a comparison of euv spectroheliograms and photospheric magnetograms

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Page 1: A comparison of EUV spectroheliograms and photospheric magnetograms

A C O M P A R I S O N O F E U V S P E C T R O H E L I O G R A M S A N D

P H O T O S P H E R I C M A G N E T O G R A M S

JOSEPH B. GURMAN* and GEORGE L. WITHBROE

Center for Astrophysics, Harvard College Observatory

and

Smithsonian Astrophysical Observatory, Cambridge, Mass. 02138, U.S.A.

and

JOHN W. HARVEY Kitt Peak National Observatory**, Tucson, Ariz. 85717, U.S.A.

(Received 17 May; in revised form 22 October, 1973)

Abstract. A comparison of EUV data from the Harvard College Observatory experiment on OSO-6 with photospheric magnetograms from Kitt Peak National Observatory indicates a bipartite rela- tionship between values of the longitudinal field strength B and Mg x intensity I averaged over square areas 35" • 35": in quiet regions IB/~ I s, where 0.0<~ k< 0.3, and in active regions IB[ "~ I. From these relationships we infer that ]B[ ~ ne 2~ in quiet regions and ]B[ ~ ne 2 in active regions. In addition, the photospheric field beneath a coronal hole is found to be virtually identical to that beneath normal quiet regions.

1. Introduction

The overall structure of the solar corona seen during eclipses and the apparent domi-

nance of magnetic pressure over gas motions in the chromosphere and lower corona

have long suggested that magnetic fields determine the structure of the upper solar atmosphere. Observations at visible, extreme ultraviolet (EUV), and X-ray frequencies

as well as theoretical considerations support this view (cf Howard, 1967, 1971). With the advent of rocket and satellite-borne instrumentation it has become possible to

compare directly features observed in the corona with the structure of the underlying

photospheric magnetic field. A comparison of OSO-4 spectroheliograms and Mt.

Wilson magnetograms indicates a monotonic relationship between photospheric

magnetic field strength and EUV intensity of spectral lines and continua formed in the chromosphere and corona (Noyes and Withbroe, 1972).

In the present study we explore in more detail the relationship between intensity and

field strength for a coronal line, Mg x 2625, through use of data from the Harvard College Observatory (HCO) experiment on OSO-6 and the 40-channel magnetograph of the McMath solar telescope at Kitt Peak National Observatory (KPNO). From

these data we infer distinctly different relationships between the strength of the photo-

spheric magnetic field and density in the overlying corona in quiet and in active regions.

* Now at Dept, of Physics and Astrophysics, University of Colorado. ** Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.

Solar Physics 34 (1974) 105-111. All Rights Reserved Copyright �9 1974 by D. Reidel Publishing Company, Dordrecht-Holland

Page 2: A comparison of EUV spectroheliograms and photospheric magnetograms

I 0 6 JOSEPH B. GURMAN ET AL.

2. Observations and Analysis

Full-disk K P N O magnetograms in the Zeeman-sensitive photospheric Fe I line at

5233.0 A were obtained by W. Livingston on 1970 February 27 (1514 UT), March 6

(1535 UT), March 7 (2057 UT), and March 9 (1516 UT). Photographic representa-

tions o f the February 27 and March 7 data are reproduced in Figure 1.

Fig. 1. Photospheric rnnagnetograrns and Mg x spectroheliograms for 27 February and 7 March 1970. In the rnagnetograms longitudinal fields B Jr of negative polarity are indicated dark; positive polarity, light. In the spectroheliograrns the darkest areas correspond to regions of greatest EUV in- tensity. Mg x data near the limb were not used. The circles surrounding the spectroheliograrns

indicate the approximate position of the limb.

Page 3: A comparison of EUV spectroheliograms and photospheric magnetograms

A C O M P A R I S O N OF E U V D A T A 1 0 7

Several HCO OSO-6 spectroheliograms in the coronal Mg x line at 625 A were also obtained during this period. Photographic representations of spectroheliograms for 1970 February 27 and March 7, constructed from digital data through use of a computer-driven cathode ray tube display, are given in Figure 1. For description of the KPNO and HCO instruments and their operation, see, respectively, Livingston and Harvey (1971), Livingston et al. (1971), and Huber et al. (1973). In order to provide a better visual comparison with the magnetograms, the limb brightening (cf. Withbroe, 1970, 1972) has been removed from the Mg x data.

As a first approximation, the intensity of the Mg x line is proportional to the square of the coronal electron density. Consequently, the excellent correlation between areas of enhanced Mg x intensity and areas of enhanced magnetic field strength suggests the existence of a relationship between coronal electron density and the strength of the photospheric magnetic field.

