EDLéN, BENGT

(b. Gusum, Sweden, 2 November 1906;

d. Lund, Sweden, 10 February 1993), physics, especially ultraviolet and astronomical spectroscopy.

A physicist who devoted more than six decades to studying the spectra of high-temperature ions, Edlén achieved wide recognition in the early 1940s when he identified the ions that give rise to the long-mysterious coronal-line spectrum. The apex of his research life, this interdisciplinary breakthrough launched him into a long career as honoree, institution builder, and arbiter of scientific promise and achievement.

The eldest of five children of Gustaf Fridolf Edlén and Maria Amalia née Rundberg, Edlén spent his early years in Gusum, a small industrial town in central Sweden where his father worked as an accountant. When eleven, he enrolled in nearby Norrköping’s upper school. During his nine years as a student there, he received particularly good marks in biology and physics. He later recalled that his instructor Harald Mohlin (whose doctoral advisor had been Knut Ångström of Uppsala University) had those in the physics class conduct laboratory studies of the Sun’s Fraunhofer spectrum. He also remembered participating in the school’s astronomy club. The combined influence of his father’s numeracy and these experiences surely helped prepare the way for his later contributions to spectroscopy.

In early 1927, after a stint of mandatory military training, Edlén matriculated at Uppsala University. He soon decided to study physics. This was a fortunate choice because Manne Siegbahn, an x-ray spectroscopist who had recently been recruited from Lund University and more recently been awarded the Nobel Prize in Physics, was busily updating the Physics Institute’s spectroscopic armamentarium. Among other things, he was developing apparatus for measuring extreme ultraviolet (EUV) spectra so that he could calibrate his x-ray wavelength scale. In early 1929, Siegbahn asked Edlén, who already had one publication on x-ray measurements to his credit, and fellow student Algot Ericson to inaugurate the institute’s first vacuum spectrograph for studying EUV spectra produced by spark discharges. Swiftly distinguishing himself, Edlén was picked to describe the instrument and some of its initial results in a paper at the Scandinavian Scientific Congress in Copenhagen that August.

Edlén and Ericson followed up on this report, which received encouraging praise from Niels Bohr, with a flurry of papers in Nature, the Zeitschrift für Physik, and the French Academy’s Comptes rendus. Their last collaborative paper—submitted May 1930 with Ericson posthumously on the byline—dealt with the EUV spectra of the isoelectronic lithium-like ions C IV (i.e., trebly ionized carbon), N V und O VI. Continuing this line of research, Edlén went on to use the institute’s EUV spectrograph to measure lines from nearly all the high-temperature ions of the elements between lithium and oxygen. Moreover, displaying the same industry and adeptness with the institute’s calculating machines as its vacuum spectrograph, he identified the atomic transitions giving rise to these lines and investigated how the energy of specific atomic states varied along isoelectronic sequences. He brought all this work together in a doctoral thesis that he defended in April 1934. The following month Edlén, who was then twenty-seven, received a doctorate, a prestigious Bjurzons Prize for his dissertation, and an appointment as a Dozent (instructor) in physics.

The one consequential diversion from Edlén’s arduous experimental inquiries during the preceding three years arose from his curiosity about the bearing of his results on astronomical spectroscopy. This curiosity may have been piqued by the example of Ira Bowen, an EUV spectroscopist at the California Institute of Technology who had caused quite a stir in the late 1920s with his

compelling attribution of the chief nebular emission lines to familiar ions undergoing “forbidden” transitions—i.e., exceptional transitions that occur in sufficient numbers to be observed in highly rarefied gases in which collisions are so infrequent that excited atoms have fairly long lifetimes. In any case, Edlén had familiarized himself with astronomical spectroscopy’s current state by auditing the course on the field that Dozent Carl Schalén offered at Uppsala in the fall of 1931. Having learned from Schalén that N IV was the source of two emission lines observed in the hot Wolf-Rayet stars, Edlén had gone on to use his own hard-won knowledge of the energy levels of multiply ionized carbon, nitrogen, and oxygen to identify the transitions giving rise to many more unknown Wolf-Rayet lines. Pleased with these findings, he had ventured into the astronomical literature for the first time by announcing them in Britain’s Observatory (1932) and discussing them fully in Germany’s Zeitschrift für Astrophysik (1933).

