Magnetometer and HF radar data often indicate the presence of magnetohydrodynamic, field line resonances in the nightside magnetosphere. These resonances have frequencies of about 1.3, 1.9, 2.6, and 3.4 mHz and are due to cavity modes or waveguide modes which form between the magnetopause and turning points on dipolelike magnetic shells. Energy from these cavity modes tunnels to the field line resonances which are seen in the F region by the HF radar and on the ground by the magnetometers. The presence of these field line resonances gives us an excellent diagnostic tool for determining the position of the mechanism leading to the energetic electrons and field-aligned currents associated with substorm intensifications and auroral brightening. Using data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, riometers, and bistatic auroral radars and data from the Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay in Canada, we have identified a number of intervals in which substorm intensifications occurred during times when field line resonances existed in the region of the magnetosphere where the intensification occurred. In the events that we have analyzed in detail, the ionospheric signatures of the substorm intensification began equatorward (earthward) of existing field line resonances. These observations give very strong evidence indicating that at least one component of the substorm mechanism must be active very close to the Earth, probably on dipolelike field lines in regions with trapped and quasi-trapped energetic particles. Furthermore, the auroral intensifications started near the position of one of the equatorward resonances, indicating that the field line resonances may play a role in triggering or producing the substorm intensifications. One possible scenario is mode conversion to kinetic Alfv6n waves in the resonance. Table 1), meridian scanning photometers, a bistatic auroral radar, and a charged couple device (CCD) imager and became fully operational in December of 1989. The CANOPUS meridian scanning photometer array (MPA) uses meridian scanning, eight-channel, filter wheel photometers at Rankin Inlet, Gillam, Pinawa, and Fort Smith. Only the data from the Gillam (GILL) and Rankin Inlet (RANK) instruments are used in this study. Five of the channels measure auroral emissions (4709, 4861 (twice), 5577, and 6300-/•), and three channels measure the background near 4800, 4935, and 6250/•. The photometer scans the meridian at two revolutions per minute with a sampling rate of 510 samples per scan per channel. Data from the scans are averaged into 17 latitudinal bins, centered on the latitude of the station. The data bins are 0.5 ø wide and 0.5 ø apart for the low-altitude (110 kin) emissions (4861, 5577, and 4709 /•) and approximately 1.0 ø wide for the high-altitude (230 kin) emission (6300/•). The magnetometer array uses three-component, ring core, flux gate instruments. Each channel is sampled at 5-s intervals. The magnetometers are aligned in geogra...

Night and early morning data from the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and from the magnetometers in the Canadian CANOPUS array often show structured spectra with distinct spectral peaks at 1.3, 1.9, and 2.6 mHz. These frequencies are very stable and vary by less than ±5% for the 6 days of data we have analysed. The radar measurements of the F-region drift velocities indicate that these spectral peaks are often associated with field line resonances of the shear Alfvén wave, and that the resonances are seen at lower latitudes as the frequency increases. The magnetometer data indicate that the magnetohydrodynamic (MHD) waves have westward phase velocities. We shall show that these observations are compatible with the formation of MHD cavity modes in the early morning and nightside magnetosphere, between the magnetopause, at approximately 14.5 RE, and turning points in the dipolar magnetosphere, outside the plasmasphere. Likely sources of energy are compressional pulses from the solar wind or Kelvin–Helmholtz instabilities in the low-latitude boundary layer.

During a fortuitous meridional conjunction of Polar and Geotail at the nightside magnetosphere throughout the course of a geomagnetic substorm, measurements of Poynting flux indicate that most of the electromagnetic energy flux density that is radiated in the form of waves at the location of Geotail at ∼18 RE is dissipated before it reaches Polar at ∼5 RE, i.e., above the auroral acceleration region. While the Poynting flux measured at Polar (and to a greater extent at Geotail) is more than sufficient to account for particle acceleration below the satellite, it still represents a small portion of the earthward directed particle energy flux density measured at Geotail. If even a small portion of the bursty bulk flow energy couples to Alfven waves, it would be energetically sufficient to account for the expected auroral energy deposition during substorms. Power dissipation via kinetic Alfven waves along auroral field lines represents a viable mechanism by which localized reconnection flows can slow down. This may explain why fast earthward flows reported at midtail (>30 RE) distances can exist with no near‐Earth counterpart and why any putative candidates of an ionospherically reflected flow burst pulse in the tail have very small amplitudes.

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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