- Email: [email protected]
Solar EUV flux variation and flare brightenings
Available online at www.sciencedirect.com
Advances in Space Research 50 (2012) 683–689 www.elsevier.com/locate/asr
Solar EUV flux variation and flare brightenings Xingming Bao a,⇑, Wenbin Xie b,1 a
Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China b Department of Physics, Jilin Normal University, Jilin 136000, China Available online 21 June 2012
Abstract ˚ and flare brightening has been examined in different sizes On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195 A of active regions by using SOHO/EIT, MDI and Ha observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Ha band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are ˚ flux peaks are well correlated to that of corresponding to the EUV flare brightenings. For the large active region, the local EUV 195 A ˚ flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active the GOES X-ray flux. The EUV 195 A ˚ flux is not well correlated to GOES X-ray flux. The EUV 195 A ˚ flare brightenings in the moderate active region, the local EUV 195 A ˚ flux region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195 A ˚ varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195 A flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux. Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: EUV brightenings; EUV intensity; Solar flares
1. Introduction Solar flares are the most violet eruption in atmosphere which release large amounts of magnetic energy in several tens of minutes. During flares bright emissions often appear in the location of images where the solar atmosphere is heated. Traditionally solar flares are observed with sudden bright emissions (typically two bright ribbons) in Ha monochromatic images. With the launch of space based instruments such as Solar Heliosphere Observatory (SOHO), Transition Region And Coronal Explorer (TRACE) and Geostationary Operational Environmental Satellites (GOES) in the recent two decades, solar flares are intensively observed in Extreme-ultraviolet (EUV) ⇑ Corresponding author. Tel.: +86 10 64807659; fax: +86 10 64888716.
E-mail address: [email protected] (X. Bao). Also in Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, China. Tel.: +86 434 3295977; fax: +86 434 3294566. 1
and X-ray band. By comparing with Ha images, EUV observations show that some flare ribbons are still discerned in EUV and X-ray images even they are not recognized in Ha images during prominence eruptions (Bao et al., 2006, 2007). This suggest that solar flares are basically coronal, rather than chromospheric phenomena (Priest and Forbes, 2002). EUV brightening is an indication of various levels of flares. The physical nature whether the EUV flare brightenings reflect local heating or increase of its local electron’s density is not well understood. Zhang et al. (2001) show that the flux of EUV bright points varied intermittently and suggest that the number of EUV bright points is more than that in SXT images is due to their temperature is lower than 2 106 K. After a systematic variability analysis of EUV emission from a dipolar loop system in the core of an active region, Nightingale et al. (1999) suggests that EUV brightenings are not related to flare heating processes and seem to be produced by compressional waves that travel with approximately the sound speed through the active
0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.06.016
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region loops, perhaps triggered by chromospheric reconnection processes. In order to explore the question if the EUV brightenings and large flares experience the same physical process, it is necessary to study quantitatively the variation of EUV flux in different sizes of active regions. Using early satellite observations, Kane and Donnelly (1971) show that EUV impulsive and hard X-ray emissions are well correlated in occurrence, time dependence and intensity and indicate that the principle mechanism for ˚ radiation during solar the impulsive emission of 10–1030 A flares is the free-bound, bound-bound, and bremsstrahlung emission from the quasi-thermal plasma produced by the collision loss of energetic electrons. Qiu et al. (2000) show that the peaking time of EUV intensity delays that of GOES X-ray several minutes and suggest that the delays EUV emission may come from cooling of soft X-ray loops. However, a question arise: when the magnetic loops is heated during flares, the temperature should increase to the EUV temperature (105 –106 K) at first, then to the X-ray temperature (> 106 K). Cheng (1990) also found the impulsive UV component prior to the hard X-ray burst and proposed electric current heating in the loop before the acceleration of hard X-ray electrons. The full disk EUV flux reflects the total EUV emission and only the brightenings contribute to the full disk EUV flux. What is the variation of local EUV flux in individual active region? In this paper, we examined the local EUV flux and EUV flare brightenings in different sizes of active regions on 2010 February 8 when two M-class flares and more than ten C-class flares occurred in solar disk. The variation of total and local EUV fluxes, GOES X-ray fluxes and local magnetic flux are compared.
