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The structure and physical properties of the quiet corona as inferred from the July 11, 1991 solar eclipse data
Adv. Space Res. Voi. 14, No. 4, pp. (4)69-(4)72, 1994 Printed in Great Britain. All rights reserved.
0273-1177/94 $6.00 + 0.00 Copyright © 1993 COSPAR
THE STRUCTURE AND PHYSICAL PROPERTIES OF THE QUIET CORONA AS INFERRED FROM THE JULY 11, 1991 SOLAR ECLIPSE DATA J. Sykom,* P. Ambro~** and O. G. Badalyan*** * Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranslai Lomnica, Slovak Republic ** Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondrejov, Czech Republic *** Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, 142092 Troitsk, Russia
ABSTRACT We were successful in observing the linear polarization of the solar corona in the white light and in the light of the green emission line 530.3 nm during the July 11, 1991 total solar eclipse (La Paz, Mexico). Preliminary analysis reveals remarkable differences in the degree of polarization for both sets of data, particularly for the distribution of polarization around the sun's limb. The upper theoretical limits of the degree of polarization are compared with our results. The real structure of the corona is compared with the calculated coronal magnetic field. The rather unusual shape and structure of the eclipse corona in a given phase of of the solar cycle are discussed in connection with a definition of coronal flattening.
INTRODUCTION One of the most powerful methods of studying the solar corona is polarimetry. Polarization of coronal radiation provides information on the physical properties and characteristics of the corona and, furthermore, it is practically a unique source of information on coronal magnetic fields. At the same time, polarization is a very important test for the presence of one or a~n another mechanism of radiation under the physical conditions in the solar corona. Unfortunately, at this moment the greater part of the results discussed in this paper relates to the white-light corona only.
EQUIPMENT AND OBSERVATIONS Thanks to the exceptionally long duration of totality (six and a half minute in La Paz) it was possible to observe the white-light and green-line coronae with just one instrument, successively. We have used a telescope with an achromatic lens, 13 cm in diameter and focal length of 195 cm. For the green-line observation a special ultra-narrow-band 0.2 nm tunable filter (Andover Co., U.S.A.) was used to isolate the light of the FeXIV 530.3 nm spectral line. In both the types of observation a polaroid rotated in step of 45*. Kodak Tri-X Pan film was used to obtain two series (1/125 s and 1/15 s) of four images and one series (1 s exposure) of three images in the white light and one set of four images (30 s exposures) of the green-line corona. We have found the Kodak D-69 developer to be very suitable to reasonably suppress the high gradient of the coronal brightness with distance from the limb. All the data were subjected to extensive photographic, photometric and computer digitization processing. Acording to the well-known theory, when using a set of four images taken with the polaroid rotating by 45*, it is possible to directly determine (4)69
(4)70
J. Sykora et al.
the coronal brightness and the degree and direction of coronal polarization. The polarization of the green corona depends considerably on the direction of coronal magnetic fields. PRELIMINARY
ANALYSIS OF THE DEGREE
OF POLARIZATION
In the case of a spherically symmetrical corona the degree of polarization of the white-light corona should not exceed 50% under conditions of Thomson scattering. However, we have observed/1/ at the m a x i m u m of the solar cycle (eclipse of February 16, 1980) polarizations up to 55% in some well-developed streamers. Do not cast doubts on validity of Thomson scattering mechanism of coronal radiation it was necessary to assume a substantial concentration of coronal matter near the plane of the sky. This practically implies existence of the streamers at the sun's limb and, at the same time, their more or less radial orientation at the moment of the 1980 eclipse.
/
/
Fig. 1. Triad of equidensities (1 s exposures) and the derived degree of polarization. To visualize better the effect of polarization one equidensity of the unpolarized corona was added and plotted as the dashed line. The result which we present in Figure 1 indicates different circumstances in the case of the July 11, 1991 eclipse. The degree of polarization reached only 405{ in the best developed streamers (or systems of streamers) in the NE- and SW-quadrants. This can be understood when analysing the distribution of activity on the sun's surface. Simply, the observed streamers are in fact far from the plane of the sky, i.e. they are not located at the sun's limb. The NE-streamer is most probably well behind the east limb, while the system of streamers in the SW-quadrant has its foot points well in front of the west limb. The 45% polarization belongs to the coronal condensation just above an active prominence at the west limb. The lowest degree of polarizations (20%) are characteristic of the north-pole and west-limb coronal holes which are, of course, best detected on images taken through neutral radial filters(for example, those taken by S. Koutchmy, by Rhodes College and the H A O group). The SE-region near the south pole can hardly be considered a coronal hole because of its 35% degree of polarization. On the other hand, the calculated structure of the coronal magnetic field (see Figure 2) indicates coronal hole properties for this region.
