Satellite Observations

DMI is heavily involved in the Danish Ørsted satellite project for which it has the scientific leadership, and DMI houses the Ørsted Science Data Center. DMI is further involved in the Argentinean-American-Danish SAC-C satellite whose magnetic mapping payload is similar to Ørsted's instrumentation. DMI participates in analysis and interpretation of measurements from the German CHAMP satellite which is equipped with GPS receivers and Danish built magnetometers and star cameras similar to those used on Ørsted. Finally, DMI has started collaboration with the team behind the Australian FedSat which carries measurement equipment similar to that used on Ørsted.

The Ørsted Satellite

The first (and so far sole) Danish satellite bears the name "Ørsted" in honor of the Danish physicist and former professor at Copenhagen University, H.C. Ørsted (1777-1851), who in 1820 discovered electromagnetism and in 1834 laid the foundation for regular geomagnetic observations in Denmark. The Ørsted satellite project - in which DMI plays a major role - became reality through international collaboration which includes Danish research institutions and private industry as well as European and overseas partners. The Ørsted satellite (photo to the right) was launched on February 23, 1999, as a piggy-back mission to the American ARGOS satellite, on top of a Delta-II launch vehicle, which sent it into a slightly elliptic slowly precessing high-inclination orbit at 630–870 km altitude.

The Ørsted satellite falls with its mass of only 61 kg into the category of small satellites. It carries a highly specialised payload which includes high-precision sensors for registration of the geomagnetic field, charged particle fluxes, and dual-frequency reception of GPS signals. DMI is responsible for the following themes within the Ørsted satellite project:

• Communication link between ground and space for satellite control and science data reception (using an antenna mounted on the roof of the DMI building, see photo below).
• Ørsted Science Data Center (ØSDC) for data processing and distribution
• Scientific project management and international scientific coordination
• Responsibility for external field studies
• Analysis of Charged Particle Detector (CPD) data and study of high-energy radiation
• Analysis of magnetic field vector data from the fluxgate magnetometer (CSC) and studies of electric currents in the upper atmosphere
• Analysis of GPS receiver (TurboRogue) data for investigations of ionospheric plasma density and for applications in weather and climate monitoring and forecasting.

The Ørsted satellite had a nominal lifetime of 14 months two of which were reserved for commissioning and 12 for science operation. However, the satellite has by now completed almost 5 years, is still in operation and continues to deliver scientifically useful data. The number of excellent research results  is already large and steadily growing.

Instruments onboard Ørsted

The main sensors onboard the Ørsted satellite are two different magnetometers which can measure the geomagnetic field with unprecedented precision. The satellite is further equipped with high-energy particle detectors and a complex GPS receiver used for atmospheric profiling. The instruments are described in the following.

Fluxgate magnetometer (CSC)

The Overhauser magnetometer (OVH)

Charged Particle Detector (CPD)

The Star Imager (SIM)

Multi-frequency GPS receiver

TurboRogue)

Results from Ørsted

The Ørsted satellite project - scientific results and achievements

The primary motivation for the Ørsted satellite mission is the precise global and regional mapping of the geomagnetic field. Since the last mission with a similar task – the Magsat satellite – some 20 years have passed during which no high-precision measurements of the geomagnetic field were conducted from space. Ørsted became therefore a timely and important project which stood at the start of the "geopotential decade".

Ørsted has contributed to several international geomagnetic field models

• Ørsted magnetometer measurements form the base for the latest International Geomagnetic Reference Field, the IGRF2000, which is the official IAGA standard, valid until 2005. The global precision is better than 10 nanoTesla (nT) which is remarkable for a field whose total strength reaches nearly 70,000 nT near the magnetic poles.
• Improved models – the "Ørsted Initial Field Model" (OIFM) and the "Ørsted Main and Secular Variation Model" (OSVM) – are widely used. The precision is better than 5 nT.
• Further improvements which account for certain external fields such as Dst – among them the "CHAMP-Ørsted Model" (CO2) and a "Comprehensive Model" (CM3) – arrive at a precision of better than 3 nT.

A graphical representation of the total magnetic field strength at the Earth's surface inferred from the IGRF2000 is shown below. The blueish-black range of colors represents a field strength above the mean field at the surface and the reddish-yellow range a field strength below the mean field. Obviously, the geomagnetic field is strongest near the magnetic poles (north-east Canada and the Australian side of the Antarctis). A secondary maximum appears in Sibiria. The most pronounced minimum is found in the south-eastern part of South America and in the South Atlantic, it is known under the name "South Atlantic Anomaly" (SAA). The main reason for the Sibirian maximum and the South Atlantic minimum is the "excentric dipole" nature of the geomagnetic field: if the field were approximated by a simple dipole the dipole would need to be placed about 700 km away from the centre of the Earth in order to render best agreement.

Ørsted magnetometer measurements are used for mapping magnetostatic anomalies

• Magnetostatic anomalies (crustal anomalies) were mapped with the help of Ørsted measurements with a local precision better than 1 nT. This is specifically valuable in regions which are otherwise difficult to access (such as oceans and desert areas).

Ørsted magnetometer measurements are used for determination of electric currents in space

The most important external currents systems which are mapped and modeled using Ørsted data include the following.

• The magnetospheric ring current is represented by a model which is parameterized with the magnetic storm index Dst and magnetic local time.
• High-latitude field-aligned currents are modeled as a function of solar wind conditions, season, magnetic storm conditions, and hemispheric attributes.
• The auroral and equatorial electrojets, intense large-scale electric current systems in the ionosphere, are modeled using Ørsted magnetometer data.

Ørsted data enhance our understanding of spacecraft computer failures due to hard radiation

• The radiation belts in the space around the Earth are populated with highly energetic particles. The intensity of particle fluxes capable to penetrate spacecraft subsystems is higher than previously thought.

Ørsted GPS observations have contributed to the development of methods for GPS based atmospheric profiling

• Methods have been developed to improve atmospheric profiling using satellite based GPS measurements and to pave the way for assimilation of results from GPS measurements into weather and climate models.

The figure below shows two profiles of the air temperature as a function of altitude. One is derived from Ørsted GPS measurements and the other from a model provided by ECMWF (European Centre for Medium range Weather Forecast). The reference location is at 37 deg southern latitude and at 209 deg eastern longitude and the reference time is 12 UT.

Scientific Publications

• Almost 100 articles have been published or are in press in international technical journals and monographs
• More than 60 papers have appeared in international proceedings volumes.
• More than 200 oral and poster presentations have been given at international conferences

Large space technology contracts were won by Danish universities and industry

• The Danish Technical University (DTU) and Danish enterprises have secured international contracts in the fields of magnetometer and star camera design and construction at a value of about 100 mio Danish Crowns.

The interest in and awareness of science and technology has increased among students at schools as well as at universities.

• The spacecraft engineering curriculum at Ålborg University (ÅU) has received huge attention from the students. A spacecraft engineering training at DTU has started. Both ÅU and DTU have in the mean time launched microsatellites built by engineering students.
• Many school classes have chosen the Ørsted satellite project as their research topic and have with great enthusiasm researched information about Ørsted in newspaper articles, via the Internet and through direct contact with the Ørsted team.

High interest for the Ørsted satellite in the general public

• Nation-wide recognition of excellent Danish achievements in space research and technology
• Ørsted has contributed to a favorable public attitude towards science and technology

Many results from the Ørsted project are described in more detail in the report "Ørsteds Resultater" by Peter Stauning,  DMI Techn. Rep. # 11-2002 (in Danish).

December 2003