The relative ionospheric opacity meter (riometer) is a radio receiver used for monitoring the intensity of cosmic radio noise. The principle adopted is that if no absorption of the radio signals occurs in the ionosphere, the signal intensity repeats every sidereal day (23 h 56 m). If the signal falls below that expected (the quiet day level) then ionospheric absorption has occurred. This absorption results predominantly from the effects of increased ionisation in the D-region of the ionosphere, which at high latitudes, is usually caused by the effects of energetic particle precipitation (electrons or ions). Far less commonly, increases in ionospheric absorption may be caused directly by the ionising radiation associated with solar flares; for this to arise the observing station must be in daylight.

When the maximum electron concentration of the F region exceeds about 4 1011 m 3, additional absorption arising from the electron-ion collisions at F-region altitudes becomes important. Thus spatial measurements of riometer absorption can also be used to track regions of enhanced F-region concentration (Rosenberg et al., 1993).

A 30 MHz riometer has operated continuously at Halley since 1972 with a three element yagi antenna pointing at the south celestial pole. As part of a joint project with the University of Lancaster, the system was replaced in 1981 by four La Jolla riometers to provide spatial coverage. Each riometer records in one of the invariant magnetic cardinal directions at 45 elevation. With this configuration of riometers, the horizontal gradient of absorption and the horizontal speed and direction of precipitation events can be determined under some circumstances. Each riometer is sampled once per second and has a sensitivity of 0.1 dB. Further details of the riometer system and the way in which such data sets can be exploited are given in Hargreaves and Jarvis (1986). Comparisons between the broad-beam systems, such as that deployed at Halley, and the narrow beam imaging system are given by Rosenberg et al. (1991). The Halley system is not suitable for observing small-scale absorption features (~10 km). Alternative methods for detecting small patches of precipitating particles utilising the experiments at Halley are described by Jarvis et al. (1990).

The variations of absorption observed at Halley and Siple Stations (75.6 S, 83.6 W, L=4.3) as a function of geomagnetic activity, local time and season are described by Rosenberg and Dudeney (1986). In addition, these authors show that the mean level of absorption over Halley is about twice as great as that over Siple; a result of Halley being in the vicinity of the South Atlantic Geomagnetic Anomaly where the field is weak, and the likelihood of precipitation of energetic particles is greatly increased.

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