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ASTRONOMY AND ASTROPHYSICS

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May 12, 2000

Structure and dynamics of an active region loop system observed on the solar disc with SUMER on SOHO D. Spadaro1 , A. C. Lanzafame2 , L. Consoli2, E. Marsch3 , D. H. Brooks4, and J. Lang5 1 2 3 4 5

Osservatorio Astro sico, Viale Andrea Doria 6, I-95125 Catania Italy. Istituto di Astronomia, Universita di Catania, Viale Andrea Doria 6, I-95125 Catania Italy. Max-Planck-Institut fur Aeronomie (MPAE), D-37191 Katlenburg-Lindau, Germany. Centro de Astro sica da Universidade do Porto, Rua das Estrelas, Porto 4150, Portugal. Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K.

Received 24 December 1999; accepted 28 April 2000

Abstract. In this paper we present and discuss spectra obtained by SUMER on SOHO from an active region loop system observed on the solar disc, close to the central meridian, on July 26th, 1996. The region was observed with a spatial resolution of about 200 by 200 in emission lines forming in the transition region and inner corona, with the aim of investigating the physical structure and dynamical behaviour of the plasma in active region magnetic loops. To this purpose we have reduced and analysed the spectral observations in order to determine the values of intensity, Doppler shift and line pro le width for the selected emission lines in all the spatially resolved elements of the examined area of the solar disc. By comparing intensity, velocity maps and photospheric magnetic elds obtained by MDI on SOHO, several magnetic loops have been identi ed, some of which contemporarily appear over a range of temperatures, while others are compact and only visible in a limited temperature range. A few loops exhibit velocity elds typical of siphon ows, the siphonlike velocities being higher in compact loops. Two compact loops seen in the transition region lines show asymmetric siphon-like velocity elds and high non-thermal velocities at the up owing footpoint. High non-thermal velocities are also associated with the falling footpoint of a larger loop. Besides such loops, other bright features are observed in the transition region lines, whose morphology cannot be identi ed as arch-like. They have no coronal counterpart, are red-shifted with respect to the median line centroid position and exhibit high non-thermal velocities.

1. Introduction

Determining the physical conditions in the solar atmosphere is crucial for understanding the physical mechanisms that produce coronal heating, the solar wind and transient phenomena such as solar ares. In recent decades our knowledge of these conditions has improved dramatically as a result of high resolution X{ray, EUV, and UV spectroscopy, achieved with instrumentation own on rockets and many orbiting spacecraft. These observations have shown, in particular, that magnetic loops are the dominant structures in the outer solar atmosphere (e.g., Bray et al. 1991) and have been extensively used in recent years to study the general properties of such coronal structures (see, e.g., Kjeldseth{Moe & Brekke 1998, for a short review of these studies). The recent solar spacecraft, the ESA/NASA Solar and Heliospheric Observatory (SOHO; Domingo et al. 1995), contains high spatial and spectral resolution extreme ultraviolet (EUV) spectrometers that together provide unprecedented wavelength coverage, dynamic range, temporal resolution and extended observation periods, when compared to previously own instrumentation, consequently producing signi cantly more complete information on the physical conditions in the solar atmosphere than before. These instruments are the Solar Ultraviolet Measurements of Emitted Radiation (SUMER; Wilhelm et al. 1995) and the Coronal Diagnostic Spectrometer (CDS; Harrison et al. 1995). Several investigations have already been carried out using SOHO spectroscopic observations of loops above acKey words: Sun: atmosphere { Sun: corona { Sun: trantive regions (e.g., Brekke et al. 1997b, Fludra et al. 1997, sition region { Sun: UV radiation Brynildsen et al. 1998, Kjeldseth{Moe & Brekke 1998, Maltby et al. 1998) in order to extend our knowledge of the physical structure, plasma dynamics and temporal behaviour of magnetic loops. Some of these investigations concentrated on loops observed at the solar limb (Brekke Send oprint requests to : D. Spadaro, e-mail: et al. 1997b, Kjeldseth{Moe & Brekke 1998) because of the [email protected] better contrast with the background which makes them

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D. Spadaro et al.: Structure and dynamics in active region loops

EIT image of the active region studied taken on 1996 July 26 at 01:40 UT in the narrow wavelength band centered Fig. 2. EIT image of the active region studied taken on 1996 around the Fe X, XI 171 lines. July 26 at 15:33 UT in the narrow wavelength band centered around Fe XII 195 line.

