The Astrophysical Journal Letters, 856:L38 (6pp), 2018 April 1
https://doi.org/10.3847/2041-8213/aab849
© 2018. The American Astronomical Society. All rights reserved.
Brightening and Darkening of the Extended Solar Corona during the Superflares of September 2017 Farid F. Goryaev1
, Vladimir A. Slemzin1 1
, Denis G. Rodkin1
, Elke D’Huys2
, O. Podladchikova2, and Matthew J. West2
P.N. Lebedev Physical Institute of the RAS (LPI), Moscow 119991, Russia;
[email protected] 2 Royal Observatory of Belgium, B-1180 Brussels, Belgium Received 2017 December 29; revised 2018 March 10; accepted 2018 March 20; published 2018 April 4
Abstract On 2017 September 6 and 10, the strongest X9.3 and X8.2 flares of the decade occurred in the active region NOAA Active Region 12673. During these flares, the Sun Watcher with Active Pixels and Image Processing (SWAP) telescope on board the Project for Onboard Autonomy 2 (PROBA2) satellite registered the unusual alternate brightening and darkening of the western corona at the heliocentric distances ≈1.2–1.7R☉. The X9.3 flare on 2017 September 6 was accompanied by coronal brightening up to 30%–45% at distances ≈1.35–1.7R☉. Numerical simulations showed that this brightening might be produced by resonant scattering of the flare radiation by the Fe IX–Fe XI ions in the coronal plasma at the temperature T∼0.8–1MK, and the densities seriously reduced in comparison with the typical values for the quiet background corona probably moving outward with velocities of 30–40km s−1. At the maximum of the flare and one hour later, two coronal mass ejections (CMEs) originated, which dimmed the coronal emission in the SWAP 174Å passband above the western limb by 20%–30%. The X8.2 flare on September 10 was accompanied by a CME, which rose up and progressively dimmed the western part of the corona up to 60%. An hour later the darkening, produced by a global rearrangement of the magnetic field structure and an evacuation of a significant part of the coronal plasma, extended over the complete western limb. A differential emission measure (DEM) analysis showed a decrease in the electron density of the background plasma with T∼1–2MK at distances 1.24–1.33R☉ by 2–3.5 times after the CME. At the same time, an additional DEM peak at T≈0.8MK appeared, which may be associated with an additional emission in the SWAP passband produced by the flare radiation resonantly scattered by the coronal plasma. Key words: methods: data analysis – Sun: corona – Sun: flares – Sun: UV radiation – techniques: image processing plasma and to analyze the relative contribution of excitation mechanisms to the EUV emission of the corona.
1. Introduction Extreme-ultraviolet (EUV) emission of the diffuse corona provides valuable information regarding the properties and parameters of coronal plasma at typical temperatures of 1–2MK. So far, most of the EUV telescopes and spectrometers have observed the corona below 1.3R☉, and only in some special cases observations has it extended up to several solar radii (see, e.g., Feldman et al. 1999; Kohl et al. 2006; Slemzin et al. 2008; Andretta et al. 2012). Since 2009 November observations of the solar corona up to 2R☉ have been performed with the Sun Watcher with Active Pixels and Image Processing (SWAP) telescope as a part of the Project for Onboard Autonomy 2 (PROBA2) mission (Halain et al. 2013; Seaton et al. 2013). The wide field of view and low stray light of SWAP allow for the study of the mechanisms of formation of EUV emission in the extended corona, associated with the collisional excitation and resonant scattering of radiation coming from the lower corona. In particular, SWAP observations have enabled the evaluation of collisional excitation and resonant scattering contributions to the EUV emission of a streamer and diffuse corona (Goryaev et al. 2014). Recently, the Sun has produced two powerful X-class flares: the X9.3 flare on 2017 September 6, and the X8.2 flare on 2017 September 10, both of which erupted from AR 12673 and were accompanied by several coronal mass ejections (CMEs). In this work, we aim to study the influence of these eruptive events on EUV emission and the state of the diffuse corona. We use the specially processed SWAP images along with numerical modeling and the differential emission measure (DEM) analysis to investigate the temperature structure of the coronal
2. Observations and Data Processing The SWAP EUV telescope provides images of the solar disk and inner corona in a single spectral bandpass centered on 174 Å over a 54′×54′ field of view with 3 17 pixels and a cadence of about two minutes. The response function of SWAP covers the four intense spectral lines of Fe ix–Fe XI ions: Fe IX λ171.08, Fe X λ174.53, Fe X λ177.24, and Fe XI λ180.41. To suppress stray light, the optical two-mirror design includes special baffles. The details of the design and the parameters of SWAP can be found in Seaton et al. (2013). In the present work, we use SWAP data for the X9.3 flare on 2017 September 6 and the X8.2 flare on 2017 September 10 (see Figures 1(a) and (c)) to investigate the effect of these superflares and associated CMEs on the EUV corona. The procedure of processing the SWAP images consisted of the following steps. The level 0 SWAP FITS files were processed to level 1 to correct for dark current, detector bias, flat-field variations, and bad pixels using the standard p2sw_prep procedure with the option to apply a point spread function (PSF) deconvolution to the data to remove stray light. The procedure for removing stray light from SWAP images is considered in detail in Halain et al. (2013). The accuracy of this procedure was also evaluated in Goryaev et al. (2014; see Figure 2(b)), where the errors were estimated within of 20% for coronal EUV emission after subtraction of the stray light up to heights of 0.7R☉. 1