magnetic field data are obtained by SOHO Michelson Doppler Imager (MDI) at approximately 1.5 hr intervals. Three active regions in solar disk on 2010 February 8 are ˚ , MDI magselected to calculate EUV flux. The EIT 195 A netic field and Ha of north hemisphere are shown in Fig. 1. The three areas are marked by rectangle A, B and C in Fig. 1 which represent three types of active regions respectively: Region A (AR 11046, N23 W15) is a moderate active region with two small sunspots, Region B (AR 11045, N25 E51) is a large active region with a sunspot group and Region C (N20 W30) is a small plage region in Ha without sunspot. Active region AR 11045 was located near meridian of the solar disk. White rectangles ˚ flux is calculated. In indicate areas where the EUV 195 A Ha image (Fig. 1c), AR11045 is a large active region and the Region A and C only displayed as weak plage. The magnetic configurations in Region A and B are complicated and that of Region C is simple bipolar in MDI magnetic field (Fig. 1b). Their total unsigned magnetic flux are listed in Table 1. The flux of Region A, B and C are 1:64 1020 Mx, 1:26 1021 Mx and 8:6 1019 Mx respectively. ˚ image at 05:24 UT on 2010 FebFig. 2 is an EIT 195 A ruary 8. White contours indicate the area with intensity larger than 50 DN s1 and the black contours indicate the area with intensity larger than 150 DN s1. Almost all
2. Observational data and reductions The Extreme-ultraviolet Imaging Telescope (EIT) on board SOHO images the Sun with field of view of 45’ and ˚, 1024 1024 pixels in four EUV emission lines: 171 A ˚ , 284 A ˚ and 304 A ˚ (Delaboudinie`re et al., 1995). 195 A ˚ emission lines is about The temperature of Fe XII 195 A ˚ filter images is 1.5 106 Kevin and the cadence of 195 A ˚ data are pro12 minutes or more. In this study, the 195 A cessed with dark current and flat field reduction. The EIT flux is calculated by the integral of digital number (DN) in selected area. In order to compare the active region with different sizes, the EIT flux is divided by the number of pixels in the selected area. The Ha images are obtained from Yunnan Astronomical Observatory (http://www. ynao.ac.cn), Chinese Academy of Sciences. Its detectors have 3073 2048 pixels with cadence of 40 s (http:// 159.226.148.131/solar/equipment_full.html). GOES has included a space weather monitor (SEM) X-ray sensor which measures the disk-integrated solar emission in two ˚ and 0.5–4 A ˚ since 1974 (Garica, bands covering 1–8 A 1994). The 1-minutes X-ray flux data used in this study is ˚ channel. The obtained by GOES-14 satellites 1–8 A
Fig. 1. Three active regions on the north hemisphere on 2010 February 8. They are marked by rectangles A, B and C respectively. Active region AR 11045 is the largest region among them. White rectangles indicate the ˚ flux is calculated. (a) EIT 195 A ˚ image at areas where the EUV 195 A 03:12:28 UT. (b) MDI magnetogram at 03:15:02 UT shows the longitudinal magnetic field. (c) Ha image from Yunnan Astronomical Observatory at 03:15:40 UT. Region A and C appeared as weak plages.
X. Bao, W. Xie / Advances in Space Research 50 (2012) 683–689 Table 1 Magnetic flux of three active regions. Region
Magnetic flux (Mx)
Region A Region B Region C
1:64 1020 1:26 1021 8:64 1019
white areas are located in north hemisphere and all black areas are located in these three regions. Some white areas are located out of the solar limb. Two horizontal lines 1 and 2 mark the position of pixels which sweep over the Region B and Region A and C respectively. The intensity distribution along the two lines is plotted with solid and dashed curves in the lower part of image. Beside the three active regions, the intensity in the quiet region is lower than 50 DN s1 . The intensity in active region 11046 reaches 200 DN s1 and the intensity in the flare region in large active region 11045 reaches 1000 DN s1 , even reaches 4000 DN s1 in some M-class flares. This indicates that the intensity of brightenings in active region is larger than 50 DN s1 .
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Zhang et al. (2001) estimate the average EIT flux is 55.2 DN s1 pixel1 . In this study, the EUV flux is calculated by summing the intensity which is larger than 50 DN s1 in the region, then dividing by the region area (number ˚ flux is DN s1 of pixels). Therefore the unit of EIT 195 A 1 pixel and reflect variation of average intensity in the ˚ images are co-aligned by applying region. The EIT 195 A a cross-correlation algorithm. The accuracy of the alignment is about 2-300 . Since the variation of intensity in the area near the side of the rectangles is not significant, the small shift (2-3”) between two consecutive frames contributes little error to their EUV flux calculations. 3. Results ˚ for 2010 The GOES X-ray flux (solid) in from 1 to 8 A February 8 is shown in Fig. 3. There are two M-class X-ray flares that occurred at 07:48 UT and 13:48 UT. The total ˚ flux (dashed) of the full disk is plotted in EIT 195 A
˚ image at 05:24 UT on 2010 February 8. The two horizontal lines 1 and 2 mark the pixels whose positions sweep over the Region B and Fig. 2. EIT 195 A Region A and C respectively. The intensity distributing along line 1 is the solid curve while that along line 2 is the dash curve in the lower part of the EIT ˚ image. The intensity peaks at Region A, B and C are marked by a, b and c. White contours indicate the area with intensity larger than 50 DN s1 195 A and the black contours indicate the area with intensity larger than 150 DN s1. The intensity in quiet region is lower than 50 DN s1, the intensity in small active regions (Region A and C) reaches to 200 DN s1 and the intensity in the large active region (region B) reaches up to 1000 DN s1 during flare time.
