*Gordon Rostoker[1]
Solar-Terrestrial Environment Laboratory, Nagoya University[1]
The ionospheric electric field in the midnight sector has been
characterized by an electric field reversal which stretches
from the early evening sector high latitude region to the midnight
meridian somewhere near the equatorward edge of the auroral oval.
This locus has been termed the Harang Discontinuity, and many
researchers use the electric field reversal as a template in
which to organize their own auroral oval data. In this talk I
shall demonstrate that one cannot simply connect the high latitude
and low latitude regions by a continuous electric field reversal
and that there are, in fact, two separate regions of electric
field reversal , i.e. the high latitude convection reversals
and a midnight sector convection reversal which becomes more
visible at times of substorm expansive phase onset. Several different
types of evidence will be presented to support the need to change
the present view of the Harang discontinuity.