First Imaging Observation of Standing Slow Wave in Coronal Fan loops
DD raft version O ctober
25, 2018
Preprint typeset using L A TEX style AASTeX6 v. 1.0
FIRST IMAGING OBSERVATION OF STANDING SLOW WAVE IN CORONAL FAN LOOPS
V. P ant , A. T iwari , , D. Y uan D. B anerjee , Indian Institute of Astrophysics, Bangalore-560 034, India Northumbria University, Newcastle Upon Tyne, NE1 8ST, UK Institute of Space Science and Applied Technology, Harbin Institute of Technology, Shenzhen 518000, China Center of Excellence in Space Sciences, IISER Kolkata, India
ABSTRACTWe observe intensity oscillations along coronal fan loops associated with the active regionAR 11428. The intensity oscillations were triggered by blast waves which were generateddue to X-class flares in the distant active region AR 11429. To characterise the nature ofoscillations, we created time–distance maps along the fan loops and noted that the intensityoscillations at two ends of the loops were out of phase. As we move along the fan loop, theamplitude of the oscillations first decreased and then increased. The out–of–phase naturetogether with the amplitude variation along the loop implies that these oscillations are verylikely to be standing waves. The period of the oscillations are estimated to be ∼
27 min,damping time to be ∼
45 min and phase velocity projected in the plane of sky ∼ − . The projected phase speeds were in the range of acoustic speed of coronal plasma atabout 0.6 MK which further indicates that these are slow waves. To best of our knowledge,this is the first report on the existence of the standing slow waves in non–flaring fan loops. a r X i v : . [ a s t r o - ph . S R ] A ug Keywords:
Sun: Corona ; Sun: Coronal loops ; Sun: Oscillations INTRODUCTIONMagnetohydrodynamic (MHD) waves are ubiquitous in solar corona. With the advent of modern spacebased instruments, di ff erent types of wave modes have been observed in the last decade. Slow MHD modes(compressional waves) were first observed in the polar coronal holes using UVCS by Ofman et al. (1997).Later, DeForest & Gurman (1998) and Ofman et al. (1999) reported propagating intensity disturbances(PDs) in polar plumes using EIT onboard SOHO. Recently, several authors have reported that small scalejets and spicules at transition region and chromosphere are associated with with PDs seen in polar plumesand polar coronal holes (Pant et al. 2015; Jiao et al. 2015; Samanta et al. 2015; Bryans et al. 2016; Yuan et al.2016). Reflections of propagating slow waves were also reported in hot and flaring coronal loops using AIA(Kumar et al. 2013, 2015) and XRT observations (Mandal et al. 2016). The authors have reported that thesewaves are triggered by the flares at the footpoint of the coronal loops. Recently, Fang et al. (2015) havemodelled reflective slow mode in flaring loops using 2.5D magnetohydrodynamic simulation in synthetic131 Å emission images.Apart from propagating slow waves, flare–excited standing slow waves have also been observed in hotand flaring coronal loops. Oscillations in Doppler velocity, detected in Fe xix , were reported in hot flaringcoronal loops using SUMER / SOHO and SXT / Yokoh (Wang et al. 2002). Time period of oscillations wasfound to be 14–18 min. These oscillations were interpreted as slow standing modes. Wang et al. (2003a,b)have performed statistical study of slow standing modes in several hot coronal loops and post flare loops,respectively. They have reported a π / xix and Fe xxi emission lines (formation T > > OBSERVATIONSOn 2012 March 7, a group of fan loops were observed near the active region AR 11428 (see Figure 1).Two X-class flares were detected consecutively at a distant active region, AR 11429 to the north west ofAR 11428. The approximate distance between AR 11428 and AR 11429 is about 455 Mm. The GOESX-ray emission (inset in Figure 1) exhibit the evolution of the flares. The X-ray flux at two channels peakedat 00:22 UT and 01:13 UT, respectively. The strength of two peaks corresponds to the fluxes of X5 andX1 classes, respectively. Both the X5 and X1 flares originated from AR11429 and the associated energypulses reach AR11428 at 00:27UT and 01:15 UT, respectively. The fan loops were initially driven to movetransversely, and subsequently the intensity perturbations along the loops became detectable. A three-hourdata set (00:00 UT-03:00 UT) taken by the Atmospheric Imaging Assembly (AIA) on board Solar DynamicsObservatory (SDO) (Lemen et al. 2012) was used for detailed analysis. The fan loops of interests are visiblein both 171 Å and 193 Å, so we only use these two channels for study. RESULTS3.1.
