The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27-29, 1992. When the interplanetary magnetic field (IMF) B• component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar-cap potential drops in the two hemispheres are similar. When B• is positive and IB•I > B•, the convection configurations are mainly determined by B• and they may appear as normal "two-cell" patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of decreases (less thn one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the "merging cell" potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The "lobe cell" provides a potential between 8.5 and 26 kV and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if IBl > IBI. To estimate the potential drop of the "viscous cells," we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state. New Zealand. 6491 6492 LU ET AL.: HIGH-LATITUDE IONOSPHERIC CONVECTION PATTERN Pedersen and Hall conductance models are obtained by combining the auroral conductance model of Fuller-Rowell and Evans [1987] with an empirical model of conductance produced by solar extreme ultraviolet radiation based on Chatanika radar observations. The statistical electric potential model is based on Millstone Hill radar observations [Foster et al., 1986]. Both conductance and potential models are parameterized by the hemispheric power index (HPI) [Foster e! al., 1986]. A very important feature of AMIE is its ability to give quantitative information about the uncertainty in the resultant patterns, so that features mapped reliably can LU ET AL.' HIGH-LATITUDE

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 (

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high‐latitude ionospheric convection and field‐aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field‐aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field‐aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field‐aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field‐aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field‐aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi‐steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

No abstract

The availability of data from magnetic observatories in machinereadable form means that we can now study secular change (internal variations of the magnetic field) by analysing time series with a rapid sampling rate rather than annual means. The major task is to identify and remove fields due to external sources: the major benefit is finer time resolution, which is particularly important because of the recent interest in sudden phenomena such as geomagnetic impulses and jerks. Apia and Amberley observatories provide good data for experimentation.Many external fields are satisfactorily excluded by taking averages of hourly values at local midnight for the five international quiet days of the month and removing annual and semi-annual lines by least squares. Quiet days based on the K-index are found t o be at least as suitable as those based on the AE and DSt indices.Time series of monthly means are developed for all components a t both observatories from 1921 (Apia) and 1923 (Amberley). Cubics provide good fits t o the long-term variations, but the residuals t o these cubics are highly significant. An 11 yr variation is noticeable, showing that we have not succeeded in removing this external source of noise. Residual curves for both observatories, which are some 30" apart, follow each other quite closely. The large swings, with a period of approximately 22 yr, that were noted t o be in phase with the double sunspot cycle by Beagley & Bullen, have not continued in phase.The magnetic field is widely believed t o have undergone a change around 1969-70. This change is now known as the geomagnetic jerk. The finer time resolution afforded by monthly means is used t o investigate the duration of this event. The resolution is limited t o 6-18 month by noise in the measurements. A rapid change in the Y-component occurs effectively instantaneously at 1970.6 for Apia and 1972.4 for Amberley. We propose our method as a D. Gubbins and L . Tomlinson useful one for a global study of the phenomenon. Another jerk, believed to occur in 1978, is revealed in the Apia record a t 1978.1.