Solar Corona : CORONIUM

Solar corona during 1991 eclipse (photo © Steve Albers).
During the total solar eclipse of August 7, 1869 (which was visible from Alaska to North Carolina), Harkness and Young discovered an emission line of feeble intensity in the green part of the spectrum of the corona. Young (1879) identified the line as the Fe Line No. 1474 on Kirchoff's scale. If this identification were correct it was surprising that only a single line of the hundreds of iron lines should be observed in the coronal spectrum and that it should appear at such great heights. Young was well aware of this great difficulty which at times served to mislead rather than to offer a clue to identification. There were many other cases of lines of different metals coinciding exactly or at least so closely that no instruments at that time were capable of resolving them. Although Young completely dispelled the idea from his own mind that the green coronal line was due to iron, he could not give up the thought that it might have some very definite relation to it.

The eclipses which followed that of 1869 gave evidence of other lines which could not be satisfactorily identified and were thought to have some relation to the green line in question. Certain groups of these lines had wavelength ratios which served to strengthen the assumption that they arose from a common substance.

Young's discovery in 1876 that Kirchhoff's "1474" line was double did not seem to help the state of affairs. It was very certain by this time that the green coronal line was not of terrestrial origin as some had thought it to be. Liveing and Dewar (1881) made a comparison of the iron spectrum in the laboratory with the solar spectrum and gave evidence which seemed to prove for a while that the coronal line was identical with one of the iron doublet. Little confidence, however, was placed in this work. Observations were made at every opportunity in order to increase the small amount of available data for theoretical and for laboratory work on identification.

Reliable spectral observations were not obtained until 1893. At the time of the eclipse of 1898, measurements of the green line showed that for the past thirty years identity had been assumed with the chromospheric line 1474 at 5316.8 while in reality the green coronal line was approximately 14 units toward the violet end of the spectrum at about 5303 . This more accurate determination of the wavelength of the line due to a hypothetical element called "coronium" was made possible by using instruments of better definition and higher dispersion.

During the next few years many observations were made especially by English and by American astronomers in an effort to obtain better determinations of the wavelength of the coronal lines and in particular the line at 5303 . The emphasis on the observations of the green line is characteristic of the entire history of coronium. Grotrian has found that the 5303 line is stronger than all other coronal lines combined. He also shows that all the coronal lines, including 5303, contain only 1/160 of the total coronal energy and therefore do not sensibly affect the colour of the corona. The 1918 determinations gave the value 5302.93 . while the ones made in 1930 yielded 5302.91 . These observations also showed that the green line arises in a region within 200,000 kilometers. of the sun's edge. The Revised Rowland Tables, 1928, page 226, gives another list of lines of probable coronal origin.

Lyot in 1930 and 1931, observed the green line without an eclipse at a high altitude at Pic du Midi and obtained a value of 5303.21 . Lyot in 1935 with a coronagraph determined a value of 02.8 for the green line.

Observations, other than those for wavelength determinations, have yielded many interesting facts. Slitless images of the strong coronal lines at 5303 and at 6374 give us structural details resembling eruptive prominences. The images also reveal condensations in the green coronium ring but these are not related to the prominences. The line at 3601 exhibits many peculiarities. During the eclipse of 1908 the line was very strong and like some others it was missing during previous eclipses. The reason for the change in line intensity does not seem to be known. Some propose the sun spot cycle as its cause. However, since the eclipse of 1908 the 3601 line has been visible without exception.

In 1918 V.M. Slipher observed a green line in the space occupied by the moon's image. This scattering was due, perhaps, to the same factors as in the case of Hydrogen Beta and Hydrogen Gamma. Coronium appears relatively abundant in those regions of the sun's limb from which the great extensions of the corona flow. Certain observations of 5303 have indicated that the substance may at times extend about a solar diameter above the sun's surface. The 1930 slitless observations show that coronium has an uneven distribution around the sun and that the eruptive prominences given by the 5303 line resemble very closely those given by the 3388 line.

