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Storm Time Plasma Transport at Middle and High Latitudes

Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. Storm-Enhanced Density (SED) is a pronounced feature of the Millstone Hill ionospheric observations, and its characteristics explain the ionospheric storm dusk effect and provide a source for the enhanced density features which have been found to populate the polar cap. It is separate from the usual positive phase effect and is associated with the transport of solar-produced plasma from lower latitudes toward noon. The SED is observed equatorward of the evening sector trough, in the region of sunward plasma convection. The integrated column density (TEC) is enhanced by a factor of 2 to 4 and is spatially extended along the sunward convection trajectory. Two-dimensional radar mapping shows the continuity of the region of SED from mid-latitudes through the cusp. The plasma carried sunward is fragmented on entering the polar cap and constitutes a source for F region patches within the polar cap. Its latitude of occurrence expands with increasing Kp, and it can be identified at L=1.3 during large storms.


As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ~750 m/s with a flux of 1014 m-2 s-1.

The east coast of the United States is a preferred longitude for the occurrence of SED due to the offset of the poles. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region.

http://www.haystack.edu/~jcf/research/sed.htm -- Revised: June 11, 1996
E-mail: jcf@hyperion.haystack.edu
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