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

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5. Active features

5.1. Transition region explosive events

There are a large number of small explosive events, just a few pixels wide, in both quiet Sun datasets. They are located mostly over or close to the bright chromospheric network pattern. Often they do not show significantly enhanced intensities relative to the surrounding emission in quieter areas. These explosive events were detected by distortions in the spectral line profile, that is through the greater value of the single-Gaussian line width needed to fit the N v, O v and Si iv line profiles. The typical size of these explosive events is of the order of 2000 km. Given the duration of each single raster exposure, the lifetime of such events is between 1-4 minutes.

Line profiles in these small active locations are heavily distorted from the single-Gaussian line profile typical of the other surrounding quieter regions. An example of the spectral line profile can be seen in Fig. 9. It is possible to distinguish a central peak at approximately the rest wavelength, and two large wings, broadly symmetrical to the central unshifted line. The shape of the wings changes dramatically for different active points, and in some cases only one wing (generally the blue-shifted one) is visible in the spectrum.

[FIGURE] Fig. 9. O v 629.73 Å line profile in an active point.

The secondary blue and red-shifted peaks have been fitted for a sample of these explosive events assuming that both the central peak and the wings have Gaussian profiles. However, in many cases the wings of the profiles are very irregular and it is not easy to fit a single-Gaussian to them: in some cases two Gaussian profiles were necessary for the blue-shifted wing, and sometimes even then, the fit was poor. In contrast, an accurate single-Gaussian fit was always possible for the central, unshifted feature, and the resulting line position and width for the central part of the active point line profiles is very similar to the values obtained in the surrounding quieter plasma. This quiescent, central spectral feature could be due to quieter plasma along the line of sight.

The resulting spectral line positions and line-of-sight velocities relative to the average quiet Sun value are displayed in Fig. 10 for a sample of active points. The x-axis values represent the blueshifted line component and the y-axis values represent the corresponding red-shifted line component. From these data, it is possible to see that both red-shifted and blueshifted line-of-sight velocity lie in the 50-80 km s-1 range, with very similar values. The total intensity associated with the two line wings in each active point is different, the blue-shifted wing being more intense in nearly all cases. It is also worth noting that in some active points (not included in Fig. 10) only the blueshifted line component was observed.

[FIGURE] Fig. 10. Velocity shifts measured from the multi-Gaussian fits for some active points from Si iv and O v spectral line profiles. Abscissae: blue-shifted component; ordinate: red-shifted component.

Unfortunately, the heavy distortion of the line profiles for these explosive events did not allow a reliable fit for the density-dependent O iv lines, so that estimates of electron densities were not possible. The behaviour of these explosive events is the same for all the transition region lines: Si iv, O v and N v. This suggests that the structures generating this activity are coherent over the different temperatures of formation of these lines. In contrast, no correlation has been found between the Mg x spectral line behaviour and the transition region activity, indicating that this activity is confined to transition region temperatures.

These kind of events are not new in solar observations. They were first observed by Brueckner & Bartoe (1983), which called them turbulent events . Later analyses showed that turbulence was not the cause of these phenomena and they were referred to as explosive events . Dere et al. (1989) report the physical characteristics of a large number of explosive events, which are broadly consistent with those described in the present study.

Fig. 11a shows the locations of the active points relative to the cell-network structure, as defined by the Si ii intensity map. The active events take place either over the network pattern or in the region very close to it. The occurrence frequency of such activity falls dramatically in the cell interiors. Some clusters of active points are found where the Si ii emission is strongest, but isolated points are found mostly at the edges of Si ii bright network. Fig. 11b attempts to correlate the bright points with the photospheric magnetic field from the Kitt Peak images. It is possible to see from Fig. 11b that the largest number of the active points is not correlated with the stronger photospheric magnetic field but is located outside of the magnetic features. Unfortunately, the sensitivity and spatial resolution of the magnetogram does not allow to detect the presence of possible weaker magnetic field features correlated with the active points. It is interesting to see that in the cases where bright points and magnetic field are correlated, the bright point is sitting on top of magnetic field of the same polarity.

[FIGURE] Fig. 11. a  Si ii intensity map with the locations of active points superimposed. b  Kitt Peak magnetogram with the locations of active points superimposed. Active point areas are circled by black lines on the intensity maps. Black means negative polarity, white positive polarity.

In the past, a few attempts have been made to correlate magnetic field and explosive events, mainly by Dere et al. (1991) and Porter & Dere (1991). They found that most explosive events are not correlated with the strongest features of the magnetic field, but are found at the boundaries of the network. Only in a few cases are transition region explosive events correlated with X-ray bright points, magnetic bipoles or a neutral line of an emerging active region.

5.2. A jet-like structure

In the SEP16 dataset a large active area was observed at (40", 95") in the field of view displayed in Fig. 2: this region can be seen as the brightest area in the whole intensity map in both N v and O v lines. This active area is [FORMULA] 3500 km long along the slit length, and around 7000 km wide in the solar X direction. From the time which the SUMER slit took to cover the observed extent of this structure (rastering toward the east limb), we can deduce that the lifetime of the activity is at least 10 minutes.

The most striking feature of this dynamic feature is a huge jet-like structure which seems to be rooted in the core of the active area and extends toward the south, slightly inclined relatively to the SUMER slit. The spectral line profile is extremely distorted, showing a large, non-Gaussian blue-shifted component superimposed on a regular profile similar to the quiet areas. Fig. 12 shows an image of this structure. The image represents an intensity map obtained by summing all the counts under the O v blue-shifted line profile, from 0 to -300 km s-1. The feature seems to comprise two loop-like, or jet-like plasmas moving very fast and originating from the same central active area. The longest feature is around 14000 km long, and is inclined at around 10o relative to the N-S direction. Unfortunately, it is not possible to distinguish between a loop, with very dynamic plasma flowing along its length, and a plasma jet. The N v line profile shows a very similar behaviour, although the longest of the two jet-like structures in Fig. 12 appears to be slightly shorter than the O v counterpart.

[FIGURE] Fig. 12. The jet-like structure observed in the SEP16 dataset.

Assuming that this feature is a jet, the slight inclination relative to the SUMER slit allows the measurement of the velocity perpendicular to the line of sight. Also assuming that the bright emission in Fig. 12 at different slit positions comes from the same plasma which has moved eastward, we have estimated the speed perpendicular to the line of sight as being between 60 and 100 km s-1. Combining this velocity with the estimate of the line-of-sight velocity obtained from the line profile, gives a rough estimate of the plasma absolute speed as between 180 and 270 km s-1. The uncertainty of the measured speed is large due to the breadth of the extended blue-shifted component. It is interesting to note that no signature of a red-shifted counterpart of the observed feature is observed anywhere in the active area.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000