Ground-based Geophysical Observations

The magnetic field of the Earth

The Earth on which we live possesses a magnetic field called the "geomagnetic field". The main part of the geomagnetic field is generated by electric currents flowing in the Earth‘s core. Minor contributions stem from local anomalies caused by reservoirs of magnetic minerals in the Earth‘s crust. Together they constitute the sources of the internal magnetic field. A small part of the total field is caused by external sources, including electric currents in the partially ionized upper atmosphere and in the fully ionized magnetosphere.

The figure to the right shows our concept of the Earth‘s interior, with the magnetic field visualized through field lines extending into space. The Earth's solid inner core contains mainly iron and nickel. It is surrounded by the liquid outer core which consists of highly ionized molten metal, an excellent electric conductor. The liquid metal is in constant motion caused by gravity effects and the Earth‘s rotation. In the presence of a very weak initial magnetic field, the convective liquid generates electric currents which lead to amplification of the initial magnetic field. This process is sustained through the "dynamo effect".  The outer core is embedded in the mantle which consists of highly viscous material and which gradually changes to solid with increasing distance from the centre. The relatively thin and solid outermost shell - termed "crust" - is built up of heterogenous mineralic material which can locally modify the main magnetic field generated in the outer core. The dynamo motion and consequently the geomagnetic field can vary with time which is observed on the Earth's surface as the so-called "secular variation" of geomagnetic field intensity and orientation.

Magnetic field lines are not directly visible but they can be mapped using appropriate means. A freely moving magnetic needle, for instance, tends to align itself with the magnetic field. If the needle is constrained to motions in the horizontal plane it will orient itself toward magnetic north - this is the concept of a compass. The angle between magnetic and geographic north is called "declination". If the needle can move freely in all directions it will almost always point obliquely up or down. Only at the magnetic poles it will be strictly vertical and at the magnetic equator strictly horizontal. The angle between the horizontal plane and the direction of the needle is called "inclination". The geomagnetic field becomes weaker with increasing distance from the Earth. Each point in the space surrounding the Earth (the "geospace") is thus characterised by a certain magnetic field intensity and direction.  

The contribution to the geomagnetic field which is of external origin stems predominantly from electric currents flowing in the upper atmosphere, at 100-150 km altitude. The upper atmosphere is partially ionized, primarily by solar ultraviolet radiation but also by other effects such as energetic charged particle precipitation. Ionized air is a good electric conductor, comparable to conductive sediments in the ground. Electric currents in the upper atmosphere can reach high intensities, particularly after outbursts of solar activity, and the associated geomagnetic disturbances can be substantial. Such events are therefore called "magnetic storms". These storms manifest themselves not only through intense geomagnetic disturbances but also through auroral displays. Under normal conditions aurora is only observed in the "auroral zone" (which includes southern Greenland), but during the most severe magnetic storms it is possible to observe aurora also in Denmark.

DMI's involvement in geomagnetism dates back to its foundation in 1872 and can be considered a continuation of H.C. Ørsted's studies on geomagnetism. Auroral observations and ionospheric research were added later. Today DMI operates a network of magnetic observatories and variometer stations in Denmark and Greenland and is involved in international satellite projects including the Ørsted satellite for which DMI has scientific responsibility.

Ground-based geomagnetic measurements in Denmark

DMI conducts geomagnetic field measurements at the Brorfelde (Zealand) geomagnetic observatory. The aerial picture to the left shows the small, nonmagnetic wooden houses of the observatory where the measurements are carried out. For the magnetic field measurements various instruments are used, among them a certain type of fluxgate magnetometers which was designed and constructed at DMI (see photo below). Using this fluxgate magnetometer, the Solar-Terrestrial Physics Division of DMI provides in real-time the most recent declination measurements from the Brorfelde observatory.

