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Astron. Astrophys. 357, 743-756 (2000)

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3. Structure of the transition region

The large spatial area covered by these observations together with the good count rates provide an excellent opportunity to study the overall nature and structure of the transition region, in relationship to the underlying network structure. It is well known that the transition region is dynamic, so these observations simply provide a snapshot of the general features. Gaussian line profiles were fitted to the strongest lines in all the spatial pixels of both datasets. Maps of intensity, spectral line position and width were created for each of the following lines: O v 629.8 Å, N v 1238.8 Å, Si iv 1402.8 Å.

The presence of a Si ii line close to the O v line enabled the determination of the chromospheric network pattern in the SEP16 dataset; the second order Mg x line at 624.9 Å provided an intensity map at coronal temperatures. These two lines however are too weak to allow an accurate Gaussian fit, and their intensity maps have been determined simply by summing all the counts in a wavelength range centered on the peak wavelength and subtracting the background emission. In the case of Mg x the contribution of a blended, unidentified chromospheric line has been subtracted from the total intensity.

The resulting intensity maps have been deconvolved from the instrumental point spread function (Wilhelm et al. (1995) (PSF) using a maximum entropy algorithm part of the standard IDLTM software, in order to improve spatial resolution on these maps.

The SEP17 intensity map (on a logarithmic scale) for Si iv is given in Fig. 1a. A portion of the field of view in the SEP16 intensity maps is presented in Fig. 2 (O v), Fig. 3a (Si ii) and Fig. 3b (Mg x). The temperature of formation of O v is around T[FORMULA] K; Si iv is formed at T[FORMULA] K; Si ii formation temperature is around 104 K; the Mg x ion peaks at Log T[FORMULA] K.

[FIGURE] Fig. 1. a  Si iv intensity maps (logarithmic scale) for the SEP17 quiet Sun raster at disk centre. b  Si iv line shift map for the SEP17 quiet Sun raster; brighter pixels correspond to red-shifted positions.

[FIGURE] Fig. 2. O v intensity map (logarithmic scale) for the SEP16 quiet Sun raster near disk centre. Contour plots represent the photospheric magnetic field: white means positive polarity, black negative polarity.

[FIGURE] Fig. 3. Si ii a and Mg x b intensity maps (logarithmic scale) for the SEP16 quiet Sun raster at disk centre. Contour plots represent the photospheric magnetic field: white means positive polarity, black negative polarity.

The intensity maps of the two transition region lines displayed in Fig. 1a and Fig. 2 reveal the presence of a multitude of fine structures in the quiet Sun. These can be identified as filamentary patterns of brighter emitting material distributed across the whole field of view. These structures seem to be clustered together in large concentrations which constitute the whole of the brighter areas in Figs. 1a and 2. However, in many cases these brighter structures are extended also over the darker areas.

These filamentary structures seem to be overlying the chromospheric network, as seen through the intensity map of the Si ii 1258.8 Å line in Fig. 3a; they account for [FORMULA] 75% of the total intensity emitted in these lines in the whole field of view, and they are at the limit or below the instrumental spatial resolution. Fig. 1a and 2 seem to suggest that these structures are mostly composed by a large number of small transition region loops crossing the network boundaries and sometimes overlying supergranular cells. No obvious signatures of these transition region structures appear to be present in the Mg x intensity map, which, on the contrary, appears to be much smoother and less structured. However the Mg x emission does appear to be strongest where there is a cluster of Si ii emission.

Figs. 2 and 3 also display the contours of the photospheric magnetic field overplotted to the intensity maps. The magnetic field image has been taken from a full-disk magnetogram obtained by the Kitt Peak observatory, and has been co-aligned with the SUMER field of view. Unfortunately, the sensitivity and the spatial resolution of the magnetogram are too poor to help in determining the correlation (if there is any) between the SUMER fine structures observed in the O v emission and the photospheric magnetic field. It is possible to see that the magnetic field is stronger where the Si ii emission is brightest and most of the filamentary structures are located, but no conclusion on possible relationship between individual structures and magnetic field can be drawn. However, it is possible to see that the some of the strongest magnetic field signatures are correlated with the larger coronal structures as seen in Mg x. In particular, the brightest Mg X emission in the field of view is correlated with areas containing enhanced magnetic field of opposite polarity.

A much higher spatial resolution and sensitivity of the magnetograph are required in order to be able to correlate individual transition region fine structures and the magnetic field.

Figs. 1b (Si iv) and 4 (O v) show maps of the fitted spectral line position for the two quiet Sun regions (SEP17 and SEP16). The bright pixels in the spectral line shift map correspond to red-shifted material. In both datasets, there is a correlation between the fine structures and the spectral line position. There is a trend of red-shifted emission along the bright filamentary structures relative to darker locations surrounding them. Given the small-scale nature of this trend, it is not easy to correlate these apparent motions with the photospheric magnetic field, also displayed in Fig. 4. From Fig. 4 it is only possible to see that the largest shifts seem to be correlated with stronger magnetic field, but the low spatial resolution of the Kitt Peak images prevents from drawing any further conclusion. On average, the relative motion between plasma in structures and plasma outside structures is [FORMULA] km s-1 for O v. More details of the analysis of the spectral line shifts are given in the next section.

[FIGURE] Fig. 4. Spectral line shift map for the O v line. Brighter pixels correspond to red-shifted positions. Contour plots represent the photospheric magnetic field: white means positive polarity, black negative polarity.

The intensity maps for the transition region emission also show that there are a number of small active locations scattered all over the field of view. During the 60 seconds exposure time (Fig. 2), bright elements (explosive event locations) are seen preferentially along the slit (or y axis), while the solar rotation and the width of the slit limit the spatial extent along the x axis. These small active structures are located above the chromospheric network. At these locations, the line profiles are distorted from the Gaussian shape which is prevalent in the quiet Sun data. The line shape in these small active locations (explosive events) shows secondary peaks, possibly corresponding to turbulence or fast plasma motions along the line of sight. A larger-scale explosive event was also observed in the present dataset (outside the field of view displayed in Fig. 2). This corresponds to a rather wide region where line profiles are very irregular, possibly a jet-like structure. These small active features, and the jet-like structure, will be discussed in detail in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000