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Enhanced coronal heating and 3D solar magnetic fields in AR 7321
Adv. Space Res. Vol. 25, No. 9, pp.1769-1772,2O@J 0 2000 COSPAR. Published by Ekevier ScienceLtd. Ali rights reserved Printed in Great Britain 0273-1177/00 $20.00 + 0.00
Pergamon www.elsevier.nl/locate/asr
Pll: SO273-1177(99)00631-6
ENHANCED CORONAL FIELDS IN AR 7321
SEATING
AND
3D SOLAR
MAGNETIC
Huaning Wang l,?-,Takashi Sakurai’, and Yihua Yan 2 ‘National Astronomical Observatory of Japan, 2-21-l Osawa, Mitaka, Tokyo 181-8588, JAPAN ‘~e~~~~gAgtro~om~ca~ Ubse~~to$y, Da&an Rd. #!&IA, C~ao~a~g District, ~e~~~~gfUUt?fZ, ~~~~A ABSTRACT The photospheric vector magnetic fields, Hcuand soft X-ray images of AR 7321 were simultaneously observed with the Solar Flare Telescope at Mitaka and the Soft X-ray Telescope of Yohkoh on 26 October 1992, when there were no drastic activities in this region. Taking the observed photospheric vector magnetic fields as the boundary condition, the 3D magnetic fields above the photosphere were computed with a new extrapolation method. Trough a comparison among 3D magnetic fields, Ha features, and soft X-ray features, the following results have been obtained: (a) Magnetic separatrices or quasi-separatrix layers play an important role in ~rom~pheric and coronal heating, but they do not ,seem to be dominant in the enhanced coronal heating. (b) The enhanced coronal heating might be due, not only to the magnetic shear, but also to the emerging magnetic flux. 0 2000 COSPAR. Published by Elsevier Science Ltd.
~~TRODU~T~O~ It is widely believed that solar magnetic fields play a very important role in chromospheric and coronal heating. Several heating mechanisms have been proposed during the last decades. All of these mechanisms can be divided into two classes: (a) the dissipation of MHD waves by phase mixing or resonant absorbtion; (h) the formation and dissipation of thin current sheets. Based on Yahkoh soft X-ray observations, Priest et al. (1998) suggested that the heating mechanism of the diffuse coronal features (DCFs) is in agreement with class (b). However, the heating mechanism of the bright coronal features (BCFs) remains unknown. Falconer et al. (1997) believe that a kind of hybrid heating processes including both (a) and (b) is a viable possibility for some of these coronal features. They demonstrated that the heating of BCFs in solar active regions, namely enhanced coronal heating, is closely related to the presence of strong magnetic shear along neutral lines in the core magnetic fields in these regions. Recently Dhmoulin and Priest (1997) suggested a new heating mechanism: In a large class of 3D Inagnetic fields without null points there are locations, called ‘quasi-separatrix layers (QSLs) ‘, where the field-line linkage changes drastically and then concentrated electric currents are formed by smooth boundary motions. These QSLs are the relevant ger~eralization of normal magnetic separatrices to ~on~gurations without null points. When a QSL is thin enough, a turbulent resistivity is triggered, and the QSL then rapidly evolves into a dynamic current sheet that releases energy by fast 3D magnetic reconnection. In the case of coronal heating, the fragmentation of the photospheric magnetic field stimulates the dissipation of magnetic energy in the corona. This kind of heating melanism, of course, needs to be examined with observations. The photospheri~ vector magnetic fields, HCYand soft X-ray images of active region NOAA 7321 were simutaneously observed with the Solar Flare Telescope at Mitaka (Sakurai et al., 1995) and the Soft X-ray Telescope of Yohkoh (Tsuneta et al. ,199l) on 26 October 1992, when there was no drastic activity in this region. Taking the observed photospheric
vector magnetic fields as the boundary condition,
3D nonlinear force-
H. N. Wanget nl.
1770
free magnetic fields above the photosphere were computed with a new extrapolation method proposed by Yan and Sakurai (1997, 1998). S ince Yohkoh data and Mitaka data were obtained in well arranged time sequencies during this day, we can study the relationship among 3D magnetic fields, Ho features and soft X-ray features in this active region.