Ground-based measurements in Greenland

The most important ground-based observations of phenomena in the upper atmosphere conducted by DMI or with participation of DMI include:

• Magnetometer measurements of geomagnetic variations resulting from electric currents in the ionosphere and magnetosphere
• Riometer measurements of ionospheric radio wave absorption resulting from energetic particle precipitation
• Incoherent scatter radar measurements of plasma density, temperature and velocity in the upper atmosphere (see photo of the Sondrestrom incoherent scattar radar to the right)
• Digisonde measurements of the ionospheric electron density

The main objective of these measurements is to investigate how solar activity - most strongly manifested through mass and radiation outbursts from sunspots - influences conditions in the Earth environment and affects the Earth's climate. DMI's measurements are used in international collaborations, among them the "International Solar-Terrestrial Physics" (ISTP) programme.  Denmark contributes to this program with important observations from Greenland where the coupling between solar wind and upper atmosphere is particularly strong. DMI's measurements are further important ingredients to GPS-based techniques in weather and climate monitoring since such techniques require proper consideration of ionospheric density fluctuations (which are largest at high latitudes).

More than 75 years of geomagnetic observations in Greenland

Construction of the Qeqertarsuaq geomagnetic observatory

On February 1, 1926 - more than 75 years ago - the first routine observations of the geomagnetic field were made on Greenlandic ground, at the newly built Qeqertarsuaq observatory (photo to the left). This happened 14 months after the "International Union of Geophysics and Geodesy" (IUGG), during its General Assembly in Madrid, had adopted two resolutions which stressed the importance of conducting geomagnetic observations in Greenland and strongly recommended the establishment of a geomagnetic observatory. The text of the resolutions reads:

"In view of the geographical position of Greenland and of the importance which continuous magnetic and electric data obtained in this region would have for the general subject of the Earth’s magnetism and electricity, it is considered highly desirable that a permanent observatory for such purposes be established at the most suitable side on the west coast of Greenland."

"That the Section deems it highly desirable to call attention to the need of additional magnetic and electric observations in high latitudes especially north of 60° N and south of 50° S."

The Minister of Interior at that time, C.N. Hauge, gave his approval to build an observatory in Qeqertarsuaq (formerly Godhavn), and he himself attended the laying of the foundation stone on September 6, 1925. The Ministry of Naval Affairs to which DMI belonged at that time agreed to lend instruments for magnetic field measurements and to provide observatory equipment. The observatory was eventually established by DMI's director, Dan la Cour, an expert in magnetic field measurements. He even participated personally in the construction work in Qeqertarsuaq. The photo to the right shows the preparation for the topping-out ceremony.

The first manager of the Qeqertarsuaq geomagnetic observatory was Dr. G. Ljungdahl from the "Kungliga Sjökarteverket" in Stockholm, a scientist who had previously worked in the field of geomagnetism. From the very beginning he participated in geomagnetic mapping and exploratory investigations of the area and selected the final location of the observatory. For the preparation of the concrete foundation he chose to have brought sand from places some 100 km away, mainly from "Kronprinsens Ejland", because the local sand was considered too magnetic and was thought to have undesired influence on the magnetic precision measurements.

Instrumentation of the geomagnetic observatory

Ljungdahl set up sensitive geomagnetic instruments in the winter of 1925/1926. The observatory was not connected to an electric power line, and power for the operation of the instruments was supplied from batteries which had to be regularly recharged at the nearby radio station (Godhavn Radio). The quality of the geomagnetic measurements benefitted much from la Cour's internationally recognized expertise in the construction of high precision instruments. La Cour, together with his young assistent Viggo Laursen, had developed a series of robust and extraordinarily precise magnetometers with low power consumption which proved to be excellently suited for the cold and harsh environment of Greenland.

These instruments were built in large numbers at the Danish geomagnetic observatory Rude Skov, and one particular type, the so-called "LaCour magnetometer" (photo to the left), was sold in several hundred copies to observatories all over the world.

A year after, in 1927, Ljungdahl was replaced by the young Johannes Olsen who later became one of DMI's most distinguished and internationally acknowledged scientist in the field of geomagnetism.

The home of the manager

An attractive house was built (photo to the right) which served as living quarters for the manager but also as laboratory space. Here the films with the magnetic registrations were developed, magnetograms were  examined, calibration constants were determined and magnetic indices were derived. Obviously, human activity in the observatory proper had to be limited to a minimum in order to avoid  any unnecessary disturbance of the magnetometer measurements.

The first managers of the observatory came from Denmark while Greenlandic support staff was hired locally. The first assistant was Ole Mølgård who for many years performed excellent work at the observatory. Housing conditions for the Danish staff were at that time much different from those for the Greenlandic staff (see photo to the left). In recent years the observatory was run exclusively by local staff.

