Instrument description

What riometer measures?

Riometer (Relative ionospheric opacity meter) measures cosmic radio noise absorption (CNA) in the D-region of ionosphere. Frequencies used for the narrow band measurement are typically reserved for the military communication, so time to time transmitters can saturate the receiver. The background signal from the sky has diurnal variation. One of the strongest radio sources on the sky is Cygnus α. The diurnal variation of the signal is know as quiet day level, which are used as a reference level for the observed absorption.

SGO Riometer Instrumentation

Today SGO has three different riometer types in operation:

The traditional LaJolla riometers (30.0 and 32.4MHz)
A novel spectral riometers (20-55MHz)
Riometer application from KAIRA LBA (20-80MHz)

The spectral riometers will replace the old instrumentation. SGO has been also involved to the operation of IRIS imaging riometer at Kilpisjärvi owned by University of Lancaster.

La Jolla riometers

The riometer receiver system consists three main parts: the receiver antenna, riometer radio and datalogging system.

Receiver radio

SGO uses analogical La Jolla riometers. The riometer measures the signal received from the antenna by compensating the signal (0-7.6VDC) and cthe compensation voltage is recorded (0-10V in 16-bit values). During the absorption events the signal level drops from the quiet time level, which need to be defined. Active flares in the Sun and lightning discharges are spikes in the raw signal. The solar flares are more common during the high solar activity. The riometer radio unit locates in the temperature controlled housing nearby the antenna.

Receiver antenna

SGO Ja Jolla riometer has dual half-wavelength dipole antenna, which forms 60 ° wide beam towards zenith of the sky. The antenna is rather sensitive for deformation and ice forming in winter conditions. Also it reacts to wet weather conditions, which is seen in the quality of the data.

Datalogger

SGO La Jolla riometer signal is time stamped with GPS PPS and sampled with 10Hz from 16-bit AD-converter. This is made in shared 8 channel AD-converter unit with SGO 40Hz induction coil magnetometers. The units are built in SGO. Parallel to riometer voltage the riometer housing temperature is recoreded to see the radio is in stabile temperature as the outdoor temperature in Finland varies from from above +30 °C to below -40 °C.