QSLS IN 3D MAGNETIC FIELDS On 26 October 1992, 150 frames of vector magnetograms were obtained with the Solar Flare Telescape at Mitaka. We selected one of them as the boundary conditions for the extrapolation of 3D magnetic fields above the photosphere. The extrapolated 3D fields have been examined with the divergence-free condition and the force-free condition (Wang et al., 1998). We compute the values of N, the norm of the displacement gradient tensor determined by footpoints of field lines (Priest and Demoulin, 1995), and consider the 3D field lines with N > 10 as QSLs.
Figure 1: QSLs obtained from 3D magnetic fields in AR7321(S26 W04) on 26 October 1992. The field of view is 200” x 200”. The strength of the photospheric line-of-sight magnetic field is mapped by the contours (solid lines, positive polarity; dotted lines, negative polarity; 80, 320, 640, 960, 1280, 1600 and 1960G levels). level. This map indicates the intersection of (a) Gray scale map of iV(Nmaz = 102) at the chromospheric QSLs with the chromospheric surface. (b) Field lines connecting the areas with N > 10. These lines form 3D QSLs above the chromosphere. Figure 1 shows the intersections of QSLs with the chromosphere and the field lines connecting the areas with N > 10. These field lines indicate the 3D structure of QSLs. When a null point appears in 3D magnetic fields, the value of N becomes very large and QSLs approach the separatrices. In fact, there is a null point in the extrapolated 3D fields, which is located between two main negative polarities and close to the chromosphere. This is the reason why the maximum value of N is about 100.
QSLS, Ha AND SOFT X-RAY FEATURES A QSL forms a 3D shell standing different
solar atmospheric
layers.
on the photospheric surface, which should have an observable effect on First of all, we make a comparison between QSLs and Ha features.
Figure 2 shows the lower portion of the field lines forming the QSLs in the chromosphere. It can be found that most of these footpoints are located in the Ha plage areas. This suggests that the QSLs play a role in the chromospheric heating. However, in Figure enhanced
the Ho bright points, El and E2, which have been identified with the footpoints of the BCF 3(b), are located in the intersection of the QSLs with the chromosphere. This means that the coronal heating might be partly related to the QSLs in the chromosphere.
Next, we compare the QSLs with the soft X-ray features observed by Yohkoh. Figure 3 shows that the BCF with footpoints El and E2 is embraced by the field lines forming the QSLs. It can be seen from Figure 3(b)
Coronal
Heating
that ali of BCF’s are surrounded by the DCFs, and both BCFs and DCFs are contained facts imply that the QSLs play a significant role in the coronal heating.
0
20
40
60 80 130 x (x132arcsec)
'20
1771
and SolarMagneticField
by the QSLs. These
i40
Figure 2: Comparison between QSLs and Her image of AR7321 on 26 October 1992, (a) The lower portion of field lines forming the QSLs under the height of z = 41t. The contour map is as the same as that in Figure 1. The footpoints of the fkld lines are uniformly distributed in the area with N > 10 in the chro~lospl~ere, where Ha plages appear. (b) Hai image. The bright points, El and E2, are the foot#point,s of t,he brightening feature in Figure 3(b)
0
70
40
60 8'3 100 x (xl32arcsec)
120
140
Figure 3: Comparison between QSLs and soft X-ray image of AR7321 on 26 October 1992. (a) QSLs and soft X-ray image. The contour map is as the same as that in F’igure 1. The footpoints of field lines are uniformly distributed in the area with N > 10 in the chromosphere. All of these field lines are not ovel the height of .z = 40”. (b) Soft X-ray image of strongIy heated coronal features in AR7321 on October 26, 1992. These features are regarded as the sites of enhanced coronal heating in solar active regions. The X-ray intensity has been normalized to the same scale (DN/s). The X-ray intensity in brightest areas exceeds 4500 DN/s, and the threshold is 1000 DN/s. El and E2 are the locations of the Ha bright points in Figure 2(b).