Results from the geomagnetic observations

The Qeqertarsuaq geomagnetic observatory has since more than 75 years supplied magnetometer data of highest quality. Measurements from Qeqertarsuaq have significantly contributed to building models of the (slowly varying) internal magnetic field and to mapping of the drift of the magnetic pole. Qeqertarsuaq is located at a latitude where the geomagnetic field is often open to the interplanetary space (this is the "cusp region"), with the consequence that the geomagnetic field is most directly and strongly influenced by variations in the solar wind. The magnetic observations from Qeqertarsuaq were used in many hundred scientific publications dealing with the coupling between solar activity, the solar wind and the Earth's upper atmosphere, among them ionospheric electric currents and aurora borealis.

The comprehensive scientific work on the characteristics and dynamics of the magnetic field in polar regions which had to a large extent been performed at DMI was an important factor among those which eventually led to the Ørsted satellite project. Geomagnetic measurements from DMI's refurbished Qeqertarsuaq observatory (photo to the left) play an important role in ongoing scientific work based on the very successful magnetometer measurements from the Ørsted satellite.

For further information about the observatory in Qeqertarsuaq and the magnetic measurements in Greenland, please contact: Civilingeniør Ole Rasmussen, responsible for DMI’s magnetic measurements. Phone: +45 39 15 74 75, e-mail: or@dmi.dk or Peter Stauning, phone: +45 39 15 74 73, e-mail: pst@dmi.dk

The Greenland chain of variometer stations

Back in the 1970's DMI began to establish a chain of magnetometers along the Greenlandic coasts (with the exception of the north coast) as a contribution to the "International Magnetospheric Study" (IMS) programme (figure below). The chain has been in continuous operation since then but has undergone several phases of modernization. The stations are so-called "variometer stations", i.e., they do not conduct absolute measurements of the geomagnetic field but temporal deviations from a reference level ("geomagnetic variations"). Therefore the stations do not possess observatory status.

The magnetometer system

The first data acquisition systems were analog recorders but since 1981 DMI has acquired variometer data in digital form. From 1981 through 1990 all stations recorded with 1 min sampling rate. In 1986 DMI began to gradually modify the acquisition systems in order to record with 20 s sampling rate. Modification was completed by 1991, and since then all stations run at 20 s sampling rate. In 1999 work began on upgrading to 1-s sampling rate. Work was completed in the summer of 2002, and since then all stations record at both, 1 s and 20-s sampling rate.

The vast majority of the sensors now employed are three axes linear core fluxgate magnetometers designed and built at DMI under the supervision of O. Rasmussen. It is the same type of instruments which is also used in Brorfelde, see photo under the magnetic field of the Earth. The magnetometers are optimised for robustness and long term stability rather than for high sensitivity. The rms noise is approximately 0.1 nT in the 1 mHz – 1 Hz band. The instruments are set up in the field such that the sensor axes are oriented toward local magnetic north (H), local magnetic east (E) and vertically down (Z). Some stations are equipped with a gimbal system which guarantees gravity-controlled vertical alignment of the sensor block.

The instruments run fully automatically most of the time and require (under normal conditions) no manual intervention except for clock adjustment and change of storage media approximately once every three month. This is usually performed by locally hired caretakers.

Scientific results

• The Greenland magnetometer chain contributed to the discovery and mapping of various high-latitude electric currents systems. Among them are the so-called "DBY current" which is a daytime ionospheric current system controlled by the east-west component of the interplanetary magnetic field, and the "NBZ current" which is a near-noon field-aligned current system at polar latitude controlled by the north-south component of the interplanetary magnetic field.
• The Greenland magnetometer chain was instrumental in the detection of "Traveling Convection Vortices" (TCV) which have become a field of research on their own. Research on TCV continues in various countries, but a satisfactory and comprehensive explanation is still missing.
• Following initial work of O. Trochichev (Petersburg, Russia) who developed the "Polar Cap" (PC) index from Vostok (Antarctica) magnetometer data, a PCN ("Polar Cap North") index was developed at DMI. It is routinely computed from Qaanaaq (THL) magnetometer data and is made available in a preliminary form to the scientific community.
• Greenland magnetometer data were used in numerous scientific publications and presentations, sometimes on their own, sometimes together with radar and satellite observations. In many studies they were essential for obtaining results and reaching conclusions.

December 2003