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Figure 1. (a) SWAP image for the X9.3 flare (squared) on 2017 September 6 (12:01:42 UT); the full frame corresponds to the SWAP field of view 54′ × 54′, and the dashed line square shows the Atmospheric Imaging Assembly/Solar Dynamics Observatory (AIA/SDO) field of view 41′×41′. (b) Polar transformation of the shown SWAP image (fixed difference to 11:30:32 UT) for 2017 September 6. (c) SWAP image for the X8.2 flare on 2017 September 10 (16:00 UT). (d) Polar transformation of the SWAP image (fixed difference to 15:20:48 UT) for 2017 September 10.
The images of the off-limb corona were then transformed to polar coordinates using an interpolation procedure (see Figures 1(b) and (d)). The solar corona in the polar representation was divided into a number of cells: 12 columns in latitude (numbered from 0 to 11, 30° in width each) and four rows (numbered from 0 to 3, 0.17R☉ in height each) from the limb to the altitude 0.68R☉. These height zones correspond to the circular rings shown in Figure 1(a). For a better signal-to-noise ratio, the intensities in all of the pixels within each cell are integrated by multiplying the R/R☉ factor for each pixel, which accounts for the variation of the pixel area with height. We did not consider the part of the corona corresponding to the zeroth row (the heights from the limb to 0.17R☉) containing the bright coronal loop structures. To see changes in the off-limb corona due to the flares and CMEs, we made fixed-difference polar images, using as a reference the first image in the series taken before the flare start (as a flare marker, we used the GOES data). Figures 1(b) and (d) show the fixed-difference polar images for the events of 2017 September 6 and 10, respectively. They show a brightening of the lower corona during the flare X9.3 at 12:01 UT (see column 3, 90°–120° in latitude) and a darkening of the corona above AR 12673 after the maximum of the flare X8.2 at 16:00 UT.
3. Results and Discussion On 2017 September 6, the major X9.3 flare at the western part of the disk (position S09W34) started at 11:53 UT and peaked at 12:02 UT according to the GOES data. This flare was then followed by the two CMEs that are given in the CACTus catalog3 for CMEs observed by LASCO-C2: (i) the CME event detected at 12:24 UT, which reached a velocity of 586km s−1, and (ii) the CME at 13:36 UT, with a velocity of 625km s−1. We recognized the onsets of these CMEs by their dimmings on the solar disk using the Solar Demon data for flares, dimmings, and EUV waves event detection.4 The onset of the dimming for the first CME occurred at about 12:07 UT, and at about 13:00 UT for the second CME. On 2017 September 10, the X8.2 flare at the limb (position S08W88) started at 15:35 UT and peaking at 16:06 UT. The flare was associated with a CME, which was detected by LASCO-C2 at about 16:00 UT at the heliocentric distance of 2.4R☉ with a velocity of 921 km s−1. The Solar Demon data show the onset of a dimming at the flare site at about 15:50 UT, and the dimming maximum was reached at about 16:03 UT. We processed the SWAP data for these events to investigate the 3 4
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http://www.sidc.be/cactus/ http://www.solardemon.oma.be/science/dimmings.php/
The Astrophysical Journal Letters, 856:L38 (6pp), 2018 April 1
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Figure 2. (a) Top: time evolution of integrated EUV intensities as the ratio to the reference value: the red curve shows the data for cell 1 (heights 0.17–0.34R☉), the blue curve shows those for cell 2 (0.34–0.51R☉), and green curve shows those for cell 3 (0.51–0.68R☉); bottom: SWAP flare and Geostationary Operational Environmental Satellite (GOES) light curves. (b) Correlation of the relative SWAP flare radiance vs. EUV emission ratios for cells 2 (blue) and 3 (green). (c) Electron density distributions. (d) Results of simulation for the relative EUV fluxes in the SWAP passband during the X9.3 flare maximum for temperature 1 MK and two density distributions: the black curves correspond to the collisional excitation, Icol (Icol + Isct ); the blue curves correspond to the resonant scattering of EUV radiation coming from the underlying corona, Isct (Icol + Isct ); the red curves correspond to the resonant scattering from the flare, Ifl (Icol + Isct ) (see the text for explanations).
