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Since 1981, a phase-sensitive digital ionospheric sounder has been in operation at Halley (76°S, 27°W), the most southerly of the British Antarctic Survey's [BAS] research stations. This instrument is a NOAA HF radar (Grubb, 1979) which was upgraded during 1991 with new digital signal processing and time sequence generation boards, standard PC architecture and high capacity optical disc storage for the raw data. This upgrade was a joint venture between BAS and Utah State University (USU), the former concentrating on the machine software and the latter on the hardware. Identical hardware and software is now in operation at the USU Bear Lake Observatory and the European Incoherent Scatter Radar site near Tromso. The instrument at Halley is referred to as the Advanced Ionospheric Sounder [AIS] (Dudeney, 1981) and primarily operates as a dynasonde (Wright and Pitteway, 1979). A rapid-fire, four-pulse transmission sequence is employed at each sounding frequency. For each pulse, two parallel receivers allow the comparison of echo phases between selected pairs of dipoles from an array of four receiving antennas. The coincidence of echoes within the four-pulse set eliminates spurious noise echoes and allows the derivation of a full set of echo parameters including sky-map echo location, wave polarisation sense and Doppler velocity. At Halley an L-shaped pattern of receiving antennas is used (Jarvis and Dudeney, 1986), but many variations are possible (Tsai et al, 1993).
The AIS operates as a dynasonde continuously throughout the year recording ionograms at least every quarter-hour. These soundings are processed computationally at Halley to estimate basic ionogram parameters which are transferred by satellite via Cambridge to NASA on a weekly basis to form part of the International Solar Terrestrial Physics (ISTP) Key Parameter set (Dudeney et al., 1995). In addition, during campaign periods, more frequent ionograms and specialist fixed-frequency sounding modes are employed for specific scientific studies. Data integrity checks are carried out at Halley by the over-wintering field personnel to ensure good system performance is maintained. A preliminary analysis is also made of ionospheric features of particular interest for further study back in the United Kingdom. Once a year the raw data are returned by ship to BAS headquarters where they are processed and entered into an in-house geospace database held on CD-ROM.
This short article describes a dynasonde data sequence recorded at Halley that illustrates the advantages which the spatial and Doppler information bring to the interpretation of standard ionograms. This example was recorded under range-spread-F conditions. Such data are often observed at Halley particularly during the equinox periods when it frequently occurs post magnetic noon. The ability to distinguish the individual sky-map locations of the echoes forming each trace in clearly striated spread-F conditions allows a time-sequence picture to be built up of the prevailing ionospheric dynamics.
The three ionograms shown on the left of Figure 1 are taken from a series of soundings recorded at 3-minute intervals on 17 March 1993. Several distinct traces within the range-spread in each ionogram are apparent which all exhibit a similar plasma frequency between 6.0 and 6.5 MHz. Two particular traces have been highlighted and labelled (traces 1 and 2) so that their progress can be followed from one ionogram to another. The corresponding sky-maps are shown on the right-hand side of Figure 1 and indicate the horizontal location of each echo relative to Halley -it is in these additional data that the full advantage of the dynasonde is seen. The groups of points (labelled 1 and 2) correspond to those highlighted in the ionograms and it is clear that they are composed of echoes from specific horizontal locations. The Doppler velocity of each echo is also illustrated on both the ionograms and the sky-maps by the grey-scale shading; the highlighted groups of echoes (1 and 2) are seen to have distinct, and different, mean Doppler velocities.