Edlén’s pace during the half decade following his doctorate must have made his student days seem like a leisurely stroll. His marriage with Ruth Grönwall in winter 1935 yielded Inga, Per, and Ruth by late 1937. Although family life must have laid claim to some of his time, Edlén seems to have been entirely preoccupied with research and career through the winter 1939. In his research he first followed up on his doctoral work with a series of articles on EUV spectra from numerous ions ranging from C II to Cu XIX. Then around 1937, having reached the point of diminishing returns from the institute’s EUV spectrograph, he turned to developing better apparatus and to seeking further applications of his results in astronomical spectroscopy. In the meantime, with Siegbahn’s departure for Stockholm as head of the Swedish Academy of Science’s new Nobel Institute for Physics in 1936 and Axel Lindh’s appointment as his successor, Edlén was given the responsibility for teaching spring-term lecture courses on spectroscopy in 1936 and 1937, atomic physics in 1938, and physical optics in 1938 and 1939.

All the while, aware that Sweden’s academic gatekeepers attached great importance to international recognition, Edlén was building relations with spectroscopists abroad through collaborations and on research trips. Seven of these relations ended up having particular significance for him—that with the Belgian astronomical spec-troscopist Pol Swings, who journeyed from Liége to Uppsala in 1934 particularly to work with him and became a close friend and collaborator; those with the German senior spectroscopist Friedrich Paschen and astronomical spectroscopist Walter Grotrian, with whom he became well acquainted during a sojourn in Berlin during 1935; those with the American EUV spectroscopists Joseph Boyce, who passed through Uppsala en route to the 1936 eclipse and invited him to come to the United States the following year, and Ira Bowen, who followed up Edlén’s visit to Pasadena in 1937 by proposing a year later that they collaborate in identifying emission lines on a spectrogram from Nova RR Pictoris (1925); and those with the American theoretical astrophysicists Donald Menzel, whom he met at Harvard during his 1937 trip, and Henry Norris Russell, who visited his laboratory in Uppsala during the International Astronomical Union’s Congress in Stockholm in 1938. Two, indeed, wrote articles that stimulated Edlén to take up the coronal-line problem in April 1939.

Seven decades earlier, during the total eclipse of 1869, Charles Young and William Harkness had been the first to detect the solar corona’s distinctive green emission line. In the intervening period, eclipse observers, who were joined by Bernard Lyot with his coronagraph during the 1930s, had gradually identified and measured some twenty emission lines that could be said with fair confidence to originate in the corona. However, none of the numerous attempts to explain this unique spectrum had been successful. The early failures to find matches between laboratory and coronal lines had given rise to the notion that an unknown element—dubbed “coronium”—produced the coronal spectrum. During the late 1920s, this speculation had fallen by the wayside for two reasons. On the one hand, the successes of chemists and physicists in filling most of the remaining gaps in the periodic table of elements had raised doubts about the very existence of coronium. On the other, Bowen’s 1927 attribution of the chief nebular lines to rare transitions in oxygen and nitrogen ions—instead of to the equally hypothetical element “nebulium”—had pointed to an alternative path for seeking a solution. After Bowen’s breakthrough, spectroscopists had presumed that the corona’s spectrum would be traced back to ions of known elements. Their putative solutions had all been found wanting because, at best, only two line matches had been proposed, not nearly enough to strike those familiar with the problem as more than lucky coincidences. Through the 1930s, therefore, the coronal-line problem was widely regarded as the last great riddle of astronomical spectroscopy.