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Fig. 3. The area (pixel number) of full disk is 1024 1024 = 1.048576 106 pixels and the actual number of pixels with intensity larger than 50 DN s1 is about 2.8 3.8 104. From Fig. 2, it shows that the intensity in quiet regions contribute little to the EUV flux, so about 9.6 9.7 105 pixels with intensity lower than 50 DN s1 are deselected and we select 105 pixels as the area of full disk. It is clearly shown that the two prominent peaks are well correspond to the two M-class X-ray flares and the ˚ flux at 21:34 UT is also fitted at same time peak of 195 A as GOES X-ray flux peak. The increase of the background ˚ flux from beginning to the end of the of the full disk 195 A day may related to the increase of EIT flare loops. ˚ flux (dashed) of AR 11045 is The local EUV 195 A shown in Fig. 4 together with GOES X-ray flux (solid). The pixels with intensity larger than 50 DN S1 are selected to calculate the EUV flux and its number is about 5 6103. The area of Region B is 120 90 = 10800 pixels which is about two times of selected pixels, that is, nearly half of pixels are deselected. Though the EUV flux of AR 11045 is larger than that of full disk, the EUV flux of the full disk is larger than that of AR 11045 considering the area of full disk is about 9 times larger than that of AR 11045. Since the AR 11045 is the largest active region on the solar disk and contribute large amounts to the total full disk EUV ˚ flux of AR 11045 is simflux. So the profile of EUV 195 A ˚ flux. According to the ilar to that the full disk EUV 195 A GOES X-ray flux, there are more than ten C-class flares and two M-class flares that occurred on 2010 February 8. During 03:00 UT-06:00 UT, there are four peaks and one sub-peak (before second peak) in X-ray flux. We noticed that there are five EUV flux peaks corresponding to X-ray peaks and sub-peaks during this period. This indicates that the X-ray sub-peak comes from AR 11045 because EUV flux is a local flux of AR 11045. The most prominent peaks appeared at 07:48 UT and 13:48 UT and 21:34 UT are indicated by three vertical bars and they coincide with the three ˚ peaks are also major X-ray flares. Some small EUV 195 A well consistent with peaks in X-ray flux peaks at several
C-class flares such as flares at 03:24 UT, 04:26 UT, 05:36 UT, and 06:12 UT. The Ha images show that consecutive white and dark loops emerged and spread out during period between 03:00 UT and 07:00 UT. ˚ movie (v1.gif) shows a coupe of flare The EIT 195 A brightenings appearing at AR 11045 during flare time, sometimes were accompanied by surges and mass ejections. During 03:00 UT-06:00 UT, a series of south directed loops rose and expanded out and fit well the five EUV flux peaks of C-class flares in Fig. 4. The first M-class flare at 07:48 UT was so bright that it appeared overexposed at center of AR 11045 (Fig. 5a). Post flare loops appeared above the flare ribbons during the second M-class flare at 13:48 UT (Fig. 5b). During the C-class flare in AR 11045 at 21:34 UT, it is noticed that the flare brightenings were not brighter than two previous M-class flares but also followed by rising of post flare loops (Fig. 5c). ˚ flux (dashed) of Region A Fig. 6 shows the EUV 195 A ˚ . The area of (AR 11046) and GOES X-ray flux in 1–8 A region A is 120 90 = 10800 pixels and the number of selected pixels with intensity larger than 50 DN s1 is between 4200 and 5000. The first flare brightening with ris˚ images ing of bright loops in AR 11046 in EIT 195 A appeared around 03:00 UT which was corresponding to ˚ flux the first peak. The left vertical bar indicates that 195 A peak at 12:06 UT is corresponding to a C-class flare. ˚ flux peak of Region A at 19:06 UT coAnother 195 A occurred a minor sub-peak of GOES X-ray flux. EIT ˚ images shown that the coronal loops reconnected 195 A with its surrounding loops. Since the width of the EUV flux peaks of Region A is larger than that GOES X-ray flux peaks, the variability between EUV flux of Region A and GOES X-ray shows a little correlation. ˚ images (v2.gif), we noticed From a series of EIT 195 A ˚ flux curve are that the sudden increases in the EIT 195 A ˚ ) bright flare loops accompanied by rising of EIT 195 (A ˚ flare brightenings (upper panel in Fig. 7). The EIT 195 A ˚ difference images can be displayed clearly in EIT 195 A (lower panel in Fig. 7).
˚ and mean EIT 195 A ˚ flux curve (dashed) of the full disk. Three major peaks of Fig. 3. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A ˚ flux fit well to the three flare peaks of GOES X-ray flux. 195 A
X. Bao, W. Xie / Advances in Space Research 50 (2012) 683–689
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˚ and mean EIT 195 A ˚ flux curve (dashed) of AR 11045 (Region B). The pixels with Fig. 4. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A intensity larger than 50 DN S1 are selected to calculate the EUV flux and its number is about 5 103. The area of Region B is 120 90 = 10800 pixels ˚ flux is almost similar to the total flux of full disk in Fig. 3 since the large part of which is about two times of selected pixels. The variation of EUV 195 A EUV flux of pixels with intensity larger than 50 DN S1 come from AR 11045. The vertical bars indicate the maximum of three major flares which co˚ band. occurred in flux peaks of X-ray and EUV 195 A
˚ images of AR 11045 during M-class flares at 07:48:45 UT (a), 13:48:45 UT (b) and C-class flare at 21:34:45 UT (c). Fig. 5. Three EIT 195 A
˚ and mean EIT 195 A ˚ flux (dashed) of AR 11046 (Region A). The area of region A Fig. 6. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A is 90 120 = 10800 pixels and the number of pixels with intensity larger than 50 DN s1 is between 4200 and 5000. Two vertical bars indicate ˚ band. simultaneous flux peaks of two flares in X-ray and EUV 195 A
˚ flux curve (dashed) of Region C on 2010 The EIT 195 A February 8 is shown in Fig. 8. Its area is 50 50 = 2500 pixels and the number of selected pixels with intensity larger than 50 DN s1 is between 500 and 650 — nearly four fifths pixels in the area is deselected. The main EUV flux peaks are indicated by vertical bars. The strongest peak ˚ flux appeared at 20:06 UT (3rd vertical of EUV 195 A
bar in Fig. 8), almost align to a sub-peak of GOES Xray flux. Due to the small magnetic flux of Region C, its ˚ flux of the full disk contribution to the total EUV 195 A ˚ is limited, so the EUV 195 A flux of Region C is almost independently of the GOES X-ray flux. We notice that ˚ each peak corresponds to brightennings in EIT 195 A images.
