 17, 27, and 99 inFigure 2 (see Table 1). always temporally coincide to the appearance of the longest B-streak. In theevent shown in Figure 4a, several short blooming elements almost merged intoone thick B-streak, while in the flares presented in Figures 4b and 4c, the thickpart of the B-streak and diffuse core are contained between two thin bloomingelements. The LDE flares have noticeably longer durations of 35–75 min anddecay times of 15–45 min; their multi-element B-streaks are visible in largernumbers of EUVI images (up to 10). Let us remind that in such events we takeinto account only one, the longest B-streak. Feasible summation of the lengthsof all visible B-streaks is not always unambiguous, and would complicate themethod proposed. The fact that we consider only one streak can be the reasonwhy the mentioned LDE events are located above the regression line in Figure 2.Based on the fact that the longest B-streaks indicate the brightest flaresources, it is interesting to consider the time difference, ∆ t , between the ob-servation of such a B-streak in a STEREO/EUVI image, t S , and a flare peakin the soft X-ray GOES data, t G . The corresponding histogram for ≥ M1.0flares is presented in Figure 5. The time difference does not exceed ± L/R S ≥ .
35, the longest B-streak was observed
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Figure 4.
Long-duration GOES X-class flares (upper row) and their thick, two or multi-ele-ment, relatively short B-streaks (bottom). The 2012-01-27, 2012-03-07, and 2014-02-25 eventsare labeled 171, 189, and 337 in Figure 2 (see Table 1). near the GOES SXR peak. Obviously, | ∆ t | should be small in short-durationflares. Pre-flare heating of brightened erupting filaments or their static extensionsproduce earlier B-streaks (∆ t < t ≥
10 min occurred a few times more often thanadvancing B-streaks with ∆ t ≤ −
4. Powerful Far-Side Flares
In this section, we present powerful far-side flares which were not visible fromEarth but were found due to B-streaks through a visual inspection of dailySTEREO/EUVI movies and images of 2007–2014. Almost no such far-side flares
SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 9 hertok et al. -20 -10 0 10 20 30 40 ∆ t = t S -t G (min)020406080100120140 N u m be r o f e v en t s Figure 5.
The distribution of the time difference between the observations of the longestSTEREO/EUVI B-streak, t S , and the peak of the SXR flux measured by GOES, t G , forcoincident ≥ M1.0 flares. were found in 2007–2009 for two reasons. Firstly, major flares occurred rarelyduring this period of low activity. Secondly, two parts of the solar surface in-visible from Earth but observed with STEREO spacecraft were still small. Thelongitudinal extent of these parts gradually increased, and early in 2011 thewhole far-side of the Sun became accessible to STEREO observations. In 2014,both STEREO probes were located almost exactly behind the Sun, and almostall far-side flares were observed by the two spacecraft simultaneously.The SXR fluxes (GOES classes) of such backside flares were estimated fromthe relative lengths
L/R S of the longest B-streaks using equation (1). We re-stricted ourselves to flares with L/R S ≥ .
2, which corresponds to the GOESclass of ∼ > M4.0. The results are presented in Table 2, where for each of thedetected events the following information is listed: the date (column 1); timeof the longest B-streak (2); the STEREO A or B probe, in whose images thestreak was measured (3); approximate coordinates of the flare (4); the NOAAnumber of active region (5) assigned before its disappearance behind the westlimb (mainly for A-probe) or after its appearance at the east limb (mainly forB-probe); the value of the
L/R S parameter (6); the estimated peak SXR flux inunits of 10 − W m − (7), and the flare GOES class (8). In accordance with thesmall statistical errors of the parameters found in evaluating equation (1), the95% confidence interval of the estimated flare class is rather narrow for shortB-streaks and broadens gradually with an increase of L/R S . For example, theconfidence interval is M2.6–M4.7 for an M3.5 flare and X10–X18 for an X13flare.Column 9 of Table 2 presents probable ranges of the GOES classes for majorfar-side flares of 2010–2012 estimated by Nitta et al. (2013a) from the calculatedtotal DN output. We included all of the 16 backside flares listed by the authors,although three of these flares had L/R S < .
2. In two of these events (2011-03-21, 02:11 and 2011-11-03, 22:41), the longest B-streaks were even shorter,
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Table 2.
A list of major far-side flares detected from B-streaks in STEREO/EUVI 195 ˚Aimages (see Section 4). Asterisks in column 6 mark the events in which the correction for anexposure time of 8 s was applied. Estimated SXRDate Time A/B Location AR
L/R S Flux Class Nitta et al. UT F G class range(1) (2) (3) (4) (5) (6) (7) (8) (9)2010-01-17 03:56 B S25 E128 11041 0.54 163 X1.6 M3.4–M9.62010-01-19 13:41 B S25 E95 11041 0.33 81.2 M8.1 –2010-01-19 20:36 B S25 E92 11041 0.37 95.6 M9.6 –2010-07-29 08:31 A N29 W136 – 0.23 48.6 M4.9 –2010-08-31 20:56 A S22 W146 11100 0.49 142.4 X1.4 M8.4–X2.52010-09-01 21:51 A S22 W162 11100 0.42 114.4 X1.1 M5.4–X1.62010-11-03 12:16 B S19 E98 11121 0.26 ∗ ∗ ∗ ∗ ∗ SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 11 hertok et al.