Time evolution of intensity oscillations
To derive the properties of the oscillations, we placed three artificial slices, S1, S2, and S3, along thefan loops as shown in Figure 2, at the locations where the intensity oscillations were clearly seen. Wechose broad artificial slices in order to capture the longitudinal oscillations despite they get displaced intransverse direction due to interaction with the blast wave. It should be worth noting at this point that onlyone footpoint, close to the active region, of fan loops was clearly visible in 171 and 193 Å. The length ofthe artificial slices correspond to the distances along the fan loop up to which clear signatures of intensityoscillations were observed. Therefore, the length of the artificial slices may not be equal to the lengthof the fan loops. We discuss the estimation of the length of the fan loop in section 3.3. For each of thethree artificial slices, we generated a time–distance map, which will be termed as an x-t map, henceforth,
Figure 1 . Full disk image of Sun at AIA 171 Å. Red and yellow box represent the location of active regions AR 11428and 11429 respectively. Region of interest (ROI) which is used for further analysis is enclosed in the box shown in red.GOES X–ray flux variation is overplotted in the figure. Curves in orange and white represents the flux correspondingto two passbands, i.e, throughout the paper. Figure 3 represents the x-t maps for slices S1, S2, and S3 for 171 and 193 Å in left,middle and right panels respectively. The signatures of intensity oscillations were clearer in AIA 171 Å ascompared to AIA 193 Å, because fan loops appeared more di ff use in AIA 193 Å. A possible reason for thisis discussed in section 3.4. The vertical lines in red in Figure 3 represent the instances when blast waves Figure 2 . Left : AIA 171 Å image of the ROI shown in Figure 1. Three curved artificial broad slices, S1, S2 andS3 are overplotted in red which were used to generate the x-t maps shown in Figure 3. The dotted curves in orangerepresents the length of the fan loops.
Right : Same as left but for AIA 193 Å. Movies 1 and 2 are corresponding toAIA 171 and 193 Å are linked with this figure. hit the fan loops system. We noticed that the second blast wave hit fan loops when the intensity oscillationsdriven by the first blast wave were still present.Figure 3 shows that the intensity oscillations were out of phase at the two ends of the artificial slices asseen in the x-t maps. The out–of–phase signature was clearly seen in both AIA 171 and 193 Å. It is clearfrom the Figure 3 that one reflection point (or antinode) of the oscillations was present near the one footpointwhich is clearly visible in the intensity images. While the other antinode was present at the other end ofthe artificial slice which may or may not be close to the other footpoint. From movies 1 and 2 linked withFigure 2, it is evident that the shape and appearance of fan loops was changed after the second blast wavehit the fan loop system. Therefore, intensity oscillations were not clearly seen in x-t maps after second blastwave hit the fan loops.Figure 4 shows the variation of intensity with time at di ff erent distances along the artificial slice S1. Y-axis represents the relative intensity normalized to the local background. Two dashed vertical lines in redrepresent the instant of time when blast wave hit fan loops. Since blast wave hit the fan loops twice, we fitted Figure 3 . Time - distance (x-t) maps corresponding to slices S1, S2 and S3 as marked in Figure 2 are shown in left,middle and right panels, respectively. Two vertical red lines represent the time at which two blast waves impacted thefan loop system. Y-axis represents the distance along the artificial slice. the sinusoidal and damped sinusoidal curve