Fig. 1.

easier to distinguish, particularly in EUV lines. These authors were able to study the distribution along the loop, the Doppler shifts and the time variability of the plasma emission in several lines forming in the temperature range characterizing the solar transition region and low corona. They also studied the spatial relationship between loops as seen in spectral lines formed at dierent temperatures. An important result of these studies on solar limb regions is that magnetic loops at dierent temperatures are not necessarily co{located within the same region, but may be signi cantly shifted from each other or have different shapes and sizes. There are also many examples of loops in one temperature range that do not exist at other temperatures. However, this result must be evaluated taking into account also the temporal variability, which seems to be the norm for magnetic loops in active regions (Kjeldseth{Moe & Brekke 1998). A loop at a given temperature, that is 'missing' in one location at a particular time, may be present there at another time. These studies also found a frequent occurrence of large line shifts in active region loops, particularly when observed in lines forming below 5  105 K. These shifts commonly correspond to velocities along the line of sight of 50{100 km s?1 , but larger shifts, corresponding to 200{300 km s?1 have also been observed. In this paper we present and discuss spectra obtained by SUMER from an active region loop system observed on the solar disc, close to the central meridian. This region has been observed with a spatial resolution of about 200 200 in emission lines forming in the 105 ?106 K temperature range, with the purpose of investigating the physical structure and dynamical behaviour of the plasma in the transition region portion of the magnetic loops. To this aim, we have reduced and analysed the spectral observations in order to determine the values of intensity,

Observed spectral lines. Tf is the temperature of maximum ion population in ionisation equilibrium. Ion  ( A) logTf Diraction Order Line Set C IV 1548.2 5.0 First A O IV 787.7 5.2 Second A SV 786.4 5.2 Second A Ne VIII 770.4 5.9 Second A NV 1238.8 5.3 First B OV 629.7 5.4 Second B Mg X 624.9 6.1 Second B Fe XII 1242.0 6.2 First B O VI 1031.9 5.5 First C

Table 1.

Doppler shift and line pro le width for the selected emission lines in all the spatially resolved elements of the examined area of the solar disc. These data have been used to locate the coronal structures within the considered active region and to identify their morphology, and also to study the spatial relationship between loops as seen in spectral lines formed at dierent temperatures. Spectral line pro les have been examined with the purpose of getting information about the presence of resolved mass motions inside the active region, as well as about non{thermal broadening of the lines, which is usually attributed to microturbulence and is probably related to the heating mechanisms working in these layers of the solar atmosphere. Since both Doppler shifts and non{thermal line broadening of the EUV emission lines formed in the transition region and corona have signi cant implications for the mass and energy balance in the outer layers of the solar atmosphere (Boland et al. 1975, Pneumann & Kopp 1978, Mariska 1992), understanding their characteristics is important to improve our knowledge of the mechanisms

D. Spadaro et al.: Structure and dynamics in active region loops

Fig. 3.

3

Images of the active region as seen in individual lines of data set A (see Tables 1 and 2).

and processes ruling the physical conditions of the solar et al. (1995, 1997) and Lemaire et al. (1997). SUMER's spectral resolution element is about 44 m A in rst order loop plasma. (22 m A in the second order) and is only weakly dependent on wavelength. The spatial resolution is close to 100 , 2. Observations both along the length of the slit and perpendicularly to it. Each detector exposure is limited to a single wavelength SUMER is a high resolution, normal incidence, stigmatic range 44  A wide in rst order. The censlit spectrometer capable of imaging the Sun in the wave- tral 22approximately  A portion of the sensitive area is coated length ranges 660 { 1600  A ( rst order) and 330 { 805  A with KBr, which providesdetector greater sensitivity above 900  A (second order). Detailed descriptions of the characteristics than is available from the bare parts of the detector on and performance of the instrument are given in Wilhelm

4

Fig. 4.

D. Spadaro et al.: Structure and dynamics in active region loops

Images of the active region as seen in individual lines of data set B (see Tables 1 and 2).

either side of this region. A comparison of a line intensity recorded on the KBr portion of the detector with its intensity recorded on a bare section of the detector provides a useful technique for distinguishing between rst and second order lines. We obtained spectra for this study using the SUMER detector A and the narrow 000 :3  12000 slit, oriented in the North/South direction (nominal spacecraft orientation). These observations were carried out on 1996 July 26, from approximately 9.00 to 11.00 UT, selecting a eld of view

initially centered at ?7200, 10300 (SOHO heliographic coordinates) with respect to the centre of the solar disc and including the active region NOAA 7978B (see Solar Geophys. Data, 625, Sept. 1996). Fig. 1 and Fig. 2 shows two images of the considered active region taken on 1996 July 26 at 01:40 UT and 15:33 UT by the EUV Imaging Telescope (EIT) on SOHO (Delaboudiniere et al. 1995) in the narrow wavelength bands centered around the Fe X, XI 171 and Fe XII 195 lines.

