An Investigation of the Dawn Dusk Asymmetry In the Aurora Occurrence Probability

According to the open magnetosphere model of Dungey (1961), magnetospheric dynamics are largely governed by the interconnection between the magnetic fields of the Earth and the interplanetary medium. Magnetic reconnection may occur at the subsolar magnetopause, depending on the interplanetary magnetic field (IMF) orientation, thus creating open field lines that are dragged to the nightside to form the magnetotail lobes. In the high latitude ionosphere the footprints of these open field lines comprise the polar cap, where aurora can be induced with an oval shape large scale structure. Using years of electron energy flux measurements from the SSJ instrument aboard several DMSP satellites, we developed a method to characterise the auroral oval morphology in a statistical way. Given an energy range and a specific threshold above which electron precipitation is defined as aurora, we investigate the probability of observing aurora in different magnetic latitude and local time sectors. We obtain an auroral occurrence probability map which reveals a dawn/dusk asymmetry in the auroral oval. Here we present a simple model (1D) to explain this asymmetry in terms of sunward transport of plasma sheet magnetic flux. We assume that the auroral occurrence probability is proportional to the magnetic flux of the plasma sheet per hour MLT. By taking into account the magnetic flux convection from the nightside to the dayside through the return flow, as well as the corotation speed of the Earth, we solve the time-stationary continuity equation to describe the MLT variation of the occurrence probability. We find that the production of plasma sheet magnetic flux via magnetic reconnection on the nightside is associated with a high probability of observing aurora, as expected. Conversely, the probability of observing aurora is lower on the dayside, where closed magnetic flux is destroyed.