During the six-minute period of data shown, groups 1 and 2 both move towards the south-west, passing slightly to the south-east of Halley at closest approach. This horizontal motion is also indicated by the Doppler velocity exhibited by each group of echoes. A negative velocity indicates movement towards Halley and a positive velocity indicates movement away. At ~340 km distant from Halley, Doppler velocities of ~270±20 ms-1 are evident (group 2 at 1942UT); at ~290 km distant, the Doppler velocity is ~200±30 ms-1 (group 1 at 1948 UT). At closest approach the Doppler velocities reduce to near zero, as expected, because the line-of-sight component seen at Halley becomes perpendicular to their overall motion. Taking the altitude of the echoes seen from Halley to be between 200 and 250 km, these line-of-sight Doppler velocities translate into an effective horizontal velocity of between ~310±30 and ~330±30 ms-1 for group 2 at 1942UT and between ~240±30 and ~260±30 ms-1 for group 1 at 1948 UT. This may be compared with the positional movement observed during the sky-map sequence; during the 6-minute sequence both group 1 and group 2 move through ~130±10 km. This translates to a velocity of ~360±20 ms-1. The horizontal motion calculated from the Doppler velocity and that calculated from the movement of the echo-location position seen on the sky-map plots between consecutive soundings are therefore in agreement within experimental error and taking into consideration the fact that the former are instantaneous velocities while the latter is the mean over a six minute period. It should be noted however, that a more rigorous calculation must also account for the fact that different sections of an ionospheric scattering region are seen as it passes across the field of view (Lanchester et al., 1993).
The velocities calculated by the two methods above may also be compared with those derived using the method of fitting a bulk flow vector to the data from each ionogram by solution of the multiple set of linear equations (one equation for each echo) that relate the sky-map position and Doppler velocity to that bulk vector (Wright and Pitteway, 1994; Jarvis, 1995). This technique produces values for the horizontal velocity component of 251, 274 and 296 (±~25) ms-1 and velocity bearings towards 51°, 71° and 59° (±~10°) west of south for the ionograms at 1942UT, 1945 UT and 1948 UT respectively. These results are again in reasonable agreement with the other techniques but tend to indicate slightly lower velocities. This may be due in part to the fact that the vector fitting technique also accounts for the vertical component of motion and does not assume that the line-of-sight Doppler velocity is due to purely horizontal movement.
It is probable that the discrete echoing regions observed in this event correspond to scattering from horizontal corrugations in the ionospheric electron density contours characteristic of the passage of travelling ionospheric disturbances or atmospheric gravity waves. This is consistent with the fact that the echoing region observed close to overhead at Halley throughout this period tends to be elongated in the north-west to south-east direction in alignment with the corrugations themselves. The movement of the patches of echoes on the sky-map will be indicative of phase front velocity. Physical interpretation of the velocity derived by the two other techniques, which both depend upon the line-of-sight Doppler velocity, is likely to depend upon the reflection mechanism.
This example has demonstrated just one of the advantages of a phase-sensitive ionosonde over a conventional analogue system. Over thirty scientific research papers have been published using data from the AIS at Halley. Increased computing power and on-line data storage capability combined with the flexibility available to program specialised sounding modes means that the instrument continues to provide much potential for key areas of ionospheric research.
References
Dudeney J.R., "The ionosphere - a view from the pole", New Scientist, 91(1271), 714-717, 1981.
Dudeney J.R., Rodger A.S., Smith A.J., Jarvis M.J. and Morrison K., "Satellite Experiments Simultaneous with Antarctic Measurements (SESAME)", Space Sci. Rev., in press, 1995.
Grubb R.N., "The NOAA SEL HF radar system (ionospheric sounder)", NOAA Technical Memo ERL SEL-55, National Oceanic and Atmospheric Administration, Boulder, Colorado.
Jarvis M.J., "First dynasonde observations of F-region plasma flow at Halley, Antarctica", J. Atmos. Terr. Phys., in press, 1995.
Jarvis M.J. and J.R. Dudeney, "Reduction of ambiguities in HF radar results through a revised receiving antenna array and sounding pattern", Radio Sci., 21, 151-158, 1986.
Lanchester B.S., T. Nygren, M.J. Jarvis and R. Edwards, "Gravity wave parameters measured with EISCAT and Dynasonde", Ann. Geophysicae., 11, 925-936, 1993.
Tsai L.-C., F.T. Berkey and G.S. Stiles, "On the derivation of an improved parameter configuration for the Dynasonde", Radio Sci., 28, 785-793, 1993
Wright J.W. and M.L.V. Pitteway, "Real-time acquisition and interpretation capabilities of the Dynasonde", Rad. Sci., 14, 815-835, 1979.
Wright J.W. and M.L.V. Pitteway, "High-resolution
vector velocity determinations from the dynasonde", J. Atmos.
Terr. Phys., 56, 961-977, 1994.
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