IONOSPHERIC CONVECTION RESPONSE TO SLOW, STRONG VARIATIONS IN A NORTHWARD INTERPLANETARY MAGNETIC-FIELD - A CASE-STUDY FOR JANUARY 14, 1988

We analyze ionospheric convection patterns over the polar regions duri ng the passage of an interplanetary magnetic cloud on January 14, 1988 , when the interplanetary magnetic field (IMF) rotated slowly in direc tion and had a large amplitude. Using the assimilative mapping of iono spheric electrodynamics (AMIE) procedure, we combine simultaneous obse rvations of ionospheric drifts and magnetic perturbations from many di fferent instruments into consistent patterns of high-latitude electrod ynamics, focusing on the period of northward IMF. By combining satelli te data with ground-based observations, we have generated one of the m ost comprehensive data sets yet assembled and used it to produce conve ction maps for both hemispheres. We present evidence that a lobe conve ction cell was embedded within normal merging convection during a peri od when the IMF B(y) and B(z) components were large and positive. As t he IMF became predominantly northward, a strong reversed convection pa ttern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern ( winter) hemisphere became weak and disordered with a dawn-to-dusk pote ntial drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interp ret this behavior in terms of a recently proposed merging model for no rthward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over t he dayside magnetosphere, producing reverse convection in the summer h emisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF B(x) compon ent present at this time could have contributed to the observed hemisp heric asymmetry. Reverse convection in the summer hemisphere broke dow n rapidly after the ratio \B(y)/B(z)\ exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potent ial drop was established in both hemispheres when the magnitude of B(y ) exceeded that of B(z), with potential drops of the order of 100 kV, even while B(z) remained northward. The later transition to southward B(z) produced a gradual intensification of the convection, but a great er qualitative change occurred at the transition through \B(y)/B(z)\ = 1 than at the transition through B(z) = 0. The various convection pat terns we derive under northward IMF conditions illustrate all possibil ities previously discussed in the literature: nearly single-cell and m ulticell, distorted and symmetric, ordered and unordered, and sunward and antisunward.