A. M. Breed

The influence of polar-cap convection on the geoelectric field at Vostok, Antarctica

Abstract Vertical geoelectric field measurements at Vostok, Antarctica ( 78.5° S , 107° E ; corrected geomagnetic latitude, 83.4°S) made during 1998 are compared with both Weimer (1996) and IZMEM (1994) model calculations of the solar-wind-induced, polar-cap potential differences with respect to the station. By investigating the correlations between these parameters for individual UT hours, we confirm and extend the diurnal range over which significant correlations have been obtained. Nineteen individual UT hours are significantly correlated with the Weimer model predictions and nine with the IZMEM model predictions. Diurnal variation in the slopes of the linear regressions allows us to comment on each model, demonstrating that Antarctic polar plateau geoelectric field measurements can be used to investigate polar convection. Seasonal variations in the diurnal electric field variations at Vostok are compared with the Carnegie global electric circuit diurnal curves, after allowance is made for the solar-wind-induced, polar-cap potential difference patterns.

Application of the Dopplionogram to Doppler‐sorted interferometry measurements of ionospheric drift velocity

The Dopplionogram was developed as a method of displaying Doppler shifts along the frequency axis of ionograms recorded using B-mode soundings of the Dynasonde, an early type of HF digital ionosonde. The basic idea of recording Doppler shifts in an ionogram format is applied and extended to the Doppler velocity mode of the Digisonde Portable Sounder-4 (DPS-4), a related and more recent type of digital ionosonde. In order to describe our mode of operation a Dopplionogram is redefined to mean a set of stepped-frequency soundings that yields a set of ionospheric Doppler shifts particular to the chosen transmission frequencies. Extension of the technique to include Doppler-sorted interferometry (DSI) analysis of the Doppler spectra facilitates a detailed analysis of ionospheric velocity variations in time and group height. This revitalized approach to DSI should prove useful for the study of ionospheric dynamics for which knowledge of the height profile of electric currents, drift velocity, and neutral winds is required. The technique is demonstrated using measurements of polar cap plasma winds obtained with a DPS-4 located at Casey, Antarctica (66.3°S, 110.5°E).

Signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above Casey, Antarctica

Signatures of the ionospheric cusp in HF digital ionosonde measurements of plasma drift made at the polar cap station Casey, Antarctica (−80.8° geomagnetic latitude), are investigated. Measurements recorded during the campaign interval February 13–17, 1996, are considered in this case study because the summer dipole tilt effect, and an interplanetary magnetic field (IMF) northward condition on February 16, were favorable for the detection of the cusp at a higher than usual latitude. On February 14 and 15 the magnitude of the IMF was about 4–6 nT, and the station probably passed just poleward of the cusp. The most general signatures of the cusp were enhanced electric field and electric field turbulence shown by increased drift velocity and velocity scatter in the convection throat, respectively. Broadband, unstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were associated with the intervals of enhanced convection turbulence. A major cusp event occurred above Casey on February 16, when the magnitude of the IMF increased from about 5 to > 10 nT. Cusp signatures during this event included the drift velocity surging to large values just before and after an interval during which the F region echoes were lost because of an absence of F region ionization and the formation of electron density patches. The loss of echoes was only partly explained by increased absorption and scatter of the transmitted radio waves. Although the spectral width of Doppler peaks increased, this, by itself, was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near-vertical interferometry. However, signatures of the cusp were easily recognized in digisonde data, and the cusps location and dynamics can be monitored using digisondes.