separately at two di ff erent time intervals. Red curve representsthe best fit sinusoidal curve. We should point out that the damping of the oscillations were not seen clearlyduring the first period of observation, which may be due to the impact of the second blast wave. Thus wedid not fit damped sinusoidal curve during the first time interval. However, we fitted a damped sinusoidalcurve (shown in orange) in the second time interval and noticed the signature of damping at some locationsalong the fan loops ( e.g, at 5 Mm). The intensity of oscillations became undetectable after 120 min becausethe shape and appearance of the fan loop changed (see movies 1 and 2). The estimated average period ofthe oscillation, P and average damping times, τ , at the location of three slices, S1, S2 and S3 in 171 and193 Å are listed in Table 1. Since only one oscillation was observed during second interval, there were largeuncertainties in the damping time. The quality factor (ratio of damping time by time period) estimated atthe location of three slices is also listed in Table 1. These oscillations are weakly damped as compared tothose reported earlier in hot coronal loops. A possible reason for weak damping is outlined in section 4. 3.2. Variation of amplitude of intensity oscillation
We noted that the relative amplitude (after normalising with background intensity) of the intensity oscil-lations along S1 in 171 Å first decreased and then increased while moving from one end at S1 (close toone footpoint) to other (may be close to another footpoint) (see Figure 4). The variation can be seen clearlyfor both curves fitted at two separate time intervals shown in red and orange. Furthermore, the variation ofthe amplitude at di ff erent distances along S1, S2 and S3 in 171 and 193 Å is also shown in Figure 4. Sys-temic decrease and increase of the amplitude of oscillations, while moving from one end of slice to another,was seen at the location of all slices. This signature clearly indicate the existence of an anti-node near thefootpoints of the fan loop.3.3. Estimation of loop length and velocity of the oscillations
The footpoint of the fan loop that was away from the active region was distributed and therefore, not seenclearly in normal intensity images of 171 or 193 Å. Thus, it was not straightforward to measure the length ofthe fan loops. Moreover, the shape and appearance of fan loops also changed with time (see movie 1 and 2).To estimate the length, we chose the frames where the fan loops were best seen in normal intensity images.We chose several points along the visible segment of the fan loops and interpolated a cubic spline betweenthem. The length of interpolated curve should be approximately equal to the projected length of the fanloops. The orange curves in Figure 2 are the fitted spline curves which represents the projected length of fanloops at the location of three artificial slices. We found the length of the fan loops at the location of S1, S2and S3 to be 62, 74 and 54 Mm, respectively (see Table 1). Note that the estimated length is the projectedlength in the plane of sky. Assuming the length of fan loop as same in 171 and 193 Å, we estimated thephase velocity of oscillations in 171 (193) Å to be 75 (85), 83 (101) and 65 (91) km s − at the location ofS1, S2 and S3, respectively. The phase velocity of oscillations, v , are comparable to the speed of sound in171 and 193 Å. Figure 4 . Top : Left:- Intensity variation after normalising to the background intensity, at di ff erent distances alongS1. Two vertical dashed line represents the instances when blast wave hit the fan loops system. Middle and bottom :Variation of amplitude of intensity oscillations for S1, S2 and S3 in 171 and 193 Å. The top row right panel is sameas the middle row left panel.