Edlén’s own interest in the coronal-line problem was engendered by two publications he read in early spring 1939. One was an essay review on the problem by Swings. His Belgian friend not only stressed the problem’s recalcitrance but also provided an up-to-date table of twenty-five coronal lines, including ten for which Lyot had determined the wavelengths and relative intensities. The more provocative one was a proposal by Grotrian in Die Naturwissenschaften suggesting that two coronal lines came from highly ionized iron. Edlén must have been struck by his German colleague’s opening lines:

Since I. S. Bowen and B. Edlén have recently shown that forbidden lines of Fe VII appeared in the spectrum of Nova RR Pictoris 1925, since W.S. Adams and A. H. Joy earlier showed that 5 coronal lines made an unambiguous appearance in the spectrum of RS Ophiuchi during a nova-like outbreak in 1933, since finally there are more and more indications that conditions for the excitation of spectral lines exist in the outer regions of the solar atmosphere that greatly exceed what would be expected in thermal equilibrium, it no longer seems completely amiss to discuss whether coronal lines are to be interpreted as forbidden lines from highly ionized atoms. (p. 214)

This was Edlén’s first serious encounter with the counterintuitive—and quite original—idea that the temperature in the sun’s atmosphere increased in rising from the photosphere through the chromosphere to the corona. Suddenly the prospect of scrutinizing his published and unpublished hot-ion spectral data for further matches with the coronal-line spectrum was quite alluring. Needing an accomplishment that would dramatize the larger significance of EUV spectroscopy, he could appreciate that following up Grotrian’s line of attack on the coronal-line problem might just give him a superb opportunity to demonstrate his own and his specialty’s worth.

Edlén’s first step was to search his unpublished hotion spectral data for forbidden transitions that matched further lines on Swings’s list. He made four new tentative matches, including two that were analogous to those proposed by Grotrian. Much encouraged, he continued by examining hot ions of iron on account of this element’s high cosmic abundance. Using isoelectronic extrapolation techniques, he soon found four more prospective matches. His success here in matching the bright green coronal line at 5,303 Å convinced him by later April 1939 that he was indeed on the right track. Through the spring, Edlén carried through more isoelectronic calculations, making seven further provisional matches. However, as his list of matches grew, he came up with several alternatives for lines that he had already tentatively matched. By early July, when he left Uppsala for a month of research with Swings in Liége and a conference on astronomical spectroscopy in Paris, he had gone well beyond Grotrian’s two suggested identifications. He had provisionally matched one or more hot-ion transitions to fifteen additional coronal lines. Pleased, he showed Swings his results. But eager to tie up remaining loose ends, he asked his Belgian friend not to tell anyone.

Upon returning to Sweden, Edlén resumed his coronal calculations. His examination of a wider range of forbidden transitions yielded another nine tentative identifications, three for previously matched lines and six for unmatched lines. At this juncture, he drew up a fresh cumulative list of identifications, provisionally matching twenty-two of the twenty-five lines listed in Swings’s review article. Surprisingly, rather than announce his progress, Edlén set aside his research on the coronal-line problem for more than a year. He later recalled feeling under great obligation to Swings to complete their collaboration on Fe III’s spectrum. He also recalled being daunted by the abundance of lines on the EUV plates that he had made with the goal of refining his wavelength determinations. He may have been bothered as well by the way in which certain lines stubbornly defied all attempts at identification, thereby challenging his customary standards for completeness. Other matters probably claimed Edlén’s attention as well. Germany had attacked Poland, and everyone, even in neutral Sweden, was worried about the future. His divorce from the mother of his three young children must also have been unsettling. In any case, after his marriage with Elfriede Mühlbach at the end of June 1940 and three months of military duty, Edlén began mulling over the coronal problem again.