Table 2.
Continued
Estimated SXRDate Time A/B Location AR
L/R S Flux Class Nitta et al. UT F G class range(1) (2) (3) (4) (5) (6) (7) (8) (9)2013-04-24 21:46 A N09 W169 11719 0.23 48.6 M4.9 –2013-05-01 02:31 B N15 E115 11739 0.32 77.7 M7.8 –2013-10-05 06:56 B S23 E125 11865 0.23 48.6 M4.9 –2013-10-31 20:25 A N12 W139 11880 0.51 151 X1.5 –2013-11-02 04:26 A N03 W139 11875 0.83 301 X3.0 –2013-11-04 05:21 A N01 W165 11875 0.21 42.7 M4.3 –2013-11-21 00:56 A S23 W123 11901 0.42 114 X1.1 –2013-11-21 16:26 A S22 W132 11901 0.25 54.7 M5.5 –2014-01-06 07:56 A S15 W113 11936 0.67 222 X2.2 –2014-01-26 08:37 B S16 E107 11967 0.42 114 X1.1 –2014-01-31 15:07 B S14 E155 11974 0.30 70.9 M7.1 –2014-02-10 18:16 A N13 W161 – 0.34 ∗ ∗ ∗ L/R S < .
1, but their blooming consisted of many elements, that is typical forLDE flares associated with large CMEs. The third flare of 2012-07-23, 02:36with
L/R S ≈ .
18 is particularly noteworthy and will be discussed below. AsTable 2 shows, our analysis of EUVI B-streaks revealed 63 far-side flares abovethe M4.0 level, including 22 X-class flares, in addition to 94 and 28 flares of thesame classes registered by GOES during the ascending and maximum phasesof the current solar cycle (until September 2014). Table 2 demonstrates thatour estimations of the GOES classes for 13 other major backside flares listed byNitta et al. (2013a) are close to their results as well. In the majority of cases, ourestimated GOES importance falls within the range specified by these authors.Figure 6 illustrates B-streaks in six of the strongest backside flares observedby STEREO/EUVI. Shown in Figure 6c is the 2012-09-20 flare with the longest
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Figure 6.
Full-Sun STEREO/EUVI 195 ˚A images of six strongest far-side flares detectedfrom B-streaks (see Table 2).
B-streak exceeding the solar diameter; in this case
L/R S ≈ .
38 that correspondsto the estimated SXR class of X13. This event turned out to be not only themost powerful far-side flare but the strongest flare of the Solar Cycle 24 until thepresent time. This conclusion and evaluation of the flare class are consistent withthe results of Nitta et al. (2013a) obtained through the calculations of the full-disk EUVI total DN output. With an example of this event, we can demonstratethat B-streaks allow not only to estimate the SXR importance of large LDEflares but also to reconstruct their probable time history. For this purpose, itis sufficient to have visible B-streaks in several images, to measure their largestrelative length, and to calculate the SXR flux using equation (1). As Figure 7ashows, the B-streaks in the 2012-09-20 event are clearly visible, at least, in eightframes of the 5-min cadence. At the growth phase, a long B-streak appears verysharply. The shape of the time profile indicates that the maximum flux occurredbetween two frames of 14:56 and 15:01. This suggests that the SXR class of this
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Figure 7.