Temperature and density of the fan loop
We estimated the temperature and density of the fan loop using automated di ff erential emission measure(DEM) technique as developed by Aschwanden et al. (2013). The temperature of the fan loops was found tobe ∼ Table 1 . Observational parameters of oscillationsAIA 171 Å AIA 193 ÅSlice P (min) τ (min) Q l (Mm) v (km s − ) P (min) τ (min) Q l (Mm) v (km s − )S1 27.5 ± ±
25 1.45 62 75 24.1 ± ±
18 0.81 62 85S2 29.6 ± ±
25 1.79 74 83 24.4 ± ±
10 1.53 74 101S3 27.6 ± ±
20 1.52 54 65 19.7 ± ±
12 1.72 54 91N ote — P represents the period of oscillations, τ represents the damping time, Q is the quality factor, defined as theratio of damping time and period of oscillations, l is the projected length of the fan loop at the location of the slice and v is the velocity of the oscillations. along the loop. Since the temperature of fan loops is low, they appear brighter in 171 Å channel and di ff usein hotter channels like 193 Å. We should point out that a fan loop may consists of several finer strands andwe have not considered that scenario here. DISCUSSION AND CONCLUSIONSWe observe intensity oscillations in a non–flaring fan loop system as seen in AIA 171 and 193 Å images.The intensity variations were out of phase close to two footpoints of fan loops and the amplitude of theintensity oscillations varied along fan loops at the location of artificial slices. The amplitude of intensityoscillations first decreased and then increased while moving from one footpoint to another along the fanloop. It should be noted that it is di ffi cult to identify the di ff erences between standing and propagatingwaves without spectroscopic signatures. Recently, Yuan et al. (2015) have performed forward modelling ofstanding slow magnetoacoustic waves in flaring loops. They have reported that the variation of amplitudealong the coronal loops is one of the signatures of the standing slow magnetoacoustic waves (see, Figure 8in Yuan et al. 2015). Moreover, small phase shift in the intensity variations with time at di ff erent distancesalong fan loop corresponding to the slice S1, as seen in Figure 4, can be due to the presence of standingslow oscillations (Taroyan et al. 2007; Taroyan & Bradshaw 2008). We estimated the time period of the1oscillations ∼
27 min and damping time ∼
45 min. We calculated the projected length of the fan loops andestimated that the velocity of oscillations are comparable to the velocity of sound in 171 and 193 Å. Thesesignatures allows us to conclude that the observed oscillations are due to standing slow waves in coronal fanloops. The fan loops under study are associated with a sunspot. Yuan et al. (2011) reported presence of longperiod oscillations in the coronal di ff used plasma near an active region. The oscillations observed in thisstudy are di ff erent from those reported by Yuan et al. (2011) because the event under study was triggeredby energy impulse of flares, while Yuan et al. (2011) studied persistent leakage of long period oscillationsfrom the underneath sunspot.It is worth mentioning that only one footpoint of fan loops was clearly seen in AIA 171 and 193 Å images.At this stage we can only conjecture two possible scenarios by which the reflection of wave from other endcan happen. Either the antinode of the oscillations is present at the other footpoint which is distributed andtherefore not seen clearly in normal intensity images or the antinode could be present at the region of sharpdensity contrast close to the other end of the fan loop. The region of sharp density change may have actedas a reflecting surface. These scenario may be experimented in future studies using computer simulations.At most of the locations along the fan loops, oscillations are found to be undamped. The reason for theabsence of damping at most of the locations is not clear to us, more observations of such events are requiredto reach conclusive views on the damping. However at few locations along the fan loop, we indeed notedweak damping. The oscillations at those locations are weakly damped as compared to those reported inOfman & Wang (2002); Wang et al. (2002, 2003b, 2015) where the damping time was comparable to thetime period of the oscillations in hot and flaring coronal loops (T > ∼ ffi cient enough. Since thermal conduction is one of the main mechanism to damp slow waves,the oscillations were weakly damped in our study.In summary, we found the signatures of standing slow magnetoacoustic waves in cool fan loops. In earlier2studies these oscillations were particularly observed in the hot coronal loops. To the best of our knowl-edge, this is the first report, on the observational signatures of the existence of weakly damped standingoscillations in cool fan loops. ACKNOWLEDGMENTSThe authors thank the referee for her / his valuable in-depth comments which have helped us to improvethe manuscript. REFERENCEShis valuable in-depth comments which have helped us to improvethe manuscript. REFERENCES