The system of currents flowing along magnetic field lines in Earth's auroral region and the acceleration of particles that produces the visible aurora are closely related to each other. This connection provides on of the most important aspects of the magnetosphere-ionosphere (M-I) coupling. Recent research has found that on the night side, auroral acceleration tends to be more intense in the winter (dark) hemisphere than in the summer (sunlit) hemisphere. The difference is most pronounced in the pre-midnight sector and is related to the upward region-1 (R1) field-aligned current (FAC) system (downward flowing electrons). A relation between the acceleration of electrons and the amount of current, the Knight relation, implies that the field-aligned current must be more intense in the winter hemisphere. This implication seems paradoxical because the electrical conductivity in the summer hemisphere is higher due to the increased photo ionization. This project will test the three possible explanations of this apparent paradox: (1) The FAC is indeed more intense in the winter hemisphere, despite the fact that the overall conductivity is lower; (2) The FAC is more localized in latitude in the winter hemisphere so that the average current density becomes larger in the winter hemisphere than in the summer hemisphere; (3) In the winter hemisphere the upward FAC is more finely structured than in the summer hemisphere, creating strong local FACs.
Magnetic field and particle precipitation data from the DMSP satellites will be used for this project. An automatic procedure to identify FAC structures will be applied to the data sets, which will create a list of nearly 300,000 FAC crossings. The FAC intensity and density will be examined to test (1) and (2), respectively. The difference between the actual and fitted data will be used as a measure of the amplitude of internal structures, which will be examined for testing (3). The events will be classified in terms of the ionospheric condition (sunlit or dark) based on the solar zenith angle at the ionospheric foot point.
The project will compare those characteristics between sunlit and dark events for both R1 and R2 currents in each local-time sector. A preliminary study has suggested a positive result for (1), but that does not exclude the possibility that the other two explanations may also apply. Particle precipitation data will be examined to test the idea that the interhemispheric asymmetry of the electron precipitation overcompensates for the asymmetry of the background conductivity due to the solar illumination. This study will be based on the largest data set ever used for studying large-scale FACs. In addition, the PI will participate in the University of Maryland's initiative for supporting students from underrepresented groups in geospace science research.