by fitting a square area around the flare site and by taking the total EUV emission from it. The size of the square was chosen so that brightness changes in the surrounding regions were essentially less than in the flare region, and therefore did not affect the results. As is seen from Figure 2(a), there is a good correlation between SWAP flare brightness and EUV emission ratios at different heights in the corona. There are three successive enhancements in SWAP fluxes due to the EUV flare radiance. The first hump, with the corresponding maximum at 12:02 UT, is similar to the profile of the GOES 1–8Å flux; the following two smaller maxima were observed by SWAP at about 12:55 UT and 13:45 UT. The profiles of the total EUV emission in cells 1, 2, and 3 agree well with those of the EUV flare flux: at first, there is an increase of EUV emission in the cells consistent with the EUV flare flux and darkening after the onset of the first CME at 12:07 UT, then the
temporal changes of EUV emission in the extended corona. The time variations of the total intensity (in DN) in the cells of Segment 3 (90°–120° in latitude) for height rows 1–3 (hereafter referred to as cells 1, 2, and 3) were considered, where the flares under study more significantly affected the corona. 3.1. EUV Brightening in the Corona during the X9.3 Flare Figure 2 shows the results of our analysis for the X9.3 flare event over the time interval 11:30–14:00 UT. Figure 2(a) displays (i) the time evolution of the integrated EUV intensities (coming from the aforementioned cells) presented as ratios of EUV fluxes to the pre-flare value at 11:30:32 UT, and (ii) the corresponding time evolution of the SWAP 174Å and GOES 1–8Å light curves. The vertical lines correspond to the onsets of the CMEs associated with the flare. The SWAP light curve was determined 3
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enhancement obtained in assumption of the given plasma conditions is much less than the measured one. We suggest two possible reasons for this discrepancy between the measurements and the results of the simulations. First, density of the coronal plasma above the western limb may be much lower than it follows from the taken density distribution ne(r). The DEM analysis showed that at the distance r≈1.25R☉ the plasma density was of ≈107cm−3, which is 3–4 times less than in the taken density model. The calculated signal enhancement matches with the measured one if we assume that the density at the point 1.6R☉ (center of cell 3) ≈(3–4) × 105cm−3. The corresponding probable density distribution is shown in Figure 2(c) (dashed line). Second, the resonance scattering component excited by radiation from the lower corona can be less than assumed in our simulations due to the Doppler dimming effect resulting from the plasma outflows from the Sun (see, e.g., Goryaev et al. 2014 and references therein). The spectral lines of the emission from the quiet corona correspond to the similar plasma temperature of ∼1MK as the remote coronal plasma, so at moderate plasma mass velocities of 30–40km s−1 the corresponding Doppler shift results in a reduction of the resonance scattering component by an order of magnitude. In contrast, the lines emitted by the flare correspond to the kinetic temperatures of several MK, so the Doppler dimming effect is not important. The results of the corresponding simulation are given in Figure 2(d) (dashed lines). The comparison shows a good agreement with measurements.
