D. Monselesan

Ionospheric convection at casey, a southern polar cap station

A digital ionosonde (Digisonde Portable Sounder 4) located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° corrected geomagnetic latitude) has been operational since early 1993 and has accumulated 3 years of plasma drift measurements, providing an excellent data set for studying the characteristics of ionospheric convection flow at a southern polar cap station. The purpose of this study is to investigate the influence of the IMF on the F region ionospheric convection over Casey and to compare it to the Heppner-Maynard satellite-derived electric field models. We find clear dependencies in the drift on the sign and strength of the IMF By and Bz components and with Kp. Antisunward flow dominates during Bz south conditions, turning to have a sunward component around noon when Bz is northward. The By component causes the entire convection system to rotate and distorts the dayside flow in the proximity of the throat, with a dawnward (duskward) component for By negative (positive). Comparison with the Bz south Heppner-Maynard BC, DE, and A patterns is favorable at most times, although we predict a rounder, more dominant dusk (dawn) cell and a smaller crescent-shaped dawn (dusk) cell for By 0). There is a dependence on Kp when Bz is south in both the model and the drifts, flow directions becoming more antisunward and velocities becoming higher on the dayside as Kp increases. This implies the polar cap is expanding under conditions of enhanced reconnection. When Bz is north, the F region drift agreement with the BCP(P) and DEP(P) models is excellent on the dawn (dusk) side for By 0) but diverges on the opposite side as the pattern flow lines twist sunward. Separation of the drifts into Bz weakly ( 3 nT) northward cases did not reveal any appreciable difference in the observed drift velocities.

Digital ionosonde measurements of the height variation of drift velocity in the southern polar cap ionosphere: Initial results

During the late austral summer of 1995-1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E, -80.8° corrected geomagnetic (CGM) latitude), in an experimental drift mode with the aim of resolving the height variation of drift velocity in the polar cap ionosphere. We devised control programs for a Digisonde Portable Sounder 4 to collect data at separate frequency-range gates corresponding to the E and F regions to investigate the differences in their motions. During a 4-day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic field ( V⊥ ) were 85 m s -1 in the E region and 485 m s -1 in the F region (using 10 m s -1 bins and echoes from all heights in each region). Vertical profiles of drift velocity were obtained by sorting echoes into 10-km group-height bins. For measurements obtained within ±3 hours of magnetic noon the average profile showed that in the lower E region V⊥ increased approximately exponentially with true height. The corresponding velocity scale height was 46.7 m s -I km -1 . The mean value of V⊥ leveled off to about 700 m s -1 above 120 km, where it remained up to the F region peak height. The vertical gradient was caused by the increase in collision frequencies at the lower heights. The F region field-aligned component of drift (V∥) showed a strong diurnal variation, with mean values of -30 m s -1 near noon and +60 m s -1 during the night at a height of 180 km. The average over the whole day reveals a net upward drift of 30 m s -1 . This behavior is attributed to the interaction between the meridional components of the generally antisunward neutral wind (U N ) and perpendicular drift ( V⊥ S ) moving plasma down the field lines during the day and up the field lines during the night, with U N and V⊥ s having net equatorward values when averaged over all day. While the E region drift direction tended to be aligned with the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in direction, suggesting a smaller-scale size and lifetime for the E region structures giving rise to the echoes. There were events lasting over 2 hours during which the drifts in the two regions were clearly resolved into different azimuths (by nearly 180° for two events). These transient directional shears show the time variability in the phase transition between an F region collisionless, magnetized plasma driven by the E X B/B 2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.

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.

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.

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.

Comparison of high-latitude ionosonde drifts with a convection model

Abstract A comparison of averaged ionospheric F-region drifts measured by a digital ionosonde at Casey, Antarctica is made with convection patterns from the Weimer (1995) electric potential model. Significant changes occur in the average diurnal flow vectors over Casey as the Interplanetary Magnetic Field angle in the By-Bz plane is varied. Steps of 30° in By-Bz angle show the evolution of the measured convection as Bz turns from southward to northward through ± By . This results in drifts that vary respectively from anti-sunward to sunward on the dayside. These changes compare favourably with the Weimer patterns as the two-cell pattern distorts and forms a three- and four-cell pattern when the IMF is respectively within ∼60° and ∼30° of Bz due north. At most IMF angles, drift directions are usually within 20° and speeds within 100 m/s of the model predictions. The best direction/speed agreement do not coincide at one IMF angle, being mostly dependent on the strength of flows and the position of convection cell centers.