The BCFs in Figure 3(b), however, are related only to a part of field lines forming the QSLs. In other words, the enhanced coronal heating is not uniformly distributed within the volume confined by the QSLs. This indicates that enhanced coronal heating cannot be atfributed only to the QSLs. Comparing
the magnetogram
at 03 : 07UT with that at 23 : 51UT, the following differences
have been found:
H. N. Wanget at.
1772
(a) The magnetic shear along the neutral-line was increased; (b) A large emerging flux region appeared in the westen side of this active region. Since there was no drastic activity during the period from 03 : 07UT to 23 : 51UT, the configuration of the magnetic fields in this active region evolves stationarily during this period. In that sense, the enhanced coronal heating might be due not only to the magnetic shear but also to the emerging magnetic flux.
SUMMARY AND DISCUSSION Taking the photospheric vector magnetic fields in Figure 2 as the boundary condition, the 3D magnetic fields above the photosphere were computed with a new extrapolation method, and then the QSLs were determined in these fields. According to the analyses in Sections 2 and 3, we conclude that (a) QSLs play a significant role in the chromospheric and coronal heating, but they are not the dominant factor in the enhanced coronal heating; (b) the enhanced coronal heating might be due to emerging magnetic flux in addition to the magnetic shear. QSLs or Separatrices are the surfaces separating cells of magnetic field lines, where concentrated electric and corona, the QSLs or currents may be formed due to smooth boundary motions. In the chromosphere separatrices define the morphology of Ho and soft X-ray features. Magnetic fields contained in the volume defined by QSLs or separatrices make significant contributions to the chromospheric and coronal heating. Since a small part of the magnetic flux in a sunspot is involved in QSLs, the X-ray emission above the sunspot is usually week. This result gives an explanation why X-ray emission is almost missing in sunspot umbra (Pallavicini et al., 1979). It is clear that the magnetic shear and emerging magnetic the exact heating mechanism in BCFs remains unknown. heating is necessary in the future.
flux are related to enhanced coronal heating, but A comprehensive study of the enhanced coronal
ACKNOWLEGEMENTS The numerical for hospitality NSFC grants.
computations were carried out on VPP300/16R during his stay as a Foreign Research Fellow.
at NAOJ. H. N. Wang is grateful to NAOJ H. N. Wang and Y, Yan are supported by
REFERENCES Demoulin, P. and Priest, E. R., The Importance of Photospheric Intense Flux Tubes for Coronal Heating, Solar Phys., 175 123 (1997). Falconer, D. A., R. L. Moore, J. G. Porter, G. A. Gary, and T. Shimizu, Neutral-Line Magnetic Shear and Enhanced Coronal Heating in Solar Active Regions, Astrophys. J.,482, 519 (1997). Pallavicini,
R., G. S. Vainana,
Astrophys.
G. Totani,
and M. Felli, The Coronal Atmosphere
above Solar Active regions,
J., 229, 375 (1979).
E. R. and P. Demoulin, Three-Dimensional Magnetic Reconnection without Null Points, I, J. Res., 100, 23443 (1995). Priest, E. R., C. R. Foley, J. Heyvaerts, 2‘. D. Arber, J. L. Culhane, and L. W. Acton, Nature of the Heating Mechanism for the Diffuse Solar Corona, Nature, 393, 545 (1998).
Priest,
~~~phys.
Sakurai,
T., K. Ichimoto,
Y. Nishino,
K. Shinoda,
M. Noguchi,
et al., Solar Flare
Telescope
at Mitaka,
P. A. S. J., 47 81 (1995).
Tsuneta, S., L. Acton, M. Bruner, J. Lemen, W. Brown, et al., The Soft X-Ray Telescope for the SOLAR-A Mission, Solar Phys., 136 37(1991). Wang, H. N., T. Sakurai, Y. Yan, and Y. Liu, The extrapolated 3D Solar Magnetic Field in AR7321, Proceedings of Numerical Astrophysics 1998, in press (1998). Yan, Y. and T. Sakurai, Analyses of Yohkoh SXT Coronal Loops and Calculated Force-Freee Magnetic Field Lines from Vector Magnetogra~, Solar Phys., 174, 65 (1997). Yan Y. and T. Sakurai, New Boundary Integral Equation Presentation for Finite Energy netic Fields in Open Space above the Sun, Astrophys. J., submitted (1998).
