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The Physical Condition of the Solar Corona

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The Physical Condition of the Solar Corona 135 THE PHYSICAL CONDITION OF THE SOLAR CORONA BY C. W. ALLEN University of London Observatory, London CONTENTS PAGE $7. Chemical composition. ............... $ 10. Dynamical structure of the corona. ........... ........... 150 Acknowledgments References ............................................... 152 Abstract. The observations from which the physical conditions of the corona may be derived have been reviewed. The density distribution obtained from the light scattering of electrons is a straight mean. However, the corona is an irregular object and some attempt is made to determine the amount and the effect of the irregularity. A comparison is drawn between estimates of temperature by line breadth, density gradient, ionization and radio emission. Recent evaluations are given of the constants in the ionization formula. The excitations of coronal lines by collisions and by solar radiation are discussed quantitatively and lead to an excitation collision cross section of approximately 2 x cm2. The estimate of the metal to hydrogen ratio in the corona is rather higher than in other astro- physical sources. The main sunspot cycle changes are (a) a variation in the intensity and distribution of streamers and (b) a variation of a 1.8 factor in electron density with a density minimum about a year after sunspot minimum. Temperature variations are only about 15% but are fairly regular. Present knowledge of the source of heating in the corona is too indefinite to allow quantitative estimates of the heat gained by the corona, but the heat lost by conduction and ultra-violet radiation is of the order of 2 x lo4erg cm-2 sec-'. The streamers and geo- magnetic storms give evidence of a flow of material outward through the corona. This appears to be 1000 times greater than the thermal evaporation and calls for outward forces in the corona that are not at present understood. 0 1. INTRODUCTION NTIL about 1942 the solar corona was a challenge to astronomers and physicists. Its spectrum was known but not understood. The bright U continuum observed close to the sun's limb was thought to be due to electron scatter, but the absence of Fraunhofer lines was unexplained. Fraunhofer lines were indeed detected weakly in the outer corona but were apparently not due to the same scattering source as the inner continuum. This left the absorption lines also unexplained. Finally there were several bright emission lines which 136 C. W. Allen were not identified and were usually called ‘ coronium ’ lines. These lines were eventually identified by EdlCn (1942) who showed them to be due to the elements A, Ca, Fe, Ni in very high states of ionization. The high temperature required to explain the ionization also explained (a)the absence of Fraunhofer lines in the bright near continuum, (b) the absence of the more familiar emission lines of H and the metals, (c) the breadth of the coronal emission lines, (d)the extended size of the corona, and (E) the new (1946) radio measurements of quiet sun intensity, As a result of these successes the corona is now as well understood physically as most astronomical objects. In ten years it has changed from an enigma to an astrophysically useful body in which one can study the activities of atoms at high temperatures. The conditions of temperature and pressure may be derived fairly directly from observations and could attain a good level of accuracy if it were not for the irregularity and variability of the corona itself. From the temperature and pressure distribution one may estimate the emission of thermal radio waves and the conditions of excitation and ionization. One finds fairly consistent agreement with observation. On the other hand the energy balance, structural forms and dynamic activity of the corona are not well understood. The problems of the corona have been reviewed from several points of view by Unsold (1938), Waldmeier (1941), Bugoslavskaya (1950), Siedentopf (1950), Mitchell (1951), Shklovskij (1951) and Woolley and Stibbs (1953). Without doubt a comprehensive analysis will be given by Waldmeier (1953 +). However, progress with the coronal problems is closely bound up with our knowledge of its physical condition, which is therefore the subject of this review. $2. OBSERVATIONSAND DESCRIPTIONOF THE CORONA For many years astronomers have used the opportunities provided by the solar eclipses to observe the colour, spectrum, brightness distribution, polarization and appearance of the corona. Many of these observations may now be made by coronagraph instead of by eclipse. The coronagraph method allows one to attain higher resolution and much better observational continuity, but it is available only for the bright inner corona. The advent of radio astronomy has provided powerful methods of studying the corona. Radio observations may be made without eclipses, and by frequency selection one may obtain information for