Case Study of the Evolution of Global Ionospheric Convection during Substorms

K.K. Hashimoto, l. Lyons, and T. Kikuchi


Evolution of global ionospheric convection is investigated for a substorm event
on February 2, 2002 with unusually good SuperDARN radar echo coverage. Global
magnetometer chains from the polar cap to the dip-equator show the development
of the DP2 electric field during the growth phase, which followed a sudden
southward turning of the IMF. Three DP2 reductions were  observed during the 90
min substorm growth phase, the first two of which were followed by further DP2
enhancement. Electron precipitation measured by the IMAGE FUV/SI13 at 16-17 MLT
at 70-75 degrees magnetic latitudes was coherent with these changes  of DP2, and
the dusk-side proton precipitation measured by the IMAGE FUV/SI12 showed more
gradual changes reflecting the overall DP2 changes. The DP2 convection changes
were seen clearly in the SuperDARN data. Prior to the second two convection
reductions, but not prior to the first, flow shear of the Harang discontinuity
was observed to develop in the post-midnight sector. The Harang discontinuity
shifted equatorward from 75 to 68 degrees magnetic latitudes as the growth phase
developed. The second and the third reductions in convection led to the pseudo
break-up and the expansion phase onset, respectively, both of which initiated
within the Harang flow-shear region and were associated with large weakening of
the Harang flow shear. Our results suggest that both auroral activity on the
night-side and auroral precipitation on the afternoon-to-dusk region are
strongly controlled by global convection, that reduction in convection leads to
substorm expansion or pseudo break-up only under the condition of a
well-developed Harang flow shear, and that an enhancement of convection prior to
full expansion-phase development leads to termination of the expansion phase
(i.e. a pseudo breakup) and reformation of the Harang  shear.

_______________ 
Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004