Plasmasphere/ Ionosphere Coupling: PBL Interactions Viewed from the Ground and 
Space

J.C. Foster


Modern observing capabilities afford a global perspective on the stormtime
disturbance of the coupled ionosphere/plasmasphere system. We use distributed
ground-based imagery of ionosphere/magnetosphere TEC (total electron content)
derived from GPS observations to produce high-resolution spatial and temporal
maps of the intensity and evolution of the perturbation of the cold-plasma
envelope surrounding Earth. During strong disturbances, a ridge of SED (storm
enhanced density, greatly elevated TEC) forms across mid latitudes in the
post-noon ionosphere. The evolution of continuous SED plumes stretching from the
US East Coast, across Canada, and from noon to midnight across high polar
latitudes is seen repeatedly in the TEC observations. The MIT Millstone Hill
incoherent scatter radar (Massachusetts) has been used to probe the altitude
structure of the ionosphere in and around the SED plume, and quantifies its
rapid sunward (westward) motion. Overflights with the Defense Meteorological
Satellite Program (DMSP) satellites locate the plume with respect to auroral
particle precipitation and electric fields, further clarifying the processes
leading to the formation of this global space weather feature. Correlating the
ground-based and low-altitude observations with space-based imagery of the
high-altitude plasmasphere (from the NASA IMAGE spacecraft) reveals that these
SED features result from the erosion of the outer layers of Earth's plasmasphere
by intense sub-auroral electric fields. Such SED features occur in conjugate
hemispheres, at all longitudes, and extend many Earth radii into space, spanning
our atmosphere from the lower ionosphere to the outer limits of the
magnetosphere. In the early phases of a strong storm, electric fields penetrate
the inner magnetosphere where they uplift and redistribute the plasma of the
low-latitude ionosphere. Eastward electric fields near dusk produce a poleward
shift of the equatorial anomalies (EA) and enhancements of plasma concentration
(total electric content, TEC) in the post-noon plasmasphere and mid-latitude
ionosphere. Strong magnetospheric electric fields are generated as
storm-injected energetic particles fill the enhanced ring current. These SAPS
(subauroral polarization stream) electric fields erode the plasmasphere boundary
layer (PBL), producing plasmaspheric drainage plumes which carry the
high-altitude material towards the dayside magnetopause. Both the SAPS electric
fields and the ionospheric storm enhanced density and associated plasmaspheric
erosion plumes are features of the PBL - a region of interaction between the
outer and inner regimes of the magnetosphere and ionosphere. The driving
electric fields affect the plasma at all altitudes along PBL magnetic field
lines, carrying the dusk-sector cold plasma sunward. The near-Earth footprint of
the plasmaspheric erosion events is seen as the mid-latitude streams of
storm-enhanced density which appear to sweep poleward across the North American
continent. At ionospheric heights, these processes produce a storm front of
dense thermal plasma which extends continuously from low latitudes into and
across the polar regions.

We use Tsyganenko magnetic field mapping to project space and ground-based
imagery of the interrelated M-I coupling features of the disturbed PBL into the
magnetospheric equatorial plane, where their close association becomes readily
apparent. The SAPS electric field occurs in that portion of the Region II
current and sunward convection cell which extends into the low-conductivity
region equatorward of the extent of electron precipitation in the dusk sector
ionosphere. Plasmasphere and ionospheric erosion occurs where the strong
poleward (radially outward) SAPS electric field overlaps the outer regions of
cold plasma in the PBL.

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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006