Internal and external influences on ionospheric electrodynamics
at low and middle latitudes
R. A. Heelis
University of Texas at Dallas
In the presence of the Earth’s magnetic field, the distribution of charged and neutral particles with altitude produces an anisotropic electrical conductivity. At low and middle latitudes the electron mobility parallel to the magnetic field is sufficiently high to allow electric potentials to map almost unattenuated in this direction. Perpendicular to the magnetic field the conductivity is distributed in two layers at altitudes identified with the E region and the F region ionosphere. In the lower layer, a large Hall conductivity supports intense currents known as electrojets that can be further enhanced by the orientation of the magnetic field. The distribution of Pedersen conductivity in both layers provides the opportunity for current loops to connect the E- and F-region through field-aligned currents. In a quasi-steady state electric potentials are then distributed to insure that the total current is divergence free everywhere.
Electric fields at low and middle latitudes are produced by externally applied potentials at high latitudes and by internally generated potentials resulting from neutral wind driven currents. To understand these electric field sources it is of course necessary to describe the quiet and disturbed time behavior of the neutral winds. However, it is also necessary to appreciate how applied potentials are dependent on the behavior of the conductivity integrated along the direction of the magnetic field, and on conditions in the solar wind and the magnetosphere.
During quiet times the actions of migrating and non-migrating tides in the E region can be identified in drift patterns that have diurnal and semi-diurnal variations. The influence of meridional and zonal winds in the F-region may also be identified by expected longitudinal variations. The influences of the variations in the solar wind and the magnetosphere are most readily seen during storm times but may also be identified during moderately quiet times. These influences are associated with expansion of the area influenced by these inputs, by modification of the neutral winds by energy inputs and by penetration of electric fields inside the plasmasphere.
In this work we will first describe the underlying physical principles that govern the behavior of electric fields at low and middle latitudes. Then, observational and computational results will be utilized illustrate these principles at work during quiet and disturbed times.