Murray. Parkinson

The Tasman international geospace environment radar (TIGER) - current development and future plans

The Tasman international geospace environment radar is a dual HF radar system with overlapping footprints designed to map ionospheric motions by detecting ionospheric scatter. The first radar was set up on Bruny Island, Tasmania at the end of 1999 and development of the second radar to be placed near Invercargill, NZ, has begun. TIGER is part of the super dual auroral radar network (SuperDARN) which currently consists of 15 radars deployed in the northern and southern hemispheres. TIGER is located more equatorward than other SuperDARN radars, enabling it to observe new phenomena, such as auroral westward flow channels (AWFCs). This paper describes TIGERs capabilities and presents examples of observations, including an AWFC. Plans to develop digital transmitters and receivers are discussed as is a proposal to extend the network to even lower latitudes by deploying two additional radars.

MEASUREMENTS OF MID-LATITUDE E-REGION, SPORADIC-E, AND TID-RELATED DRIFTS USING HF DOPPLER-SORTED INTERFEROMETRY

Abstract Modern HF digital ionosondes have been used for Doppler-sorted interferometry (DSI) to automatically measure F -region plasma drifts by detecting the Doppler shift and angle-of-arrival of echoes. We report on the use of a Digisonde 256 receiving on a seven-element antenna array located at the Australian mid-latitude station Beveridge (37.5° S, 144.9 ° E, −48.0 ° CGM latitude) to extend the routine application of drift measurements to the E -region, and especially sporadic- E (Es) . Obtaining good quality E -region drift measurements required many more soundings independent in the frequency and time domains than are usually made for F -region measurements. This is because the E region isoionic surfaces were usually more horizontally stratified than those in the more disturbed F -region, and so did not return as many of the oblique echoes upon which the accuracy of the technique depends. Smoothness of the ionosphere was less of a problem when performing Es measurements because of the patchy, cloud-like property of the layers. When making F -region drift measurements the motions are usually thought of as being uniform throughout the volume of ionosphere sampled for echoes. Application of the technique to E -region measurements is interesting because of the enhanced irregular neutral winds associated with gravity waves which grow in amplitude with height. These irregular winds control the motion of the plasma via high collision frequencies, and lead to the formation of Es layers at mid-latitudes. Therefore, the possibility of sharp vertical gradients in plasma drifts must be considered when making and analysing measurements. During numerous campaigns conducted throughout 1994/95, horizontal drifts in the range of ≈30−250 m/s were measured for Es patches lasting up to 6h. The drift measurements sometimes indicated the presence of wave-like motions in the ionosphere with periods in the range 5–90 min. A TID was observed to propagate towards the station from the south, and clearly showed an association between long-period motions in the F -region (≈90 min), and the arrival of a patch of slant- Es and slant- F irregularities. A predominant drift direction was towards the NW or W, and was sometimes seen to veer through S towards the SE during intervals of ≈ 1–5 h. There is evidence of these veering events occurring diurnally and semidiurnally. Our results can be understood in terms of the formation of & under the influence of gravity waves and tides.

ON THE ROLE OF ELECTRIC FIELD DIRECTION IN THE FORMATION OF SPORADIC E-LAYERS IN THE SOUTHERN POLAR CAP IONOSPHERE

Abstract Measurements of the occurrence of sporadic E (Es)-layers and F-region electric fields were obtained with a modern, HF digital ionosonde located at Casey, Antarctica (66.3 °S, 110.5 °E, 81 °S CGM latitude) during the late austral summer of 1995/96. The occurrence of Es-layers was inferred from the presence of appropriate traces in normal swept-frequency ionograms, and the electric fields were inferred from F-region “drift-mode” velocities assuming that the plasma convection velocities given by E × B B 2 were measured, on average, by the interferometer. The theory of formation of high-latitude Es-layers predicts that electric fields directed toward the south west (SW) should be particularly effective at producing thin layers in the southern hemisphere. Our measurements made at a true polar cap station are consistent with this expectation, and are contrasted with observations made by incoherent scatter radars in the northern hemisphere, which also show the importance of SW electric fields, whereas the same theory predicts that NW electric fields should be important at northern latitudes. We reconcile the interhemispheric differences with simple calculations of ion convergence driven by the electric fields specified by the IZMIRAN electrodynamic model (IZMEM) in both hemispheres. The importance of the interplanetary magnetic field in the control of high-latitude Es formation is emphasised as an important adjunct to space weather modelling and forecasting.