Properties of the Quiet Corona
(4)'/1
If there are sometimes problems with the interpretation of the exceptionally high degree of whitelight corona polarization, then the problems with observed values of polarization in the green coronal line FeXIV 530.3 nm are, so to say, of a higher order. First, the theory of polarization in this line is very complicated. The most representative calculations /2/, allow degrees of polarization up to only 20%. Second, experimental measurements of polarization in the green line are far more delicate than they are in continuum. The published experimental results range from 70% to 0%. Almost the same dispersion of values was recorded by different observers during the same eclipse. Our preliminary calculations of the green corona polarization in the N, E, S and W radial directions does not indicate values exceeding 15-20%. The most interesting is probably the indication of almost zero green-line polarization at position angles 280°-300 °, where polarization of the white-light corona is the highest, i.e. 45% (see Figure 1). Further, very careful analysis of the green-line data is now in progress. COMPARISON OF THE OBSERVED CORONAL STRUCTURES
AND
CALCULATED
It is certainly interesting to see how the calculated structure of the coronal magnetic field can be fitted to observed structures during the July 11, 1991 total solar eclipse. As usual, we have carried out the calculations by using the measurements of solar magnetic fields made at the John Wilcox Observatory of the Stanford University. One of the authors (P.A.) is very much obliged to Dr. J. Todd Hoeksema for providing him with the set of harmonic coefficients prior to their pubM~K( JUt. 11.1Wl tO1.tl~ t t ~ ~aDtB L :1m * 8:~0
h:4.~
~0AIA AJI¢ 2 L l m l 10 JUt. 25,1Mt
Fig. 2. Comparison of the calculated coronal magnetic field with a real structure of the July 11, 1991 solar corona. Fof deteails refer to text. Hypothetical position of the heliograpl~c current sheet (HCS) is outlined considering distribution of the coronal structures around the sun's limb. lication. In our calculations we prefer the radius of the source surface Rs = 4.5 I~. The reasons for this will be given in a separate paper. W e would like to comment only that namely for this value of the source surface radius a very good correlation between the calculated field line structures and the large-scale coronal structures of the middle and outer corona has been found. As for the proper
(4)72
J. Sykora et al.
structure of the solar corona and structure of the coronal magnetic fields (see Figure 2) we consider it necessary to emphasize that these are not, of course, identical features, i.e. the presence of the lines of force does not necessarily mean the presence of radiating plasma. Further, any magnetic field line of force is only a form of representing the magnetic field, explicitly it tells us anything about the intensity of the field. The intensity of the field can be inserted into the model pictures indirectly, for example by starting the calculation of the lines of force from a certain intensity of the field at an initial point of those lines of force. In this way some structures can be made visible and others can be suppressed. Two variants of calculations were performed under the same boundary conditions. In the left part of Figure 2 the calculation begins at r = 1.0 R® at points regularly distributed throughout the heliographic network (ten by ten degrees). The closed and open structures are formed according to the real character of the coronal magnetic fields. At the right, the calculation begins at the source surface (4.5 R®) and proceeds to radius r = 1.0 R®. In this case all the lines of force are open and their foot points in the photosphere probably indicate the position and contours of coronal holes. Both presentations are equally significant and it is desirable to superimpose them. It is only reasonable to compare this composite picture with the observed structure of the coronal features. The sketch of the July 11, 1991 eclipse coronal structure (at the bottom of Figure 2) has been provided by Dr. Serge Koutchmy. REMARK
O N T H E DEFINITION OF S O L A R C O R O N A F L A T T E N I N G
