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Astron. Astrophys. 361, 759-765 (2000)

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3. Observational properties

The observations we analyzed were obtained by HSOS, YOHKOH/SXT, SOHO/EIT and MDI on November 1, 1997. The plasma ejection, manifested as an H[FORMULA] surge, EUV and X-ray jet, was observed at the boundary of the main sunspot of NOAA 8100 at position S18E10. Fig. 1 shows the general appearance of the H[FORMULA] surge. In the figure, Panel `A' is a line-of-sight magnetogram at 03:16 UT, superposed by a surge as a contour at 03:42 UT. In the magnetogram, the brighter patches are the positive polarity fields, and the darker patches, the negative ones. Panel `B' is a portion of the YOHKOH SXT image at 04:15 UT; the X-ray jet is indicated by an arrow. The white patch underneath the jet is an X-ray bright point. Panel `C' is an H[FORMULA] filtergram at 03:42 UT taken at - 0.24 Å from the line center, the brighter patche under the base of the surge is a surge-flare. Panel `D' is an H[FORMULA] Dopplergram at 02:37 UT, where the brighter patches are downward motion, and darker ones, upward motion. This figure shows the appearance of the plasma ejection at different wavelengths and in the different layers of the Sun accordingly (e.g., H[FORMULA] in the lower chromosphere, X-ray in the corona at temperature above 2.5 MK).

[FIGURE] Fig. 1a-d. General appearance of the surge. a  line-of-sight magnetogram at 03:16 UT, superposed by the surge as contour at 03:42 UT; b : a portion of the Yohkoh SXT image of 04:15 UT, the arrow shows an X-ray jet; c : an H[FORMULA] filtergram at 03:42 UT taken at - 0.24 Å from the line center; d : an H[FORMULA] Dopplergram at 02:37 UT.

3.1. Morphology and velocity

Fig. 2 is time sequence of H[FORMULA] filtergrams (the first and third columns) and Dopplergrams (the second and fourth columns). The surge appeared before 02:21 UT (there were no H[FORMULA] data at the early phase of the surge), and lasted two and a half hours. It consisted of several components. At 02:29 UT, two threads (shown by two arrows) had bright points at their bases, the third one on the right was smaller than the other two and had no bright point at its base. The two larger threads twisted together at 02:37 UT, which could be seen more clearly in H[FORMULA] Dopplergrams. It was clearly seen that the two twisted components were blue shifted, and a red shifted component (see the arrow at 02:37 UT) appeared on the right at the same time. This indicated that while the major part of the material of the surge moved upward, a fraction of the material moved downward. At 03:23 UT, the two blue-shift components merged at their tops, then at 03:39 UT, the once amalgamated tops of the two threads separated. We suggest that this phenomenon presents the untwisting process. During the decay phase of the H[FORMULA] surge, more fractions of threads became red-shift (denoted by two arrows at 03:56 and 04:38 UT), and an X-ray jet appeared (see following). In order to demonstrate the velocity patterns of the H[FORMULA] surge more clearly, we present in Fig. 3 four selected Dopplergrams at critical times when key evolutions were seen. The two blue shifted components twist (at 02:37 UT), untwist (at 03:23, 03:39 UT) and rotate (03:56 UT).

[FIGURE] Fig. 2. Comparison of time sequences of H[FORMULA] filtergrams (the first and third columns) and Dopplergrams (the second and fourth columns). The two arrows at 02:29 UT show the two main components of the H[FORMULA] surge. Arrows at 02:37, 03:56, and 04:38 UT in Dopplergrams indicate red-shift components of the surge.

[FIGURE] Fig. 3. Velocity patterns seen in H[FORMULA] Dopplergrams.