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

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

Radio studies of the southern hemisphere high-latitude ionosphere

Abstract Radio studies are crucial for the study of the high-latitude ionosphere and this paper is a brief review of the use of radio probing techniques being used in Antarctica. It is important to study the Antarctic ionosphere in order to understand conjugate effects and the result of differences in north-south symmetry such as the greater offset between the geographic and geomagnetic poles which occurs in the southern hemisphere. The main radio techniques being used in Antarctica are HF auroral radars, digital ionosondes and satellite beacon observations. Current developments of the SHARE HF radar system, installation of additional digital ionosondes, and the installation of GPS receivers which can provide TEC data, will all provide significant advances in our ability to study the Antarctic ionosphere. Details of the structure and convection of the ionosphere, and its response to magnetospheric phenomena, will all be studied in much more detail than before. Undoubtedly collaborative studies with conjugate northern hemisphere systems will have the highest priority but collaborative studies using the different Antarctic instruments will reveal new insights into phenomena such as polar arcs, patches and blobs. Advances in the use of digital ionosondes to study the ionosphere will also be important at lower latitudes. As an example, observations of storm effects at sub-auroral latitudes, obtained using a digital ionosonde operating as an oblique HF radar, are presented.

Electric field measurements at a southern mid-latitude station obtained using an HF digital ionosonde

Abstract It is important to understand the convection of the inner magnetosphere to fully describe the response of the low- to mid-latitude thermosphere-ionosphere system to geomagnetic storms. Realistic numerical simulations of mid-latitude electric fields suffer from limited knowledge of lower thermospheric winds and ionospheric conductivity on a global scale. Even empirical models of mid-latitude electric fields suffer from the paucity of measurements made by the handful of incoherent scatter radars concentrated in the American-European sector, and the intermittent satellite measurements made in other regions. Thus it would be very useful to show the extent to which Doppler velocity measurements made with the numerous digital ionosondes deployed around the globe can be used to infer F -region electric fields. The monthly average diurnal variation of Doppler velocity measured by a recently commissioned Digisonde at Bundoora (145.1°E, 37.7°S, geographic; 49°S magnetic) is seen to resemble the average diurnal variation of ion drift measured by the incoherent scatter radar at Millstone Hill (71.5°W 42.6°N; 57°N). Moreover, the Bundoora measurements exhibit the nighttime westward perturbation drifts found in Dynamics Explorer-2 ion drift measurements.

Comparison of neutral winds derived from airglow and ionosonde measurements

Abstract Meridional winds in the thermosphere determined from optical and ionosonde measurements have been compared using data from campaigns in winter (July 1994), equinox (March 1995) and summer (January 1996). The correlation coefficient for the entire data set is 0.76, a little lower than the value of 0.83 for the March 1995 data set, used in a previous study (Dyson et al., 1997). In all three data sets a Burnside factor of 1.7 has been used on the basis of the more detailed findings of the March study (Dyson et al., 1997) that showed the agreement between the winds worsened if a lower Burnside factor was used, but improved if the atomic oxygen density in the MSIS model was increased by about 20%.

Formation of sporadic E-layers by magnetospheric electric fields in the southern polar cap ionosphere

Abstract The occurrence of ionospheric electric fields and sporadic E ( Es )-layers were measured with a modern, HF digital ionosonde located at Casey, Antarctica (81°S, corrected geomagnetic latitude) during the late austral summer of 1995/96. The observations show that Es -layers had a peak occurrence near to magnetic midnight under the influence of south-west electric fields generated by the solar wind-magnetosphere interaction. Our results are consistent with the standard theory of thin Es -layer formation by electric fields applied to the southern hemisphere, and they complement the basic prediction that north-west electric fields will produce thin Es layers at high latitudes in the northern hemisphere. Hence the magnitude and orientation of the interplanetary magnetic field must be a major factor in the control of Es -occurrence at high latitudes.