- Email: [email protected]
Determination of intensity variations along coronal loops
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719 www.elsevier.com/locate/jastp
Determination of intensity variations along coronal loops A. Borgazzi, A. Costa1 Instituto de Astronomı´a y Fı´sica del Espacio, CONICET-UBA, CC. 67, suc. 28, 1428 Buenos Aires, Argentina Available online 2 November 2005
Abstract In this work we describe dynamical features of brightenings along coronal loops using a method derived in a previous paper. We register the behavior of space and time intensity variations along a compound of coronal magnetic tubes imaged by the Transition Region and Coronal Explorer (TRACE) in the well known coronal line at 17.1 nm. The studied loops were observed in the south-west limb on October 1st, 2001. We determine a coherent behavior of the intensity along neighboring magnetic tubes. During a first time interval we register a downflow with a brightening average speed of 111 km s1 and during a second time interval we detect an upward displacement of the brightening with an average speed of 50 km s1 . r 2005 Published by Elsevier Ltd. Keywords: Solar corona; Post-flare loops
1. Introduction The fact that most of the classical static and stationary theoretical models describing the behavior of the plasma inside coronal loops cannot generally explain the disagreement between observational data and theoretical predictions of temperature and density profiles (Aschwanden et al., 2000) suggests that, in the frame of these models, radiative losses cannot be compensated by thermal conduction. Therefore, different heating mechanisms must be assumed in order to sustain the whole picture (Aschwanden et al., 1999, 2000; Walsh and Galtier, 2000). Thus for non-stationary models that consider no linear effects, the deposition of energy caused by the presence of nanoflares or of slow Corresponding author. Fax: +55 12 3945 6810.
E-mail addresses: [email protected] (A. Borgazzi), [email protected] (A. Costa). 1 Member of the Carrera del Investigador Cientı´ fico (CONICET). 1364-6826/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.jastp.2005.03.008
magnetoacoustic waves (Nakariakov et al., 2000; Tsiklauri and Nakariakov, 2001), seem more promising. On the other hand, Harra et al. (2004) found that the appearance of transition region loops (up to 1 MK) is related to small-scale flaring in the corona resembling the cooling of these loops, with material draining down from the loop-top. Moreover, theoretical time-dependent models that consider individual loops, where the plasma evolves in response to a cyclical process of heating and cooling of the plasma, have difficulties in fitting, observations (Klimchuk and Mariska, 1988). Thus, more dynamic theoretical scenarios that combine spatial or temporal averages of individual unresolved loops are needed to make the theory and observations compatible, and to decide between the two possible interpretations (flow or wave models). Kjeldseth-Moe and Brekke (1998) proposed a dynamic scenario, where the observed individual active region loops are considered as composed of strands of finer structures or magnetic tubes, each one with a resolution that is below the resolution of SOHO
ARTICLE IN PRESS 1716
A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
instruments. The thin magnetic tubes could have gas flowing in different directions with different temperatures and with different speeds. This scenario suggested the existence of coarse-grained, unresolved fine-structured tubes, mostly magnetically isolated and of a global quasi-isothermal appearance that could theoretically justify the large velocities observed. Moreover, Schrijver (2001) described frequent catastrophic cooling and evacuation of quiescent solar coronal loops in active regions (occurring almost once every two days), which could be triggered when the loop heating is reduced by 1.5 orders of magnitude. Relatively cool material seemed to move downwards at speeds of up to 100 km s1 . He also suggested that this scenario is compatible with the existence of several thin cool strands embedded within an initial loop. However, he established that the filamentary structure must behave in a coherent manner. The technique developed for the analysis of loop brightenings in Costa and Stenborg (2004) (hereafter Paper I) was applied to a non-resolved loop structure observed with the Mirror Coronagraph for Argentina (MICA) (Stenborg, 2000) land-based telescope in the well known green coronal line at 530.3 nm. We tracked the plasma along the loop under study, provided that the shift of the intensity excess detected along it is a trace of the motion of the density excess. Otherwise, the shift of the intensity excess may represent the speed at which the plasma inside the loop changes its temperature to make it visible at the wavelength of observation. In this work, we analyze the coherence of the behavior of a structure composed of several magnetic threads. We apply the method discussed in Paper I to a loop system observed in the EUV, as opposed to the white light data we analyzed previously. The event that caused the magnetic structure started around 04:40 UT in the vicinity of the NOAA AR 9628 ð22 S; 91 WÞ and AR 9632 ð21 S; 73 WÞ and was imaged by TRACE (Handy et al., 1999). As our aim is to describe the motion of the brightening along the loop as a function of time, we selected images corresponding to a time interval ranging from 17.31 to 18.60 UT (times are expressed as hours and fractions of hours). TRACE is sensitive to 1–2 MK plasma emission with a much higher spatial resolution (0.5 arcsec). We used the 17.1 nm filter and a cadence of the sequence of images of about 1 min. The images were normalized and treated with a median filter in order to avoid white spots on the images. The latter are produced by the incidence of high energetic particles on the CCD camera.
