Geomagnetic and Auroral Research In Denmark

1
Geomagnetic and Auroral Research In Denmark
Ørsted to Ørsted
Geomagnetism at DMI after 1901 The 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.
Professor H. C. Ørsted and the Ørsted
Satellite The 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)
Development of high-precision magnetic
instruments at DMI The construction of magnetic
measuring instruments was initially headed by
DMIs 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.
The Danish Meteorological Institute and
Geomagnetism Compas-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.
World-wide sale of Geomagnetic Instruments The
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.
Viggo Laursen, Johs. Egedahl, and Dan B. la Cour
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.
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.
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.
QHM instrument
International Polar Geomagnetic Observations Dan
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.
Auroral Spectral Observations and Paintings In
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.
DMI Geomagnetic Observatory in Qeqertarssuaq
(Godhavn), Greenland
DMI Geomagnetic Observations from Greenland La
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.
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.
DMI Ionospheric Observations from Greenland As
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.
Harald Moltke Top of Sulur.
Moltkes Paintings of Auroras An 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
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.
DMI and the Ørsted satellite project The 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.
The references quoted may be found at the web
address http//www.dmi.dk/fsweb/soljord/nordlys_f
orsk/nordlys_publ.html A catalog of Harald
Moltkes auroral paintings may be found
at http//www.dmi.dk/dmi/jubilaeum/island.html
Active aurora over Iceland at 2030 on 11
November 1899 painted by Harald Moltke. The
instrument in the foreground is a spectrograph.
Peter Stauning. Danish Meteorological Institute.
September 2002. pst_at_dmi.dk