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Modern Solar Facilities – Advanced Solar Science, 253–256 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Magnetic field structure and dynamics in coronal hole - active region interface zone I. A. Bilenko Sternberg State Astronomical Institute, Moscow, Russia Email: bilenko@sai.msu.ru Abstract. We study the changes of magnetic field structure in active regions (AR), coronal holes (CH), and coronal hole - active region (CH-AR) interface zones. Zones of coronal hole - active region interface are characterized by relatively fast fluctuations of the photospheric magnetic fields. Interaction between the closed AR magnetic fields and the open CH fields can cause or facilitate some eruptive events. 1 Introduction It is often assumed that CH boundaries are sources of slow solar wind. CH boundaries have also been suggested as important factors in coronal eruptive events, especially when they are associated with ARs. Webb et al. (1978) found that most of the transients are related to large-scale changes in CH areas and tend to occur on the borders of evolving equatorial holes. Kahler & Moses (1990) found that the boundary changes are characterized by events of length scales < 2 × 104 km and time scales of < 3 h. They interpret the boundary changes in terms of magnetic reconnection. There are different types of CH boundaries (Kahler & Hudson 2002). The boundaries of CHs are quite irregular, reflecting the structure of adjacent photospheric magnetic fields. Small-scale changes in the structure of a CH boundary could be seen from SOHO/EIT images to occur on a time scale of a few hours. However, the main structure may be observed for several months and appears to rotate quasi-rigidly. The maintenance of such a rigid open-field structure against differential AR rotation could be achieved by continuous reconnection between open and closed field lines at the boundary of the CH (Wang et al. 1996). It is interesting to compare the evolution of the photospheric magnetic field at CH-AR interface zones as seen by MDI (Scherrer 1995) to that of the solar corona observed by EIT (Delaboudinière et al. 1995) and Yohkoh SXT (Tsuneta et al. 1991). 2 Analysis and results CHs are known to be the regions of open magnetic field, low coronal density, and low temperature . They are associated with weak, predominantly unipolar photospheric magnetic fields (Bohlin 1997). In Figure 1 the process of formation and evolution of a positive-polarity CH near the AR NOAA 9026 during the period 2000, 5–7 June is presented. The top row shows magne- 254 I. A. Bilenko: Magnetic fields in coronal hole - active region interface zone tograms from SOHO/MDI overlaid with the CH boundaries observed in the He i 10830 Å line (from Kitt Peak National Solar Observatory/KP-NSO). The middle row represents the AR and CH evolution in X-ray observed by Yokoh/SXT, and the bottom row shows their evolution in the Fe xii 195 Å line observed by EIT. In this area, there was no CH in the He i line detectable on June 5. In some small regions of reduced density overlapping, large faint arc structures were seen. In X-ray a large region of reduced density (X-ray coronal hole) surrounded the AR. It forms sharp arc boundary structures. The arcs connect the AR negative-polarity fields with the positive-polarity photospheric fields at the boundary of the X-ray CH. The general magnetic configuration changed little until June 6. On June 6, X11 (13:30 UT), X23(14:58 UT) class flares, and several minor M and C class events occurred in NOAA 9026. These events resulted in changes of magnetic field configuration. The Fe xii arc structures were destroyed, probably they reconnected with closed magnetic fields of the AR and an open magnetic structure appeared (cf. Fig. 1 middle row, second image). A new CH in the chromospheric He i line was observed (second image in the top row in Fig. 1). The post-flare X-ray arc increased and connected boundaries of the new positive-polarity CH and a negative one on the other side of the AR, forming a large helmet structure above the AR. Such structures are thought to be the sources of some eruptive events. Six CMEs were observed during June 6 and three CMEs on the next day including two halo CME. Possibly, the halo CMEs were produced in that magnetic structure. Some brightenings were observed in the footpoints of the X-ray arc at the CH boundary. Changes in coronal arcs occur, and the brightness could be attributed to changes in the photospheric magnetic field (Howard & Ŝvestka 1977). An analogous process was observed during the period 1–7 November 1998. An X-ray