D. Spadaro et al.: Structure and dynamics in active region loops

Fig. 5. Images of the active region as seen in individual lines of data set C (see Table 1 and 2).

Table 1 lists the spectral lines considered in this paper together with their formation temperature and the spectral order of diraction on the detector. The lines are grouped in three sets, each of them includes the lines simultaneously observed by a single detector exposure, both in the rst and second order. For each line set, raster images have been constructed, stepping the slit using the telescope pointing mechanism, in a direction perpendicular to its length and taking a detector exposure for each step. These are described in Table 2, which reports, for each raster, the step size, the number of steps, the scanned eld of view (FOV), the detector single exposure time, the start and the end times of the observation, in UT. During the acquisition of a raster, the telescope pointing was adjusted using the standard compensation for solar rotation (Wilhelm et al. 1995), in order to follow the active region of interest during its apparent motion on the solar disc.

3. Method of analysis

We have corrected the SUMER spectral data for the detector sensitivity pattern ( at- eld correction) and geometric distortion prior to analysis. The detector sensitivity pattern in the SUMER detectors is periodically estimated by observing the Lyman continuum with the spectrometer grating defocused. Each exposure in our observing sequence was corrected to account for the detector sensitivity pattern by dividing the observed spectral data by the

at image taken closest in time to the observation, which, in our case, is the at- eld exposure taken on 1996 July 25. Since the detector sensitivity pattern changes with time, there is an additional random component to the line pro-

5

les which cannot be accounted for. Such a component, however, is expected to have negligible eects on spatially averaged line widths. We have also checked that this procedure actually reduces the data noise by performing a least-square Gaussian tting to the O VI 1031.9 line pro le before and after the at- eld correction, taking the estimated error in the t as an indication of the data noise. Considering pro les spatially averaged over 20 pixels, we estimate a reduction due to the at eld correction in the data noise up to 15%. Each exposure was also corrected for geometrical distortions using the DESTRETCH routine, written by T. Moran and available through the SUMER software. This maps the data to a rectangular grid derived from laboratory measurements prior to launch. The additional broadening caused by the interpolation required for the geometric distortion correction is expected to be important for the narrower lines, but should not aect signi cantly the transition region and coronal lines studied in this paper. Furthermore, any additional broadening is counterbalanced by the correction for the pin-cushion distortion introduced by the detector, which is especially important for the determination of the spectral line centroid at various spatial locations. To increase the signal-to-noise ratio in the data, we have smoothed the data over 5 neighbouring pixels along the slit length, which corresponds to 5 arcsec along the slit, and over 7 raster positions, which corresponds to 5.6 arcsec perpendicular to the slit. Spectral tting was based on a maximum likelihood program developed by Lang et al. (1990) for use with the Spacelab 2 experiment CHASE. The observed spectrum is tted with a background and up to ten Gaussian shaped lines as Ik = b0 + b1 xk + b2 xk + 2

X L

=1

i

hi

(  ) x ? x0 2 (1) exp ? w k

;i

i

where I is the observed count rate in pixel k, L is the number of lines, each with half-width w and centroid position x0 . Values of the parameters b0 , b1 , b2, h , x0 and w are obtained by the maximum-likelihood method. The program has been further developed for SOHO by D.H. Brooks (see Brooks et al. 1999), primarily to improve its ability to resolve close lying lines and to deal with large and numerous datasets. More details are given therein. The new routine is implemented as ADAS602 in the Atomic Data and Analysis Structure (Summers 1993). The program allows entry of spectral data in the form of a multi-dimensional array. In the case of SUMER, the input array dimensions are maintained and the code allows automatic cycling through each spatial (and, if present, temporal) resolution element. The program provides an output structure together with an array containing the tted data for further processing, e.g. tted image display, evaluation of non-thermal broadening or line shift at each spatial resolution element. k

i

;i

i

i

;i

6

D. Spadaro et al.: Structure and dynamics in active region loops

Table 2.

