Geomagnetic and Auroral Research In Denmark

Ørsted – to – Ørsted

Professor H. C. Ørsted and the Ørsted SatelliteThe first Danish satellite, Ørsted, launched 23 February 1999, has now been in orbit for three and a half years and is still performing well. The satellite conducts a high-precision geomagnetic research mission with strong international cooperation. The satellite is named after the Danish professor Hans Christian Ørsted (1777-1851), who in 1820 observed and reported that electrical currents in a wire would cause deflection of a compass needle. Thus, he became the discoverer of electromagnetism. H. C. Ørsted further, in 1826, explained the magnetic variations associated with auroras by postulating the existence of electric currents along the arcs in the upper atmosphere. While at that time a revolutionary idea, the existence of such currents is now commonly agreed; some types are named auroral electrojets. Professor Ørsted founded and was from 1829 first director of the Danish Polytechnical High-School (Technical University). Further he founded the first geomagnetic observatory in Denmark at ”Gyldenløves Bastion” in Copenhagen. Routine observations started here in 1842 and were continued at this site for more than 20 years. (Reproduction of painting of H.C. Ørsted with permission from Nationalhistorisk Museum, Frederiksborg, Hillerød)

The Danish Meteorological Institute and GeomagnetismCompas-based navigation was important for civilian and military purposes. The Danish Meteorological Institute, founded in 1872, was given the task to make geomagnetic surveys in Denmark and to monitor the daily variation of the magnetic declination. These tasks were conducted by Adam F. W. Paulsen (1833-1907) who later became director of DMI. In 1889 a new magnetic observatory was built in Copenhagen by DMI to continue the geomagnetic observations started by H. C. Ørsted.

Pioneering Auroral Research.Adam Paulsen tried to estimate the heights of auroras near Godthaab by using triangulation on simultaneous observations of the lower border of auroral arcs from two observatories separated by 5.8 km. Heights were estimated for 22 auroras and were found to range from 0.6 km to 68 km [Paulsen, 1893,1894]. Now we know that these results – among the first of their kind - were wrong, since auroras are never observed below 60 km by reliable techniques. Adam Paulsen also observed the deflections of a compass needle during auroral activity and discussed the existence of horizontal as well as vertical electrical currents associated with auroras. He combined the auroral investigations into an auroral "ray"' theory [Paulsen, 1894, 1896] where he postulated that the auroral emissions were created by excitation of air molecules by invisible "cathode rays" emitted from a negative electrode in space - a result not far from modern auroral concepts since two years later J.J. Thomson (England) discovered that the cathode rays were actually electrons. In fact, auroras are mainly produced by energetic electron bombardment.

Danish Geomagnetic and Auroral Research.The brilliant works of H.C.Ørsted initiated a strong interest in geomagnetic and auroral research in Denmark. In Denmark the weather observing stations were requested to also report the occurrences of aurora. An expedition headed by Adam Paulsen was sent to Nuuk (Godthaab), Greenland, by the Danish Meteorological Institute during the ”First International Polar Year” 1882/83 to establish a temporary geophysical observatory for meteorological, geomagnetic and auroral studies.

DMI Auroral Expedition to Iceland Adam Paulsen was director of the Danish Meteorological Institute from 1884 to 1907. In 1899 he arranged an auroral expedition to Akureyri at the north coast of Iceland to observe the occurrency and spectra of auroral emissions and to investigate the relations between atmospheric electricity and auroras. The expedition left Copenhagen in the fall 1899 on M/S Botnia bound for Akureyri. With the expedition was also a young physicist, Dan Barfod la Cour (1876-1942), who was responsible for the spectrographic observations, and an artist painter, Harald Moltke (1871-1960), who was given the task to provide color reproductions of the observed auroras.

Auroral Spectral Observations and PaintingsIn Akureyri they established an observing site at ”Sulur” mountain. The scientific results from the two expeditions were remarkable. Adam Paulsen soon after the expedition reported the detection of 16 not previously observed lines in the ultraviolet part of the auroral spectrum [Paulsen, 1900,1901]. An accurate scaling of these lines showed that they came from oxygen and nitrogen emissions which supported Paulsens theory on the excitation of ordinary air molecules by invisible cathode rays. But the works by Harald Moltke were perhaps even more remarkable. He produced a set of beautiful oil colour paintings of auroras - possibly the finest artistic reproductions of auroras ever made. The original paintings are kept at the Danish Meteorological Institute but reproductions have been given to numerous observatories and institutions all over the world.