Edlén finally decided in early 1941 to announce the matches that he regarded as most solid. He had not yet finished up the spectrum of Fe III. Nor had he extracted any useful information from his new EUV plates or come up with even provisional matches for some lines. But in order to continue getting support from Uppsala University, he needed to bring an end to the hiatus in his scientific production that stretched back to the Paris conference. The best way to do so, Edlén evidently concluded, was to get out a preliminary report on his coronal research. He arranged for the submission of a four-page account of this research in March to the Swedish Academy of Sciences for publication in the Arkiv för Matematik, Astronomi och Fysik. In the account, titled “An Attempt to Identify the Emission Lines in the Spectrum of the Solar Corona,” he argued that fifteen coronal lines, including Grotrian’s two, could be attributed to forbidden transitions of iron, nickel, and calcium ions. As warrant for these identifications, he pointed out that these lines accounted for more than 97 percent of the coronal emission spectrum’s intensity, that the average intensity ratio of the iron and nickel lines was about ten to one in accord with the cosmic abundances of these two elements, that the ions involved had a relatively narrow range of ionization potentials, and that lines arising from ions with similar ionization potentials behaved, according to Lyot’s observations, similarly to one another. Edlén estimated that the average ionization potential of the ions involved was 400 electron volts, which indicated—along with Lyot’s estimates of the thermal broadening of certain lines—a coronal temperature well over 1,000,000 degrees. Such a high temperature would explain, he remarked, both the uniqueness of the coronal spectrum and “the failure of all previous attempts to connect it with any known atomic or molecular spectrum.” Over a year later, he followed up this preliminary report with a comprehensive paper on the coronal-line problem in the Zeitschrift für Astrophysik. There, besides greatly amplifying his discussion of his investigative pathway and the implications of his putative findings, he proposed four further line identifications. It attests to Edlén’s care in scrutinizing his tentative matches that nearly all nineteen identifications (including Grotrian’s two) he listed—eight iron lines, six nickel lines, three calcium lines, and two argon lines— turned out to be quite robust.

Meanwhile, Edlén was eager to get his solution to the coronal-line problem into circulation. Its acceptance as a breakthrough on this long-standing conundrum would greatly enhance his career prospects. The trouble was that the times were not propitious. It was fairly easy to make his identifications known in neutral Sweden and neighboring Denmark by following up his paper in the Academy’s Arkiv with a lecture, seminars, and a semipopular article. But it was quite a different matter to place his results before the international experts whose judgments would be crucial. Europe was sinking ever deeper into war, and many ordinary channels of communication were constricted or closed. Moreover, scientists in the leading scientific nations were increasingly preoccupied with military matters. He overcame these obstacles by mailing advance copies of one or both of his papers to scientists who had earlier published on the coronal-line problem whom he knew personally—the Americans Bowen, Boyce, Menzel, and Russell; the Belgian Swings (who by this time was in America), and the Germans Grotrian and Paschen.

Once started on its way, Edlén’s solution was not only rapidly disseminated but enthusiastically received. This warm welcome in the midst of World War II was a result both of its Swedish provenance and its robustness. Had Edlén been a citizen of one of the belligerent powers, his interpretation of the coronal emission spectrum might well have had great difficulty spreading beyond that power’s alliance system. But as a Swedish physicist with good contacts in the United States and Germany alike, Edlén easily reached interested audiences on both sides of the battle lines. Once out, of course, his solution’s reception depended upon its ability to satisfy the criteria that astronomical spectroscopists had set for the problem in the course of evaluating and rejecting earlier proposed solutions. His case that many of the known coronal lines could be attributed to forbidden transitions of highly excited iron, nickel, calcium, and argon ions swiftly came to be seen as not only compelling but also full of promise for future research into the corona’s physical state. The quick international recognition accorded his breakthrough gave Edlén a decisive edge in the competition that resulted in his appointment at the age of thirty-eight to Lund University’s chair of physics in 1944. His solution of what was then astronomical spectroscopy’s last great riddle won him numerous honors in the ensuing decades, most notably the Royal Astronomical Society’s Gold Medal (1945), the National Academy of Sciences’ Henry Draper Medal (1968), and election to the Académie des sciences as a foreign member (1972).

Well positioned and much honored, Edlén continued his research in spectroscopy for the remainder of his long career. His passion for the field was so deep that, when asked once about his favorite pastimes, he replied “spectroscopy and gardening.” Besides his research, Edlén presided effectively over a major postwar expansion of Lund University’s Physics Institute until his retirement in 1973. Among his many national and international committee responsibilities over the decades, the most important was his long service on the Swedish Academy’s Nobel Committee for Physics (1961–1976). While serious in demeanor, he was certainly not without a sense of humor. For instance, when a colleague told him near the end of his life of a foreign scientist’s recent inquiry about the academy’s failure to recognize his coronal breakthrough with a Nobel Prize, he commented that he would much rather that someone asked that than “Why did Edlén receive the Nobel?” Edlén died in 1993, less than half a year after several happy celebrations of the fiftieth anniversary of his comprehensive 1942 paper on the coronal spectrum.