Force-Free
Mag-
Pergamon www.elsevier.nl/locate/asr
Pll: SO273-1177(99)00631-6
ENHANCED CORONAL FIELDS IN AR 7321
SEATING
AND
3D SOLAR
MAGNETIC
Huaning Wang l,?-,Takashi Sakurai’, and Yihua Yan 2 ‘National Astronomical Observatory of Japan, 2-21-l Osawa, Mitaka, Tokyo 181-8588, JAPAN ‘~e~~~~gAgtro~om~ca~ Ubse~~to$y, Da&an Rd. #!&IA, C~ao~a~g District, ~e~~~~gfUUt?fZ, ~~~~A ABSTRACT The photospheric vector magnetic fields, Hcuand soft X-ray images of AR 7321 were simultaneously observed with the Solar Flare Telescope at Mitaka and the Soft X-ray Telescope of Yohkoh on 26 October 1992, when there were no drastic activities in this region. Taking the observed photospheric vector magnetic fields as the boundary condition, the 3D magnetic fields above the photosphere were computed with a new extrapolation method. Trough a comparison among 3D magnetic fields, Ha features, and soft X-ray features, the following results have been obtained: (a) Magnetic separatrices or quasi-separatrix layers play an important role in ~rom~pheric and coronal heating, but they do not ,seem to be dominant in the enhanced coronal heating. (b) The enhanced coronal heating might be due, not only to the magnetic shear, but also to the emerging magnetic flux. 0 2000 COSPAR. Published by Elsevier Science Ltd.
~~TRODU~T~O~ It is widely believed that solar magnetic fields play a very important role in chromospheric and coronal heating. Several heating mechanisms have been proposed during the last decades. All of these mechanisms can be divided into two classes: (a) the dissipation of MHD waves by phase mixing or resonant absorbtion; (h) the formation and dissipation of thin current sheets. Based on Yahkoh soft X-ray observations, Priest et al. (1998) suggested that the heating mechanism of the diffuse coronal features (DCFs) is in agreement with class (b). However, the heating mechanism of the bright coronal features (BCFs) remains unknown. Falconer et al. (1997) believe that a kind of hybrid heating processes including both (a) and (b) is a viable possibility for some of these coronal features. They demonstrated that the heating of BCFs in solar active regions, namely enhanced coronal heating, is closely related to the presence of strong magnetic shear along neutral lines in the core magnetic fields in these regions. Recently Dhmoulin and Priest (1997) suggested a new heating mechanism: In a large class of 3D Inagnetic fields without null points there are locations, called ‘quasi-separatrix layers (QSLs) ‘, where the field-line linkage changes drastically and then concentrated electric currents are formed by smooth boundary motions. These QSLs are the relevant ger~eralization of normal magnetic separatrices to ~on~gurations without null points. When a QSL is thin enough, a turbulent resistivity is triggered, and the QSL then rapidly evolves into a dynamic current sheet that releases energy by fast 3D magnetic reconnection. In the case of coronal heating, the fragmentation of the photospheric magnetic field stimulates the dissipation of magnetic energy in the corona. This kind of heating melanism, of course, needs to be examined with observations. The photospheri~ vector magnetic fields, HCYand soft X-ray images of active region NOAA 7321 were simutaneously observed with the Solar Flare Telescope at Mitaka (Sakurai et al., 1995) and the Soft X-ray Telescope of Yohkoh (Tsuneta et al. ,199l) on 26 October 1992, when there was no drastic activity in this region. Taking the observed photospheric
vector magnetic fields as the boundary condition,
3D nonlinear force-
H. N. Wanget nl.
1770
free magnetic fields above the photosphere were computed with a new extrapolation method proposed by Yan and Sakurai (1997, 1998). S ince Yohkoh data and Mitaka data were obtained in well arranged time sequencies during this day, we can study the relationship among 3D magnetic fields, Ho features and soft X-ray features in this active region.