3.2. Magnetic fields

In Fig. 4 we present the time sequence of line-of-sight magnetograms showing the magnetic field evolution on the base of the H[FORMULA] surge. Iso-gauss contours with levels [FORMULA]50 and 100 G are superposed on the grey-scale maps to demonstrate the field evolution more clearly. The upper three rows are the magnetograms from HSOS, and the lowest row is taken from SOHO/MDI full disk magnetograms. We find that the magnetograms from HSOS and SOHO/MDI are consistent in morphology, but the spatial resolution of the HSOS magnetograms is higher than that of SOHO/MDI. The positive (negative) polarity fields are represented by brighter (darker) color in the grey scale maps and white (dark) contours in the iso-gauss drawings. The close curves at 06:27 and 09:39 UT represent two emerging magnetic bipoles respectively. These bipoles are associated with the H[FORMULA] surge. Usually, a new emerging magnetic bipole manifests itself in one of two ways. One way is the appearance of arch filament systems in H[FORMULA] and/or H[FORMULA] filtergrams; the second is co-temporal growing and separating of opposite polarity flux in line-of-sight magnetograms. In this figure, we determine the emerging bipoles by the second way from a continuous 30 hour time-sequence of SOHO/MDI magnetograms. Three arrows at 03:15 UT show magnetic flux patches of an emerging magnetic bipole outlined by the curve at 06:27 UT. The positive magnetic flux (shown by arrow `1') was cancelled by a pre-existing large-scale negative flux during its emergence, and finally disappeared near 09:39 UT. The other two flux patches grew continuously since they did not encountered a flux of opposite polarity. Two arrows at 04:31 UT show the foot points of the H[FORMULA] surge. We can see clearly the evolution of magnetic fields on the base of the H[FORMULA] surge. One end (shown by arrow `4') of the H[FORMULA] surge is rooted at the cancellation site mentioned above. Arrow `6' at 05:07 UT shows a positive flux patch which might have already cancelled with the nearby nagetive polarity field before appearing in the photosphere. This cancellation seems to correspond to another component of the H[FORMULA] surge indicated by arrow `5'. It is shown by the time-lapse H[FORMULA] filtergrams that the H[FORMULA] surge appeared before 02:21 UT. However, the time sequence of HSOS/MDI magnetograms demonstrated that the positive flux of the new emerging bipole had not appeared in the photosphere until 03:16 UT, which was about one hour later than that of the first appearance of the H[FORMULA] surge. Surge appears at the early stage of flux emergence, and is often (Kurokawa & Kawai, 1993) considered the first signature of an emerging flux region. From the spatial relationship we believe that the H[FORMULA] surge analysed is intrinsically correlated with the new emerging flux outlined at 06:27 UT. This surge presents a nice example that surge appears before the new emerging flux broken up in the photosphere. If we accept the idea that surge results from magnetic reconnection between new and pre-existing flux, then in this example the reconnection must have taken place below the photosphere. In a sense, we could conclude the H[FORMULA] surge studied manifested an interaction, which took place at quite low atmosphere, between the newly emerged flux and the pre-existed large-scale magnetic fields of opposite polarity of AR 8100. Fig. 5 shows a vector magnetogram taken near 03:16 UT, superposed by the surge (surge-flaring) as dark (white) thick contour at 03:23 UT. The line-of-sight components are presented by both grey scale map and iso-gauss contours with levels [FORMULA]50, 100 and 400G; the transverse components are presented by short line segments with length proportional to the field strength and alignment parallel to the field direction. Two arrows show the foot points of the H[FORMULA] surge.

[FIGURE] Fig. 4. Time sequences of line-of-sight magnetograms showing the magnetic fields evolution on the base of the H[FORMULA] surge, superposed by iso-gauss contours with levels [FORMULA]50 and 100 G. The positive (negative) polarity fields are represented by brighter (darker) color in the grey scale maps and white (dark) contours in the iso-gauss drawings. The upper three rows show the magnetograms from HSOS, and the lowest row from SOHO/MDI. The close curves at 06:27 and 09:39 UT present two emerging magnetic bipoles which associated with the H[FORMULA] surge. Three arrows at 03:15 UT show three magnetic flux patches of the emerging magnetic bipole closed by the curve 06:27 UT. Two arrows at 04:31 UT mark the foot points of the H[FORMULA] surge. Arrow at 05:07 UT shows the magnetic field at a surge foot. The field of view i s about 50 by 50 arcsec.

[FIGURE] Fig. 5. A set of vector magnetogram taken near 03:16 UT, superposed by the surge (surge flaring) as dark (white) thick contour at 03:23 UT. The line-of-sight components are presented by grey scale map and by iso-gauss contours with levels [FORMULA]50, 100 and 400G; the transverse components are presented by short line segments with length proportional to the field strength and alignment parallel to the field direction. Two arrows show the foot points of the H[FORMULA] surge. The field of view is about 105 by 120 arcsec.

