The UK Solar System Data Centre (UKSSDC) stores a collection of RIOMETER traces in physical rolls of paper. These data were produced at various stations in Northern Europe, during the time period 1975-1995.

A RIOMETER (Relative Ionospheric Opacity Meter) is an instrument used to quantify the amount of electromagnetic wave ionospheric absorption in the atmosphere. As the name implies, a riometer measures the "opacity" of the ionosphere to radio noise emanating from distant stars and galaxies.

In the absence of any ionospheric absorption (i.e. no outbursts from the sun, no ionospheric disturbances, and no interference from terrestrial sources), this radio noise, averaged over a sufficiently long period of time, forms a Quiet-Day Curve (QDC). Increased ionization in the ionosphere will cause absorption of radio signals (both terrestrial and extraterrestrial), and a departure from the quiet-day curve. The difference between the quiet-day curve and the riometer signal is an indicator of the amount of absorption, and is measured in decibels. Riometers are generally passive radio antenna operating in the VHF radio frequency range (~30 MHz).

Riometers are used to study the ionospheric D region (60 km to 90 km above the surface of the Earth) by measuring radio wave absorption. The smooth drop in radio energy from dawn to noon and the increase from noon to dusk is well-understood in terms of the sun’s ionizing effect on the D region and the radio wave absorption that results. However, the depth of daytime absorption varies considerably, especially in the winter. This variation is believed to be caused by changes in the chemical composition of the D region, which are in turn due to winds blowing between high and middle latitudes. This process is not fully understood, so observing absorption and comparing it to other measurements may yield clues to what is happening in this part of the atmosphere.

Riometers also show the impact of solar flares and charged particles on the D region. These effects are related to solar and geomagnetic disturbances, and are of interest in the study of space weather. Space weather effects at midlatitudes are rather poorly understood.

Observations of normal D region absorption and solar flares are limited to daytime hours. Absorption due to particle precipitation can occur at night in conjunction with auroral disturbances, but these events are most common at high latitudes. At midlatitudes, auroral absorption may only occur a few times each year, depending on the latitude and the current progress of the 11-year solar sunspot cycle.