Both the spectroheliograms and the magnetograms consist of digital arrays, but disparities in resolution, array dimension, and array orientation with respect to solar north, as well as differences of several horn's in the times of data acquisition, had to be reconciled before close comparison of these data could be undertaken. We then aver- aged the higher-resolution (2.38") magnetic data over the area of each (35" • 35") EUV spectroheliogram element to obtain the magnetic parameter

< [BI >,j = ~ ~ IBim=, (1) m /t

for all magnetogram elements m, n falling within each spectroheliogram element i, j. The resulting array of <[BI> we call a magnetoraster. Because of the noise N~ + 15 G per element in the magnetic data, averaging [BI leads to a rise in the mean field by an amount equal to the mean value of INI; our results suggest that the mean value of INI is approximately 10 6.

Figures 2 illustrates how the strength of the photospheric longitudinal magnetic field, as measured by the parameter <IB]>, varies as a function of the intensity of the Mg x 2625 line for each of four magnetorasters constructed by the above method. The length of the error bars indicates the standard deviation from the mean. As in Figure 1, the Mg x data have been corrected for the influence of limb brightening. In addition, to avoid possibly misleading effects from the application of Mg x limb- brightening corrections that may be invalid near the limb, Fe ~ )~ 5233 limb darkening, and the loss of spatial resolution near the limb, we used only those points with/~ = =cos0>~0.8, where 0 is the angle between the line of sight and the vector from the center of the Sun to the surface point.

The most obvious feature of the four graphs in Figure 2 is the division of each curve into two distinct regions. In the region with l og / (Mg x)< 1.9, the curves relating log <lBl> and log I (Mg x) have small slopes. The data suggest that

log< IBI > ~ K log I (Mg x) + constant, (2)

where K has a value between about 0.0 and 0.3. For log I (Mg x)>~ 1.9 the data indi-

Page 4: A comparison of EUV spectroheliograms and photospheric magnetograms

108 JOSEPH B. GURMAN ET AL.

2.5

2.0

1.5

1.0 m

(-9 �9 J

2.(

t.5

1.0

[ f

FEB 28

J t MAR 7

{ {--,I~-! J I I

1.5 2.0

r

MAR 7

I I M A R 9

2.5 2.5 / / / /5 210

LOG INTENSITY MgXX625(erg sec -I cm2sr -I) Fig. 2. The relationship between the strength of the photospheric magnetic field [BJ and intensity of Mg x 2625 for four different sets of data. Each point with error brackets is the mean value of IBI

corresponding to Mg x intensities I over the range log(I)--0.05 and log(/)+ 0.05.

cate that log( IBI) ~ log I (Mg x) + constant, (3)

confirming the earlier findings of Noyes and Withbroe (1972), who compared OSO-4 spectroheliograms and Mt. Wilson magnetograrns.

3. Discussion

In another investigation (Withbroe and Gurman, 1973) we used observations of nine EUV lines ranging from N v 21239 to Fe xvz 2335 to develop a series of empiri- cal models for the chromospheric-coronal transition layer and lower corona in quiet and in active regions. From these models we have derived a relationship between the electron density in the lower corona and the intensity of the Mg x 2 625 line:

I ( M g x ) 1.l • 10 -16 2

Page 5: A comparison of EUV spectroheliograms and photospheric magnetograms

A COMPARISON OF ELW DATA 109

where I is the intensity at the center of the disk in ergs cm-2s-lsr -~ and n e is the electron density at the base of a hypothetical corona in which the density decreases exponentially with increasing height. For the range of Mg x intensities given in Figure 2, this simple equation yields intensities within 10% of those computed with the models.

Thus, the relationships (2) and (3) correspond to

IBI 2k n e , ( 4 )

where 0.0 <k <0.3 and log/(Mg x) < 1.9, and

2 IBI ~ ne (5)

for greater Mg x intensities. The latter relationship was first suggested by Newkirk (1972) on the basis of analyses of OSO-4 and Mt. Wilson data by Withbroe and Noyes (1971) and Noyes and Withbroe (1972). Since the division between quiet and active areas of the Sun occurs at about logI(Mg x)~2.0, relationships (4) and (5) refer respectively to quiet and to active regions. While magnetograph noise leaves the proportionalities (2) and (4) in some doubt, the data do suggest that the relationship between the coronal electron density and strength of the underlying photospheric magnetic field strength is significantly different in quiet and in active regions.

Pneuman (1973) suggests that the density structure of the corona is closely linked to the structure of the magnetic field and in particular depends on whether the coronal field configuration in a given area is open or closed. He demonstrates that closed field configurations result in high coronal densities and open field configurations produce low densities. Since open and closed field configurations are usually associated respect- ively with areas of weak and strong fields, the bipartite relationship between ]B] and ne could result indirectly from some relationship between the strength of the photo- spheric magnetic field and the configuration of the coronal field. Under this hypothesis, the relationship [BI ~n~ z would occur in regions with closed configurations and the other relationship between IB] and ne (Equation (4)) would occur in regions with open configurations.

