Mesospheric mean winds and tides observed by the Imaging Doppler Interferometer (IDI) at Halley, Antarctica

This paper describes the first ever mesospheric wind observations from Halley, Antarctica, over a full year. The recent implementation of an Imaging Doppler Interferometer at Halley is providing a new, high quality and continuous dataset to investigate the dynamics of the Antarctic mesosphere. The mean winds show clear seasonal variations, with reversals in both zonal and meridional components near the equinoxes. The dominant tidal modes have periods of 12 h and 24 h but with significant variations in amplitude during the year. Waves with longer periods are also apparent at certain times of year. The seasonal variations and amplitudes of the winds and tides are compared with other high-latitude sites in the southern and northern hemispheres. It is found that the overall pattern of winds at Halley is broadly similar to that seen at similar geographic latitudes, but with noticeable differences which may be related to it being a southern hemisphere site

Occurrence and characteristics of high-latitude mesospheric echoes at MF: observations by Halley and Tromso dynasondes

The dynasonde at Halley, Antarctica, is operated throughout the year making ionogram soundings every five minutes. The extensive database of observations from 1993 and 1994 has been processed to investigate ionogram echoes from mesospheric heights. The occurrence of these echoes shows a very clear seasonal trend with far more echoes observed during the winter. The echoes are shown to be predominantly dependent on the absence of direct solar illumination. The characteristics of the echoes are compared with similar observations from a northern hemisphere site at Tromso, Norway. The observations from Halley and Tromso are discussed in relation to Polar Mesosphere Summer Echoes (PMSE) which are seen at higher radio frequencies above Tromso. It is found that ionogram mesospheric echoes have different occurrence statistics from PMSE and there is no statistical evidence to suggest that the echoes observed during the summer are different from those observed at other times of year

Toward ionospheric tomography in Antarctica: first steps and comparison with dynasonde observations

Total electron content (TEC) measurements obtained at two Antarctic stations over nine months beginning early in 1994 have been analysed as a first step to performing ionospheric tomography. Two receiving systems were deployed at the Faraday and Halley research stations operated by the British Antarctic Survey to monitor signals from a random selection of passes of satellites in the Navy Navigational Satellite System. The resultant measurements of total electron content have been inverted and combined with ionosonde measurements of true height and foF2 to yield two-dimensional contour maps of ionospheric electron density. In spite of the poor geometry of the observations, some 130 satellite passes were found to be suitable for reconstruction using the techniques developed for ionospheric tomography. The contour maps of plasma density have been compared with independent observations of the vertical electron density profile measured by the dynasonde ionospheric sounder located at Halley. An example is presented of a deep trough investigated by the technique, illustrating the potential of the tomographic method for study of an extended spatial region of the ionosphere over inhospitable terrain

Initial observations of mesospheric winds using IDI radar measurements at the Bear Lake Observatory

The adaptation of Imaging Doppler Interferometry (IDI) to the dynasonde deployed at the Bear Lake Observatory (41.9°N, 111.4°W) in northeastern Utah enables the routine measurement of mesospheric dynamics. Influenced by the direction of the prevailing wind, a clear seasonal variation is seen in the mesospheric echo numbers. The technique also provides a monitor for the amplitude of the main tidal modes as well as for studying planetary waves with periods of several days. The observations are found to be consistent with those from different instruments based at the same site thus confirming the IDI characterisation of the mesospheric wind field

Dynasonde observations of electron concentration gradients above Tromsø

The Dynasonde is a digital ionosonde which measures the amplitude and phase of radio echoes reflected from the ionosphere. In addition to the frequency and virtual range of each echo, one parameter of particular interest is the direction of arrival which allows horizontal structures in the ionosphere to be studied. Under suitable ionospheric conditions, namely where structures or strong gradients are present in the electronconcentration, echoes may be observed out to horizontal distances of several hundred kilometres. We describe an automated method for latitudinally mapping these gradients in F-region peak electronconcentration using the Dynasonde. Good agreement is found when the results are compared with a co-located incoherent scatter radar lending credibility to observations of electronconcentrationgradients by a Dynasonde alone. This is significant for sites where no supporting instruments exist. Although Dynasondes account for only a small fraction of digital ionosondes in use today, the techniques described in this paper could be extended to other digital ionosondes with similar directional capabilities. The combination of routine soundings, and the worldwide distribution of such instruments, would provide a powerful means to monitor ionospheric structures on a regular basis

New initiatives in observing the antarctic mesosphere

Studies of the Antarctic mesosphere, and its contrasts with the Arctic, are expected to hold the key to improved understanding of the mesopause region and energy and momentum transfer processes linking the mesosphere to the atmosphere below and the magnetosphere above. It is with this objective that a number of new initiatives are being undertaken at the British Antarctic Survey (BAS) research stations at Halley (76°S, 27°W) and Rothera (68°S, 68°W), both through new uses of existing instrumentation and through collaborative programmes with other groups. A brief review of recent developments and their scientific rationale is presented

Characteristics of the high-latitude trough

The EISCAT radar has provided data for a comprehensive study of the high-latitude trough in electron concentration, which occurs in the auroral zone. In this paper the characteristics of the trough are illustrated, the method of its formation is outlined and important features of the trough are described. A large upward velocity along the geomagnetic field line is shown to play a significant role in the formation of the trough. The large ion-neutral difference velocities which initiate the formation of the trough may also drive the plasma into a non-thermal state which should be taken into account during the analysis of incoherent scatter data

Observations of large field-aligned flows of thermal plasma in the auroral ionosphere

Large upward field-aligned ion flows have previously been observed in the high latitude ionosphere in response to frictional heating of the local ion population. Results from a recent experiment using the EISCAT radar show similar features but allow, for the first time, determination of the field-aligned profiles of plasma parameters during these events. The upflows occur during frictional heating. The flows are shown to be transient plasma upwellings, from regions where the ion temperature has been elevated by the motion of a convection shear over the observed field line