The July 11, 1991 solar eclipse took place at the phase of the solar cycle when according to the long-term statistics (Figure 5 i n / 3 / ) the flattening of the solar corona should be approximately = +0.150. However, applying the classical LudendoriT's definition/4/of the solar corona flattening we have received an indication of practically circular shape of the corona (E = +0.012 for r = 1.6 R® and even ~ = -0.048 for r = 2.0 R®). This is, of course, because the streamers appeared in projection at unusually high solar latitudes, nearly at the poles. Taking into account this position of the streamers the inclination of about 50° of the heliospheric current sheet (HCS) can be assumed. Only under assumption that the drawn position of HCS plays a role of the solar equator we obtain a "desirable" flattening E -- +0.169 at r = 1.6 R®. A way out of this curious situation seems to be only in stating that LudendoriT's definition of the solar corona flattening is far from to give the realistic results in some special cases. This is probably true in the case of eclipses when the HCS inclination is remarkable. ACKNOWLEDGEMENTS
The solar eclipse expedition and the processing of the data were possible thanks only to extensive scientific and financial supports. Different parts of the project were accomplished under Slovak Academy of Sciences grants Nos. 494 and 59 and grant No. 30301 of the Czechoslovak Academy of Sciences. Access to the digitizing facilities of the Institute for Transport of Informations (Moscow) and Institute d'Astrophysique (Paris) is gratefully acknowledged. Substantial financial support was provided by Jozef Menc~k and Son Co. (Poprad, C.S.F.R.) and by the Geographic Society (U.S.A.). REFERENCES
1.
O.G. Badalyan, M.A. Livshits and J. S~kora, Polarization of the white-light corona and its large-scale structure in the period of solar cycle maximum, Solar Phys., in press, (1992). 2. L.L. House, Ch.W. Querfeld and D.E. Rees, Coronal emission-line polarization from the statistical equilibrium of magnetic sublevels. II. FeXIV 5303 A, Astrophys. J. 255, 753 (1982). 3. S. Koutchmy, J.B. Zirker, R.S. Steinolfson and J.D. Zhugzda, Coronal Activity, in: Solar Interior and Atmosphere, eds. W.C. Livingston and M.S. Matthews, Univ. Arizona, Tucson, 1991, p. 1044. 4. S.A. Mitchell, Eclipses Of the Sun, Handbuch der Astrophysik, Band ~, 339-340 (1929).
0273-1177/94 $6.00 + 0.00 Copyright © 1993 COSPAR
THE STRUCTURE AND PHYSICAL PROPERTIES OF THE QUIET CORONA AS INFERRED FROM THE JULY 11, 1991 SOLAR ECLIPSE DATA J. Sykom,* P. Ambro~** and O. G. Badalyan*** * Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranslai Lomnica, Slovak Republic ** Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondrejov, Czech Republic *** Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, 142092 Troitsk, Russia
ABSTRACT We were successful in observing the linear polarization of the solar corona in the white light and in the light of the green emission line 530.3 nm during the July 11, 1991 total solar eclipse (La Paz, Mexico). Preliminary analysis reveals remarkable differences in the degree of polarization for both sets of data, particularly for the distribution of polarization around the sun's limb. The upper theoretical limits of the degree of polarization are compared with our results. The real structure of the corona is compared with the calculated coronal magnetic field. The rather unusual shape and structure of the eclipse corona in a given phase of of the solar cycle are discussed in connection with a definition of coronal flattening.
INTRODUCTION One of the most powerful methods of studying the solar corona is polarimetry. Polarization of coronal radiation provides information on the physical properties and characteristics of the corona and, furthermore, it is practically a unique source of information on coronal magnetic fields. At the same time, polarization is a very important test for the presence of one or a~n another mechanism of radiation under the physical conditions in the solar corona. Unfortunately, at this moment the greater part of the results discussed in this paper relates to the white-light corona only.