Active aurora over Iceland at 20:30 on 11 November 1899 painted by Harald Moltke. The instrument in the foreground is a spectrograph.

DMI auroral expedition to Iceland 1899-1900. Top of Sulur 1899. Standing in the middle Adam F.W. Paulsen, director of DMI. Sitting to the left Dan la Cour (later director of DMI). Sitting to the right Harald Moltke. Behind him Ivar Jantzen also from DMI. The other persons in the photo are local assistants.

Harald Moltke: Top of Sulur.

Peter Stauning. Danish Meteorological Institute. September 2002. pst@dmi.dk

Moltkes Paintings of AurorasAn example of Moltkes paintings is displayed in the figure. The characteristic mountain ridges near Akureyri are displayed in the background of the painting. In the foreground on a pedestal is shown the spectrograph. The instrument comprises a prism to refract the auroral light that enter through a narrow slit at the front. A lens system focuses the refracted image of the slit onto a photographic plate mounted at the rear of the instrument. A vivid display of curly break-up auroral arcs is seen. When observing the aurora (in darkness), Harald Moltke sketched the shape on cartoon, and noted the colors, movements and other characteristics of the aurora as well as the view direction and the time. In daylight the following morning he painted most carefully the observed auroral display guided by his sketches and notes

Geomagnetism at DMI after 1901The geomagnetic observations in Denmark, initiated by H. C. Ørsted, were resumed from an observatory established in 1889 in the Botanical Garden in central Copenhagen by the Meteorological Institute. Routine observations started in 1891. With the advent of electrical tramways in the city the disturbances in Copenhagen became excessive and a new observatory and magnetic laboratory was established in Rude Skov north of Copenhagen in 1907. At this location an era of highly skilled magnetic instrument developments started.

Development of high-precision magnetic instruments at DMIThe construction of magnetic measuring instruments was initially headed by DMI’s director, Adam Paulsen, himself. In 1920 Dan B. la Cour was appointed head of the Division of Geomagnetism. From 1923 to 1942 he was also director of the DMI. La Cour was a brilliant instrument constructor and developed with assistance from Viggo Laursen and Johannes Olsen (1894-1991) several types of magnetic precision instruments which came in use at hundreds of observatories all over the world. Among other the Quartz Horizontal-Force Magnetometer (QHM) [la Cour, 1936] and the Magnetometric Zero Balance (BMZ) [la Cour, 1927, 1942] instrument for absolute measurements of the horizontal and vertical components, respectively, of the Earths magnetic field.They, furthermore, developed the la Cour variograph [la Cour and V. Laursen, 1930] for recordings of relative variations in the three magnetic components. The instrument uses a focused light beam reflected at mirrors mounted at small magnets either suspended on torsioned quarts fibre (horizontal components) or installed on a balance (vertical component) to produce traces on photographic paper mounted at a rotating cylinder. The deflections of the traces from a marked baseline are proportional to the geomagnetic variations. These instruments were built in large quantities and high quality for sale from the DMI and they have been standard observatory instruments of worldwide geomagnetic observations through several decades.

International Polar Geomagnetic ObservationsDan la Cour worked intensely for the preparation of the meteorological, geomagnetic and auroral observations that were to be conducted from a large number of temporary stations in both the northern and southern polar regions at the 50 year anniversary for the ”First Polar Year” (1882/83). He became President for the Commission for this ”Second International Polar Year” 1932/33. The Second Polar Year was a great accomplishment. After the year had passed Dan la Cour and his colleague and successor in the geomagnetic work, Viggo Laursen, worked hard to make the numerous observations made during the polar year available to the international community. In recognition of his extensive contributions la Cour was in 1936 elected President of the International Union for Geodesy and Geophysics (IUGG). He held that position until his death in 1942.His co-worker, Viggo Laursen, compiled in 1950 more than 1000 publications that had resulted from the polar year. Like Dan la Cour he was an active officer of IAGA, was vice-president during 1957-60 and became its President during 1960-63. In recognition of Viggo Laursens efforts for the ”International Geophysical Year” (IGY) in 1957/58 one of the four World Data Centers for Geomagnetism, WDC-C1, was placed at the Danish Meteorological Institute.