both the high and low corona. Distri- bution of radio intensity across the sun’s disc and out into the corona map be determined by radio interferometry or by use of eclipses. Geomagnetic and ionospheric observations give evidence of the variable particle and ultra-violet radiations that come from the sun and therefore through the corona. These must be taken into consideration in assembling our ideas of the physical corona. The corona is.found to have a spectral distribution very similar to that of the sun (Ludendorff 1925, Grotrian 1931). The emission spectrum lines contribute only about 1 yo of the total light and therefore do not influence the measurements of colour, polarization and brightness. The brightness decreases rapidly from the limb outward. There is no well-defined outer boundary to the corona, and the limit to which the corona can be detected depends rather fortuitously on the extent of the streamers (coronal rays). These can normally be seen to about four radii from the sun’s centre but hzve been traced as far as twelve radii (Laffineur et al. 1952). On the PHYSICAL SOCIETY PROGRESS REPORTS, VOL. 17 (c. w. ALLEN) Fig. 1. Drawing of the corona of 1901 May 18 (from iVent. R. Astr. Soc., 1027, 64, Appendix, Plate 11). Reproduced by coitrtesy of the Royal Astrononiicnl Soci:/y. Fig. 2. Photograph of the corona of 1932 August 31 (photograph by P. A. McNally, reproduced with permission from ' Astronomy ' by R. H. Baker (New York : Van Nostrand, 1938), p. 296). The Physical Condition of the Solar Corona 137 other hand the lower boundary between the chromosphere and the corona is rather sharp. The physics of this boundary is a difficult problem which appears again in connection with the existence of prominences. It is evident from polarization measurements that the corona is essentially an extended solar atmosphere which scatters sunlight and also emits certain spectrum lines. The Fraunhofer absorption lines are completely obliterated in the process of scattering the light. However, superimposed on the corona proper is a halo of light diffracted by interplanetary (zodiacal light) particles. This component of the radiation does contain the Fraunhofer lines and therefore was called by Grotrian (1934) the F corona to distinguish it from the purely continuous K corona. The F corona contributes a greater proportion of the total brightness as the distance from the sun increases. For studies of the coronal physical condition it is necessary to ensure that brightness observations are suitably corrected for the F corona. The appearance of the corona shows that it cannot be regarded as a quiet atmosphere in equilibrium under gravity alone. The structural features can best be demonstrated by sketches such as shown in fig. 1 (Plate) for the eclipse of 1901 May 18, and photographs as in fig. 2 (Plate) for the eclipse of 1932 August 31. The appearance differs markedly from one eclipse to another and is far more complicated at time of sunspot maximum. The illustration in fig. 1 is a minimum eclipse which, however, shows the main features as follows : (a) Polar plumes are well developed at the N and S poles. At sunspot maximum these plumes are confused by other features. (b) -4 set of equatorial plumes can be detected on the E side. These radiate from active areas and usually-but not always-from sunspots. At sunspot maximum this type of plume may be found at any solar latitude. (c) Smooth streamers can be seen emerging from most of the sun's surface in fig. 1, and a remarkable one is shown in fig. 2. Further from the sun's surface they tend to gather into rather straight rays. The base of the streamers often encloses a mitre-shaped area containing arches and prominences. (d) A set of arches surmounting a prominence may be seen in the south-east quadrant of fig. 1. The polar plumes are the only evidence that the sun possesses a general dipole magnetic field which penetrates into the corona. The plumes probably consist of ionized gas constrained to move along lines of force of the dipole field. It is tempting to conclude that the equatorial plumes also represent the lines of magnetic force due to sunspots (or magnetically active areas). The difficulty with the general application of this idea is that the plumes appear to run parallel to the streamer edges and would suggest that the lines of the magnetic field follow the streamer edges into the distant outer corona. Magnetic fields of this shape are not possible. Coronal observations give no indications of the magnitudes of the magnetic fields at the polos, but fields associated with equatorial plumes may be derived from sunspot observations (Giovanelli 1947, Grotrian 1948). 0 3. DENSITYDISTRIBUTION The distribution of density in the solar corona may be determined rather directly fromphotometry of coronal brightness. It is now evident that the coronal continuum is caused by sunlight scattered by an electron atmosphere. The chief arguments for this are that the colour of the corona is the same as the sun, and that 138 C.
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