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˚ images (upper panel) and running difference images (lower panel) of AR 11046. Each brightening occurred at increase of EIT Fig. 7. A series of EIT 195 A ˚ flux in Fig. 6. 195 A
˚ and mean EIT 195 A ˚ flux (dashed) of Region C. The area of Region C is Fig. 8. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A 50 50 = 2500 pixels and the number of pixels with intensity larger than 50 DN s1 is between 500 and 650. The vertical bars indicate the main flux peaks ˚ and they varied independently with peaks of X-ray flux. of 195 A
4. Discussion and conclusions On 2010 February 8, three active regions were present on the solar disk. This gives us a good chance to study EUV flux variability of different sizes of active regions and its relation to EUV flare brightenings and GOES X-ray flux. The main full disk EUV flux comes from active regions, especially from large active regions. The sudden increase of EUV flux co-occurred with the EIT flare brightenings. During flare time, the intensity increased significantly (tens ˚ flux, we notice times). By comparing their local EUV 195 A that the larger the magnetic flux of a active region is, the ˚ flux the active region emits. The investigamore EIT 195 A ˚ flux of different active regions shows tion of EUV 195 A that for both large and small active region, the flare brightenings are corresponding to the peaks of local EUV flux. For a large active region like AR 11045, the two major ˚ flux fit well to two GOES X-ray Mpeaks of EUV 195 A class flares with a few minutes delay. This result is consist to previous study (Qiu et al., 2000). So many EUV peaks
of flares demonstrates that the brightenings in large active region is frequent, thus the large active region is dynamic in the low corona. For a moderate active region like AR 11046, the peaks ˚ is also well correlated to that of observed in EUV 195 A GOES X-ray flux. When the EUV flare brightening appeared, obvious increase can be identified in the EUV ˚ flux curve. As for the weak plage region like Region 195 A ˚ flux is still C, the variation of its own local EUV 195 A obvious though it is ignorable when comparing to the total ˚ flux. Due its low magnetic flux, it is more accuEUV 195 A rate to reflect EUV flare brightening in such a small region by using total EUV flux than total flux of full disk. Comparing the large active region, the moderate and small acctive region is relatively quiet compared to the large active region (AR 11045). ˚ more preOur study suggests that local EUV flux 195 A cisely reflects the EUV brightening for a active region than the full disk EUV flux and GOES X-ray flux. Due to the ˚ images, limited cadence time (12 minutes) of EIT 195 A
X. Bao, W. Xie / Advances in Space Research 50 (2012) 683–689
the variation of EUV flux does not match the 1 minute cadence of X-ray flux in GOES. With the launch of AIA (Atmospheric Imaging Assembly) on board SDO (Solar Dynamics Observatory), a more detailed study on the variation of EUV flux with high cadence within 1 minute will be carried out to examine more detailed features of EUV flare brightenings (Woods et al., 2011). Acknowledgement This work is supported by Joint Funds of Astronomy of National Natural Science Foundation of China under grant No.11078012 and Collaborating Research Program of Key Laboratory of Solar Activity National Astronomical Observatories of Chinese Academy of Sciences under grant No.KLSA2011 10. SOHO is a project of international cooperation between ESA and NASA. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.asr.2012.06.016.
