ASTRONOMY & ASTROPHYSICS OCTOBER II 1996, PAGE 205

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Astron. Astrophys. Suppl. Ser. 119, 205-210 (1996)

Magnetic configuration of a sunspot region using the Hβ line

Hongqi Zhang

Beijing Astronomical Observatory, Chinese Academy of Sciences Beijing 100080, China

Received June 6, 1995; accepted February 12, 1996

Abstract. — The configuration of the magnetic field in an active region is analysed using a set of Hβ chromosphericmagnetograms and narrow pass-band filtergrams taken at various positions on the line. We found that the chromo-spheric magnetic field is parallel to the chromospheric features and inhomogeneous. The photospheric magnetic fielddisappears abruptly in the edge of the penumbra; the chromospheric field extends beyond the penumbral boundarywith an inclination of about 10◦−20◦ to the solar surface. Chromospheric superpenumbral filaments show a goodcorrelation with the chromospheric magnetic field and their configurations are related to the photospheric magneticstructures nearby the sunspot. Part of the magnetic flux of the sunspot returns to the neighbouring enhanced networks,which is almost vertical to the solar surface.

Key words: Sun: magnetic fields; chromosphere; sunspots

1. Introduction

Most models of the configuration of the magnetic fieldin solar active regions were established using observa-tions of the photospheric magnetic field and their ex-trapolation. The configuration of the magnetic field alsocan be deduced from magnetograms obtained in spectrallines formed in different solar atmospheric layers. Mea-surements of the chromospheric magnetic field were madeby Severny & Bumba (1958), Giovanelli (1980), Giovanelli& Jones (1982), Zhang et al. (1991 and 1993), Dara et al.(1993). The influence of the photospheric blended lines inthe wing of the Hβ line for measurements of the chromo-spheric magnetic field has been analyzed by Zhang (1993).Analyses of the chromospheric magnetic field are few rela-tive to photospheric ones, since most chromospheric linesare not highly sensitive to the magnetic field and chromo-sphere departures from local thermodynamic equilibrium.

The preliminary analyses based on the measurementof the magnetic field at various wavelengths in the wingof a single chromospheric line, such as the Hβ line, weremade by Zhang (1993, 1994). The band pass of the filterin the Huairou magnetograph is 0.12 Å. It is much nar-row than the width of the Hβ line. As we analyse a seriesof chromospheric magnetograms and corresponding filter-grams obtained at the various wavelengths from the wingto the center of the chromospheric Hβ line, we can es-timate the possible formation layers of the magnetic fieldand the relationship with the monochromatic images fromthe photosphere to chromosphere (Zhang 1996).

In this paper, we present the configuration of the mag-netic field in the active region based on photosphericvector magnetograms and chromospheric magnetogramswith different wavelengths in the Hβ line obtained at theHuairou Solar Observing Station of the Beijing Astronom-ical Observatory. In Sect. 2, we show the observationaldata. In Sect. 3, we analyse the formation of the Hβ lineand the relationship with chromospheric magnetograms.In Sect. 4, we discuss the possible spatial configuration ofthe magnetic field in sunspots and enhanced networks.

2. Observational data

A series of chromospheric and photospheric filtergramsand corresponding magnetograms in an αp active regionwere observed in the end of August 1993, at the HuairouSolar Observing Station of the Beijing Astronomical Ob-servatory. The preliminary results of this active regionwere presented by Zhang (1994). Figure 1 shows a pho-tospheric image and a photospheric magnetogram in thisregion (S3.2, W57) on August 27, 1993. We can find thatthe sunspot of this region is positive and a part of themagnetic field shows negative polarity on the west side ofthe sunspot, due to the projection effect of the magneticfield. Enhanced networks of negative polarity are locatedon the east side of the sunspot.

