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Martyn, David Forbes

MARTYN, DAVID FORBES

(b. Cambuslang, near Glasgow, Scotland, 27 June 1906; d. Camden, New South Wales, Australia, 5 March 1970)

ionospheric physics.

Martyn was the eldest child of Harry Somerville Martyn, an ophthalmic surgeon, and his wife, Elizabeth Craig Allan, née Thom. He grew up in comfortable middle–class circumstances on the outskirts of Glasgow, where his father had his practice. Graduating B.Sc. from the Royal College of Science, London, in 1926 with a major in physics, he then took up a research studentship at the University of Glasgow, investigating the triode oscillator circuit both experimentally and theoretically; he successfully traced the instability in frequency that had puzzled many observers to a hitherto neglected flow of current to the valve’s grid. For this work he was awarded a Ph.D. by the University of London in October 1928.

Martyn remained at Glasgow for a further year before accepting an appointment as a research officer—one of four notable new appointments made at this time—with the Australian Radio Research Board (RRB). His ensuing move to Australia and into the burgeoning field of upper atmosphere research was of fundamental importance to his future career. Martyn’s early work for the board was largely experimental. It soon became evident, however, that his real strength was as a theorist with a quite remarkable command of the behavior of systems of charged particles in electromagnetic fields.

Martyn was initially attached to the RRB group working in T. H. Laby’s laboratory at the University of Melbourne, where he undertook a systematic investigation, with R. O. Cherry, of the fading of signals from local radio stations. Following his transfer to the Sydney group under the direction of J. P. V. Madsen, this work evolved into a more general investigation of sky waves reflected from the ionosphere and hence into the properties of the ionosphere itself.

Martyn also continued, however, to work directly on radio propagation. In late 1932 the Australian government initiated a major inquiry into the nation’s broadcasting services, and Martyn made significant technical contributions to the debate that resulted over the use of long–wave versus medium–wave carriers. Soon afterward, in the course of surveying available data on the propagation of medium waves in the ionosphere, he developed a very useful theorem, now known as Martyn’s theorem, by which expressions relating to waves incident obliquely on the ionosphere could be obtained from those for vertical incidence.

In addition, with V. A. Bailey, Martyn developed in full quantitative detail a theory to explain the newly discovered “Luxembourg effect,” in which signals from a long–wave broadcasting station in Luxembourg had been found to interact with the carrier wave of other European stations. The initial idea almost certainly came from Bailey, who had been a student of J. S. Townsend at Oxford, and later a collaborator in his authoritative studies on the motions of electrons in gases. Their explanation amounted to an application of Townsend’s methods and data to the ionosphere. A powerful signal such as the Luxembourg one would, they argued, affect the mean velocity of agitation of electrons in the ionosphere in the vicinity of the station, and hence their collision frequency. The absorbing power of that region of the ionosphere would thus vary with the modulation level of the station, and the resulting modulation would be impressed on any other carrier wave traversing the region.

By mid 1935 Martyn’s colleague O. O. Pulley had successfully commissioned a new semiautomatic pulse–echo recorder of his own design that provided both a continuous display of ionospheric layer heights and ionization densities at frequent intervals. The data so obtained formed the basis of an influential paper by Martyn and Pulley on conditions in the upper atmosphere. Perhaps their most striking conclusion was that the temperature of the F2 layer was 1000°C or more. In addition they provided a stimulating discussion, drawing on evidence from a wide range of sources, of possible processes of electron detachment and loss that might explain the persistence of high electron concentrations in the F region even at night.

Shortly afterward Martyn, in collaboration with Pulley and J. H. Piddington, perfected a “pulse-phase” technique for probing the ionosphere that combined the advantages of E. V. Appleton’s frequency-change method and the pulse-echo method invented by Gregory Breit and Merle A. Tuve. The new technique yielded continuous data on the state of polarization of reflected waves and hence on changes taking place in the reflecting layers of the ionosphere. Martyn was at once able to take advantage of it in a debate with other leading workers in the field over the validity of the “Lorentz” polarization term in relation to ionospheric reflections, concluding that Sellmeier’s rather than Lorentz’s dispersion formula was applicable in this case.

During this same period Martyn also became interested in the connections between ionospheric disturbances, as detected on the continuous recorders now available to him, and events occurring on the sun. This marked his introduction to questions of solar physics and the beginning of his long association with workers at Australia’s Mount Stromlo observatory.

As early as 1930 Martyn, in an unpublished RRB report, had pointed out the advantages of using ultra-high frequencies for both long-distance communications (suggesting the use of the moon as a reflector) and obtaining information about objects from which the beam was reflected. During a visit to Britain in 1936 he at once guessed the nature of the secret work on radiolocation being done by R. A. Watson Watt and his colleagues, and on his return to Australia successfully urged that UHF work be initiated there too. He was the natural choice when, in February 1939, the British government invited Australia, along with the other dominions, to send a scientist to England to learn the secrets of radar. Later in the same year he was appointed head of Australia’s new Radiophysics Laboratory charged with developing an independent Australian radar capability.