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References Bao, X., Zhang, H., Lin, J. Formation of the coronal mass ejection leading edge on 2003 Febrary 18 event. Chin. J. Astron. Astrophys. 6, 741–750, 2006. Bao, X, Zhang, H., Lin, J., G.A. Stenborg, G.A. Coronal mass ejections and the associated activities on the solar disk observed on October 26, A&A 463, 321–331, 2003. Cheng, C.C. Ultraviolet observations of the preimpulsive phase in a solar flare – enhanced turbulence and heating. Astrophys. J. 349, 362–368, 1990. Delaboudinie`re, J.-P., Artzner, G.E., Brunaud, J., Gabriel, A.H., et al. EIT: extreme-ultraviolet imaging telescope for the SOHO mission. Solar Phys. 162, 291–312, 1995. Garica, H.A. Temperature and emission measure from GOES soft X-ray measurements. Solar Phys. 154, 275–308, 1994. Kane, S.R., Donnelly, R.F. Impulsive hard X-Ray and ultraviolet emission during solar flares. Astrophys. J. 164, 151–163, 1971. Nightingale, R.W., Aschwanden, M.J., Hurlburt, N.E. The variability of EUV Brightenings in coronal loops observed with TRACE. Solar Phys. 190, 249–265, 1999. Priest, E.R., Forbes, T.G. The magnetic nature of solar flares. Astron. Astrophys. Rev. 10, 313–377, 2002. Qiu, J., Wang, H., Chae, J., et al. Extreme-ultraviolet flare loop emissions in an eruptive event. Solar Phys. 194, 269–283, 2000. Woods, T.N., Hock, R., Eparvier, F., et al. New solar extreme-ultraviolet irradiance observations during flares. Astrophys. J. 739, 59–71, 2011. Zhang, J., Kundu, M.R., White, S.M. Spatial distribution and temporal evolution of coronal bright points. Solar Phys. 198, 347–365, 2001.
Advances in Space Research 50 (2012) 683–689 www.elsevier.com/locate/asr
Solar EUV flux variation and flare brightenings Xingming Bao a,⇑, Wenbin Xie b,1 a
Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China b Department of Physics, Jilin Normal University, Jilin 136000, China Available online 21 June 2012
Abstract ˚ and flare brightening has been examined in different sizes On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195 A of active regions by using SOHO/EIT, MDI and Ha observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Ha band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are ˚ flux peaks are well correlated to that of corresponding to the EUV flare brightenings. For the large active region, the local EUV 195 A ˚ flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active the GOES X-ray flux. The EUV 195 A ˚ flux is not well correlated to GOES X-ray flux. The EUV 195 A ˚ flare brightenings in the moderate active region, the local EUV 195 A ˚ flux region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195 A ˚ varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195 A flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux. Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: EUV brightenings; EUV intensity; Solar flares
1. Introduction Solar flares are the most violet eruption in atmosphere which release large amounts of magnetic energy in several tens of minutes. During flares bright emissions often appear in the location of images where the solar atmosphere is heated. Traditionally solar flares are observed with sudden bright emissions (typically two bright ribbons) in Ha monochromatic images. With the launch of space based instruments such as Solar Heliosphere Observatory (SOHO), Transition Region And Coronal Explorer (TRACE) and Geostationary Operational Environmental Satellites (GOES) in the recent two decades, solar flares are intensively observed in Extreme-ultraviolet (EUV) ⇑ Corresponding author. Tel.: +86 10 64807659; fax: +86 10 64888716.
E-mail address: [email protected] (X. Bao). Also in Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, China. Tel.: +86 434 3295977; fax: +86 434 3294566. 1
and X-ray band. By comparing with Ha images, EUV observations show that some flare ribbons are still discerned in EUV and X-ray images even they are not recognized in Ha images during prominence eruptions (Bao et al., 2006, 2007). This suggest that solar flares are basically coronal, rather than chromospheric phenomena (Priest and Forbes, 2002). EUV brightening is an indication of various levels of flares. The physical nature whether the EUV flare brightenings reflect local heating or increase of its local electron’s density is not well understood. Zhang et al. (2001) show that the flux of EUV bright points varied intermittently and suggest that the number of EUV bright points is more than that in SXT images is due to their temperature is lower than 2 106 K. After a systematic variability analysis of EUV emission from a dipolar loop system in the core of an active region, Nightingale et al. (1999) suggests that EUV brightenings are not related to flare heating processes and seem to be produced by compressional waves that travel with approximately the sound speed through the active
0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.06.016
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X. Bao, W. Xie / Advances in Space Research 50 (2012) 683–689
region loops, perhaps triggered by chromospheric reconnection processes. In order to explore the question if the EUV brightenings and large flares experience the same physical process, it is necessary to study quantitatively the variation of EUV flux in different sizes of active regions. Using early satellite observations, Kane and Donnelly (1971) show that EUV impulsive and hard X-ray emissions are well correlated in occurrence, time dependence and intensity and indicate that the principle mechanism for ˚ radiation during solar the impulsive emission of 10–1030 A flares is the free-bound, bound-bound, and bremsstrahlung emission from the quasi-thermal plasma produced by the collision loss of energetic electrons. Qiu et al. (2000) show that the peaking time of EUV intensity delays that of GOES X-ray several minutes and suggest that the delays EUV emission may come from cooling of soft X-ray loops. However, a question arise: when the magnetic loops is heated during flares, the temperature should increase to the EUV temperature (105 –106 K) at first, then to the X-ray temperature (> 106 K). Cheng (1990) also found the impulsive UV component prior to the hard X-ray burst and proposed electric current heating in the loop before the acceleration of hard X-ray electrons. The full disk EUV flux reflects the total EUV emission and only the brightenings contribute to the full disk EUV flux. What is the variation of local EUV flux in individual active region? In this paper, we examined the local EUV flux and EUV flare brightenings in different sizes of active regions on 2010 February 8 when two M-class flares and more than ten C-class flares occurred in solar disk. The variation of total and local EUV fluxes, GOES X-ray fluxes and local magnetic flux are compared.