Image Rasters Obtained on 1996 July 26

Line set Step Size Steps FOV Exposure Start End (arcsec) (arcsec2 ) (s) (UT) (UT) A 0.76 151 115  120 10 09:03:41 09:29:09 B 0.76 151 115  120 10 09:29:34 09:55:02 A 0.76 66 49  120 12 09:55:27 10:08:53 B 0.76 66 49  120 12 10:09:18 10:23:04 C 0.38 81 30  120 5 10:23:43 10:30:32 C 0.38 81 30  120 5 10:38:45 10:45:56

Besides the determination of the intensity and nonthermal width of the line at each spatial location, the Gaussian line tting allows determination of the position of the line centroid within a fraction of a pixel, i.e. below the pixel resolution of the instrument. In the case of the N V 1238:8, O V 629:7 and O VI 1031:9 lines, given their high signal (100-400 cts/px), the centroid position is estimated by the program to an accuracy better than 0.3 pixels ( 3 km s?1 ). Table 3. Characteristics of the loops identi ed (see text and Table 4). The loop footpoints are indicated with f1 and f2 . D is the approximate distance between the footpoints.

#

Footpoints D (km)

1 2 3 4 5 6 7 8 9 10

F E A C A K G M O Q

f1

f2

B D H I J L C N P R

23 600 19 000 40 400 48 800 36 100 22 000 25 100 10 100 5 500 8 900

4. Results and discussion 4.1. Line intensities and relation with photospheric magnetic elds Fig. 3, Fig. 4 and Fig. 5 show monochromatic images of the active region loop system at dierent temperatures, grouped according to the three line sets de ned in Sect. 2. Within each set, the images are ordered from top left with increasing temperature of line formation. The images are obtained from the line tting. Isolated white pixels indicate locations where the line intensity was too low to obtain an acceptable t . The complex active region observed is composed of two main active areas, one in the South-East quadrant, the other close to the West edge of the spectrograms. Line intensities are enhanced in these two main areas and, for

the higher temperature lines, also in the structures interconnecting them. Table 4. Approximate footpoint locations for loops listed in Table 3. The identi cation of the footpoints is based on the photospheric magnetic eld strength or the velocity eld derived from transition region lines (see text and Table 3). B is the approximate magnetic eld strength peak in the footpoint area; v and  are the peak line-of-sight velocities and the peak non-thermal velocities estimated at the footpoints, respectively.

Location B (Gauss) v (km s?1 )  (km s?1 ) A -900 -13/10 32 B -200 -5 31 C -300 -6 31 D -300 5 31 E 600 25{30 F 500 6 25{30 G 500 25{30 H 300 -8 32 I 400 5 25{30 J 400 9 35 K 400 -8 34 L 15 50 M -16 43 N 8 30 O -9 38 P 10 25{30 Q -15 37 R 14 17  Both loops 3 and 5, have one footpoint in area A, the former with a red shift and the latter with a blue shift.

The images reported in Fig. 3, Fig. 4 and Fig. 5 illustrate the changing structure of the loops in the active region over a temperature range from 105 K (C IV line) to 1:6  106 K (Fe XII line). Hence, they are useful for investigating the spatial relationships among the bright structures as seen in the various spectral lines, in addition to verifying whether they are segments of one or more loops with a strati cation in temperature. 'Classical' models of transition region and coronal loops, mainly based on X{ray images by e.g., Skylab, Yohkoh, describe them as stationary structures that are

D. Spadaro et al.: Structure and dynamics in active region loops

7

Fig. 6. Ne VIII  770.4 line intensity (contour plot) over-plotted on the MDI magnetogram. Dark and bright regions indicate opposite polarity. The magnetic eld values range from -1 400 Gauss in the darkest area to +940 Gauss in the brightest area. Each identi ed loop is labelled with a number and the approximate locations of the corresponding footpoints are indicated with letters (see also Table 3 and Table 4)

hot on the top ( 106 K) and have a thin transition to the chromosphere ( 104 K) near their footpoints (Rosner et al. 1978, Withbroe 1981, Mariska 1992, Kano & Tsuneta 1996). According to this picture, we expect to observe structures which are elongated and arch{shaped when seen in the low coronal lines (Ne VIII, Mg X, Fe XII), and to nd their corresponding footpoints when observing in the transition region lines (C IV, O IV, N V, O V, O VI). We note that such arch{like structures indeed appear in the raster images obtained in the coronal lines, which are similar to the images of the active region acquired by EIT (see Fig. 1 and Fig. 2), but it is dicult to single out individual footpoints directly related to them in the transition region line images. In fact the bright structures seen in the lines forming at higher temperatures do not exhibit an obvious connection with the bright structures visible at lower temperatures. To investigate this aspect in more detail, we have compared the intensity maps with high resolution magnetograms of the same active region obtained by the Michelson Doppler Imager (MDI; Scherrer et al. 1995) on board SOHO, which allows investigation of the spatial relationship between the plasma emission in transition region and