CH was present during the whole period. The magnetic configuration indicated by arcs was stable until November 3. A great number of M and C class events occurred in the AR NOAA 8375. These events changed completely the magnetic field configuration. A new positivepolarity CH appeared in He i on November 3 on the unipolar region near NOAA 8375. A new large structure of arcs was formed which connected the boundaries of X-ray CHs at the leading and the following parts of the AR. Possibly, the halo CME on November 4 originated in that structure, and such an eruptive event resulted in the formation of an open magnetic field structure on the following negative-polarity unipolar region, and a He i chromospheric CH appeared on November 5. A huge helmet arc structure overlapping an AR and joining positive-polarity and negative-polarity CH boundaries was created. The two halo CMEs which were observed on November 5 were possibly associated with that magnetic structure. The structure of photospheric magnetic network elements in the interface zone differs strongly from that in the other part of the adjacent CH or the quiet region. Figure 2 shows the magnetic field evolution in NOAA 9026, in the adjacent CH, and in the CH-AR interface zone. MDI magnetograms at 96 m cadence are used to study magnetic field changes. Daily CH maps in He 10830 were obtained from the Kitt Peak National Solar Observatory. The MDI data were converted to magnetic field strength according to H(Gauss)=1.232 + 2.816(pixel value). The positive-polarity magnetic field average strength (thick line) and the negative one (thin line) in the CH (Fig. 2b), in AR (Fig. 2c), and in the zone between them (Fig. 2a) for magnetic fields greater than 20 Gauss are presented. In the zone of CH-AR interface fluctuations are significant, especially after the coronal hole appearance. In this region positive-polarity magnetic fields dominate as in the CH. In Figure 2d, the evolution of the ratio of the average magnetic field strength of positive-polarity and the absolute value I. A. Bilenko: Magnetic fields in coronal hole - active region interface zone 255 Figure 1. Formation and evolution of the CH during the period 2000, 5–7 June. Top row: magnetograms from KP-NSO overlaid with CH boundaries observed in He i 10830 Å (KP-NSO); middle row: AR and CH evolution in X-ray (SXT); bottom row: evolution in Fe xii 195 Å (EIT). of the negative-polarity for all values of magnetic fields in the AR (3), in the CH (2), and in the zone of CH-AR interface (1) is shown. In the CH and CH-AR zone the imbalance is much stronger than that in the AR, and the maximum coincides with the time of the CH creation above the photospheric region with unipolar, positive-polarity magnetic field. Some simultaneous variations are observed at the time of X-class flares on June 6. 3 Conclusion The photospheric magnetic field structure in the CH part connected with the AR differs significantly from that in the other parts of the CH. A CH - AR interface zone is characterized 256 I. A. Bilenko: Magnetic fields in coronal hole - active region interface zone Figure 2. Evolution of averaged line-of-sight magnetic field strength in CH - AR interface zone of NOAA 9026 (a), in the CH (b), and in the AR itself (c) for magnetic fields greater than 20 Gauss; thick: positive polarity, thin: negative polarity; Panel d: magnetic field strength imbalance in CH (2), in zone of CH-AR interface (1), and AR (3) for all magnetic fields. Abscissae: day in June 2000; ordinates: absolute field strengths in Gauss (a, b, c), relative units (d). by relatively fast fluctuations of the photospheric magnetic fields. Emerging or cancellation of magnetic flux, intensive motion of small photospheric magnetic fields may yield increased instability, and shear or twist the footpoints of the coronal magnetic fields. Such dynamic processes increase the magnetic energy in the solar corona. Interaction between the closed AR fields and the open CH fields can cause or facilitate some eruptive events. And vice versa, magnetic fields eruption can form large-scale patterns, which contain open magnetic fields. CH were formed within these open-field regions. The deepest layers of the solar atmosphere are involved in such processes. Acknowledgements. SOHO is a project of international cooperation between ESA and NASA. NSO/Kitt Peak data used here were produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/SEL. We are grateful to the Yohkoh team for supplying the SXT data. References Bohlin, J. D. 1997, in Coronal Holes and High Speed Wind Streams, ed. J. B. 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