A statistical study of the interplanetary magnetic field control of sporadic E-layer occurrence in the southern polar cap ionosphere

The influence of the interplanetary magnetic field (IMF) on the occurrence of sporadic E (Es)-layers in the southern polar cap ionosphere has been investigated. We statistically analysed ionogram and Doppler velocity observations made using a HF digital ionosonde located at Casey, Antarctica (66.3 degrees S, 110.5 degrees E; 81 degrees S magnetic latitude) during the two summer campaign intervals I January to 18 February, and 1 November to 31 December 1997. The ionogram and Doppler Velocity measurements were used to determine the Es-occurrence and electric field vectors (assuming E x B/B-2 drift), respectively. Concurrent IMF data were obtained from measurements made on board the Wind spacecraft. First, the gross properties of the IMF dependence of Es-formation were obtained: the occurrence rate was higher for negative B-y and/or positive B-z, and lower for positive B-y and/or negative B-z. To reconcile these gross properties with the electric field theory of Es-layer formation, the detailed diurnal variation of both Es-occurrence and the ionospheric electric field were obtained for different orientations of the IMF. The main statistical results are that: (1) the B-y component mainly controls the occurrence of the midnight Es-layers through its influence on the corresponding South West electric field; and (2) the B-z component mainly controls the occurrence of the evening Es-layers. However, the change in the occurrence rate for evening Es-layers was not related to the strength of the associated North West and North East electric fields. The total occurrence of Es-layers depended more on B-y than on B-z, owing to the dominance of B-y-controlled midnight Es-layers in the occurrence distribution. Nevertheless, the dependence of Es-occurrence on B-z was important. We suggest that the increase in Es-occurrence for positive B-z might be explained by the intermittent production of lower F-region ionisation by polar showers and squalls, which also increase in frequency and intensity for positive B-z. The importance of metallic ion transport within the ionosphere is also considered

Total electron content in Australia corrected for receiver/satellite offset bias and compared with IRI and PIM predictions

Abstract A procedure is presented for removing receiver and satellite (transmitter) offset bias from total electron content (TEC) data obtained using Global Positioning System (GPS) satellites from 1991 to 1998. Samples of TEC data from 1991 to 1995 for Australia have been corrected for bias. Corrected TEC values show substantial agreement with model predictions from the International Reference Ionosphere (IRI) and the Paramaterized Ionospheric Model (PIM).

The effect of fluctuating ionospheric electric fields on Es-occurrence at cusp and polar cap latitudes

Theory predicts that in the high-latitude southern hemisphere, southwest (SW) electric fields will produce convergent ion flow and thereby create thin sporadic E (Es)-layers at node heights > 120 km, whilst northwest (NW) fields will produce downward ion flow and create thicker Es-layers at heights < 110 km. To investigate this theory, Digisonde ionograms (giving the Es-occurrence) and drift measurements (giving electric field estimates) at two Antarctic stations were statistically analyzed. As previously found for the polar cap station Casey (81 degreesS magnetic), more of the Es-traces were associated with SW fields than NW fields. However, new results for the cusp station Zhongshan (73 degreesS) show that fewer Es-layers occur there, and NW fields play a slightly more important role than SW fields, similar to the results found at auroral latitudes in the northern hemisphere. To further our understanding of the occurrence distributions, we study the fluctuating properties of the electric fields at the two stations. It is found that the electric fields at Zhongshan fluctuate more than those at Casey. Thus we suggest that the field fluctuation is also an important consideration helping to explain the differences in the Es-occurrence at the two stations. This suggestion is confirmed by our numerical simulations which show that Es-layers are more effectively formed by steady SW fields than by steady NW fields, and less effectively by fluctuating SW fields than by fluctuating NW fields

Structure and dynamics of polar patches above Casey, Antarctica.

Abstract A Lowell Digital Ionosonde (DPS-4) has been used at the Australian Antarctic polar cap station Casey (−80.6° CGM latitude) in a study of ionospheric polar cap patches. The instrument ran a series of campaigns consisting of 3-minute cycles of ionograms and drift velocity measurements during 1997–1999. Patches identified from group range and critical frequencies on the ionogram records were compared with drift measurements and other geophysical data sets. Correlations were observed with Total Electron Content derived from GPS satellite observations with a receiver also located at Casey. Comparisons with plasma drift velocities, (derived from DPS drift measurements), indicate that patches are often associated with apparent fluctuations in horizontal drift velocity. Peaks in the horizontal velocity appear to correlate with the patch edges. Possible explanations of these observations are discussed. This paper considers case studies of patch observations in April 1998.