coronal lines and the photospheric magnetic elds. The coalignment of the MDI and SUMER images is based on the instrumental heliographic coordinates, which are estimated with an accuracy better than 1000. This is sucient for our purposes, since the structures identi ed are signi cantly larger than the instrumental pointing accuracy. Figs. 6 { 9 show some representative examples of these comparisons, relevant to the low coronal lines of Ne VIII and Mg X and to the transition region lines of C IV and O V. We note that the elongated, arch-shaped, bright structures observed in the coronal lines appear to connect regions with more intense magnetic elds and opposite polarities. Therefore, they can be regarded as magnetic ux tubes extending into the corona and lled by hot plasma, according to the 'classical' picture of coronal loops. Looking at these structures, it is possible to identify six loops, two of which are located in the South-East active area (labelled with 1 and 2 in the gures), three connect this area to the West side one (3, 4 and 5), and another is rooted in the Westerly area and extends towards the North edge of the spectrograms (6). We have also indicated with letters in the gures the footpoints of these loops (see also Ta-

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D. Spadaro et al.: Structure and dynamics in active region loops

Mg X  624.9 line intensity (contour plot) over-plotted on the MDI magnetogram. Numbers and letters have the same meaning as in Fig. 6.

Fig. 7.

bles 3 and 4), which correspond to areas of more intense As far as the spatial relationship between the C IV photospheric magnetic eld (both positive and negative). and O V bright structures and the photospheric magnetic Note that both loops 3 and 5 have their Eastern footpoint elds is concerned, these structures are in some cases close rooted in the same area (A). to or coincident with the areas of more intense magnetic elds, con rming that they can be regarded as the footThe line intensity distribution along loops 3, 4 and points of the loop structures detected at higher temper5 is not uniform and, particularly in the case of the atures (note their connection with the regions of positive Ne VIII 770:4 intensity map, the asymmetry between the magnetic polarity), while in other cases this does not oc'legs' of the loop is evident. Moreover, while loop 5 is sig- cur (note, in this respect, the intense magnetic area with ni cantly brighter in the Mg X line, the other two (3, 4) negative polarity, which does not exhibit a corresponding exhibit higher emission in the Ne VIII line. This could be higher emission in the transition region lines). interpreted as a signature of dierences in the maximum temperature reached by the solar plasma inside the obWe also note two bright structures near the upper edge served interconnecting loop structures, although we can- of the South-East active area, visible both in C IV and not rule out that the considered active region structures O V lines, which can be regarded as the legs of a further physically evolve over the time interval elapsed between loop structure (7) connecting two regions with opposite the acquisition of the two rasters being compared ( 25 magnetic polarities. Since the gap in the transition region min, see Table 2), with consequent variations in the inten- emission between the two legs coincides with the bright sity distribution of the examined spectral lines along the spots clearly visible in the Ne VIII intensity map and aploops. Kjeldseth{Moe & Brekke (1998), in fact, found clear proximately with a small area of higher Mg X emission, we evidence of rapid time variability in loops above active re- interpret this as evidence of a temperature gradient existgions: loops may appear or disappear in certain emission ing along the identi ed loop structure, in which the plasma lines, may show rapid variations in the distribution of their is at lower temperatures close to the magnetic footpoints emission along their lengths, or may change shape or ex- and at higher temperatures con ned to the intermediate pand outward, all on time scales of 10{20 min. portion (top) of the loop.

D. Spadaro et al.: Structure and dynamics in active region loops

9

C IV  1548.2 line intensity (contour plot) over-plotted on the MDI magnetogram. Numbers and letters have the same meaning as in Fig. 6.

Fig. 8.