threads. As a first step, we selected the structure we wanted to analyze. By visual inspection of the associated movie (along the whole time sequence), we detected a structure that seemed to remain stationary. Our aim was to characterize the temporal evolution of brightenings flowing along the loops, i.e., loops for which internal motions are observed instead of bulk movements of the structure. The second step consisted of performing several cuts of the selected structure in the first image. Fig. 1 shows some of these cuts done on the first image of the series (i.e., at 17.31 UT). The white straight lines on the loop-like structure depict the positions where the cuts (hereafter xðj; tÞ and yðj; tÞ, x and y the image coordinates of the cut j, j 1–17, and t the time) were done. Each of the cuts covers several resolved tubes, and this is reflected when the intensity along the cut is displayed. In order to avoid underlying macroscopic and coherent space displacements of the magnetic tubes, we imposed the condition that the intensity along each cut, and in all the temporal sequence, must exhibit a quadratic-like shape, with a maximum at the center of the cut and not at its edges or wings (see Fig. 2). Otherwise, as a third step, if the shape of the wings changed (meaning space displacements of the magnetic tubes), the series was reduced in time to guarantee that the magnetic structures were stable. In our case, all the cuts of the temporal interval (the whole series) fulfil this requirement. The accuracy of the procedure to guarantee the identification of stable loop structures (with an error of the width of the identified loop) was shown in Paper I.
2. Method In this section, we describe the method developed in Paper I, in its application to TRACE images, to characterize the behavior of a compound of magnetic
Fig. 1. First TRACE image (17.31 UT) showing the cuts. The first top cut is taken to be the origin from where distances are measured.
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
Intensity (Normalized Units)
0.60 0.50 0.40 0.30 0.20 0.10 0.00 0
10 20 30 Position along the cut
40
Fig. 2. TRACE intensity profile along a cut.
1717
sequence, we found that the intensity of each cut displayed two relative maxima (see Fig. 3). This was a difference with respect to the results obtained with MICA telescope where the unresolved loop showed only one maximum for each cut as a function of time. Thus, as our method registers only maximum intensity features, we divided the whole time interval in two sub-intervals, in order to have two absolute maxima. Fig. 3 shows the intensity as a function of time for all the cuts and the whole time interval. The two chosen subintervals were: 17.31–18.00 and 18.00–18.60 UT, each one containing one maximum. The fifth step was to obtain for each cut the position along the loop and the time where the maximum occurs. In order to accomplish this, we chose the spatial origin coincident with the position of the first cut (the top of the magnetic loop structure), i.e., d 1 ¼ 0. The positions of the following cuts are computed as X dj ¼ hj;j1 , j41
TRACE - Intensity of each cut
0.70
0.60
0.50
0.40
0.30
0.20 17.0
17.5
18.0 Time [UT]
18.5
19.0
where hj;j1 is the shortest distance between the two corresponding cuts. When the distance along the loop structure, where the maximum of each cut occurs, is displayed as a function of time, the resulting curve represents the tracking of the brightening along the loop. In summary, the time of the maximum (tj ) is considered to be the time of the feature that characterizes the brightening in each sub-interval. But, as the time where the maximum occurs may be slightly different from one cut to the other, indicating that the brightening moves along the loop, we must quantify the displacement of such a brightening. In Fig. 4, we plotted the position of the maximum intensity for each cut along the loop (i.e., d j ) as a function of time at which the maximum occurs (i.e., tj ). This characterizes the displacement of the brightening, as a function of time tj , for the two sub-intervals.
Fig. 3. Maximum of intensity of each cut as a function of time.