second increase and darkening after the second CME at 13:00 UT, and again the enhancement in the third maximum. The relative increase of EUV emission in the maximum of the first hump is about 10%–15% for cell 1 (the red curve in Figure 2(a)), up to 30% for cell 2 (blue curve), and 40%–45% for cell 3 (green curve). For instance, the enhancement of the averaged-over cell 3 signal in absolute value at the maximum of the flare in the SWAP passband equals ≈0.14 DN pix−1 s−1, in comparison with the reference value before the flare of 0.31 DN pix−1s−1 with the standard deviation of 0.01 DN pix−1 s−1. An analysis of the EUV emission enhancements in cells 1–3 (see Figure 2(a) and (b)) showed the increase of correlation between SWAP flare radiance and the EUV emission of the corona when the distance from the limb increases. The corresponding correlation coefficients for cells 1, 2, and 3 constitute 0.18, 0.87, and 0.93, respectively. The results of this analysis for cells 2 and 3 are given in Figure 2(b). The linear correlation between the EUV flare light curve and EUV emission from the coronal cells above the limb closest to the flare site suggests that EUV brightening in the corona is linked with the EUV radiation from the flare in such a way that the contribution of this additional flux increases with the height above the limb. In order to verify this inference, we carried out numerical simulations of the coronal EUV emission in the SWAP 174Å passband during the X9.3 flare in the corresponding geometry. As is known (see, e.g., Goryaev et al. 2014), the coronal EUV emission is produced by thermal collisional excitation and nonthermal resonant scattering of the radiation from the underlying solar corona. Here we consider the additional EUV emission of the extended corona in the SWAP passband due to the resonant scattering from a flare source. We used atomic data from the CHIANTI database (Dere et al. 1997; Landi et al. 2013) to calculate the EUV intensities in the SWAP Fe-ion lines by integrating along the line of sight. The intensities of the EUV flux coming from the underlying corona and the flare were evaluated using both SWAP images and CHIANTI data. For instance, the exciting intensities in the 171.08Å line were determined to be of 1.03×1013 and 1.37×1014 (in phot cm−2 s−1 str−1) for the underlying corona and the flare, respectively. The intensities were then convolved with the SWAP responses, Qλ (see Goryaev et al. 2014), to give the corresponding signals (in DN pix−1s−1) for three excitation mechanisms: (i) collisional excitation (Icol), (ii) resonant scattering from the solar disk (Isct), and (iii) resonant scattering from the flare (Ifl). To estimate coronal plasma conditions, we applied a DEM analysis using the technique by Plowman et al. (2013) and the EUV images of the AIA on board SDO in 94, 131, 171, 193, 211, and 335 Å EUV AIA channels. We inferred that at the time of the flare the western limb corona closest to the flare site was at the temperature ≈1MK. First we carried out simulations of the coronal emission, assuming that the coronal density distribution ne(r) was similar to that derived for the quiet background (diffuse) corona in 2010 October by Goryaev et al. (2014; solid line in Figure 2(c)). Figure 2(d) displays the corresponding results for the relative contributions of three excitation mechanisms (solid lines): collisional excitation (black), the underlying corona (blue), and the flare (red). The flare contribution constitutes ≈3% for cells 2 and 3 and can be directly compared with the ratios of EUV fluxes in Figure 2(a). This comparison shows that the calculated relative signal
3.2. Global Darkening of the Western Half-plane during the X8.2 Flare The results of the SWAP data processing for the X8.2 flare are given in Figure 3 for the time period 15:20–19:00 UT. SWAP observed progressive darkening at the western limb in the overlying corona. Figure 3(a) shows the time variations of both EUV emission ratios (normalized to the pre-flare value at 15:20:48 UT) in the extended corona, and SWAP flare and GOES X-ray fluxes. There is no correlation between SWAP flare radiance and GOES 1–8Å emission. Though the flare started at 15:35 UT, the GOES flux began to grow noticeably only at 15:50 UT and practically simultaneously with the onset of the associated CME. After peaking at 16:06 UT, the GOES flux smoothly decreased by an order of magnitude at 18:00 UT. In its turn, within the time interval 15:35–15:50 the SWAP flare brightness increased by a factor of 1.5–2, and was then followed by a sharp drop after 15:50 UT, reaching the minimum at 16:20 UT. Thereafter, SWAP flare radiance returned to grow with three subsequent maxima. At the same time, EUV emission of the corona in the three cells under study demonstrate a sharp drop after the onset of the CME at 15:50 UT. The low level of EUV emission was then preserved for many hours without recovering to the initial state of activity. Nevertheless, it may be remarked that the EUV coronal emission in the cells, especially for cells 2 and 3, manifest some visible correlation with the SWAP flare radiance. One may suggest two possible explanations for the darkening of the western corona seen in SWAP passband above the limb: (i) a decrease of the partial ionic densities of the Fe IX–Fe XI ions due to the heating of the plasma, and (ii) a global depletion of the coronal plasma captured by the outgoing CME. In the first case, the corona has to relax to the usual ionization state after the flare has stopped. The estimation of the recombination time gives the values of about 4
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Figure 3. (a) The same as in Figure 2(a) for the X8.2 flare. (b) AIA/SDO image for 2017 September 10 (16:00 UT) with coronal regions (boxes) used for the DEM analysis. (c) and (d) DEM temperature distributions in boxes 1–3 before the CME (14:30 UT) and after (16:00 UT). The arrow points out the DEM maximum at 0.8–1MK (see the text).