Moreover, we notice in the examined active region, particularly in the O V intensity map, some bright compact sources which do not have corresponding counterparts in lines emitted above 8  105 K, in analogy to the results found by Fludra et al. (1997) who observed active regions on the solar disc with CDS. 4.2. Velocity elds Further indications of the physical properties of the examined magnetic ux structures can be obtained from the analysis of the velocity elds characterising the selected area of the solar disc, and of the non-thermal broadening of the emission lines. In fact, the spectral data allow us to determine line-of-sight velocity and non-thermal velocity maps of the active region loop system. The values of velocity along the line of sight have been deduced for each line from the Doppler shift of the centroid of the line pro les with respect to the median of the line centroid in the considered region of the solar disc. The median is used as wavelength reference, since there is no absolute wavelength reference in the spectrograph, and since no chromospheric lines such as those used by Brekke et al. (1997a) suitable to calibrate the wavelength scale have been observed.

The values of the non{thermal velocity have been deduced from the width of the line pro les, after removing the instrumental broadening (Wilhelm et al. 1997, Chae et al. 1998) and the contribution to the FWHM given by the thermal Doppler broadening corresponding to the line formation temperature (e.g., Mariska 1992, Chae et al. 1998). We are thereby assuming that the turbulent bulk motions (or unresolved wave motions) of the emitting plasma cause a further Doppler broadening of the line pro le (Mariska 1992). We report and discuss here the results concerning the analysis of the O V 629:7 line pro les. Similar results are obtained for the other transition region lines (Lanzafame et al. 1999). The coronal lines are characterised by an intensity which is about one order of magnitude lower, so that the line-of-sight velocity and non-thermal velocity values can only be determined with a signi cantly lower accuracy. By comparing the O V line shift with the intensities in the transition region line of O V and the coronal lines of Ne VIII and Mg X (see Figs. 10 { 12), we can detect signi cant plasma ows at the footpoints of loops 3, 4 and 5. For the footpoints at the Western edge of the South-East main active region, the O V 629:7 centroid shifts with respect to the median centroid position on the whole ob-

10

D. Spadaro et al.: Structure and dynamics in active region loops

O V  629.7 line intensity (contour plot) over-plotted on the MDI magnetogram. Numbers and letters have the same meaning as in Fig. 6.

Fig. 9.

served area can reach values corresponding to (10 ? 15) km s?1 , while smaller velocities, (5 ? 10) km s?1 , are detected near the other footpoints. Moreover, we note that for each interconnecting loop the signs of the plasma velocity at the footpoints are opposite, so exhibiting evidence of siphon ows along the magnetic structures, i.e. plasma ows from one footpoint to the other driven by a pressure dierence between the two loop footpoints or an asymmetric heat deposition along the loop (e.g., Cargill & Priest 1980, Priest 1981). This conclusion is based on the assumption that, for our on-disc observations, the plasma

ow close to the loop footpoints is nearly parallel to the line of sight, while in the intermediate section of the loop it is almost perpendicular. Possible evidence of siphon ows is also seen in loop 1 inside the South-East main active region, even if this complex structure is not wholly within the spectrograms, with velocities of ?5 km s?1 (up ow) at one footpoint and +6 km s?1 (down ow) at the other. For loops 2 and 7 the plasma velocities appear signi cant only in one footpoint. It is worth noting that the footpoints with up ow velocities also have a higher ( 35 km s?1 , see Fig. 13) than the typical O V 629:7 non-thermal velocity of the region observed (25-30 km s?1 ).

Loop 6 also appears characterised by a siphon ow, with a small blue shift with respect to the median at the Southern footpoint (8 km s?1 ) and a large red shift with respect to the median at the Northern footpoint, with a peak value of 15 km s?1 . The Northern footpoint also has a high non-thermal velocity (40-50 km s?1 , see Fig. 13). The velocity eld at the Southern footpoint of this loop suggests the presence of a bundle of relatively large magnetic ux tubes converging at that footpoint. The analysis of the O V velocity elds also helps in identifying another three compact loops, two of which are visible in the N V and O V intensity maps but do not reach coronal temperatures. The two loops which do not reach coronal temperatures are far from the main active areas. The rst (8) is close to the centre of the image and shows an asymmetric siphon ow velocity with a blue shift of up to 16 km s?1 at the Eastern (up ow) footpoint and a red shift of 8 km s?1 at the Western one. The Eastern footpoint again has a higher O V non-thermal velocity ( 40 km s?1 , see Fig. 13). Loop (9), smaller and less bright, is a little closer to the upper edge of the image and also shows a nearly symmetric siphon ow velocity with a red shift of up to 10 km s?1 at the Northern footpoint and a blue shift of 9 km s?1 at the Southern footpoint, which is characterised by a slightly higher non-thermal

D. Spadaro et al.: Structure and dynamics in active region loops

11

Centroid line shift from median for O V 629.7 (contour plot) superimposed on the O V 629.7 intensity map. Solid (dot-dashed) contours indicate blue (red) shifts with respect to the median of 3, 5, 10 and 15 km s?1 . The numbers labelling the identi ed loops and indicating the corresponding footpoints are also shown.