3. Results and discussion The fourth step consisted of determining the temporal and spatial coordinates where a maximum occurred for each of the cuts along the whole series (see Fig. 3). That is, once the spatial position of the cuts was chosen based on the first image (considered to be the first time of the sequence), the maximum of intensity for each cut was computed for every image in the sequence (see Fig. 2). Note that while the maximum intensity profile of each cut varies along the time series (Iðj; ti ÞaIðj; tk Þ, j, the number of the cut, ti and tk the time of two different images), each pair of spatial coordinates of a given cut have the same value (xðj; ti Þ ¼ xðj; tk Þ and yðj; ti Þ ¼ yðj; tk Þ). When following the temporal behavior of the intensity profile of each cut along the whole image
Fig. 4 shows the location of the brightening, as a function of time measured along a virtual axis, that extends from the top of a whole complex loop, composed of several thin magnetic tubes, to the bottom of the same structure for the two time sub-intervals chosen. The first cut is taken to be the top of the axis. From Fig. 4a, we can see that a coherent maximum of activity occurs on a first time range, from 17.40 to 17.55 UT, over the whole sub-interval considered (17.31–18.00 UT). That is, all the threads that compose the structure covered by the cuts exhibit their maximum of activity resembling a coherent sliding-down of the flow (in approximately 9 min) from the top of the structure. The average velocity is 111 10 km s1 .
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
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had its maximum (i.e., more than one thread had its maximum brightening at the same time).
Distance along the loop in Mm
80
60
4. Conclusions
40
20
0 17.35
17.40
(a)
17.45 17.50 Time[UT]
17.55
17.60
Distance along the loop in Mm
80
60
40
20
0 18.10
(b)
18.20
18.30 18.40 Time [UT]
18.50
18.60
Fig. 4. Location of the brightening as a function of time and measured along the loop. The spatial origin is chosen to be the position of the first cut: (a) first sub-interval; (b) second subinterval.
Fig. 4b also shows a coherent maximum of activity resembling the upward motion of flow in approximately 18 min (from 18.20 to 18.50 UT) over the sub-interval ranging from 18.00 to 18.60 UT. The average velocity in the second case is 50 8 km s1 . The error was estimated following the steps discussed in Paper I. Note that the method detects maximum activity along the loop; thus, it displays the tracking of the brightening along the loop as a function of a particular time interval (this time interval is formed by the times where the maximum intensity of the different cuts occurred). For the whole time interval (the two sub-intervals), the fact that more than one brightening occurred at the same time indicates that more than one cut in the same image
As was mentioned, most of the classical theoretical models have difficulties in determining the physical conditions under which models are compatible with observations. Thus, the description of time and velocity features can contribute to the understanding of the evolution of the heating and cooling mechanisms, as is demonstrated by the frequent catastrophic cooling and evacuation of quiescent solar coronal loops in active regions or by the correlation of cooling transition region loops with small-scale flaring occurring in the corona. Nevertheless, the analysis of much more cases is needed in order to get a statistical description of the characteristics and the parameters involved in this type of phenomenon, i.e., the range of times in which the brightenings vary, characteristic velocities of the brightenings, height of the loop structures, etc. An upcoming paper presenting further analysis is at work. To gain insight into this problem, in a previous paper (Paper I), we have implemented a diagnostic method that allows the discrimination between the motion of the magnetic structure and the motion of the internal plasma. In this work we applied the method to images obtained with TRACE that have a better spatial resolution and, thus, we could track the brightenings along a compound of threads or resolved loop structure. We could describe two different situations. One, where the emission develops initially near the loop-top and later slides down with an average speed of about 111 10 km s1 , occurring in a lapse time of about 9 min. This result is in agreement with the high-speed picture sustained by several authors, e.g. Kjeldseth-Moe and Brekke (1998), Schrijver (2001). The mean velocity, 111 km s1 , is in accordance with the catastrophic evacuation description proposed by Schrijver (2001). The other behavior that we report shows that the brightenings are upwardly directed with an average speed of about 50 8 km s1 , occurring in a lapse of time of about 18 min. Moreover, a remarkable result of our observations is that the different apparently isolated threads evolve in a coherent way. That is, all the threads that composed the structure covered by the cuts exhibited their maximum of activity in two time ranges resembling the coherent downward and upward motion of flow (of 9 and 18 min, respectively, over the whole interval of 1.5 h). This can also be inferred from the fact that the different data points in Fig. 4(a,b) did not necessarily occur along the same thread. Moreover, the scenario of various coherent-behaving threads is in accordance with the description of an individual non-resolved loop-like structure