Figure 2 shows a set of Hβ filtergrams at differ-ent wavelengths with an interval of 0.04 Å, between−0.40 Å and −0.12 Å from the line center. In the fil-tergram at Hβ−0.40 Å, the chromospheric features arenot distinct. It is similar to the photospheric image. We

206 Hongqi Zhang: Magnetic configuration of a sunspot region using the Hβ line

Fig. 1. A unipolar sunspot region on Aug. 27, 1993. A photospheric image (left) and a longitudinal magnetogram (right), white(black) is positive (negative) polarity. North is at the top, west is at the right

notice that as the observed wavelength in the set of filter-grams approaches the Hβ line center, chromospheric fea-tures become significant. The dark filaments extend andtheir lengths increase from outside the sunspot penumbrain the west of the sunspot. These filaments constitute thesuperpenumbral features. On the east side of the sunspot,there is no evidence of filament-like features. It is possi-ble that the spatial resolution of these images is lower, sochromospheric fine features cannot be found.

Figure 3 shows a set of chromospheric longitudinalmagnetograms. Filament-like magnetic features in themagnetograms show good correlation with features in thechromospheric filtergrams. The lengths of chromosphericmagnetic filament-like features f1 and f2 increase withdark filaments as observed wavelengths shift to the Hβline center. These chromospheric features thus reflect in-formation on the magnetic field formed in the correspond-ing atmospheric layers.

3. Data analysis

The formation of the Hβ line in the chromospheric mag-netic field was analysed by Zhang (1996). The emergentlight of Hβ Stokes parameters mainly comes from bothlayers, chromosphere and photosphere, but there is littlecontribution from the temperature minimum zone. Theemergent Stokes parameters near the Hβ line center formmainly in the chromospheric layer and those in the wingform in the photospheric layer. As the observed wave-length of the Huairou Magnetograph shifts from the wingof the Hβ line to the line center, the emergent light formedin the photosphere decreases gradually and that formedin the chromosphere increases. From magnetograms ob-served at various wavelengths from the wing of the line tothe center, we can estimate the possible configuration ofthe magnetic field.

The real formation layers of Stokes parameters of theHβ line are more complex. The formation heights of dif-

ferent kinds of solar structures observed in the Hβ imagesmay be different, such as sunspot umbrae, penumbrae anddark filaments, even though these features are observed atthe same wavelength in the wing of the chromospheric line.

The disturbance of photospheric blended lines in thewing of the Hβ line is significant for the measurement ofthe magnetic field in sunspot umbrae. The evident influ-ence of the blended line FeI λ4860.98 Å in the V profileof the umbra was discussed by Zhang (1996). The reversalstructures relative to the photospheric field can be foundin the Hβ chromospheric magnetograms near such wave-lengths. The influence of the chromospheric mass motionis a notable problem for the measurement of the chromo-spheric magnetic field. The Doppler motion of chromo-spheric mass shifts the line. If the reversal structures inchromospheric magnetograms are only caused by the massmotion, we should obtain these reversal structures at a se-ries of wavelengths in the wing of the shifted Hβ line. Thisis not consistent with our results presented above. How-ever, due to the violent relative motion between the photo-spheric and chromospheric masses in the sunspot umbrae,the disturbance of the photospheric blended lines probablybecomes more significant.

From a series of magnetograms obtained at variouswavelengths in the wing of the wide chromospheric lines,such as Hα and Hβ lines, we can understand the gradualchange of the magnetic field from the photosphere to thechromosphere, except in sunspot umbrae, where the dis-turbance of photospheric blended lines is significant. Wenotice that the magnetic sensitivity of the line changeswith wavelength. For example, near the Hβ line centerthe sensitivity is weaker than that in the wing of the line.

Hongqi Zhang: Magnetic configuration of a sunspot region using the Hβ line 207

Fig. 2. A set of chromospheric filtergrams of the unipolar sunspot region at wavelengths −0.40, −0.36, −0.32, −0.28, −0.24,−0.20, −0.16 and −0.12 Å from the Hβ line center at 0258 – 0313 UT on Aug. 27, 1993

208 Hongqi Zhang: Magnetic configuration of a sunspot region using the Hβ line

Fig. 3. A set of chromospheric longitudinal magnetograms of the unipolar sunspot region at wavelengths −0.40, −0.36, −0.32,−0.28, −0.24, −0.20, −0.16 and −0.12 Å from the Hβ line center at 0258 – 0313 UT on Aug. 27, 1993. White (black) is positive(negative) polarity