EQUIPMENT AND OBSERVATIONS Thanks to the exceptionally long duration of totality (six and a half minute in La Paz) it was possible to observe the white-light and green-line coronae with just one instrument, successively. We have used a telescope with an achromatic lens, 13 cm in diameter and focal length of 195 cm. For the green-line observation a special ultra-narrow-band 0.2 nm tunable filter (Andover Co., U.S.A.) was used to isolate the light of the FeXIV 530.3 nm spectral line. In both the types of observation a polaroid rotated in step of 45*. Kodak Tri-X Pan film was used to obtain two series (1/125 s and 1/15 s) of four images and one series (1 s exposure) of three images in the white light and one set of four images (30 s exposures) of the green-line corona. We have found the Kodak D-69 developer to be very suitable to reasonably suppress the high gradient of the coronal brightness with distance from the limb. All the data were subjected to extensive photographic, photometric and computer digitization processing. Acording to the well-known theory, when using a set of four images taken with the polaroid rotating by 45*, it is possible to directly determine (4)69
(4)70
J. Sykora et al.
the coronal brightness and the degree and direction of coronal polarization. The polarization of the green corona depends considerably on the direction of coronal magnetic fields. PRELIMINARY
ANALYSIS OF THE DEGREE
OF POLARIZATION
In the case of a spherically symmetrical corona the degree of polarization of the white-light corona should not exceed 50% under conditions of Thomson scattering. However, we have observed/1/ at the m a x i m u m of the solar cycle (eclipse of February 16, 1980) polarizations up to 55% in some well-developed streamers. Do not cast doubts on validity of Thomson scattering mechanism of coronal radiation it was necessary to assume a substantial concentration of coronal matter near the plane of the sky. This practically implies existence of the streamers at the sun's limb and, at the same time, their more or less radial orientation at the moment of the 1980 eclipse.
/
/
Fig. 1. Triad of equidensities (1 s exposures) and the derived degree of polarization. To visualize better the effect of polarization one equidensity of the unpolarized corona was added and plotted as the dashed line. The result which we present in Figure 1 indicates different circumstances in the case of the July 11, 1991 eclipse. The degree of polarization reached only 405{ in the best developed streamers (or systems of streamers) in the NE- and SW-quadrants. This can be understood when analysing the distribution of activity on the sun's surface. Simply, the observed streamers are in fact far from the plane of the sky, i.e. they are not located at the sun's limb. The NE-streamer is most probably well behind the east limb, while the system of streamers in the SW-quadrant has its foot points well in front of the west limb. The 45% polarization belongs to the coronal condensation just above an active prominence at the west limb. The lowest degree of polarizations (20%) are characteristic of the north-pole and west-limb coronal holes which are, of course, best detected on images taken through neutral radial filters(for example, those taken by S. Koutchmy, by Rhodes College and the H A O group). The SE-region near the south pole can hardly be considered a coronal hole because of its 35% degree of polarization. On the other hand, the calculated structure of the coronal magnetic field (see Figure 2) indicates coronal hole properties for this region.
Properties of the Quiet Corona
(4)'/1
If there are sometimes problems with the interpretation of the exceptionally high degree of whitelight corona polarization, then the problems with observed values of polarization in the green coronal line FeXIV 530.3 nm are, so to say, of a higher order. First, the theory of polarization in this line is very complicated. The most representative calculations /2/, allow degrees of polarization up to only 20%. Second, experimental measurements of polarization in the green line are far more delicate than they are in continuum. The published experimental results range from 70% to 0%. Almost the same dispersion of values was recorded by different observers during the same eclipse. Our preliminary calculations of the green corona polarization in the N, E, S and W radial directions does not indicate values exceeding 15-20%. The most interesting is probably the indication of almost zero green-line polarization at position angles 280°-300 °, where polarization of the white-light corona is the highest, i.e. 45% (see Figure 1). Further, very careful analysis of the green-line data is now in progress. COMPARISON OF THE OBSERVED CORONAL STRUCTURES
AND
CALCULATED
It is certainly interesting to see how the calculated structure of the coronal magnetic field can be fitted to observed structures during the July 11, 1991 total solar eclipse. As usual, we have carried out the calculations by using the measurements of solar magnetic fields made at the John Wilcox Observatory of the Stanford University. One of the authors (P.A.) is very much obliged to Dr. J. Todd Hoeksema for providing him with the set of harmonic coefficients prior to their pubM~K( JUt. 11.1Wl tO1.tl~ t t ~ ~aDtB L :1m * 8:~0
h:4.~
~0AIA AJI¢ 2 L l m l 10 JUt. 25,1Mt
Fig. 2. Comparison of the calculated coronal magnetic field with a real structure of the July 11, 1991 solar corona. Fof deteails refer to text. Hypothetical position of the heliograpl~c current sheet (HCS) is outlined considering distribution of the coronal structures around the sun's limb. lication. In our calculations we prefer the radius of the source surface Rs = 4.5 I~. The reasons for this will be given in a separate paper. W e would like to comment only that namely for this value of the source surface radius a very good correlation between the calculated field line structures and the large-scale coronal structures of the middle and outer corona has been found. As for the proper