DMI Geomagnetic Observations from GreenlandLa Cour established a geomagnetic observatory in Qeqertarsuaq (Godhavn), Greenland, to continue the DMI polar geomagnetic observations started by Adam Paulsen. The observatory buildings were completed in 1925. Routine geomagnetic observations started on 1 February 1926. During the IGY three geomagnetic observatories were in operation in Greenland, at Thule close to the geomagnetic pole, at Godhavn, and at Julianehaab (later moved to Narsarsuaq).Pioneering research resulted from the polar magnetic observations. Leif Svalgaard, manager of the Geomagnetic Observatory in Qaanaaq (Thule), noticed, for instance, that the vertical component of the magnetic field at Thule displayed either of two different characteristic patterns for the variations during the day. This feature, now named the Svalgaard-Mansurov effect, is due to the consistent toward/away direction of the interplanetary magnetic field within a solar wind sector [Svalgaard, 1973]. On initiative from Johannes Wilhjelm and Eigil Friis-Christensen the geomagnetic observations from Greenland have since 1972 been extended by establishing temporary ”variation stations” along the west coast of Greenland. The array of variometer stations was later extended to the east coast of Greenland and to the ice cap. At present 3 permanent (absolute measuring) and 15 temporary (variation) magnetic observatories are operated in Greenland stretching from the auroral zone to the magnetic pole. They are equipped with modern flux-gate magnetometers and form the most comprehensive array of geomagnetic observatories existing in the polar regions.They have so far supplied data for hundreds of scientific publications and provide now a most useful support of the Ørsted geomagnetic research mission.

The references quoted may be found at the web address:http://www.dmi.dk/fsweb/soljord/nordlys_forsk/nordlys_publ.html A catalog of Harald Moltke’s auroral paintings may be found at:http://www.dmi.dk/dmi/jubilaeum/island.html

Viggo Laursen, Johs. Egedahl, and Dan B. la Cour

Present DMI Magnetometer development and sale .The DMI magnetometry work was later carried on by Emil Kring Lauridsen, Ole Rasmussen and Michel Genevey, who have developed and produced for sale the modern flux-gate magnetometers. An ingenious suspension mechanism for the tri-axial sensor element, careful selection of materials with negligible temperature variations, and precise and reliable electronic circuits have contributed to give an instrument of superior quality. With a production of over one hundred flux-gate magnetometers the total sale of DMI high-precision magnetometers to observatories all over the world exceeds one thousand instruments.

World-wide sale of Geomagnetic InstrumentsThe green coloured regions in world map below indicate the countries where geomagnetic observatories have been equipped with la Cour magnetometer instruments built at DMI. The instrument shown to the right is a QHM instrument for high-precision measurements of the horizontal component of the magnetic field.

QHM instrument

DMI Geomagnetic Observatory in Qeqertarssuaq (Godhavn), Greenland

DMI Ionospheric Observations from GreenlandAs the international radio communication expanded after World War II the need for global ionospheric observations was recognized. In Denmark pioneering theoretical work on the propagation of radio radio waves had been made by Professor P.O. Pedersen from the Technical University [P.O. Pedersen, 1927]. His successor, Professor J. Rybner, in collaboration with DMI, established in 1951 the first Danish ionosonde observing station in Qeqertarsuaq (Godhavn), Greenland, almost co-located with the geomagnetic observatory. In 1957 the operation of the US ionosonde station in Narsarsuaq, Greenland, was transferred to DMI, and in 1966 the ionosonde station at the US research site, Camp Tuto, was moved to Qaanaaq (Thule) and thereafter operated by DMI. In recent years a sizeable amount of polar geophysical observations have been conducted by DMI in Greenland and from other north-polar observatories. The location of the geophysical observatories operated by DMI or equipped with DMI instruments is indicated in the above polar map giving status as of 1990. The largest installation of this kind is the ”Incoherent Scatter Radar” (ISR) in Kangerlussuaq (Sondrestrom). The radar was installed in 1983 by SRI International for US National Science Foundation (NSF). The ISR radar (see photo) is operated jointly by SRI and DMI and serve as the base for a range of atmospheric and ionospheric investigations. These observations now provide useful support for the geomagnetic mission as well as the Ørsted GPS mission.

DMI and the Ørsted satellite projectThe Ørsted satellite project was conceived around 1990. The Ørsted project was given financial support and the construction work started in 1993. The technical management was commissioned to CRI (now TERMA A/S), while the Project Scientist and Science Data Center functions became (and still are) DMI responsibilities.