Fig. 10.

velocity. Another compact loop (10) can nally be identi ed at the Eastern edge of the South-East main active region, with an up ow velocity of 15 km s?1 at the NorthEast footpoint and a down ow velocity of 14 km s?1 at the South-West footpoint. In this loop the non-thermal velocity is higher (37 km s?1 , see Fig. 13) in the footpoint with plasma up ow and signi cantly smaller than average (17 km s?1 ) in the footpoint with plasma down ow. The spatial location of such footpoints suggests that loop 10 also emits in Ne VIII  770.4, with a brightening very close to loop 2. For loops 8, 9, and 10, therefore, the velocity eld appears to be a better indicator of the magnetic ux tubes than the line intensities. This can be understood in terms of the plasma motion being con ned by the magnetic eld. The emissivity of the transition region lines can be signi cant also outside the main magnetic ux tubes. It is interesting to note that the down owing leg of the examined magnetic loop structures exhibiting siphon ows is usually brighter than the up owing one, particularly in the transition region lines. Note that, generally, higher intensities correspond to lower velocities. Coronal lines have the same behaviour, with the only exception being loop 6, whose up owing leg is brighter than the down owing one in the lines of Ne VIII and Mg X (see Fig. 11 and Fig. 12).

This does not agree with the results of several hydrodynamic models of steady-state siphon ows in coronal loops (e.g., Noci et al. 1989, Spadaro et al. 1990a, 1990b, Orlando et al. 1995a, 1995b), which predict higher emission for the up owing leg. These models need to be revisited in the light of these and other SOHO observational results, as already pointed out, for instance, by Peres (1997). Our results, on the other hand, are in agreement with the observations of predominant and persistent red-shifted emission of lines formed at transition region temperatures, obtained during the last two decades by many authors using several UV solar instruments with lower spatial resolution (see, e.g., Brekke et al. 1997a and references therein). They imply that siphon ows inside closed magnetic structures could be the concurrent cause of the systematic redshifted emission observed in the transition region of both the Sun and late type stars (e.g., Wood et al. 1996, 1997). The principal characteristics of the loop structures identi ed and discussed above are summarised in Table 3 and Table 4. Besides such structures, the O V 629:7 intensity map reveals many other features whose morphology cannot be identi ed as arch-like. Similar features are also seen in the other transition region line intensity maps. The bright O V areas North-East of the central compact loops

12

D. Spadaro et al.: Structure and dynamics in active region loops

Centroid line shift from median for O V 629.7 (contour plot) superimposed on the Mg X 624.9 intensity map. Solid (dot-dashed) contours indicate blue (red) shifts with respect to the median of 3, 5, 10 and 15 km s?1 . The numbers labelling the identi ed loops and indicating the corresponding footpoints are also shown.

Fig. 11.

are particularly interesting since they are associated with marked red shifts with respect to the median line centroid (up to 15 km s?1 ) and enhanced non-thermal velocities (up to 40 km s?1 ). These are likely to be dynamical structures developing on scales comparable to the SUMER spatial resolution, and thus may give a signi cant contribution to the solar emission in transition region lines. 4.3. Spatial correlations The complete set of data for intensity, line-of-sight velocity and non-thermal velocity obtained for the O V line can be used to investigate possible correlations among these line parameters in the examined active region loop system. We therefore present in Fig. 14 plots of each parameter in relation to each other for the observed area of the solar disc. These scatter plots do not exhibit a de nite correlation among the considered line parameters, and this could be due to the highly inhomogeneous properties of the selected portion of the solar disc. However, we note some concentrations of points which appear to describe particular trends, with a non-linear behaviour. Furthermore, the bi-modal character of the relation between the line-of-sight