(4)72
J. Sykora et al.
structure of the solar corona and structure of the coronal magnetic fields (see Figure 2) we consider it necessary to emphasize that these are not, of course, identical features, i.e. the presence of the lines of force does not necessarily mean the presence of radiating plasma. Further, any magnetic field line of force is only a form of representing the magnetic field, explicitly it tells us anything about the intensity of the field. The intensity of the field can be inserted into the model pictures indirectly, for example by starting the calculation of the lines of force from a certain intensity of the field at an initial point of those lines of force. In this way some structures can be made visible and others can be suppressed. Two variants of calculations were performed under the same boundary conditions. In the left part of Figure 2 the calculation begins at r = 1.0 R® at points regularly distributed throughout the heliographic network (ten by ten degrees). The closed and open structures are formed according to the real character of the coronal magnetic fields. At the right, the calculation begins at the source surface (4.5 R®) and proceeds to radius r = 1.0 R®. In this case all the lines of force are open and their foot points in the photosphere probably indicate the position and contours of coronal holes. Both presentations are equally significant and it is desirable to superimpose them. It is only reasonable to compare this composite picture with the observed structure of the coronal features. The sketch of the July 11, 1991 eclipse coronal structure (at the bottom of Figure 2) has been provided by Dr. Serge Koutchmy. REMARK
O N T H E DEFINITION OF S O L A R C O R O N A F L A T T E N I N G
The July 11, 1991 solar eclipse took place at the phase of the solar cycle when according to the long-term statistics (Figure 5 i n / 3 / ) the flattening of the solar corona should be approximately = +0.150. However, applying the classical LudendoriT's definition/4/of the solar corona flattening we have received an indication of practically circular shape of the corona (E = +0.012 for r = 1.6 R® and even ~ = -0.048 for r = 2.0 R®). This is, of course, because the streamers appeared in projection at unusually high solar latitudes, nearly at the poles. Taking into account this position of the streamers the inclination of about 50° of the heliospheric current sheet (HCS) can be assumed. Only under assumption that the drawn position of HCS plays a role of the solar equator we obtain a "desirable" flattening E -- +0.169 at r = 1.6 R®. A way out of this curious situation seems to be only in stating that LudendoriT's definition of the solar corona flattening is far from to give the realistic results in some special cases. This is probably true in the case of eclipses when the HCS inclination is remarkable. ACKNOWLEDGEMENTS
The solar eclipse expedition and the processing of the data were possible thanks only to extensive scientific and financial supports. Different parts of the project were accomplished under Slovak Academy of Sciences grants Nos. 494 and 59 and grant No. 30301 of the Czechoslovak Academy of Sciences. Access to the digitizing facilities of the Institute for Transport of Informations (Moscow) and Institute d'Astrophysique (Paris) is gratefully acknowledged. Substantial financial support was provided by Jozef Menc~k and Son Co. (Poprad, C.S.F.R.) and by the Geographic Society (U.S.A.). REFERENCES
1.
O.G. Badalyan, M.A. Livshits and J. S~kora, Polarization of the white-light corona and its large-scale structure in the period of solar cycle maximum, Solar Phys., in press, (1992). 2. L.L. House, Ch.W. Querfeld and D.E. Rees, Coronal emission-line polarization from the statistical equilibrium of magnetic sublevels. II. FeXIV 5303 A, Astrophys. J. 255, 753 (1982). 3. S. Koutchmy, J.B. Zirker, R.S. Steinolfson and J.D. Zhugzda, Coronal Activity, in: Solar Interior and Atmosphere, eds. W.C. Livingston and M.S. Matthews, Univ. Arizona, Tucson, 1991, p. 1044. 4. S.A. Mitchell, Eclipses Of the Sun, Handbuch der Astrophysik, Band ~, 339-340 (1929).