the DEM distribution at temperatures log10 T =6.0–6.5 sharply decreased due to the evacuation of coronal plasma with the propagating CME, as seen in Figure 3(d). At the same time, a second maximum in the DEM distribution appeared at temperatures »log10 T =5.7–6.0, which may be presumably associated with the resonant scattering of the flare radiation by the depleted coronal plasma. We suggested that this maximum is due to (i) an increased EUV emission from the flaring region, and (ii) an effective resonant scattering of the EUV emission by the Fe ions in the SWAP 174 and AIA 171 passbands (especially in the Fe IX ion line 171.08 Å) as compared to other AIA EUV channels. A standard DEM procedure is based on the local thermodynamic equilibrium approximation, assuming that plasma emission is produced by the thermal collisional excitation. Thus, the DEM procedure interprets any additional
10–20 minutes for densities and temperatures observed in the inner corona, whereas the observed darkening effect lasted for many hours. In order to verify the second possibility, we applied the DEM analysis for three coronal regions (hereafter referred as to boxes 1, 2, and 3) shown in Figure 3(b). Box 2 is located just over the flare at the distance 0.27R☉ from the limb, and boxes 1 and 3 at the distances 0.24R☉ and 0.33R☉, respectively, from the flare site. An example of the DEM(T) (divided by log10T ) distributions in the boxes before the flare (at 14:30 UT) and after the CME lift-off (at 16:00 UT) is given in Figures 3(c) and (d). The analysis showed that most of the ambient coronal plasma before the flare corresponds to the temperature range log10 T =6.0–6.5 (see Figure 3(c)). During and after the flare, 5
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the resonant scattering effect is comparable with the observed increase of EUV fluxes. The EUV brightening turned into a darkening of the corona up to 30%–40% due to the evacuation of the coronal plasma by the two associated CMEs, which crossed the corona after the flare. In 1.5–2hr after the flare onset, the corona relaxed to the initial state. The X8.2 flare on 2017 September 10 at the western limb started nearly simultaneously with the associated CME. At the moment of the GOES flux maximum, the CME crossed the considered layer of the corona, producing progressive darkening in SWAP EUV emission, and extended almost over all of the western limb and lasted for many hours afterward. In order to explain and interpret the western coronal darkening and the absence of brightening in the SWAP passband, we carried out a DEM analysis using AIA/SDO EUV data to study the temperature structure of the corona. The analysis showed that before the flare most of the ambient plasma was at temperatures of 1–3MK, corresponding to the usual quiet Sun conditions. Then, the plasma density at these temperatures sharply depleted due to the evacuation of the coronal plasma with the CME. At the same time, the DEM analysis displayed the appearance of a second temperature component at 0.5–1MK. The additional DEM maximum likely was not associated with a colder plasma, but was shown to be very likely related to the resonant scattering of the EUV radiation of the flaring region by the low density plasma. Electron densities, derived from the DEM distributions, decreased after the onset of the flare by 2–3.5 times, reaching the minimum at 16:00–16:30 UT, and then relaxed to larger values, but did not reach the initial values. Our analysis showed that the darkening effect for the event on 2017 September 10 was related to the evacuation of coronal material with the CME.