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
described in Paper I; in this case the event was observed in the hottest (T M ¼ 1:8 106 K) Fe XIV green line (lM ¼ 530 nm) and we detected only downflows. The fact that here we have also observed upward-directed brightenings suggests that this is not the case of cooling transition region loops triggered by small-scale flaring from the corona (Harra et al., 2004). Moreover, the apparently coronal isolation of the magnetic threads suggests that coherence in the behavior of the whole event could be due to similar chromospheric conditions. An open question that is not yet resolved is related to the type of models that could better fit observations, i.e., wave-based or flow-based models. The presence or absence of high Doppler shifts could suggest flow dynamics or magnetoacoustic waves, respectively. Also, signs of wave patterns are suggested when the propagation speed is always less than the sound speed and when the moving features repeat quasi-periodically (Tsiklauri and Nakariakov, 2001). On the one hand, in the frame of flow models, as was mentioned, our results are in accordance with a kind of catastrophic evacuation or high-speed sliding down flow picture sustained by several authors e.g. Kjeldseth-Moe and Brekke (1998), Schrijver (2001). Limit cycle solutions of a compound of coherent non-resolved loops that assume an evaporation–condensation mechanism could also adjust the observations of the two types of patterns described here. On the other hand, this type of solutions have also been interpreted by several authors as downward- or upwardpropagating waves (Ofman et al., 1999; Nakariakov et al., 2000). Moreover, Ofman et al. (1999) proposed that the 5–15 min periodicity could be due to the driving of these perturbations by photospheric motions or by nonlinear transformation of the solar 5 min oscillations in the chromospheric region. Also, it might be possible that waves are triggered at the footpoints into the different threads by the same source and that then they propagate according to the sound speed of each thread.
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References Aschwanden, M.J., Alexander, D., Hulburt, N., Newmark, J.S., Neupert, W.N., Klimchuck, J.A., Gray, G.A., 1999. Threedimensional stereoscopic analysis of solar active region loops. Astrophysical Journal 515, 842–867. Aschwanden, M.J., Alexander, D., Hurlburt, N., Newmark, J.S., Neupert, W.N., Klimchuk, J.A., Gary, G.A., 2000. Three-dimensional stereoscopic analysis of solar active region loops. II. SOHO/EIT observations at temperatures of 1.5–2.5 MK. Astrophysical Journal 531, 1129–1149. Costa, A., Stenborg, G., 2004. Characterization of intensity variations along FE XIV coronal loops. A case study. Solar Physics 222 (2), 229–245 (Paper I). Handy, B.N., Acton, L.W., Kankelborg, C.C., 1999. The transition region and coronal explorer. Solar Physics 184, 229–260. Harra, L.K., Mandrini, C.H., Matthews, S.A., 2004. What causes solar active region loops to exist at transition region temperatures? Solar Physics 223, 57–76. Kjeldseth-Moe, O., Brekke, P., 1998. Time variability of active region loops with the coronal diagnostic spectrometer (CDS) on SOHO. Solar Physics 182, 73–95. Klimchuk, J.A., Mariska, J.T., 1988. Heating-related flows in cool solar loops. Astrophysical Journal 328, 334–343. Nakariakov, V.M., Verwichte, E., Berghmans, D., Robbrecht, E., 2000. Slow magnetoacoustic waves in coronal loops. Astronomy and Astrophysics 362, 1151–1157. Ofman, L., Nakariakov, V.M., DeForest, C.E., 1999. Slow magnetoacoustic waves in coronal plumes. Astrophysical Journal 514, 441–448. Schrijver, C.J., 2001. Catastrophic cooling and high-speed downflow in quiescent solar coronal loops observed with TRACE. Solar Physics 198, 325–345. Stenborg, G., Ph.D. Thesis, Goettingen University, Germany, 2000. Tsiklauri, D., Nakariakov, V.M., 2001. Wide-spectrum slow magnetoacoustic waves in coronal loops. Astronomy and Astrophysics 379, 1106–1112. Walsh, R.W., Galtier, S., 2000. Intermittent heating in a model of solar coronal loops. Solar Physics 197, 57–62.
Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719 www.elsevier.com/locate/jastp
Determination of intensity variations along coronal loops A. Borgazzi, A. Costa1 Instituto de Astronomı´a y Fı´sica del Espacio, CONICET-UBA, CC. 67, suc. 28, 1428 Buenos Aires, Argentina Available online 2 November 2005
Abstract In this work we describe dynamical features of brightenings along coronal loops using a method derived in a previous paper. We register the behavior of space and time intensity variations along a compound of coronal magnetic tubes imaged by the Transition Region and Coronal Explorer (TRACE) in the well known coronal line at 17.1 nm. The studied loops were observed in the south-west limb on October 1st, 2001. We determine a coherent behavior of the intensity along neighboring magnetic tubes. During a first time interval we register a downflow with a brightening average speed of 111 km s1 and during a second time interval we detect an upward displacement of the brightening with an average speed of 50 km s1 . r 2005 Published by Elsevier Ltd. Keywords: Solar corona; Post-flare loops
1. Introduction The fact that most of the classical static and stationary theoretical models describing the behavior of the plasma inside coronal loops cannot generally explain the disagreement between observational data and theoretical predictions of temperature and density profiles (Aschwanden et al., 2000) suggests that, in the frame of these models, radiative losses cannot be compensated by thermal conduction. Therefore, different heating mechanisms must be assumed in order to sustain the whole picture (Aschwanden et al., 1999, 2000; Walsh and Galtier, 2000). Thus for non-stationary models that consider no linear effects, the deposition of energy caused by the presence of nanoflares or of slow Corresponding author. Fax: +55 12 3945 6810.
E-mail addresses: [email protected] (A. Borgazzi), [email protected] (A. Costa). 1 Member of the Carrera del Investigador Cientı´ fico (CONICET). 1364-6826/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.jastp.2005.03.008
magnetoacoustic waves (Nakariakov et al., 2000; Tsiklauri and Nakariakov, 2001), seem more promising. On the other hand, Harra et al. (2004) found that the appearance of transition region loops (up to 1 MK) is related to small-scale flaring in the corona resembling the cooling of these loops, with material draining down from the loop-top. Moreover, theoretical time-dependent models that consider individual loops, where the plasma evolves in response to a cyclical process of heating and cooling of the plasma, have difficulties in fitting, observations (Klimchuk and Mariska, 1988). Thus, more dynamic theoretical scenarios that combine spatial or temporal averages of individual unresolved loops are needed to make the theory and observations compatible, and to decide between the two possible interpretations (flow or wave models). Kjeldseth-Moe and Brekke (1998) proposed a dynamic scenario, where the observed individual active region loops are considered as composed of strands of finer structures or magnetic tubes, each one with a resolution that is below the resolution of SOHO
ARTICLE IN PRESS 1716
A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
instruments. The thin magnetic tubes could have gas flowing in different directions with different temperatures and with different speeds. This scenario suggested the existence of coarse-grained, unresolved fine-structured tubes, mostly magnetically isolated and of a global quasi-isothermal appearance that could theoretically justify the large velocities observed. Moreover, Schrijver (2001) described frequent catastrophic cooling and evacuation of quiescent solar coronal loops in active regions (occurring almost once every two days), which could be triggered when the loop heating is reduced by 1.5 orders of magnitude. Relatively cool material seemed to move downwards at speeds of up to 100 km s1 . He also suggested that this scenario is compatible with the existence of several thin cool strands embedded within an initial loop. However, he established that the filamentary structure must behave in a coherent manner. The technique developed for the analysis of loop brightenings in Costa and Stenborg (2004) (hereafter Paper I) was applied to a non-resolved loop structure observed with the Mirror Coronagraph for Argentina (MICA) (Stenborg, 2000) land-based telescope in the well known green coronal line at 530.3 nm. We tracked the plasma along the loop under study, provided that the shift of the intensity excess detected along it is a trace of the motion of the density excess. Otherwise, the shift of the intensity excess may represent the speed at which the plasma inside the loop changes its temperature to make it visible at the wavelength of observation. In this work, we analyze the coherence of the behavior of a structure composed of several magnetic threads. We apply the method discussed in Paper I to a loop system observed in the EUV, as opposed to the white light data we analyzed previously. The event that caused the magnetic structure started around 04:40 UT in the vicinity of the NOAA AR 9628 ð22 S; 91 WÞ and AR 9632 ð21 S; 73 WÞ and was imaged by TRACE (Handy et al., 1999). As our aim is to describe the motion of the brightening along the loop as a function of time, we selected images corresponding to a time interval ranging from 17.31 to 18.60 UT (times are expressed as hours and fractions of hours). TRACE is sensitive to 1–2 MK plasma emission with a much higher spatial resolution (0.5 arcsec). We used the 17.1 nm filter and a cadence of the sequence of images of about 1 min. The images were normalized and treated with a median filter in order to avoid white spots on the images. The latter are produced by the incidence of high energetic particles on the CCD camera.