Hongqi Zhang: Magnetic configuration of a sunspot region using the Hβ line 209

4. Configuration of the chromospheric magneticfield

4.1. Perspective effect of the magnetic field

Since the active region on August 27, 1993 was locatednear the west limb of the solar disk (S3.2, W57), the an-gle between the longitudinal and the horizontal magneticfields (on the solar surface) is about 30◦ in the plane whichcontains the line-of-sight and the normal to the solar sur-face. This means that some features in the longitudinalmagnetograms reflect mainly information on the horizon-tal components of the magnetic field, while the transver-sal components of the magnetic field reflect mainly thevertical ones relative to the solar surface. Figure 4 showsa photospheric vector magnetogram in the active region.The pattern in the chromospheric magnetograms obtainedat Hβ−0.40 Å is similar to the photospheric one. A set ofchromospheric magnetograms obtained at various wave-lengths from the wing of the Hβ line to the center can beused to infer the variation of the line-of-sight componentsof the magnetic field with height in the chromosphere.

Fig. 4. A vector photospheric magnetogram of the unipolarsunspot region on Aug. 27, 1993. The solid (dashed) contourscorrespond to positive (negative) fields of 20, 40, 80, 160, 320,640, 960, 1280 Gauss

4.2. Inclination angles of the superpenumbral magneticfield

The observational results show that the magnetic field dif-fuses from the sunspot. The question is how do the mag-netic lines of force incline to the solar surface in or beyondthe edge of the sunspot penumbrae. Different results wereobtained by various authors (e.g. Giovanelli 1982; Schmidtet al. 1986; Jahn 1989; Solanki & Schmidt 1993). Figure4 shows the photospheric vector magnetogram in the ac-tive region. The transversal magnetic field extends fromthe sunspot center and connects with a magnetic struc-ture of opposite polarity on the west side of the sunspot,

due to the projection effect of the magnetic field. In thewest side of the sunspot, the photospheric magnetic fielddisappears abruptly at the edge of the penumbra, whilechromospheric magnetic features exhibit rapid spreadingfrom the spot with no evidence that the magnetic fluxreturns to the neighbouring region as the observed wave-length goes to the Hβ line center. If the Hβ line centerforms at 1600 km (Zhang 1996) and comparing with thephotospheric vector magnetogram, the inclination angleof the magnetic field at the boundary of the sunspot isabout 10◦−20◦ relative to the solar surface, from the su-perpenumbral interior boundary to the outer one.

On the east side of the sunspot, the chromosphericmagnetic neutral line is close to the sunspot, and it doesnot show any significant shift with height. The photo-spheric transversal magnetic field connects the magneticfeatures N1 and S1 of opposite polarity. There are noevident superpenumbral features. We can infer that themagnetic lines of force bridge over the magnetic neutralline. The magnetic lines of force come from the sunspotand return to the solar surface near the outer edge of thesunspot. The magnetic field takes the return-flux model(Osherovich 1982).

4.3. Magnetic field in enhanced networks

Comparing with the photospheric vector magnetogram(Fig. 4) we find that the noise of the chromospheric mag-netograms is higher than the photospheric ones. The finefeatures of the magnetic networks are not distinct. Sincethis active region was located near the solar limb, the net-work magnetic field shows evident transversal componentsin the photospheric vector magnetogram. The chromo-spheric magnetic features of enhanced networks on theeast side of the sunspot exhibit almost the same patternat different wavelengths from the wing to the center ofthe Hβ line. The reason is that, in the chromosphere, thelarge-scale network magnetic field near this active regiondoes not contain an obvious horizontal component. There-fore, the large-scale enhanced network magnetic field nearthis active region is almost vertical to the surface. Eventhe small-scale magnetic features in networks consist offibril-like magnetic features as demonstrated by Zhang etal. (1991). Zhang et al. indicated that some magnetic linesof force extend from the network magnetic elements to theupper solar atmosphere and some lines of force return tothe neighbouring regions in the lower atmosphere.

