DANISH SPACE WEATHER ACTIVITIES

Eigil Friis-Christensen and Therese Moretto Danish Space Research Institute Juliane Maries Vej 30 DK-2100 Copenhagen Denmark

ABSTRACT
HISTORICAL BACKGROUND
PRESENT DANISH ACTIVITIES
GROUND BASED FACILITIES
OBSERVATIONS IN SPACE
SPACE WEATHER AND CLIMATE
SUMMARY
REFERENCES

ABSTRACT  

Activities related to the Earth’s electromagnetic environment are carried out in Denmark mainly at the Danish Meteorological Institute, DMI and at the Danish Space Research Institute, DSRI.

The Solar-Terrestrial Physics Division of DMI conducts observations of magnetic disturbances and precipitation of high-energy particles in the auroral and polar regions. The observational network includes the Greenland chain of ground-based magnetometers and a number of riometers and imaging riometers detecting precipitation of energetic charged particles.

DSRI has traditionally been engaged in space plasma experiments on board ESA satellites as well as on other agency missions. For some time DSRI has had its emphasis on astrophysics but this year a solar system physics department has been formed with the objective of conducting research with regard to the various forces in the solar system, including their effect on the Earth and its environment.

HISTORICAL BACKGROUND  

Denmark has maintained a programme of observations and research in Solar-Terrestrial Physics for more than a century, most of which has been based at the Danish Meteorological Institute, DMI. The programme has its background in issues that arose from the needs of society related to applications, which today we call space weather. Magnetic observations were regarded important because of the responsibility of the state to update maps for navigational purposes to include current values of the declination. In particular, Greenland and the surrounding ocean was characterised by significant deviations between magnetic and geographic North as well as fast changes to this angle, which called for frequent updates of the maps.. In addition, when making observations in these regions close to the magnetic pole, it was realised that a "correct" value of the geomagnetic field was difficult to obtain. The observed values include large and variable contributions from external currents. First in the 1970s was it actually demonstrated that these contributions have two main components, namely one part coming from currents in the ionosphere, another corresponding to currents along the magnetic field lines which are nearly vertical at these high latitudes.

Right from the beginning of magnetic measurements, of course, these unexplained variations in the geomagnetic field were subject to scientific investigations. The Danish Meteorological Institute participated in the First International Polar Year 1882-1883 by conducting expeditions to Greenland to observe the magnetic field and its variations from hour to hour. The leader of the expedition was A. Paulsen, who later became Director of the Institute.

Another Director of DMI, D. la Cour, became a very active president of the Commission for the Second International Polar Year 1932-33. His design and manufacturing of geomagnetic instruments were widely recognised and his instruments were used for many years by magnetic observatories in large number of countries.

Another aspect of solar-terrestrial research was pursued at the Danish Technical University (DTU). Professor P.O. Pedersen (after whom the 'Pedersen conductivity' is named) in 1927 published a book "The propagation of radio waves along the surface of the Earth and in the atmosphere". In this book he predicted the physical properties of the ionosphere based on observations of radio wave propagation. Radio communication between Denmark and Greenland was of course very important, and it was soon realised that this communication was extremely vulnerable to conditions related to solar activity. This led to the establishment of a research and monitoring programme, the Ionosphere Laboratory, at DTU. The Laboratory operated several ionospheric observatories in Greenland and soon formed a ionosphere research programme which included experiments with balloons and rockets, and, eventually, participation in the satellite program of the European Space Research Organisation, ESRO. With the establishment of the Danish Space Research Institute (DSRI) in 1968 the space related part of the Ionosphere Laboratory of DTU was transferred to DSRI while the remaining ground-based activities were transferred to DMI.

Both having their roots in rather practical needs DMI and DSRI continue to be the main institutions in Denmark associated with space weather related issues, including a comprehensive solar-terrestrial research programme based on measurements from ground and from space.

PRESENT DANISH ACTIVITIES  

At the Solar-Terrestrial Physics Division and the Atmosphere Ionosphere Research Division at the Danish Meteorological Institute the present activities within Space Weather Research comprise:

• Relationships between processes on the Sun, the high-latitude ionosphere and the inner magnetosphere
• Real-time availability of selected measurements and indices from Greenland
• Implementation of data-driven dynamic neural network models for forecasting selected space weather parameters
• Studies of space weather effects on atmospheric weather and climate

Since the International Geophysical Year in 1957-58 DMI is the host of the World Data Centre, WDC-C1 for Geomagnetism. The archives contain World-wide geomagnetic data in the form of minute and hourly average values. WDC is part of an international effort, co-ordinating the derivation and distribution of various geomagnetic activity indices for users of various kinds, including space weather activities. In this international effort DMI has taken on the responsibility of producing the Polar Cap index, PC, which is based on the magnetic observatory, Thule, in Greenland at very high magnetic latitude. The PC index is defined as a measure of the Polar Cap electric field, which is an important parameter closely correlated with the energy input to the polar ionosphere, which, in turn, is indicative of the amount of energy transferred to the magnetosphere from the solar wind.