The observation of the 5303 line has been confined primarily to the solar corona with one exception, discounting the unconfirmed reports of certain French and Italian scientists that coronium has been detected in terrestrial volcanic gases. In 1933, Adams and Joy obtained definite evidence of the presence of 5303, 6374, and three other coronal lines in the spectrum of the nova-like star, RS Ophiuchi. Since the star is of the 11th visual magnitude, low dispersion had to be used with a consequent low degree of accuracy in wavelength determination but sufficiently high enough for identification purposes. The structure of 5303 appeared much like He II at 4686. However, by April 1934, the continuous spectrum had increased and the coronal spectrum had disappeared. Many forbidden lines of Fe II appeared and the spectrum had assumed a general form similar to that of July 1923, before the outburst.

At about the same time, H.D. and H.W. Babcock (1934) found the emission line of 5303 present in the flash spectrum taken without an eclipse. Its presence was detected in the high chromosphere - a region characterized by the lines of He I. Hylleraas, (1931) however, has found that there is no quantitative agreement that the coronal spectrum is due to helium.

Since the discovery of the green line in 1869, we find many theories and conjectures as to the element which is responsible for the production of the coronal line. The idea of iron, as the contributing element, was always held with suspicion and definitely dropped as soon as accurate wavelength determinations were made. J.W. Nicholson (1911) who had constructed certain atomic models for "nebulium", employed a similar method and dealt with the problem in terms of a dynamical theory of a hypothetical element. The formula used was of the type lambda = a(n+mu)^3, where n is an integer. The new substance which he called "protofluorine", differs from nebulium only in the fact that it has a central positive charge of 5e, while nebulium has a positive central charge of 4e, e being the electronic charge. The new element was to be electrically neutral with 5 electrons located at an equal distance from the center and travelling in circular orbits. Nicholson soon found that 5303 was not a protofluorine line and that his new element was not the coronium of the astrophysicist. An increase in the accuracy of the wavelength values of other lines used in developing the theory did everything but improve the fit of the formula. Some lines used by him are of doubtful coronal origin or even existence and some have even been identified as chromospheric lines due to Fe II and Ti II. The model used by Nicholson was a type very different from the Rutherford model. The work of Nicholson possesses only historical or academic interest.

Pannekoek, in 1924, suggested that because of the presence of Ca II in the far solar atmosphere Ca III might be the source of the coronal lines. However, J.A. Anderson (1924) shows that while 12 weak lines can be matched with Ca III, the 5303 line has no match.

An attempt to show that the lines might be due to argon was made by I.M. Freeman in 1928.

Freeman shows correspondence of over half of the coronal lines with argon within the accuracy of the wavelength values and about 10 more coronal wave numbers were given by intercombination of Messier's term values for argon. The possibility of accidental coincidence was bothersome. Line 53O3 could not be accounted for. It was believed that a line could probably be produced by two different changes in the configuration of the radiating center and the great intensity of 5303 could be connected with this possibility with a doublet structure with a delta lambda of 0.1 .

During the eclipse of 1929 a search was made, with no success, to determine whether 5303 was a doublet. Russell and Bowen (1929) made a critical study of Freeman's work and found that the number of coincidences was a little larger than might be expected from pure chance within the very wide limits of errors which had been admitted. The discordance for the better measured lines greatly exceeded the limits of observational error and they concluded that the correlation with argon was without foundation.

E.M. Lindsay (1929) investigated over 500 forbidden line possibilities of Fe II, Ca II, Ti II, Ti III, Ar II (which does not exist in the chromosphere), and several of Ti I but no coincidence was found which might not be a matter of chance. Investigation of second type collisions involving metastable 1 delta terms of Ca II also gave negative results.

In 1931, the coronal line 6374.2 was found by Hopfield to be identical, within the limits of experimental error, with the oxygen line at 6374.29. This would seem to indicate the presence of oxygen in the solar corona. DeBruin announced in 1932 that it was possible to calculate the line 5303 from laboratory data. He claimed that all of the strongest lines attributed to coronium in the visible spectrum could be connected with the terms of O I. He thought that the Zeeman effect of 6374 might give the nature of the new terms and that possibly the sp4 configuration played an important part. The calculated value of v yielded a wavelength of 5302.70 . which was close to the observed values. This was obtained by using the combination R - 2p1S0. According to this 5577 and 5303 had their origin in the metastable level of neutral oxygen, level 1S. He definitely concluded that the mysterious coronium turns out to be O I. Ferichs (1932) and Dingle (1932) immediately challenged such a statement on the grounds that the correlation was merely accidental and Dingle pointed out that the R-term was based on the assumption that requires verification of existence and therefore the problem remains unsolved.