threads. As a first step, we selected the structure we wanted to analyze. By visual inspection of the associated movie (along the whole time sequence), we detected a structure that seemed to remain stationary. Our aim was to characterize the temporal evolution of brightenings flowing along the loops, i.e., loops for which internal motions are observed instead of bulk movements of the structure. The second step consisted of performing several cuts of the selected structure in the first image. Fig. 1 shows some of these cuts done on the first image of the series (i.e., at 17.31 UT). The white straight lines on the loop-like structure depict the positions where the cuts (hereafter xðj; tÞ and yðj; tÞ, x and y the image coordinates of the cut j, j 1–17, and t the time) were done. Each of the cuts covers several resolved tubes, and this is reflected when the intensity along the cut is displayed. In order to avoid underlying macroscopic and coherent space displacements of the magnetic tubes, we imposed the condition that the intensity along each cut, and in all the temporal sequence, must exhibit a quadratic-like shape, with a maximum at the center of the cut and not at its edges or wings (see Fig. 2). Otherwise, as a third step, if the shape of the wings changed (meaning space displacements of the magnetic tubes), the series was reduced in time to guarantee that the magnetic structures were stable. In our case, all the cuts of the temporal interval (the whole series) fulfil this requirement. The accuracy of the procedure to guarantee the identification of stable loop structures (with an error of the width of the identified loop) was shown in Paper I.
2. Method In this section, we describe the method developed in Paper I, in its application to TRACE images, to characterize the behavior of a compound of magnetic
Fig. 1. First TRACE image (17.31 UT) showing the cuts. The first top cut is taken to be the origin from where distances are measured.
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
Intensity (Normalized Units)
0.60 0.50 0.40 0.30 0.20 0.10 0.00 0
10 20 30 Position along the cut
40
Fig. 2. TRACE intensity profile along a cut.
1717
sequence, we found that the intensity of each cut displayed two relative maxima (see Fig. 3). This was a difference with respect to the results obtained with MICA telescope where the unresolved loop showed only one maximum for each cut as a function of time. Thus, as our method registers only maximum intensity features, we divided the whole time interval in two sub-intervals, in order to have two absolute maxima. Fig. 3 shows the intensity as a function of time for all the cuts and the whole time interval. The two chosen subintervals were: 17.31–18.00 and 18.00–18.60 UT, each one containing one maximum. The fifth step was to obtain for each cut the position along the loop and the time where the maximum occurs. In order to accomplish this, we chose the spatial origin coincident with the position of the first cut (the top of the magnetic loop structure), i.e., d 1 ¼ 0. The positions of the following cuts are computed as X dj ¼ hj;j1 , j41
TRACE - Intensity of each cut
0.70
0.60
0.50
0.40
0.30
0.20 17.0
17.5
18.0 Time [UT]
18.5
19.0
where hj;j1 is the shortest distance between the two corresponding cuts. When the distance along the loop structure, where the maximum of each cut occurs, is displayed as a function of time, the resulting curve represents the tracking of the brightening along the loop. In summary, the time of the maximum (tj ) is considered to be the time of the feature that characterizes the brightening in each sub-interval. But, as the time where the maximum occurs may be slightly different from one cut to the other, indicating that the brightening moves along the loop, we must quantify the displacement of such a brightening. In Fig. 4, we plotted the position of the maximum intensity for each cut along the loop (i.e., d j ) as a function of time at which the maximum occurs (i.e., tj ). This characterizes the displacement of the brightening, as a function of time tj , for the two sub-intervals.
Fig. 3. Maximum of intensity of each cut as a function of time.