At the Solar System Physics Group at the Danish Space Research Institute the main space weather related activities comprise research efforts within the following topics:

• Geomagnetic field modelling including the radiation belts, anomaly field, and the location of the auroral oval
• Geo-effectiveness of storms - in particular efforts to broaden the scope of geomagnetic diagnosing
• Solar/solar wind sources of storms – in particular the identification of the source structures at the Sun and the influence of the state of the heliosphere
• Space climate - long term variations in solar activity
• Solar activity influence on the Earth’s atmosphere
• Cosmic rays, cloud formation, atmospheric chemistry

GROUND BASED FACILITIES  

A main asset of Danish space weather activities is the unique network of geophysical stations in Greenland. It includes three magnetic observatories with accurate absolute observations appropriate for main field studies. In addition, it comprises 14 magnetic variation stations, which provide relative measurements adequate for external field studies, e.g. the dynamics of field-aligned and ionospheric currents.
 

Figure 1. Map of the geophysical stations in Greenland  

Several other types of geophysical observations complement the magnetic measurements. These include the U.S. incoherent scatter radar facility operated since 1982 and a riometer network consisting of single beam stations, multibeam stations and imaging riometer installations. Finally, also a network of U.S. digital ionosondes is operated on a routine basis.

OBSERVATIONS IN SPACE  

The value of the ground-based network for space weather research is greatly enhanced by simultaneous measurements in space. The Danish satellite project, Ørsted, was conceived as a dedicated geomagnetic research satellite with high accuracy measurements suitable for studies of the magnetic field of the Earth’s interior. Another important aspect was the possibility to make accurate and systematic measurements of the external contribution to the measured magnetic field, which will disclose new features of solar-wind magnetosphere interactions and the distribution of field-aligned currents. In addition, the satellite carries a charged particle detector, which will be operated in conjunction with dedicated ground-based facilities in order to elucidate the nature of the precipitation of energetic particles.

The Ørsted satellite is planned for launch in January, 1999. Approximately one year later an Argentine-U.S. satellite, SAC-C will be launched carrying a "copy" of the Ørsted magnetic experiment. In December 1999 a German satellite, CHAMP, will be launched also with geomagnetic instruments built by the Danish team.

The Danish Space Research Institute is following-up on this demonstrated European leadership by proposing to ESA the "ultimate" geomagnetic research mission consisting of a constellation of six satellites, "SWARM", measuring simultaneously the geomagnetic field at different locations and at different local times. This enterprise will imply a quantum leap in modelling of the geomagnetic field, which is crucial for a number of space weather activities, because it will allow for modelling that considers both the internal and the time varying external fields at the same time.
 

Figure 2. The various components to the magnetic field as measured close to the Earth and their typical time-scales.

In addition to the increased accuracy of the geomagnetic field models the missions can provide directly usable parameters of the state of the magnetosphere. A set of particularly important parameters is the location and width of the auroral oval and the intensity of the associated field aligned currents. This may be obtained in the following way:

• Ørsted: will make 4 traverses per orbit (ca. 100 minutes), at two local times
• Dual configuration, Ørsted / SAC-C: 8 traverses per orbit, at 4 local times
• Multiple satellite (6) configuration e.g. SWARM: 24 traverses per orbit, at 4 local times

In addition, SWARM would be perfect in deriving a global satellite-based geomagnetic activity index because of the global coverage of the observations.

SPACE WEATHER AND CLIMATE  

A controversial topic is whether the solar activity, which manifests itself in the variations in space weather also has an effect on the lower atmosphere, the troposphere. Many studies indicating a correlation between solar activity and climate parameters have been reported. This indicates that there might be a physical link. Recently, Danish researchers have found that the total cloud cover as observed from satellites is very well correlated with the cosmic ray intensity, which, in turn, is modulated by the solar wind in a systematic way. The detailed mechanism for this effect is not explained yet. However, if future research reveals the physical mechanism responsible for the observed correlation, the importance of the state of the heliosphere, including space weather, will, quite obviously, be greatly enhanced.

Figure 3. The cosmic ray flux and total cloud cover, Svensmark and Friis-Christensen, 1997.

SUMMARY  

The solar-terrestrial research programme in Denmark has derived from genuine space weather related needs in the past. Today the two institutions, DMI and DSRI, are in a good position to provide space weather services based on the acquired expertise and the ground- and space facilities available.

REFERENCES  

Svensmark, H., and E. Friis-Christensen, Variation of cosmic ray flux and global cloud coverage - A missing link in Solar-Climate relationships, J. Atm. Sol-Terr. Phys., 59, 1225, 1997.

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