magnetic field data are obtained by SOHO Michelson Doppler Imager (MDI) at approximately 1.5 hr intervals. Three active regions in solar disk on 2010 February 8 are ˚ , MDI magselected to calculate EUV flux. The EIT 195 A netic field and Ha of north hemisphere are shown in Fig. 1. The three areas are marked by rectangle A, B and C in Fig. 1 which represent three types of active regions respectively: Region A (AR 11046, N23 W15) is a moderate active region with two small sunspots, Region B (AR 11045, N25 E51) is a large active region with a sunspot group and Region C (N20 W30) is a small plage region in Ha without sunspot. Active region AR 11045 was located near meridian of the solar disk. White rectangles ˚ flux is calculated. In indicate areas where the EUV 195 A Ha image (Fig. 1c), AR11045 is a large active region and the Region A and C only displayed as weak plage. The magnetic configurations in Region A and B are complicated and that of Region C is simple bipolar in MDI magnetic field (Fig. 1b). Their total unsigned magnetic flux are listed in Table 1. The flux of Region A, B and C are 1:64 1020 Mx, 1:26 1021 Mx and 8:6 1019 Mx respectively. ˚ image at 05:24 UT on 2010 FebFig. 2 is an EIT 195 A ruary 8. White contours indicate the area with intensity larger than 50 DN s1 and the black contours indicate the area with intensity larger than 150 DN s1. Almost all
2. Observational data and reductions The Extreme-ultraviolet Imaging Telescope (EIT) on board SOHO images the Sun with field of view of 45’ and ˚, 1024 1024 pixels in four EUV emission lines: 171 A ˚ , 284 A ˚ and 304 A ˚ (Delaboudinie`re et al., 1995). 195 A ˚ emission lines is about The temperature of Fe XII 195 A ˚ filter images is 1.5 106 Kevin and the cadence of 195 A ˚ data are pro12 minutes or more. In this study, the 195 A cessed with dark current and flat field reduction. The EIT flux is calculated by the integral of digital number (DN) in selected area. In order to compare the active region with different sizes, the EIT flux is divided by the number of pixels in the selected area. The Ha images are obtained from Yunnan Astronomical Observatory (http://www. ynao.ac.cn), Chinese Academy of Sciences. Its detectors have 3073 2048 pixels with cadence of 40 s (http:// 159.226.148.131/solar/equipment_full.html). GOES has included a space weather monitor (SEM) X-ray sensor which measures the disk-integrated solar emission in two ˚ and 0.5–4 A ˚ since 1974 (Garica, bands covering 1–8 A 1994). The 1-minutes X-ray flux data used in this study is ˚ channel. The obtained by GOES-14 satellites 1–8 A
Fig. 1. Three active regions on the north hemisphere on 2010 February 8. They are marked by rectangles A, B and C respectively. Active region AR 11045 is the largest region among them. White rectangles indicate the ˚ flux is calculated. (a) EIT 195 A ˚ image at areas where the EUV 195 A 03:12:28 UT. (b) MDI magnetogram at 03:15:02 UT shows the longitudinal magnetic field. (c) Ha image from Yunnan Astronomical Observatory at 03:15:40 UT. Region A and C appeared as weak plages.
X. Bao, W. Xie / Advances in Space Research 50 (2012) 683–689 Table 1 Magnetic flux of three active regions. Region
Magnetic flux (Mx)
Region A Region B Region C
1:64 1020 1:26 1021 8:64 1019
white areas are located in north hemisphere and all black areas are located in these three regions. Some white areas are located out of the solar limb. Two horizontal lines 1 and 2 mark the position of pixels which sweep over the Region B and Region A and C respectively. The intensity distribution along the two lines is plotted with solid and dashed curves in the lower part of image. Beside the three active regions, the intensity in the quiet region is lower than 50 DN s1 . The intensity in active region 11046 reaches 200 DN s1 and the intensity in the flare region in large active region 11045 reaches 1000 DN s1 , even reaches 4000 DN s1 in some M-class flares. This indicates that the intensity of brightenings in active region is larger than 50 DN s1 .
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Zhang et al. (2001) estimate the average EIT flux is 55.2 DN s1 pixel1 . In this study, the EUV flux is calculated by summing the intensity which is larger than 50 DN s1 in the region, then dividing by the region area (number ˚ flux is DN s1 of pixels). Therefore the unit of EIT 195 A 1 pixel and reflect variation of average intensity in the ˚ images are co-aligned by applying region. The EIT 195 A a cross-correlation algorithm. The accuracy of the alignment is about 2-300 . Since the variation of intensity in the area near the side of the rectangles is not significant, the small shift (2-3”) between two consecutive frames contributes little error to their EUV flux calculations. 3. Results ˚ for 2010 The GOES X-ray flux (solid) in from 1 to 8 A February 8 is shown in Fig. 3. There are two M-class X-ray flares that occurred at 07:48 UT and 13:48 UT. The total ˚ flux (dashed) of the full disk is plotted in EIT 195 A
˚ image at 05:24 UT on 2010 February 8. The two horizontal lines 1 and 2 mark the pixels whose positions sweep over the Region B and Fig. 2. EIT 195 A Region A and C respectively. The intensity distributing along line 1 is the solid curve while that along line 2 is the dash curve in the lower part of the EIT ˚ image. The intensity peaks at Region A, B and C are marked by a, b and c. White contours indicate the area with intensity larger than 50 DN s1 195 A and the black contours indicate the area with intensity larger than 150 DN s1. The intensity in quiet region is lower than 50 DN s1, the intensity in small active regions (Region A and C) reaches to 200 DN s1 and the intensity in the large active region (region B) reaches up to 1000 DN s1 during flare time.