BIBLIOGRAPHY

WORKS BY EDLÉN

“Zur Deutung der Spektren der heißen Sterne.” Zeitschrift für Astrophysik71 (1933): 378–390.

Wellenlängen und Termsysteme zu den Atomspektren der Elemente Lithium, Beryllium, Bor, Kohlenstoff, Stickstoff und Sauerstoff. Nova Acta Regiae Societatis Scientiarum Upsaliensis, ser. 4, vol. 9, no. 6. Uppsala, Sweden: Almqvist & Wiksell, 1934. Edlén’s dissertation.

“Edlén, Bengt.” In Uppsala Universitets Matrikel, Höstterminen 1936, edited by Thoralf Fries and Ernst von Döbeln. Uppsala: Almqvist & Wiksells, 1937. Edlen’s curriculum vitae to 1936.

“An Attempt to Identify the Emission Lines in the Spectrum of the Solar Corona.” Arkiv för Matematik, Astronomi och Fysik28B, no. 1 (1941): 1–4.

“Die Deutung der Emissionslinien im Spektrum der Sonnenkorona.” Zeitschrift für Astrophysik 22 (1942): 30–64.

“The Identification of the Coronal Lines.” Monthly Notices of the Royal Astronomical Society 105 (1945): 323–333. “Edlén, Bengt.” In Uppsala Universitets Matrikel 1937–1950, edited by Åke Dintler and J. C. Sune Lindqvist. Uppsala: Almqvist & Wiksells, 1953.

“Edlén, Bengt.” In Lunds Universitets Matrikel 1967–68, edited by Eva Gerle. Lund: Gleerup, 1968.

OTHER SOURCES

Friedman, Robert Marc. “Siegbahn, Karl Manne Georg.” In Dictionary of Scientific Biography, edited by Frederic L. Holmes. New York: Charles Scribner’s Sons, 1990.

Grotrian, Walter. “Zur Frage der Deutung der Linien im Spektrum der Sonnenkorona.” Naturwissenschaften 27 (1939): 214.

Hufbauer, Karl. “Breakthrough on the Periphery: Bengt Edlén and the Identification of the Coronal Lines, 1939–1945.” In Center on the Periphery: Historical Aspects of 20th-Century Swedish Physics, edited by Svante Lindqvist. Canton, MA.: Science History Publications, 1993.

———. “Artificial Eclipses: Bernard Lyot and the Coronagraph,1929–1939.” Historical Studies in the Physical and Biological Sciences 24 (1994): 337–394.

Litzén, Ulf. “Atomspektroskopi, klassisk mark i Lund: Bengt

Edléns forskning under 1950-talet.” In Virvlande visioner: Fysiken i Lund under det senare 1900-talet, edited by Hans Ryde. Lund, Sweden: Ugglan, 2002.

Martinson, Indrek. “Bengt Edlén’s Scientific Work.” In Trends in Physics 1981: Papers presented at the Fifth General Conference of the European Physical Society, Istanbul, Turkey, 17–11 September 1981, edited by I. A. Dorobantu. Bucharest, Romania: Central Institute of Physics, 1982.

Milne, Edward Arthur. “Address … on the Award of the Gold Medal to Professor Bengt Edlén, Professor of Physics in the University of Lund.” Monthly Notices of the Royal Astronomical Society105 (1945): 138–145.

Swings, Pol. “Une grande énigme de la spectroscopie astronomique actuelle: Le spectre de raies d’émission de la couronne solaire.” Scientia 65 (1939): 69–78.

———. “Edlén’s Identification of the Coronal Lines with Forbidden Lines of Fe X, XI, XIII, XIV, XV; Ni XII, XIII, XV, XVI; Ca XII, XIII, XV; A X, XIV.” Astrophysical Journal98 (1943): 116–128.

Karl Hufbauer