Solar-Terrestrial Physics Division
A RIOmeter (Relative Ionospheric Opacity - meter) is, in principle, a sensitive, calibrated radio receiver which measures the signal strength of the weak radio signals received from the sky. Most often a vertical antenna is being used. The measuring frequency is normally in the HF-VHF frequency range, typically around 30 MHz.
The signals are radio noise (cosmic noise) generated by stars and galaxies. These signals are virtually invariant in time so the source strength depends only on the direction in space. On their path to the receiving antenna the signals pass through the upper, ionized atmosphere, the ionosphere. Here part of the wave energy is temporarily stored in the forced motion of the free electrons. However, the energy is re-radiated and the wave restored unless the electrons collide with heavier particles like ions and atoms and thereby interrupt their forced motion. Thus the radio wave absorption intensity depends on the number density of free electrons as well as the frequency of electron collisions with the heavy constituents.
By recording the varying signal strength of the received cosmic noise and thus observing the ionospheric absorption one may determine the combined effects of changes in the electron densities and in their collision frequencies. Such changes may occur as the result of solar flare activity or geomagnetic storm and substorm processes. These disturbance events may generate radiation of high-energy electrons and ions, notably protons. Precipitation of the energetic radiation into the upper atmosphere creates additional ionization, typically at 50-100 km of altitude. The electron collision frequencies are high at these altitudes so, consequently, the resulting absorption of radio waves passing through this region will be enhanced.
Such events of precipitation of energetic particles and absorption (black-out) of radio waves in the upper atmosphere in association with solar flares and geomagnetic storms are particularly frequent and intense in the auroral and polar regions. The riometer observations from an extended net of stations are particularly well suited to explore the large-scale temporal and geographical development of such events. These disturbance events reflect the varying interaction between the solar wind and the Earth's magnetosphere as well as the "Space Weather" conditions which may affect satellites and ground systems.
The Solar-Terrestrial Physics Division of the Danish Meteorological Institute operates riometers at 20 stations in the polar region, mainly in Greenland. The location of these stations are indicated in the map of DMI Geophysical Observatories.