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Fig. 3. The area (pixel number) of full disk is 1024 1024 = 1.048576 106 pixels and the actual number of pixels with intensity larger than 50 DN s1 is about 2.8 3.8 104. From Fig. 2, it shows that the intensity in quiet regions contribute little to the EUV flux, so about 9.6 9.7 105 pixels with intensity lower than 50 DN s1 are deselected and we select 105 pixels as the area of full disk. It is clearly shown that the two prominent peaks are well correspond to the two M-class X-ray flares and the ˚ flux at 21:34 UT is also fitted at same time peak of 195 A as GOES X-ray flux peak. The increase of the background ˚ flux from beginning to the end of the of the full disk 195 A day may related to the increase of EIT flare loops. ˚ flux (dashed) of AR 11045 is The local EUV 195 A shown in Fig. 4 together with GOES X-ray flux (solid). The pixels with intensity larger than 50 DN S1 are selected to calculate the EUV flux and its number is about 5 6103. The area of Region B is 120 90 = 10800 pixels which is about two times of selected pixels, that is, nearly half of pixels are deselected. Though the EUV flux of AR 11045 is larger than that of full disk, the EUV flux of the full disk is larger than that of AR 11045 considering the area of full disk is about 9 times larger than that of AR 11045. Since the AR 11045 is the largest active region on the solar disk and contribute large amounts to the total full disk EUV ˚ flux of AR 11045 is simflux. So the profile of EUV 195 A ˚ flux. According to the ilar to that the full disk EUV 195 A GOES X-ray flux, there are more than ten C-class flares and two M-class flares that occurred on 2010 February 8. During 03:00 UT-06:00 UT, there are four peaks and one sub-peak (before second peak) in X-ray flux. We noticed that there are five EUV flux peaks corresponding to X-ray peaks and sub-peaks during this period. This indicates that the X-ray sub-peak comes from AR 11045 because EUV flux is a local flux of AR 11045. The most prominent peaks appeared at 07:48 UT and 13:48 UT and 21:34 UT are indicated by three vertical bars and they coincide with the three ˚ peaks are also major X-ray flares. Some small EUV 195 A well consistent with peaks in X-ray flux peaks at several
C-class flares such as flares at 03:24 UT, 04:26 UT, 05:36 UT, and 06:12 UT. The Ha images show that consecutive white and dark loops emerged and spread out during period between 03:00 UT and 07:00 UT. ˚ movie (v1.gif) shows a coupe of flare The EIT 195 A brightenings appearing at AR 11045 during flare time, sometimes were accompanied by surges and mass ejections. During 03:00 UT-06:00 UT, a series of south directed loops rose and expanded out and fit well the five EUV flux peaks of C-class flares in Fig. 4. The first M-class flare at 07:48 UT was so bright that it appeared overexposed at center of AR 11045 (Fig. 5a). Post flare loops appeared above the flare ribbons during the second M-class flare at 13:48 UT (Fig. 5b). During the C-class flare in AR 11045 at 21:34 UT, it is noticed that the flare brightenings were not brighter than two previous M-class flares but also followed by rising of post flare loops (Fig. 5c). ˚ flux (dashed) of Region A Fig. 6 shows the EUV 195 A ˚ . The area of (AR 11046) and GOES X-ray flux in 1–8 A region A is 120 90 = 10800 pixels and the number of selected pixels with intensity larger than 50 DN s1 is between 4200 and 5000. The first flare brightening with ris˚ images ing of bright loops in AR 11046 in EIT 195 A appeared around 03:00 UT which was corresponding to ˚ flux the first peak. The left vertical bar indicates that 195 A peak at 12:06 UT is corresponding to a C-class flare. ˚ flux peak of Region A at 19:06 UT coAnother 195 A occurred a minor sub-peak of GOES X-ray flux. EIT ˚ images shown that the coronal loops reconnected 195 A with its surrounding loops. Since the width of the EUV flux peaks of Region A is larger than that GOES X-ray flux peaks, the variability between EUV flux of Region A and GOES X-ray shows a little correlation. ˚ images (v2.gif), we noticed From a series of EIT 195 A ˚ flux curve are that the sudden increases in the EIT 195 A ˚ ) bright flare loops accompanied by rising of EIT 195 (A ˚ flare brightenings (upper panel in Fig. 7). The EIT 195 A ˚ difference images can be displayed clearly in EIT 195 A (lower panel in Fig. 7).