3.3. EUV and X-ray jet

Among various newly discovered dynamic phenomena, one of the most interesting findings are EUV and X-ray jets (Schmahl, 1981; Chae et al. 1999; Shibata et al. 1992b; Strong et al. 1992). According to Shibata et al. (1992b), X-ray jets are transitory X-ray emission enhancements with an apparent collimated motion. Chae et al. (1999) reported that EUV jets occur repeatedly at the regions where pre-existing magnetic flux is `cancelled' by newly emerging flux of opposite polarity. Fig. 6 shows SOHO/EIT 195 Å running difference images from 01:43 to 04:33 UT and an MDI magnetogram (middle-left panel). Arrow `1' at 04:51 UT showed the interface of newly emerging positive flux and negative sunspots. The interaction of these magnetic fields triggered an EUV bright point and EUV jet, which were denoted by two arrows `2' and `3' at 04:16 UT EIT image. Comparing H[FORMULA] filtergrams with EUV images, We find that the H[FORMULA] surge is spatially coincident with the EUV jet, although the EUV jet appeared later than the H[FORMULA] surge. From the EIT movie, we find that the material of the EUV jet moves along a large-scale loop. Fig. 7 is time sequences of SXT images showing an X-ray bright point and jet appeared at 04:15:05 UT. A SOHO/MDI magnetogram at 04:51 UT was presented to show the magnetic fields configuration associated with the X-ray bright point (arrow `2') and jet. Mandrini et al. (1996) found evidence showing that X-ray bright point brightenings were due to magnetic reconnection between a new bipole and pre-existing plage fields; Schmieder et al. (1997) presented that X-ray loops (bright points) appeared at the location of a emerging flux. From the time series of SXT images studied, we found that there were two X-ray bright points. The first appeared near 03:38 UT, meanwhile the twisted components of the H[FORMULA] surge untwisted. The second occurred at the same site as the first, where sunspot negative flux was "cancelled" (shown by arrow `1' at 04:51 UT) by a positive flux of the new emerging flux region described in Sect. 3.2. Near 04:15 UT, the decay phase of the H[FORMULA] surge, an X-ray jet took place in the company of the second X-ray bright point. Unfortunately, there were no H[FORMULA] data at the onset phase of the X-ray jet, we did not know the evolution of the H[FORMULA] surge at the time of the X-ray jet appearance. We suggest that the two X-ray bright points were a consequence of an explosive reconnection in the corona during the continuous flux cancellation (Nolte et al. 1979). The H[FORMULA] surge and X-ray jet may well represent a scenario of two-step reconnection, proposed first by Wang & Shi (1993) for flare observations, and confirmed by Chae et al. (1999) for EUV explosive events. The first-step reconnection takes place in the photosphere, or lower atmosphere. It is slow but continuous since the conductivity in the photosphere is much lower than that of the fully-ionized plasma in the upper chromosphere and corona. This slow reconnection manifests as flux cancellation observed in the photospheric magnetograms (Livi et al. 1985; Martin et al. 1985). The reconnection in the lower atmosphere may convert the magnetic energy into heat and kinetic energy; while, at the same time, it can transport the magnetic energy and complexity into the rather large-scale magnetic structure higher in the corona. The second-step reconnection can only take place when some critical status is achieved in the large-scale magnetic structure higher in the corona. This reconnection is explosive in nature, which is directly responsible for the energy released in the transient solar activities. Fig. 8 is an X-ray image at 04:15:05 UT superposed by the contours of H[FORMULA] filtergram at 03:42 UT. Despite the low spatial resolution of the X-ray image, it is clear that the H[FORMULA] surge and X-ray jet are co-spatial, although the length of the H[FORMULA] surge appears longer than that of the X-ray jet.

[FIGURE] Fig. 6. SOHO/EIT running difference images and an MDI magnetogram (middle-left panel). The arrow at 04:51 UT showed the interface of two opposite polarity magnetic fields. The interaction of these magnetic fields triggered an EUV bright point and EUV jet, which were denoted by two arrows at EIT image at 04:16 UT. The field of view is about 500 by 500 arcsec.

[FIGURE] Fig. 7. Time sequences of SXT images showing the X-ray bright point and X-ray jet that appeared at 04:15:05 UT. A SOHO/MDI magnetogram at 04:51 UT is presented to show the magnetic fields configuration associated with an X-ray bright point and jet. Arrow `1' shows a magnetic cancellation site which responded to the X-ray bright point (arrow `2').

[FIGURE] Fig. 8. The SXT image taken from 04:15:05 UT, superposed on the H[FORMULA] surge at 03:42 UT.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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