QSLS IN 3D MAGNETIC FIELDS On 26 October 1992, 150 frames of vector magnetograms were obtained with the Solar Flare Telescape at Mitaka. We selected one of them as the boundary conditions for the extrapolation of 3D magnetic fields above the photosphere. The extrapolated 3D fields have been examined with the divergence-free condition and the force-free condition (Wang et al., 1998). We compute the values of N, the norm of the displacement gradient tensor determined by footpoints of field lines (Priest and Demoulin, 1995), and consider the 3D field lines with N > 10 as QSLs.
Figure 1: QSLs obtained from 3D magnetic fields in AR7321(S26 W04) on 26 October 1992. The field of view is 200” x 200”. The strength of the photospheric line-of-sight magnetic field is mapped by the contours (solid lines, positive polarity; dotted lines, negative polarity; 80, 320, 640, 960, 1280, 1600 and 1960G levels). level. This map indicates the intersection of (a) Gray scale map of iV(Nmaz = 102) at the chromospheric QSLs with the chromospheric surface. (b) Field lines connecting the areas with N > 10. These lines form 3D QSLs above the chromosphere. Figure 1 shows the intersections of QSLs with the chromosphere and the field lines connecting the areas with N > 10. These field lines indicate the 3D structure of QSLs. When a null point appears in 3D magnetic fields, the value of N becomes very large and QSLs approach the separatrices. In fact, there is a null point in the extrapolated 3D fields, which is located between two main negative polarities and close to the chromosphere. This is the reason why the maximum value of N is about 100.
QSLS, Ha AND SOFT X-RAY FEATURES A QSL forms a 3D shell standing different
solar atmospheric
layers.
on the photospheric surface, which should have an observable effect on First of all, we make a comparison between QSLs and Ha features.
Figure 2 shows the lower portion of the field lines forming the QSLs in the chromosphere. It can be found that most of these footpoints are located in the Ha plage areas. This suggests that the QSLs play a role in the chromospheric heating. However, in Figure enhanced
the Ho bright points, El and E2, which have been identified with the footpoints of the BCF 3(b), are located in the intersection of the QSLs with the chromosphere. This means that the coronal heating might be partly related to the QSLs in the chromosphere.
Next, we compare the QSLs with the soft X-ray features observed by Yohkoh. Figure 3 shows that the BCF with footpoints El and E2 is embraced by the field lines forming the QSLs. It can be seen from Figure 3(b)
Coronal
Heating
that ali of BCF’s are surrounded by the DCFs, and both BCFs and DCFs are contained facts imply that the QSLs play a significant role in the coronal heating.
0
20
40
60 80 130 x (x132arcsec)
'20
1771
and SolarMagneticField
by the QSLs. These
i40
Figure 2: Comparison between QSLs and Her image of AR7321 on 26 October 1992, (a) The lower portion of field lines forming the QSLs under the height of z = 41t. The contour map is as the same as that in Figure 1. The footpoints of the fkld lines are uniformly distributed in the area with N > 10 in the chro~lospl~ere, where Ha plages appear. (b) Hai image. The bright points, El and E2, are the foot#point,s of t,he brightening feature in Figure 3(b)
0
70
40
60 8'3 100 x (xl32arcsec)
120
140
Figure 3: Comparison between QSLs and soft X-ray image of AR7321 on 26 October 1992. (a) QSLs and soft X-ray image. The contour map is as the same as that in F’igure 1. The footpoints of field lines are uniformly distributed in the area with N > 10 in the chromosphere. All of these field lines are not ovel the height of .z = 40”. (b) Soft X-ray image of strongIy heated coronal features in AR7321 on October 26, 1992. These features are regarded as the sites of enhanced coronal heating in solar active regions. The X-ray intensity has been normalized to the same scale (DN/s). The X-ray intensity in brightest areas exceeds 4500 DN/s, and the threshold is 1000 DN/s. El and E2 are the locations of the Ha bright points in Figure 2(b).
