The Advanced Ionospheric Sounder

The Advanced Ionospheric Sounder (AIS) (Dudeney et al., 1982), a National Oceanic and Atmospheric Administration ( NOAA) high frequency radar (Grubb, 1979), operating as a dynasonde (e.g. Wright et al., 1988a, 1988b; Wright, 1990; Wright et al., 1990), measures the time of flight, amplitude and phase of radio signals transmitted from the ground and reflected from the ionosphere, as a function of frequency and time. The equipment was deployed at Halley in January 1981 (Dudeney, 1981), but major developments to the hardware and the operating software have been made since then. For example, in 1991 the aging computer system was replaced by a powerful PC-architecture machine logging the data to optical disk, and the operating system was rewritten.

The AIS operates over the frequency range 0.1-30 MHz, numerous frequency sounding patterns may be selected under software control. For transmitting, a 30 m high vertical delta antenna is used for frequencies up to 3 MHz; above this frequency a log-periodic curtain mounted between two 45 m high masts separated by 134 m is employed.

There are two receivers which are switched under computer-control to 6 non-resonant dipoles laid out in an L-shaped array. The advantages of this array are increased discrimination for polarisation measurement, unambiguous echo-location, and an increased range of velocities over which unambiguous Doppler shifts can be determined (Jarvis and Dudeney, 1986).

The AIS normally records coherent echoes from the ionosphere over an area approximately 100 km radius from Halley when the local ionosphere is smooth and horizontally stratified. When there is significant tilting of the iso-ionic contours and/or ionospheric irregularities are present the fov increases to about ▒500 km in the north-south plane and ▒300 km in the east-west plane, at F-region altitudes.

The AIS can be programmed to produce almost any desired sequence of soundings in the time and frequency domain, but experience has resulted in a small suite of `recipes' which are routinely used, and known as I-, K-, B- and P- modes. I-mode reproduces the swept frequency ionosonde recording, but with the additional ability to determine angle of arrival, polarisation and Doppler velocity of the received signals. K-mode gives a time series, at high time-resolution, of a small number (normally less than 10) of fixed frequencies. B-mode is a combination of I- and K-modes, in which the I-mode frequency sweep is split into small ramps, each made up of a small increment in frequency. These are repeated more than once (as in K-mode) before proceeding to the next ramp. This provides significantly higher sensitivity for Doppler velocity measurement than does I-mode. For I-, K- and B- modes, standard peak-fitting techniques are used to identify echoes. For P-mode soundings, the full data-set sampled at 5 Ás intervals is recorded to allow more sophisticated post-processing. Each of these modes is described in further detail by Wright ( 1975) and by Wright and Pitteway (1979, 1982). At Halley, the routine programme is B-mode every 15 minutes, with other sounding types interleaved as required for special studies.

The accuracy with which the AIS can determine the location of echoes is affected by many parameters, but Wright (1990) has suggested that 1 accuracy in the angle-of-arrival measurements may be possible, although ~3 is more typical (equivalent to about 15 km horizontal range uncertainty for F-region heights). Wright et al. (1988b, 1990) have compared the electron density distribution determined from the AIS at Troms°, Norway with profiles determined by the co-sited European Incoherent Scatter Radar (EISCAT). Sometimes, particularly during disturbed periods, considerable discrepancies can occur. These can be largely understood in terms of differences between the range and temporal resolution of the instruments, and their fovs. However, it should be noted that ionospheric sounders are the only ground-based instruments which provide an absolute measurement of ionospheric electron density.

Data analysis techniques which allow plasma flow velocities to be determined in the vicinity of Halley from AIS data have recently been developed (Jarvis, 1992). Data of this nature are particularly important for extending PACE flow measurements equatorward, and for ion-neutral coupling studies involving comparisons with Fabry-Perot interferometer data.

Over the last decade the AIS has been used to investigate numerous topics concerning solar wind-magnetosphere-ionosphere coupling with considerable emphasis on studies of the main ionospheric trough. The topics of trough morphology studied include:

These studies have led to new insight into trough formation at mid and high geomagnetic latitudes, which are presented in Rodger et al. (1992).

Auroral arcs and their relationship to the ionospheric structures detected by the AIS have been studied (North and Jarvis, 1988), and the AIS has been shown to be a sensitive detector of very localised fluxes of energetic particle precipitation (Jarvis et al., 1990).

Combined AIS and magnetic pulsation data from Halley have been used to investigate the ionospheric response to pulsations both at Pc1 (Aslin et al., 1991) and Pc3-4 (Jarvis and Gough, 1988) periods. The latter study has shown that detection of ULF waves through ionospheric sounding can provide a sensitive indicator of the plasmapause position.

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