3. Results and discussion The fourth step consisted of determining the temporal and spatial coordinates where a maximum occurred for each of the cuts along the whole series (see Fig. 3). That is, once the spatial position of the cuts was chosen based on the first image (considered to be the first time of the sequence), the maximum of intensity for each cut was computed for every image in the sequence (see Fig. 2). Note that while the maximum intensity profile of each cut varies along the time series (Iðj; ti ÞaIðj; tk Þ, j, the number of the cut, ti and tk the time of two different images), each pair of spatial coordinates of a given cut have the same value (xðj; ti Þ ¼ xðj; tk Þ and yðj; ti Þ ¼ yðj; tk Þ). When following the temporal behavior of the intensity profile of each cut along the whole image
Fig. 4 shows the location of the brightening, as a function of time measured along a virtual axis, that extends from the top of a whole complex loop, composed of several thin magnetic tubes, to the bottom of the same structure for the two time sub-intervals chosen. The first cut is taken to be the top of the axis. From Fig. 4a, we can see that a coherent maximum of activity occurs on a first time range, from 17.40 to 17.55 UT, over the whole sub-interval considered (17.31–18.00 UT). That is, all the threads that compose the structure covered by the cuts exhibit their maximum of activity resembling a coherent sliding-down of the flow (in approximately 9 min) from the top of the structure. The average velocity is 111 10 km s1 .
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
1718
had its maximum (i.e., more than one thread had its maximum brightening at the same time).
Distance along the loop in Mm
80
60
4. Conclusions
40
20
0 17.35
17.40
(a)
17.45 17.50 Time[UT]
17.55
17.60
Distance along the loop in Mm
80
60
40
20
0 18.10
(b)
18.20
18.30 18.40 Time [UT]
18.50
18.60
Fig. 4. Location of the brightening as a function of time and measured along the loop. The spatial origin is chosen to be the position of the first cut: (a) first sub-interval; (b) second subinterval.
Fig. 4b also shows a coherent maximum of activity resembling the upward motion of flow in approximately 18 min (from 18.20 to 18.50 UT) over the sub-interval ranging from 18.00 to 18.60 UT. The average velocity in the second case is 50 8 km s1 . The error was estimated following the steps discussed in Paper I. Note that the method detects maximum activity along the loop; thus, it displays the tracking of the brightening along the loop as a function of a particular time interval (this time interval is formed by the times where the maximum intensity of the different cuts occurred). For the whole time interval (the two sub-intervals), the fact that more than one brightening occurred at the same time indicates that more than one cut in the same image
As was mentioned, most of the classical theoretical models have difficulties in determining the physical conditions under which models are compatible with observations. Thus, the description of time and velocity features can contribute to the understanding of the evolution of the heating and cooling mechanisms, as is demonstrated by the frequent catastrophic cooling and evacuation of quiescent solar coronal loops in active regions or by the correlation of cooling transition region loops with small-scale flaring occurring in the corona. Nevertheless, the analysis of much more cases is needed in order to get a statistical description of the characteristics and the parameters involved in this type of phenomenon, i.e., the range of times in which the brightenings vary, characteristic velocities of the brightenings, height of the loop structures, etc. An upcoming paper presenting further analysis is at work. To gain insight into this problem, in a previous paper (Paper I), we have implemented a diagnostic method that allows the discrimination between the motion of the magnetic structure and the motion of the internal plasma. In this work we applied the method to images obtained with TRACE that have a better spatial resolution and, thus, we could track the brightenings along a compound of threads or resolved loop structure. We could describe two different situations. One, where the emission develops initially near the loop-top and later slides down with an average speed of about 111 10 km s1 , occurring in a lapse time of about 9 min. This result is in agreement with the high-speed picture sustained by several authors, e.g. Kjeldseth-Moe and Brekke (1998), Schrijver (2001). The mean velocity, 111 km s1 , is in accordance with the catastrophic evacuation description proposed by Schrijver (2001). The other behavior that we report shows that the brightenings are upwardly directed with an average speed of about 50 8 km s1 , occurring in a lapse of time of about 18 min. Moreover, a remarkable result of our observations is that the different apparently isolated threads evolve in a coherent way. That is, all the threads that composed the structure covered by the cuts exhibited their maximum of activity in two time ranges resembling the coherent downward and upward motion of flow (of 9 and 18 min, respectively, over the whole interval of 1.5 h). This can also be inferred from the fact that the different data points in Fig. 4(a,b) did not necessarily occur along the same thread. Moreover, the scenario of various coherent-behaving threads is in accordance with the description of an individual non-resolved loop-like structure