4.4. Correlation between chromospheric features andmagnetic field

In comparison of the chromospheric longitudinal magne-tograms with corresponding filtergrams, we find that, asthe observational wavelengths change from the wing of theHβ line to the center, the pattern in chromospheric mag-netograms gradually departs from the photospheric one,

210 Hongqi Zhang: Magnetic configuration of a sunspot region using the Hβ line

and chromospheric filament-like magnetic features appear.The superpenumbral dark filaments show a good rela-tionship with the chromospheric magnetic features, e.g.in the chromosphere, dark superpenumbral features showa stronger magnetic horizontal field. The correlation be-tween the magnetic field and the monochromatic imagesin the chromosphere is better than the photospheric ones.

5. Discussion

It is evident that the spatial configuration of the chromo-spheric magnetic field in the sunspot region is connectedwith the surrounding magnetic structures observed in thechromospheric magnetograms. The chromospheric super-penumbral magnetic field on the west side of the sunspot isshallow and changes on both sides of the boundary surfaceof the superpenumbra relative to the surrounding atmo-sphere. The opposite case can also be found. A part ofpenumbral magnetic flux on the east side of the sunspotreturns to the enhanced network near the boundary of thesunspot and forms a return-flux model. This means thatthe extension of the magnetic flux from the sunspot iscomplex. A similar situation was also discussed by Zhanget al. (1991) from chromospheric Hβ magnetograms andHα filtergrams. The magnetic canopy model (Giovanelli1982) and the return-flux model (Osherovich 1982; Fl̊a etal. 1982) also reflect the distribution of the sunspot mag-netic field.

Moreover, since the magnetic field chenges obviouslyon both sides of the superpenumbral boundary surfacethe annular electric current probably forms in the sunspotpenumbral interface to the non-magnetic atmosphere.

6. Summary

Following analyses of the configuration of the field in anαp active region with a series of chromospheric filtergramsand corresponding chromospheric magnetograms obtainedat different wavelengths in the Hβ line, the main resultsare:

(1) The distribution of magnetic fields in different solaratmospheric layers can be obtained with a single chromo-spheric line, such as the Hβ line, because emergent Stokesparameters in the Hβ line wing form mainly in the lowersolar atmosphere while the emergent Stokes parametersnear the Hβ line center form mainly in the higher atmo-sphere.

(2) A good correlation between sunspot superpenum-bral dark filaments and chromospheric magnetic featuresis found. As the observational wavelength changes fromthe wing of the Hβ line to the center, changes of super-penumbral dark filaments accompany the increase of thechromospheric filament-like magnetic features, e.g. chro-mospheric features reflect the distribution of the magneticfield. The chromospheric filament-like magnetic field ex-tends at small angles to the solar surface.

(3) The extension of the sunspot chromospheric mag-netic field depends on the distribution of the surround-ing photospheric magnetic field. In our presented activeregion, a part of the sunspot magnetic flux returns tothe neighbouring enhanced networks consistent with thereturn-flux model.

(4) In enhanced networks of this active region, thelarge-scale magnetic field is almost vertical to the solarsurface and connects with the neighbouring sunspot field.

Acknowledgements. The author is grateful to staff at HuairouSolar Observing Station for their support in these observations.He also thanks Dr. S. Koutchmy for helpful comments and sug-gestions. This research was supported by the Chinese Academyof Sciences and National Science Foundation of China.

References

Dara H.C., Koutchmy S., Alissandrakis C.E., 1993, A&A 277,648

Fl̊a T., Osherovich V.A., Skumanich A., 1982, ApJ 261, 700Giovanelli R.G., 1980, Solar Phys. 68, 49Giovanelli R.G., Jones H.P., 1982, Solar Phys. 79, 267Jahn K., 1989, A&A 222, 264Osherovich V.A., 1982, Solar Phys. 77, 63Schmidt H.U., Spruit H.C., Weiss N.O., 1986, A&A 158, 351Severny A.B., Bumba V., 1958, Observatory 78, 33Solanki S.K., Schmidt H.U., 1993, A&A 158, 351Zhang H., 1993, Solar Phys. 146, 75Zhang H., 1994, Solar Phys. 154, 207Zhang H., 1996, (Submitted to A&A)Zhang H., Ai G., Sakurai T., Kurokawa H., 1991, Solar Phys.

136, 269Zhang H., Ai G., Li W., Chen J., 1993, Solar Phys. 146, 61