In 1933, Boyce, McKellar, and Menzel tentatively identified 5303, 3388, 4231, and 4776 with analogous high states of O II. Goudsmit and Ta-You Wu (1934) on the basis that coronal lines differ from nebular lines in broadness, predicted that the difference was the result of unusually short lifetimes in corresponding quantum states and that the coronal lines may involve transitions between doubly excited states of such an atom as helium.

This short summary of the more important events in the history of the unknown element "coronium" gives ample evidence that a great amount of effort was put forth in an attempt to arrive at the solution of the problem. In 1937 there were about 18 lines which were thought to be of the coronal group of which one or two were very suspicious.

The mystery was finally solved with the work of Grotrian and Edlen in 1939. Edlen used the law

delta nu (2P3/2 - 2P1/2) = const (Z-sigma)4

in connection with the isoelectronic sequence 3s2 3P, Al I, Si II, P III, S IV, Cl V, etc., to identify the lines of Fe XIV and Ni XVI in the solar corona. The spectrum of the inner corona is a continuum upon which are superposed broad emission lines whose identification remained a until 1939. One of the strongest of these lines is 5302 due to thirteen-times ionized iron.

Table 1 which is abstracted from a table by Edlen, illustrates the method of identification. The first two columns give the atomic number and the ion; the third column gives the term separation, 2P3/2 - 2P1/2 = delta nu tilde, in the wave number units cm-l, and the fourth column gives the fourth root of zeta where zeta equals two thirds of delta nu tilde.

Notice that the differences run very smoothly. If we assume that the green coronal line (whose wave number is 18,852 cm-1) belongs to Fe XIV, the resultant mean value of the difference tabulated in the last column is quite consistent with the run of the table. The ions in parentheses are those whose term splittings had not yet been observed; c denotes an observed coronal line. Fe X and Ni XII can be identified in the same way from the 3s2 3p5 isoelectronic sequence. The intensity ratios of the iron and nickel lines give further support to the suggested identifications. These transitions occur between terms of the ground configuration and are called forbidden lines. Edlen identified the other coronal lines in a more complicated but equally accurate way.

__________________________________________________

Atomic  Ion           Term       B     Difference
 No.               Separation
__________________________________________________

  13    Al I          112.04   2.939
                                        0.781
  14    Si II         287.3    3.720
                                        0.675
  15    P III         559.6    4.395
                                        0.622
  16    S IV          950.2    5.017
                                        0.599
  17    C IV        1,492      5.616
                                        0.579
  18    Ar VI       2,210      6.195
                                        0.564

  19    K VII       3,131      6.759
                                        0.560
  20    Ca VIII     4,305      7.319
                                        0.552
  21    Sc IX       5,759      7.871
  22    (Ti X)
  23    (V XI)                          0.543
  24    (Cr XII)
  25    (Mn XIII)
  26    Fe XIV     18,852.5c  10.588
                                        0.538
  27    (Co XV)     ----       ----
  28    Ni XVI     27,762c    11.664
__________________________________________________

REFERENCES

  1. Russel,H.N., Bowen,I.S.: 1929, Astrophys.J., 69, 196.
  2. New SOHO observations of coronal loops
  3. Ultraviolet Spectral Atlas of the Sun, 1190-1730 Angstroms
  4. Welcome to SERTS (Solar Extreme-ultraviolet Rocket Telescope and Spectrograph)
  5. GSFC Laboratory for Astronomy and Solar Physics
  6. Solar Astronomy WWW Servers (from SERTS)
  7. Welcome to ORPHEUS (Solar Heliospheric Obs.(SOHO), Coronal Diagnostic Spectrometer(CDS), SERTS)
  8. Ulysses Home Page
  9. The Sun course
  10. Swings,P.: 1943, ApJ. 98, 119. (Edlen's identification of the Coronal Lines)

Next historical event