This hypothesis could be investigated by comparing Mg • spectroheliograms with the structure of coronal magnetic fields calculated from photospheric magnetograms by, for example, the Schmidt (1964) technique. Since, however, data being acquired by the HCO experiment on ATM (Reeves et al., 1972) are far superior in spatial resolu- tion to the OSO-6 data, we feel it would be preferable to defer such an effort until the better data become available.

4. The Field below a Coronal 'Hole'

EUV and X-ray observations of the corona (e.g., Underwood and Muney, 1967; Burton, 1968; Tousey et al., 1968; Reeves and Parkinson, 1970) reveal the frequent occurrence of extended, well-defined areas of very low emission often referred to as coronal 'holes'. These features can also be found in white-light eclipse photographs (e.g., Withbroe et al., 1971) and K-coronameter data (Altschuler and Perry, 1972;

Page 6: A comparison of EUV spectroheliograms and photospheric magnetograms

110 JOSEPH B. GURMAN ET AL.

Altschuler et al., 1972; Perry and Altschuler, 1973). Recent analyses (e.g., Altschuler et aL, 1972; Munro and Withbroe, 1972; Withbroe and Wang, 1972) indicate that the coronal electron temperature and density are significantly lower in 'holes' than in normal quiet regions. The temperature is lower by 50% or more; the density is lower by a factor of 2 to 5.

A feature of this type was observed near the time of the 1970 March 7 solar eclipse (Withbroe et al., 1971). This 'hole' is outlined by the solid line in the February 27 spectroheliogram in Figure 1. The 'hole' does not show up particularly well in this figure because the contrast in the print was selected to illustrate the correlation between active regions in the magnetograms and in the spectroheliograms; the hole is much more prominent in overexposed prints. For more complete data on the hole and its surroundings see Withbroe et al. (1971).

An examination of the February 27 magnetogram in Figure 1 indicates that, com- pared with other quiet areas, there is no outstanding difference in the longitudinal photospheric field in the vicinity of the hole. We compared quantitatively the strength of the photospheric magnetic field beneath the hole with fields in other quiet regions, and found no statistically significant difference in the magnitudes of the fields. Alt- schuler et al. (1972), however, have shown that the coronal field in 'holes' diverges. The primary considerations therefore appear to be not the strength of the photo- spheric field underlying the hole but the distribution of the photospheric field across the surface in the vicinity of the hole, and whether or not this distribution yields a

field configuration that diverges in the corona.

Acknowledgements

We wish to express our appreciation to A. K. Dupree, R. H. Munro, and S. P. Smith for their helpful comments and suggestions, to W. Livingston for the magnetograms, and to J. L. Smith for the computer programs used in this analysis. This work was suppoited by the National Aeronautics and Space Administration through contract

NAS 5-9274 and Grant N G R 22-007-211.

References

Altschuler, IV[. D. and Perry, R. M.: 1972, Solar Phys. 23, 410. Altschuler, M. D., Trotter, D. E., and Orrall, F. W. : 1972, Solar Phys. 26, 354. Burton, W. M. : 1968, in K. O. Kiepenheuer (ed.), 'Structure and Development of Active Regions',

1AU Syrup. 35, 293. Howard, R. : 1967, Annual Rev. Astron. Astrophys. 5, 1. Howard, R. (ed.): 1971, 'Solar Magnetic Fields', 1AU Syrup. 43. Huber, IV[. C. E., Dupree, A. K., Goldberg, L., Noyes, R. W., Parkinson, W. H., Reeves, E. M., and

Withbroe, G. L. : 1973, Astrophys. J. 183, 291. Livingston, W. and Harvey, J. : 1971, in R. Howard (ed.), 'Solar Magnetic Fields', IA U Symp. 43, 51. Livingston, W., Harvey, J., and Slaughter, C. : 1971 in H. Sedden and M. J. Smyth (eds.), 'Automa-

tion in Optical Astrophysics', IAU Colloq. 11, 52. Munro, R. H. and Withbroe, G. L.: 1972, Astrophys. J. 176, 511. Newkirk, G.: 1972, in C.P. Sonett, P.J. Coleman, Jr., and J. M. Wilcox (eds.), Solar Wind,

Washington, U.S. Govt. Printing Office, p. 11.

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A C O M P A R I S O N OF E U V D A T A 111

Noyes, R. W. and Withbroe, G. L.: 1972, Space Sci. Rev. 13, 612. Perry, R. M. and Altschuler, M. D. : 1973, Solar Phys. 28, 435. Pneuman, G. W. : 1973, Solar Phys. 28, 247. Reeves, E. IV[. and Parkinson, W. H.: 1970, Astrophys. J. Suppl. 21, 1. Reeves, E. M., Noyes, R. W., and Withbroe, G. L.: 1972, Solar Phys. 27, 251. Schmidt, H. U. : 1964, in W. N. Hess (ed.), AAS-NASA Symposium on the Physics of Solar Flares,

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