The BCFs in Figure 3(b), however, are related only to a part of field lines forming the QSLs. In other words, the enhanced coronal heating is not uniformly distributed within the volume confined by the QSLs. This indicates that enhanced coronal heating cannot be atfributed only to the QSLs. Comparing
the magnetogram
at 03 : 07UT with that at 23 : 51UT, the following differences
have been found:
H. N. Wanget at.
1772
(a) The magnetic shear along the neutral-line was increased; (b) A large emerging flux region appeared in the westen side of this active region. Since there was no drastic activity during the period from 03 : 07UT to 23 : 51UT, the configuration of the magnetic fields in this active region evolves stationarily during this period. In that sense, the enhanced coronal heating might be due not only to the magnetic shear but also to the emerging magnetic flux.
SUMMARY AND DISCUSSION Taking the photospheric vector magnetic fields in Figure 2 as the boundary condition, the 3D magnetic fields above the photosphere were computed with a new extrapolation method, and then the QSLs were determined in these fields. According to the analyses in Sections 2 and 3, we conclude that (a) QSLs play a significant role in the chromospheric and coronal heating, but they are not the dominant factor in the enhanced coronal heating; (b) the enhanced coronal heating might be due to emerging magnetic flux in addition to the magnetic shear. QSLs or Separatrices are the surfaces separating cells of magnetic field lines, where concentrated electric and corona, the QSLs or currents may be formed due to smooth boundary motions. In the chromosphere separatrices define the morphology of Ho and soft X-ray features. Magnetic fields contained in the volume defined by QSLs or separatrices make significant contributions to the chromospheric and coronal heating. Since a small part of the magnetic flux in a sunspot is involved in QSLs, the X-ray emission above the sunspot is usually week. This result gives an explanation why X-ray emission is almost missing in sunspot umbra (Pallavicini et al., 1979). It is clear that the magnetic shear and emerging magnetic the exact heating mechanism in BCFs remains unknown. heating is necessary in the future.
flux are related to enhanced coronal heating, but A comprehensive study of the enhanced coronal
ACKNOWLEGEMENTS The numerical for hospitality NSFC grants.
computations were carried out on VPP300/16R during his stay as a Foreign Research Fellow.
at NAOJ. H. N. Wang is grateful to NAOJ H. N. Wang and Y, Yan are supported by
REFERENCES Demoulin, P. and Priest, E. R., The Importance of Photospheric Intense Flux Tubes for Coronal Heating, Solar Phys., 175 123 (1997). Falconer, D. A., R. L. Moore, J. G. Porter, G. A. Gary, and T. Shimizu, Neutral-Line Magnetic Shear and Enhanced Coronal Heating in Solar Active Regions, Astrophys. J.,482, 519 (1997). Pallavicini,
R., G. S. Vainana,
Astrophys.
G. Totani,
and M. Felli, The Coronal Atmosphere
above Solar Active regions,
J., 229, 375 (1979).
E. R. and P. Demoulin, Three-Dimensional Magnetic Reconnection without Null Points, I, J. Res., 100, 23443 (1995). Priest, E. R., C. R. Foley, J. Heyvaerts, 2‘. D. Arber, J. L. Culhane, and L. W. Acton, Nature of the Heating Mechanism for the Diffuse Solar Corona, Nature, 393, 545 (1998).
Priest,
~~~phys.
Sakurai,
T., K. Ichimoto,
Y. Nishino,
K. Shinoda,
M. Noguchi,
et al., Solar Flare
Telescope
at Mitaka,
P. A. S. J., 47 81 (1995).
Tsuneta, S., L. Acton, M. Bruner, J. Lemen, W. Brown, et al., The Soft X-Ray Telescope for the SOLAR-A Mission, Solar Phys., 136 37(1991). Wang, H. N., T. Sakurai, Y. Yan, and Y. Liu, The extrapolated 3D Solar Magnetic Field in AR7321, Proceedings of Numerical Astrophysics 1998, in press (1998). Yan, Y. and T. Sakurai, Analyses of Yohkoh SXT Coronal Loops and Calculated Force-Freee Magnetic Field Lines from Vector Magnetogra~, Solar Phys., 174, 65 (1997). Yan Y. and T. Sakurai, New Boundary Integral Equation Presentation for Finite Energy netic Fields in Open Space above the Sun, Astrophys. J., submitted (1998).
Force-Free
Mag-