No abstract

Measurements of precipitating particles and the electric field on board EXOS D spacecraft for the period January–June 1990 have been used for estimation of the diameter of the polar cap along dawn‐dusk meridian and the polar cap voltage. Identification of the polar cap boundaries has been made on the basis of specific features of precipitating ions. The data on the polar cap boundary location obtained for different geophysical conditions have been used to derive the statistical relationship between the polar cap diameter and PC index. The analysis has shown an approximately linear relationship between the polar cap diameter and the PC index for values PC < 3, the diameter tending to be asymptote when the PC index reaches large positive values. Cross polar cap voltage derived from EXOS D data is in good correlation with interplanetary quantities including the interplanetary magnetic field (IMF) southward component. The best correlation is obtained for the merging electric field υBT sin2 θ/2, with a coefficient of correlation higher than 0.82. Almost the same correlation is observed between polar cap voltage and PC index. The effect of “saturation” is not traced in the voltage dependencies on the PC index and interplanetary quantities up to values BT ≤ 10 nT.

The newly introduced index PC for magnetic activity in the polar cap has been examined to establish to which extent it can serve as an indicator of auroral electrojet activity. PC is derived from a single nearpole station, as a 15‐min average index. We have derived it for two stations, one in the northern hemisphere (Thule) and one in the southern hemisphere (Vostok). The simplicity of the PC index enables us to make a large data base for statistical investigations. We have thus used 7 years of PC values for the two stations to analyze the relationship between PC and the auroral zone indices AE, AU, and AL statistically. We find a very high correlation between PC and AE during winter and equinox, the linear correlation coefficient being ≈0.8–0.9 for Thule and ≈0.7–0.8 for Vostok. During summer the correlation is less because the PC index is then disturbed by polar cap currents controlled by the northward and east‐west components of the interplanetary magnetic field. We therefore stress the importance of having PC available from both the northern and southern hemisphere. From event studies we find that PC is sensitive both to DP 2 type electrojet activity and to substorm intensifications of the westward electrojet in the midnight or postmidnight sector but less sensitive to substorm intensifications of the westward electrojet in the premidnight sector. We conclude that PC can serve as a fast available indicator of DP 2 and DP 1 activity in the polar regions, excluding intrusions of the westward electrojet in the premidnight sector.

We analyze ionospheric convection pat terns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assirnilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and B, components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (

[1] Interyear consistency is demonstrated in the bi-monthly average diurnal vertical electric field measured over $720 ''fair-weather'' days collected during a 5-year interval (1998 to 2002) at Vostok (78.5°S, 107°E; magnetic latitude 83.6°S), Antarctica. After correcting for the influence of polar-cap convection, seasonal peaks in the average electric field values occur around July-August with a diurnal maximum at $ 2050 UT while lowest average magnitudes are measured near November-December when the associated diurnal maximum occurs at $1830 UT. These variations are consistent with expected seasonal changes in global thunderstorm activity. Comparisons of ground-level vertical electric field measurements (Vm À1 ) with Weimer-1996 model cross-polar cap potentials above Vostok (kV) for individual UT hours yield significant correlations over all hours but with reduced standard errors around local magnetic noon ($1300 UT). This implies a more direct linkage between solar wind parameters and the cross-polar cap potential near magnetic noon, for this site (magnetic latitude: 83.6°S). An all hours-all seasons linkage factor of 0.76 ± 0.06 Vm À1 per kV is determined, broadly consistent with an average ionosphere-ground potential difference of $250 kV and a measured average vertical electric field of 185 Vm À1 . Evidence is presented supporting a seasonal variation in this linkage factor, with generally lower magnitudes in the austral winter (May to August).Citation: Burns, G. B., A. V. Frank-Kamenetsky, O. A. Troshichev, E. A. Bering, and B. D. Reddell (2005), Interannual consistency of bi-monthly differences in diurnal variations of the ground-level, vertical electric field,