Solar-Terrestrial Physics Division
An Imaging Riometer (Relative Ionospheric Opacity - meter) is, in principle, a set of highly sensitive, calibrated radio receivers which measure the signal strength in various directions of the weak radio signals received from the sky. Usually the riometer system uses an array of antenna elements (e.g. dipoles) which are combined through a so-called "Butler Matrix" to form a number of regularly spaced narrow beams. The figure shows the IRIS antenna system in Sondrestrom. Typical antenna systems comprise 49 or 64 beams distributed in a regular pattern over a field of view extending within 45 degrees from vertical. The measuring frequency will be in the HF-VHF range, typically at 38 MHZ within the protected frequency band reserved for radio astronomy.
The radio noise signals from stars and galaxies are, as outlined in the information page for normal-beam riometer observations, virtually invariant in time so the source level depend only on the direction in space. These signals are used for sounding of the variable absorption intensities in the upper atmosphere to detect the effects of various disturbances relating primarily to solar flare activity and to geomagnetic storm and substorm processes. Such disturbances may create radiation of high-energy particles, notably electrons of energies from tens to hundreds of keV, and protons of energies up to tens of MeV. When precipitated into the Earths upper atmosphere this radiation may create additional ionization at altitudes of 50-100 km which, in turn, cause absorption of penetrating radio waves. The imaging riometer system measures the time-varying absorption intensities in many different directions simultaneously. From these data one may derive sequences of images of the instantaneous absorption intensity distributions.
The imaging riometer technique provides observations with a time resolution of around 1 sec of the dynamic absorption intensity distribution over an area of typically 250 x 250 km referred to an ionospheric level of 90 km. Such observations, in particular when combined with data from other sources like radar or satellite instruments, are presently the best available tool for investigations of disturbance processes in the upper atmosphere and ionosphere relating to the precipitation of energetic particles generated during solar flares and geomagnetic storms. These processes reflect the interactions between the solar wind and the Earths magnetosphere and the "Space Weather" conditions.
The Danish Meteorological Institute, in collaboration with University of Maryland, USA, and the National Institute of Polar Research, Japan, conducts imaging riometer observations from three stations: Kangerlussuaq (Sondrestrom) and Danmarkshavn in Greenland, and Longyearbyen at Svalbard. The location of these stations are indicated in the map of DMI Geophysical Observatories.
