Useful Aspects of Chromospheric Magnetic Field Data

USEFUL ASPECTS FIELD DATA

OF C H R O M O S P H E R I C

MAGNETIC

T. Sakurai 1, Debi Prasad Choudhary 1,2, and P. Venkatakrishnan 2

1National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan 2 Udaipur Solar Observatory, Physical Research Laboratory, Udaipur, 313 001, India

ABSTRACT Some useful aspects of chromospheric magnetic field data are discussed. Ca II 854.2 nm chromospheric magnetograms obtained at NSO Kitt Peak were analyzed. The magnetograms refer to a height of 800 km above the photosphere. The correlation plot between the observed chromospheric field and the extrapolated potential field often indicated deviations (non-potentiality) in regions of strong magnetic field exceeding i 5 0 0 G. This feature can be used as an indicator of flare productivity of the region. The chromospheric magnetograms can also be used to resolve the azimuth ambiguity of photospheric vector magnetograms. INTRODUCTION Most of the observational studies of the active region magnetism so far have been based on the magnetic field measurements at the photospheric level. The purpose of this paper is to suggest the importance of additional information provided by the measurement of the chromospheric magnetic fields. Routine observational data are available from the National Solar Observatory, Kitt Peak, using the 854.2 nm line of Ca II, and from Huairou Solar Observing Station, Beijing Astronomical Observatory, using the H~ line. REPRESENTATIVE HEIGHT OF CHROMOSPHERIC MAGNETOGRAMS At NSO Kitt Peak, both the photospheric magnetograms of Fe I 868.8 nm line and the chromospheric magnetograms of Ca II 854.2 nm line are routinely obtained. The formation height of Ca II line is estimated to be about 800 km above the photosphere (Harvey et al., 1999). In order to confirm this formation height, we extrapolated the photospheric magnetogram assuming a potential field model, and compared it with the observed chromospheric magnetogram. Figure 1 shows one of such examples. Panel (a) is the correlation plot between the observed photospheric field and the observed chromospheric field. Panels (b), (c), and (d) are, respectively, the potential field evaluated at a height of 200 km, 800 km, and 1400 km. It can be seen that panel (c) matches best with panel (a). Therefore, the formation height of 800 km estimated from a model atmosphere is adequate. Similar analysis has been carried out on several other regions and we obtained the same results. CHROMOSPHERIC MAGNETOGRAMS AND FLARE PRODUCTIVITY In the study of flare build-up process, one usually invokes the magnetic shear in the photospheric vector magnetograms, which is defined as the azimuth difference between the observed transverse magnetic field vector and the computed potential-field vector (Hagyard et al., 1984). Sakurai et al. (1992) quantitatively demonstrated a flare-related shear decrease in a 2B/M4.4 flare based on soft X-ray images and vector

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UsefulAspects of ChromospherwMagneticFieMData magnetograms. Wang et al. (1994) studied five flares of GOES X-ray class X and found, on the contrary, that the magnetic shear increased along a substantial portion of the neutral line during the flares. With this controversial situation in mind, Li et al. (2000a, b) made a thorough study of eight active regions, and found that the flare-producing active regions can be classified into two groups. In five among eight regions, the shear remained moderate while vigorous emerging flux activity was seen. In the other three active regions, strong magnetic shear was observed. Even in the latter group, the shear along the neutral line showed little changes. However, the shear in the flaring regions indicated by Ha flare ribbons decreased unambiguously. Their interpretation is that, if the region is accompanied with vigorous emerging flux, the energy injection and disturbances from flux emergence triggers the flare before the shear develops significantly. Only if the emerging flux activity is moderate or has been declined, the shear may develop into a critical stage and leads to a flare. On the other hand, the connection between flare activity and chromospheric magnetic fields has been pointed out. Based on the magnetic fields measured with the HZ line at Huairou Solar Observing Station, Zhang (1993) and Liu et al. (1995) found that the flare-producing active regions show chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric magnetic field. Choudhary et al. (2001) recently studied the evolution of chromospheric magnetic fields and found its useful correlation with flare activity. Using correlation plots similar to Figure 1, they compared the observed chromospheric field and computed potential field at the chromospheric height (800 km above the photosphere) in 137 active regions. In the weak-field range, within 4- 300 G, most of the observed field was close to the potential field. For field values exceeding + 500 G, the observed field showed deviations from the potential field, namely the chromospheric field was found to be stressed and 'non-potential.' This degree of non-potentiality was correlated with the flare productivity of the regions. They studied eight long-lived active regions, which made multiple disk passages, and found the strong-field non-potentiality during their initial phase. During the decaying phase, the chromospheric field converged to potential field configuration. RESOLUTION OF AZIMUTH AMBIGUITY Vector magnetograph observations leave the ambiguity of 180 ~ in azimuth of the transverse components of the field, Bt. In order to resolve this ambiguity, one needs some additional assumption. Wu and Ai (1990) proposed to use the div B = 0 condition,