Probable time history of the 1–8 ˚A flux of two famous far-side flares estimatedfrom STEREO/EUVI B-streaks (see the text and Table 2). The crosses with the right arrowscorrespond to the images in which B-streaks are not yet visible. The dashed lines represent apossible ascending part of the time profiles. The open square in panel (b) refers to an imagecorrected for an exposure time of 8 s. flare was still higher than the longest B-streak implies and seems to be aboutX20.Two other far-side events induced interest as sources of outstanding spaceweather disturbances. The 2012-07-23 flare mentioned above is widely debatedand considered as an utmost extreme eruptive event ( e.g. , Ngwira et al. , 2013;Liu et al. , 2014; Temmer and Nitta, 2015). The corresponding interplanetaryCME (ICME) arrived at STEREO-A, which was located favorably, in a veryshort transit time of 19–21 hr and brought to 1 AU a record strong inter-planetary magnetic field of 109 nT. If directed toward Earth, this ICME mightcause a geomagnetic storm comparable to the famous Carrington storm of 1859.Meanwhile, the corresponding flare was fairly moderate; Nitta et al. (2013a)estimated its soft X-class to be in the M8.2–X2.5 range, and our estimationfrom the observed B-streak length leads to even lower values of about M5.4. Theresearchers of this event suggested that the extreme characteristics of the eventwere caused by a high initial CME speed (up to 3000 km s − ), week ICME dragdeceleration in the solar wind determined by preceding solar eruptions, and byan enhancement of the magnetic field due to an interaction between two CMEs,which closely followed each other. Currently we are working on an alternativeinterpretation suggesting that the extremely high speed of CME/ICME and itsstrong interplanetary magnetic field were due to a large eruptive magnetic fluxand a weak ICME expansion in propagation from the Sun to 1 AU, in accordancewith patterns described by Chertok et al. (2013) and Grechnev et al. (2014). Themagnetic flux was estimated from the SDO/HMI magnetogram on 2012-07-12,when the parent active region was located near the center of the visible disk.The 2014-01-06, 07:56 flare occurred at a heliolongitude of W113 and pro-duced a 2.5% ground-level enhancement (GLE) of cosmic rays, only the secondone in the unusual Solar Cycle 24. Thakur et al. (2014) analyzed this protonevent and kinematics of an accompanying CME and came to a conclusion thatit was consistent with a particle acceleration by a CME-driven shock. On the SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 14 oft X-ray Class of Far-Side Flares other hand, a sufficiently long (
L/R S ≈ .
67) multi-element B-streak existedin STEREO-A images for nearly one hour during this flare. A probable timehistory presented in Figure 7b demonstrates that the flare was a powerful LDEof ≈ X2 class. According to Belov et al. (2007), an X2 class flare can well beassociated with a source of SEP with proton fluxes of J( >
10 MeV) ≈
120 pfu,J( >
100 MeV) ≈ . ≈
2% that are close to the observations.These parameters estimated from B-streaks are typical of flares related to smallGLEs.
5. Summary and Concluding Remarks
We have demonstrated how a spurious instrumental effect, which strongly in-terferes solar flare imaging, can be used to obtain a useful information. Here wewere dealing with the so-called blooming, arising from a significant saturationof the STEREO/EUVI 195 ˚A images in CCD cells corresponding to the coresof sufficiently strong flares. The saturated CCD cells lose their ability to ac-commodate any additional charge, causing them to spill over adjacent cells. Asa result, one or several bright nearly horizontal streaks (B-streaks) are formed.We have shown that in spite of many unaccounted factors, the maximum relativelength of the EUV B-streak,
L/R S , correlates with the peak flare SXR flux, F G ,measured by GOES. We have analyzed about 350 flares of 2007–2014 which wereobserved simultaneously by GOES and one of STEREO spacecraft, and foundan empirical relation between L/R S and F G . This allowed us to propose a simpleand prompt method for estimating the SXR class of far-side flares observed withSTEREO but invisible for GOES.The method consists of a direct measurement of the length of the longest B-streak in units of the solar radius in the routine EUVI 195 ˚A images or movies,accessible in near real time at http://stereo-ssc.nascom.nasa.gov/browse/ , and theuse of equation (1). It is necessary only to make sure that all the measurementswere converted to the exposure time of 8 s. Prior to this, Nitta et al. (2013a)proposed a somewhat more laborious method for estimations of the SXR fluxand class of far-side flares based on calculations of the EUVI full-disk totaldigital number output. It should be noted that if there is a temporal overlapbetween the flares in different active regions, then our method makes it possibleto measure their B-streaks and to estimate their classes independent of eachother.Applying our method to the STEREO observations during the ascending andmaximum phases of Solar Cycle 24 allowed us to find about 65 major backsideflares with the EUVI B-streak lengths of L/R S ≥ . et al. (2013a) for 2010–2012, our estimations are close to their results. Among theseevents, the 2012-09-20 flare turned out to be the strongest one in the current solarcycle. Its probable importance of about X13 was estimated from L/R S ≈ . SOLA: blooming_R2.tex; 21 August 2018; 7:28; p. 15 hertok et al. strong and prompt geomagnetic storm. Judging from the B-streak, the westbehind-the-limb flare of 2014-01-06 was sufficiently intense and long-lasting tobe connected with a source of the observed proton event including GLE.Using the two LDE events as examples, we have demonstrated that by mea-suring the maximum B-streak length in a number of consecutive images one canreconstruct a probable time history of a flare. It is clear that these and otherfeatures found due to B-streaks require further detailed study. Hopefully, theproposed simple method of prompt estimations of the SXR class of the far-sideSTEREO flares from artifact B-streaks would be useful for solar studies andsolar-terrestrial forecasting.
Acknowledgements
We are grateful to an anonymous reviewer for constructive comments,which helped us to improve the manuscript. The authors thank the GOES and STEREO teamsfor their open data used in our study. This research was supported by the Russian Foundationof Basic Research under grant 14-02-00367 and the Ministry of education and science of RussianFederation under projects 8407 and 14.518.11.7047.
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