The high-latitude electric field is one of the most important parameters that influence energy deposition in the form of Joule heating in the upper atmosphere. Numerical models often use statistical or climatological electric-field models which ignore an important component of Joule heating that is due to rapid, small-scale fluctuations in the electric field, or small-scale variability (SSV). Studies attempting to quantify this type of variability using satellite or groundbased incoherent scatter radar measurements have created variability maps associated with statistical electric-field patterns. The variability described in these studies is a different type of variability, namely, climatological model variability (CMV) that describes how well the model reproduces the measurements. The relationship between these two variabilities, and the physical significance of CMV, is presently unknown. This project aims to resolve the ambiguity between CMV and SSV by using data from southern and northern hemisphere SuperDARN radars to develop maps of both CMV (similar to previous studies) and SSV. The latter will be obtained from observations sampled at 1-2 min that are continuous for >10 min. The resulting maps will allow for a direct comparison of the relative magnitudes of CMV and SSV over the entire high-latitude region, providing a clear picture of the physical significance of SSV and CMV in electric fields. By further utilizing the widespread and continuous SuperDARN measurements for specific events, the project obtain additional information that allows the variability measurements to be located relative to large-scale convection and magnetospheric boundaries, providing clues to its origins. The end product of the research should be the resolution of the ambiguity that exists between SSV and CMV, and clarification of the physical significance that CMV plays in the high-latitude ionosphere. Since large-scale convection maps of the high-latitude electric potential are perhaps the most commonly used data product of the SuperDARN HF network, the southern hemisphere statistical patterns resulting from this study will improve the accuracy of the southern hemisphere convection patterns during times when data coverage is not widespread. The availability of hemisphere-specific statistical patterns and variability maps resulting from this project will facilitate studies of the interhemispheric dependencies of various ionospheric and magnetospheric parameters.
A major component of this project was the development of a more realistic model of the large-scale, high-latitude convection electric field that includes dependencies on hemisphere, dipole tilt angle, the interplanetary magnetic field (IMF) magnitude and clock angle, and the solar wind velocity. The model was developed using a large dataset of electric field measurements from the SuperDARN array of radars and it can vary over a continuous range of these parameters. The model is used to specify the high-latitude electric field boundary in general circulation models of the Ionosphere-Thermosphere system, to provide realistic estimates of the convection electric field in regions where "instantaneous" measurements are unavailable, and for investigating the climatology of large-scale convection in both northern and southern hemispheres. Using this model is was found that differences in the statisical convection patterns of the northern and southern hemisphere are mostly accounted for when dipole tilt is considered and the IMF By component is flipped. Note that in situations with both a non-zero dipole tilt and IMF By, the large-scale convection patterns in the northern and southern hemispheres can be very different. The main differences are attributed to lobe cell merging in the hemisphere that is tilted toward the Sun. Some residual differences are attributed to the larger offset of the magnetic pole and the larger variation in the magnetic field in the Southern hemisphere. Another major aspect of this project was the investigation and characterization of the small-scale fluctuations in the SuperDARN convection electric field measurments; so-called electric field variability. It was found that the magnitude of the observed variability was largest on the dayside, in the auroral zone and for winter-like conditions. Properties of the observed variability are consistent with turbulent flow and it is likely that plasma instabilities caused by gradients or shears in the background flow are a likely source. Finally, maps of small-scale variability that depend on hemisphere, dipole tilt angle, and IMF were derived and it is shown that variability is largest in the winter-like auroral zone and correlated with downward field-aligned currents. Preliminary work in combining the small-scale variability with the large-scale convection patterns shows promise for a more realistic model of high-latitude plasma convection.