velocity and the non-thermal velocity is evident (bottom panel). Such concentrations suggest the presence of dierent relations among the line parameters inside the dierent structures identi ed in the monochromatic images. The plot of the line-of-sight velocity vs. line intensity (top panel) also shows a weak positive trend, suggesting a predominance of red shifts in areas with higher line intensity. This is in agreement with the discussion of Sect. 4.2, where we noted that the down owing leg of the identi ed magnetic loop structures exhibiting siphon ows is usually brighter than the up owing one, particularly in the transition region lines. The middle panel (non-thermal velocity vs. line intensity) also exhibits a weak positive trend. As far as the v- scatter plot (bottom panel) is concerned, we can interpret its characteristics from inspection of Fig. 10 and 13 (see Sect. 4.2). In both the main active areas and the other compact loops, with the only exception of loop 6, the non-thermal velocity is higher for loop footpoints characterised by blue shifts in the O V line (up ows). This explains the negative correlation between the line parameters in one of the two branches appearing in the scatter plot. As for the slowly increasing branch, it mainly originates from the bright O V features NorthEast of the central compact loops discussed in Sect. 4.2

D. Spadaro et al.: Structure and dynamics in active region loops

13

Centroid line shift from median for O V 629.7 (contour plot) superimposed on the Ne VIII 770.4 intensity map. Solid (dot-dashed) contours indicate blue (red) shifts with respect to the median of 3, 5, 10 and 15 km s?1 . The numbers labelling the identi ed loops and the letters indicating the corresponding footpoints are also reported.

Fig. 12.

and from footpoint L of loop 6, which are characterised by The siphon-like velocities are higher in the compact loops marked red shifts and enhanced non-thermal velocities. which do not reach coronal temperatures. An isolated compact loop, for instance, shows an asymmetric siphonlike velocity eld, with higher ow velocity ( 16 km s?1 ) 5. Conclusions and non-thermal velocity ( 40 km s?1 ) at the rising footAnalysis of ultraviolet spectral data of an active region point. High non-thermal velocities (40 ? 50 km s?1 ) are observed by SUMER on SOHO on the solar disc has been also associated with the down owing footpoint of a larger carried out in order to investigate the physical structure loop. The observations also show that the down owing legs and dynamical behaviour of active region magnetic loops. are usually brighter than the up owing ones, particularly Comparing the intensity, line-of-sight velocity maps in transition region lines. This implies that siphon ows and photospheric magnetic elds deduced from MDI high could be the concurrent cause of the systematic red shift resolution magnetograms, we have identi ed ten magnetic observed in the transition region of both the Sun and late loops. Some relatively large loops, connecting regions of type stars. opposite magnetic polarity, are contemporarily observed Besides several loop structures, the transition region over the whole range of temperatures spanned by the se- line intensity maps show other bright features whose lected UV emission lines. Others are compact and do not morphology cannot be identi ed as arch-like. They have exhibit emission at coronal temperatures. Moreover, our no coronal counterpart, are characterised by line pro les spectral data appear to show some evidence of time vari- which are usually red-shifted with respect to the median ability in the physical conditions of the active region struc- line centroid position, and exhibit non-thermal velocities tures. about 50% higher than the average non-thermal velocSigni cant plasma ows, with velocities of up to ity measured in the examined active region. These bright 16 km s?1 , have been detected at the footpoints of both structures, together with the compact loops mentioned large and compact loops; they have the typical charac- above, show that a signi cant part of the solar emission in teristics of siphon ows along the magnetic eld lines. transition region lines comes from the compact structures

14

D. Spadaro et al.: Structure and dynamics in active region loops

Fig. 13. Non-thermal velocity for O V 629.7 (contour plot) superimposed on the O V 629.7 intensity map. Contours are at 30, 35, 40 and 45 km s?1 . The numbers labelling the identi ed loops and indicating the corresponding footpoints are also reported.

which do not reach coronal temperatures, rather than ex- Brekke P., Kjeldseth{Moe O., Harrison R.A., 1997b, Sol. Phys. 175, 511 clusively from the footpoints of large coronal loops. Acknowledgements. The authors wish to thank the SUMER team based at NASA GSFC for its kind help during the observations. Support from EIT and MDI in providing ultraviolet images and high resolution magnetograms is also acknowledged. The SUMER project is nancially supported by DARA, CNES, NASA and the ESA Prodex program (Swiss contribution). SOHO is a mission of international cooperation between ESA and NASA. The authors wish to thank the referee, whose comments have allowed a sounder version of the paper. This work has been partially supported by Agenzia Spaziale Italiana under contracts ARS-96-09 and ARS-98-117. DHB acknowledges support from the Portuguese Foundation for Science and Technology through grants PESO/P/PRO/1196/97 and PESO/P/INF/1197/97.

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