Unfortunately, Martyn’s period in this post was an unhappy one. He chafed under a peculiar administrative arrangement that had him sharing responsibility for the running of the laboratory with his former mentor, Madsen, and he alienated officials with whom he was required to deal. During 1941 his duties were more and more assumed by F. W. G. White, and Martyn subsequently transferred to the armed services to head a newly established operational research group with special responsibilities for problems associated with radar. He married Margot Adams of Sydney in 1944. They had no children.

By the war’s end Martyn had found a new scientific home at the Mount Stromlo Observatory, where he remained until appointed in 1956 as officer in charge of the RRB station at Camden. New South Wales, a station that two years later became an independent Upper Atmosphere Section specially created for him within Australia’s Commonwealth Scientific and Industrial Research Organization.

Soon after arriving at Mount Stromlo Martyn developed a theory of temperature radiation at radio frequencies from the sun. This led him to predict coronal radiation corresponding to a black body at 1,000,000ºC at wavelengths of about 1 meter, and also significant limb brightening at centimeter wavelengths. Both predictions were quickly confirmed, the former by Martyn’s one-time Radiophysics Laboratory colleague J. L. Pawsey.

The ionosphere, however, remained Martyn’s chief preoccupation, and in four major papers in 1947 and 1948 he developed a modified “dynamo” theory, based on the identification of large solar and lunar atmospheric tides, to account for marked deviations of the experimentally determined values of the ionization in the F2 region from that predicated by Sydney Chapman’s classical (and essentially static) theory. Horizontal winds due to these tides would give rise, he argued, to motions of electrons along the lines of the earth’s magnetic field that would in general have a vertical component that could account for the observed semidiurnal variations in both the ionization of the upper atmosphere and the earth’s magnetic field itself. Evidence for the existence of the tides was derived from long runs of data that had by then been recorded at various ionospheric observatories, especially those at Mount Stromlo and the Carnegie Institution of Washington’s observatory at Huancayo, Peru.

A serious difficulty remained for the dynamo theory, however, in accounting for variations in the earth’s magnetic field, namely, that the conductivity of the upper atmosphere appeared to be too low to explain the observed changes. In 1948 Martyn suggested that inhibition of the transverse (Hall) current by polarization of the medium might allow a higher-than-expected conductivity. Though the idea was criticized initially. Martyn and his one-time RRB colleague W. G. Baker worked out the theory in detail and showed that it would account most satisfactorily for the unexpectedly high conductivity. They also showed that the conductivity should be greater still in a narrow zone near the magenetic equator, giving rise to an intense equatorial electrojet. This would explain quantitatively the then recently discovered strong enhancement of the daily magnetic variations near the magnetic equator. Such an electrojet was at about the same time observed directly using a rocket―borne magnetometer. As a result of this work the dynamo theory, first proposed by Balfour Stewart in 1882, came at last to be regarded as securely established.

During the 1950’s Martyn also attempted to extend his analysis of the upper atmosphere to account for some of the features of auroras and the magnetic “Storms” associated with them. In particular, he tired to explain on electrodynamic grounds how the streams of charged particles responsible for the aurorae penetrated the atmosphere to the level at which aurorae were observed to occur—their speed appeared to be much too low to permit this—and why aurorae occurred mainly in particular high―latitude zones. Subsequent observational work, however, indicated that the mechanism he suggested was less important than he had supposed.

Martyn was elected a fellow of the Royal Society of London in 1950. Together with M. L. E. Oliphant he was the driving force behind the formation of the Australian Academy of Science in 1954. He was the academy’s first secretary (physical sciences) and was its president from 1969 until the time that, in poor and deteriorating health, he took his own life. He was also very active in various international scientific bodies, most notably the International Union of Radio Science (URSI) and, in his later years, the United Nations Scientific and Technical Committee on the Peaceful Uses of Outer Space, of which he was chairman from 1962 until his death.

BIBLIOGRAPHY

I. Original Works. A complete list of scientific publications accompanies the memorial essays by H. S. W. Massey in Biographical Memoirs of Fellows of the Royal Society of London, 17 (1971), 497–510, and J. H. Piddington and M. L. Oliphant in Records of the Australian Academy of Science, 2 , no. 2 (1971), 47–60. See also Martyn’s “Personal Notes of the Early Days of Our Academy,” ibid., 1 , no. 2 (1967), 53–72. There is a small collection of his papers, chiefly administrative, at the Austrialin Academy of Science, and a considerably larger one, including extensive files of correspondence with G. H. Munro and others, at the CSIRO Archives, Canberra. Files from his period at the Radiophysics Laboratory are at the CSIRO Division of Radiophysics, Sydney.

II. Secondary Literature. Martyn’s work with the Radiophysics Laboratory and the Radio Research Board are discussed in two books by W. F. Evans, History of the Radiophysics Advisory Board, 1939–1945 (Melbourne, 1970), and History of the Radio Research Board, 1926–1945 (Melbourne, 1973). His role in the formation of the Australian Academy of Science is recorded in the academy’s silver jubilee volume. The First Twenty―five years (Canberra, 1980).

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