ARTICLE IN PRESS A. Borgazzi, A. Costa / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1715–1719
described in Paper I; in this case the event was observed in the hottest (T M ¼ 1:8 106 K) Fe XIV green line (lM ¼ 530 nm) and we detected only downflows. The fact that here we have also observed upward-directed brightenings suggests that this is not the case of cooling transition region loops triggered by small-scale flaring from the corona (Harra et al., 2004). Moreover, the apparently coronal isolation of the magnetic threads suggests that coherence in the behavior of the whole event could be due to similar chromospheric conditions. An open question that is not yet resolved is related to the type of models that could better fit observations, i.e., wave-based or flow-based models. The presence or absence of high Doppler shifts could suggest flow dynamics or magnetoacoustic waves, respectively. Also, signs of wave patterns are suggested when the propagation speed is always less than the sound speed and when the moving features repeat quasi-periodically (Tsiklauri and Nakariakov, 2001). On the one hand, in the frame of flow models, as was mentioned, our results are in accordance with a kind of catastrophic evacuation or high-speed sliding down flow picture sustained by several authors e.g. Kjeldseth-Moe and Brekke (1998), Schrijver (2001). Limit cycle solutions of a compound of coherent non-resolved loops that assume an evaporation–condensation mechanism could also adjust the observations of the two types of patterns described here. On the other hand, this type of solutions have also been interpreted by several authors as downward- or upwardpropagating waves (Ofman et al., 1999; Nakariakov et al., 2000). Moreover, Ofman et al. (1999) proposed that the 5–15 min periodicity could be due to the driving of these perturbations by photospheric motions or by nonlinear transformation of the solar 5 min oscillations in the chromospheric region. Also, it might be possible that waves are triggered at the footpoints into the different threads by the same source and that then they propagate according to the sound speed of each thread.
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References Aschwanden, M.J., Alexander, D., Hulburt, N., Newmark, J.S., Neupert, W.N., Klimchuck, J.A., Gray, G.A., 1999. Threedimensional stereoscopic analysis of solar active region loops. Astrophysical Journal 515, 842–867. Aschwanden, M.J., Alexander, D., Hurlburt, N., Newmark, J.S., Neupert, W.N., Klimchuk, J.A., Gary, G.A., 2000. Three-dimensional stereoscopic analysis of solar active region loops. II. SOHO/EIT observations at temperatures of 1.5–2.5 MK. Astrophysical Journal 531, 1129–1149. Costa, A., Stenborg, G., 2004. Characterization of intensity variations along FE XIV coronal loops. A case study. Solar Physics 222 (2), 229–245 (Paper I). Handy, B.N., Acton, L.W., Kankelborg, C.C., 1999. The transition region and coronal explorer. Solar Physics 184, 229–260. Harra, L.K., Mandrini, C.H., Matthews, S.A., 2004. What causes solar active region loops to exist at transition region temperatures? Solar Physics 223, 57–76. Kjeldseth-Moe, O., Brekke, P., 1998. Time variability of active region loops with the coronal diagnostic spectrometer (CDS) on SOHO. Solar Physics 182, 73–95. Klimchuk, J.A., Mariska, J.T., 1988. Heating-related flows in cool solar loops. Astrophysical Journal 328, 334–343. Nakariakov, V.M., Verwichte, E., Berghmans, D., Robbrecht, E., 2000. Slow magnetoacoustic waves in coronal loops. Astronomy and Astrophysics 362, 1151–1157. Ofman, L., Nakariakov, V.M., DeForest, C.E., 1999. Slow magnetoacoustic waves in coronal plumes. Astrophysical Journal 514, 441–448. Schrijver, C.J., 2001. Catastrophic cooling and high-speed downflow in quiescent solar coronal loops observed with TRACE. Solar Physics 198, 325–345. Stenborg, G., Ph.D. Thesis, Goettingen University, Germany, 2000. Tsiklauri, D., Nakariakov, V.M., 2001. Wide-spectrum slow magnetoacoustic waves in coronal loops. Astronomy and Astrophysics 379, 1106–1112. Walsh, R.W., Galtier, S., 2000. Intermittent heating in a model of solar coronal loops. Solar Physics 197, 57–62.