OBz OBz OBy Oz = Oz ~ Oy"

(1)

We take the xy-plane as the photosphere, with the z-axis directed upward. By multiplying with this equation can be written as

( OBz

: OBz (

+

OBz/OZ,

(2)

I

The left-hand side of this equation is negative, while the right-hand side changes sign if the direction of Bt is reversed. In the right-hand side, the sign of OBz/OZ can be known if one uses both photospheric and chromospheric magnetograms to derive OBz/OZ. The limitation in this method is that it is only applicable to magnetograms near the disk center. Observed magnetograms are represented in the coordinate system in which the XY-plane is the plane of the sky and the Z-axis is directed along the line of sight. When regions off the disk center are considered and Eq. (2) is transformed to the observing coordinates (X, Y, Z), it involves quantities OBx/OZ and OBy/OZ, which are not available. We can construct a method which removes the restriction in Wu and Ai (1990)'s method. If in the local heliocentric frame the force-free condition is satisfied, we obtain the x- and y-components of force-balance equation as

(0

0)

1 0 (B2y+B2z) = By-~y+Bz--~z Bx 20x 2100y ( B 2 + B 2~, : ( ~_~ + Bz_~ zO)

(3)

(4)

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T. Sakurai et at With the help of these equations, we can write down the div B = 0 condition in a form which only involves observable quantities, as 0 =

= + +

+

(n. B) divB

(n.B)(OB ~., x

10Bz nz OX

+ ~OBy~ .,~, / + ( n x B ) z ( O B y

OY

[nx(n. B) - n y ( n • B)z]

10Bz

[ny(n. B ) + n x ( n • B)z] nz OY ( n . B ) OBz n2z OZ "

(5)

where n = (nx, ny, nz) is the unit vector normal to the solar surface z = 0, and (n x B ) z -- n x B y - n y B x . Because the value of OBz/OZ is obtained from photospheric and chromospheric magnetograms, we can select a better sign of Bt based on Eq. (5). If the region is not too far from the disk center, the expected values of OBz/OZ derived from two possible signs of Bt will have opposite signs, and there will be little ambiguity in picking up the correct solution. SUMMARY It has been known that chromospheric magnetograms often indicate a more active nature of the magnetic field in the chromosphere compared to the photosphere. In this paper we have discussed some useful aspects of the chromospheric magnetograms in quantitative studies. Continuation of routine observations of chromospheric magnetograms is highly encouraged. ACKN OWLED G EMENTS One of the authors (DPC) acknowledges the financial assistance and kind hospitality provided by the National Astronomical Observatory of Japan during his stay at Mitaka. The photospheric and chromospheric magnetograms used in this paper are provided by the National Solar Observatory, Kitt Peak, USA. REFERENCES Choudhary, D. P., T. Sakurai, and P. Venkatakrishnan, Chromospheric Magnetic Field of Solar Active Regions, Astrophys. Y., 560, 439 (2001) Hagyard, M. J., J. B. Smith Jr., D. Teuber, and E. A. West, A Quantitative Study Relating Observed Shear in Photospheric Magnetic Fields to Repeated Flaring, Solar Phys., 91, 115 (1984) Harvey, J., T. Bippert-Plymate, D. Branston, C. Playmate, F. Recely, and H. Jones, Large-Scale Chromospheric Magnetic Fields, Bull. American Astron. Soc., 194, 94.06 (1999) Li, H., T. Sakurai, K. Ichimoto, and S. UeNo, Magnetic Field Evolution Leading to Solar Flares I. Cases with Low Magnetic Shear and Flux Emergence, Publ. Astron. Soc. Japan, 52, 465 (2000a) Li, H., T. Sakurai, K. Ichimoto, and S. UeNo, Magnetic Field Evolution Leading to Solar Flares II. Cases with High Magnetic Shear and Flare-Related Shear Changes, Publ. Astron. Soc. Japan, 52, 483 (2000b) Liu, Y, N. Srivastava, D. Prasad, W. Li, and G. X. Ai, A Possible Explanation of Reversed Magnetic Field Features Observed in NOAA AR 7321, Solar Phys., 158, 249 (1995) Sakurai, T.; K. Shibata, K. Ichimoto, S. Tsuneta, and L. W. Acton, Flare-Related Relaxation of Magnetic Shear as Observed with the Soft X-ray Telescope of Yohkoh and with Vector Magnetographs, Publ. Astron. Soc. Japan, 44, L123 (1992) Wang, H., M. W. Ewell Jr., H. Zirin, and G. Ai, Vector Magnetic Field Changes Associated with X-Class Flares, Astrophys. J., 424, 436 (1994) Wu, L., and G. Ai, Acta Astrophys. Sinica, 10, 371 (1990) Zhang, H., Structures of Chromospheric Magnetic Fields in the Solar Flare Producing Active Region 5747, Solar Phys., 144, 323 (1993)

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