Table 1 Line-of-sight EMs (in 1025cm−5) and Electron Density Numbers (in 107cm−3) in Boxes 1, 2, and 3 Derived from the DEM Analysis Time, UT 14:30 16:00 18:00 19:00
Box 1 EM Ne 7.16 1.83 5.11 4.15
2.30 1.16 1.95 1.75
Box 2 EM Ne 13.3 1.02 1.64 2.29
3.09 0.86 1.08 1.28
Box 3 EM Ne 5.24 1.32 0.45 0.68
1.86 0.93 0.54 0.67
emission in the 171 Å line provided by non-collisional processes (such as resonant scattering) as if the virtual plasma component appeared at temperatures corresponding to the thermal excitation of the Fe IX–Fe XI ion lines. We verified this inference by simulating a DEM calculation with an artificial increase of EUV emission in the AIA 171 Å passband alone. This simulation resulted in the appearance of the additional DEM maximum at 0.8–1MK. Using the DEM distributions, we evaluated both the emission measure (EM) values by integrating the DEM(T) functions over the temperature range Dlog10 T =6.0–6.5 and the corresponding mean electron density numbers Ne = EM L in boxes 1–3 along the line of sight. The corresponding results are given in Table 1. We estimate the uncertainties of the DEM distributions as ∼40%, so accuracies for Ne values are of ∼20%. Assuming a spherically symmetric corona, the effective plasma size L along the line of sight was calculated from the condition that the electron density decreases with the hydrostatic scale of height for the temperature of ∼2MK. Table 1 shows that at 16:00, after the onset of the flare, the electron density in boxes 1 and 3 dropped down by 2 times, and in box 2 by 3.6 times. Then, the densities relaxed to larger values, but they did not recover to the initial values for all boxes. As the result, we infer that the darkening was caused by the evacuation of the coronal material with the CME. The DEM analysis revealed additional EUV emission, which perhaps may be associated with resonant scattering in the corona at low plasma densities.
This work was supported by the PROBA2 Guest Investigator program. The authors are grateful to Dr. D. Berghmans for support of the work. We acknowledge the use of data from AIA/SDO, CACTus CME catalog, and Solar Demon detection system. CHIANTI is a collaborative project involving George Mason University, the University of Michigan (USA), and the University of Cambridge (UK).
4. Summary and Conclusions ORCID iDs
We studied the effect of the most powerful flares in the current solar cycle on EUV emission of the extended solar corona in the SWAP 174Å passband. The X9.3 flare on 2017 September 6 showed a significant increase by an order of magnitude in EUV flux registered by SWAP, and was then followed by two successive CMEs. The analysis based on SWAP data demonstrated the high correlation between the EUV flare radiance and EUV emission of the extended corona at distances 1.17–1.68R☉. The analysis also showed that the brightening of the corona at the western limb over the distances of 1.34–1.68R☉ increased up to 30%–45% as compared to the initial level before the onsets of the associated CMEs. Numerical simulations of the coronal emission in the SWAP passband showed that the resonant scattering of the X9.3 flare EUV flux may contribute noticeably to the coronal EUV emission, if the plasma in the scattering volume at r=1.6R☉ has the temperature of ∼1MK, a density of ∼105cm−3, and outward velocities of 30–40km s−1. This result suggests that
Farid F. Goryaev https://orcid.org/0000-0001-9257-4850 Vladimir A. Slemzin https://orcid.org/0000-00025634-3024 Denis G. Rodkin https://orcid.org/0000-0002-5874-4737 Elke D’Huys https://orcid.org/0000-0002-2914-2040 Matthew J. West https://orcid.org/0000-0002-0631-2393 References Andretta, V., Telloni, D., & Del Zanna, G. 2012, SoPh, 279, 53 Dere, K. P., Landi, E., Mason, H. E., et al. 1997, A&AS, 125, 149 Feldman, U., Doschek, G. A., Schühle, U., et al. 1999, ApJ, 518, 500 Goryaev, F., Slemzin, V., Vainshtein, L., et al. 2014, ApJ, 781, 100 Halain, J.-P., Berghmans, D., Seaton, D. B., et al. 2013, SoPh, 286, 67 Kohl, J. L., Noci, G., Cranmer, S. R., et al. 2006, A&ARv, 13, 31 Landi, E., Young, P. R., Dere, K. P., et al. 2013, ApJ, 763, 86 Plowman, J., Kankelborg, C., & Martens, P. 2013, ApJ, 771, 2 Seaton, D. B., Berghmans, D., Nicula, B., et al. 2013, SoPh, 286, 43 Slemzin, V., Bougaenko, O., Ignatiev, A., et al. 2008, AnGeo, 26, 3007
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