˚ and mean EIT 195 A ˚ flux curve (dashed) of the full disk. Three major peaks of Fig. 3. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A ˚ flux fit well to the three flare peaks of GOES X-ray flux. 195 A
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˚ and mean EIT 195 A ˚ flux curve (dashed) of AR 11045 (Region B). The pixels with Fig. 4. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A intensity larger than 50 DN S1 are selected to calculate the EUV flux and its number is about 5 103. The area of Region B is 120 90 = 10800 pixels ˚ flux is almost similar to the total flux of full disk in Fig. 3 since the large part of which is about two times of selected pixels. The variation of EUV 195 A EUV flux of pixels with intensity larger than 50 DN S1 come from AR 11045. The vertical bars indicate the maximum of three major flares which co˚ band. occurred in flux peaks of X-ray and EUV 195 A
˚ images of AR 11045 during M-class flares at 07:48:45 UT (a), 13:48:45 UT (b) and C-class flare at 21:34:45 UT (c). Fig. 5. Three EIT 195 A
˚ and mean EIT 195 A ˚ flux (dashed) of AR 11046 (Region A). The area of region A Fig. 6. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A is 90 120 = 10800 pixels and the number of pixels with intensity larger than 50 DN s1 is between 4200 and 5000. Two vertical bars indicate ˚ band. simultaneous flux peaks of two flares in X-ray and EUV 195 A
˚ flux curve (dashed) of Region C on 2010 The EIT 195 A February 8 is shown in Fig. 8. Its area is 50 50 = 2500 pixels and the number of selected pixels with intensity larger than 50 DN s1 is between 500 and 650 — nearly four fifths pixels in the area is deselected. The main EUV flux peaks are indicated by vertical bars. The strongest peak ˚ flux appeared at 20:06 UT (3rd vertical of EUV 195 A
bar in Fig. 8), almost align to a sub-peak of GOES Xray flux. Due to the small magnetic flux of Region C, its ˚ flux of the full disk contribution to the total EUV 195 A ˚ is limited, so the EUV 195 A flux of Region C is almost independently of the GOES X-ray flux. We notice that ˚ each peak corresponds to brightennings in EIT 195 A images.
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˚ images (upper panel) and running difference images (lower panel) of AR 11046. Each brightening occurred at increase of EIT Fig. 7. A series of EIT 195 A ˚ flux in Fig. 6. 195 A
˚ and mean EIT 195 A ˚ flux (dashed) of Region C. The area of Region C is Fig. 8. The 1 minute curve (solid) of GOES X-ray flux in range of 1–8 A 50 50 = 2500 pixels and the number of pixels with intensity larger than 50 DN s1 is between 500 and 650. The vertical bars indicate the main flux peaks ˚ and they varied independently with peaks of X-ray flux. of 195 A
4. Discussion and conclusions On 2010 February 8, three active regions were present on the solar disk. This gives us a good chance to study EUV flux variability of different sizes of active regions and its relation to EUV flare brightenings and GOES X-ray flux. The main full disk EUV flux comes from active regions, especially from large active regions. The sudden increase of EUV flux co-occurred with the EIT flare brightenings. During flare time, the intensity increased significantly (tens ˚ flux, we notice times). By comparing their local EUV 195 A that the larger the magnetic flux of a active region is, the ˚ flux the active region emits. The investigamore EIT 195 A ˚ flux of different active regions shows tion of EUV 195 A that for both large and small active region, the flare brightenings are corresponding to the peaks of local EUV flux. For a large active region like AR 11045, the two major ˚ flux fit well to two GOES X-ray Mpeaks of EUV 195 A class flares with a few minutes delay. This result is consist to previous study (Qiu et al., 2000). So many EUV peaks
of flares demonstrates that the brightenings in large active region is frequent, thus the large active region is dynamic in the low corona. For a moderate active region like AR 11046, the peaks ˚ is also well correlated to that of observed in EUV 195 A GOES X-ray flux. When the EUV flare brightening appeared, obvious increase can be identified in the EUV ˚ flux curve. As for the weak plage region like Region 195 A ˚ flux is still C, the variation of its own local EUV 195 A obvious though it is ignorable when comparing to the total ˚ flux. Due its low magnetic flux, it is more accuEUV 195 A rate to reflect EUV flare brightening in such a small region by using total EUV flux than total flux of full disk. Comparing the large active region, the moderate and small acctive region is relatively quiet compared to the large active region (AR 11045). ˚ more preOur study suggests that local EUV flux 195 A cisely reflects the EUV brightening for a active region than the full disk EUV flux and GOES X-ray flux. Due to the ˚ images, limited cadence time (12 minutes) of EIT 195 A
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the variation of EUV flux does not match the 1 minute cadence of X-ray flux in GOES. With the launch of AIA (Atmospheric Imaging Assembly) on board SDO (Solar Dynamics Observatory), a more detailed study on the variation of EUV flux with high cadence within 1 minute will be carried out to examine more detailed features of EUV flare brightenings (Woods et al., 2011). Acknowledgement This work is supported by Joint Funds of Astronomy of National Natural Science Foundation of China under grant No.11078012 and Collaborating Research Program of Key Laboratory of Solar Activity National Astronomical Observatories of Chinese Academy of Sciences under grant No.KLSA2011 10. SOHO is a project of international cooperation between ESA and NASA. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.asr.2012.06.016.
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