The auroral oval morphology has been investigated in previous studies presenting maps of average auroral precipitation. However, such distributions tend to emphasize auroral intensity rather than the actual extent of the auroral oval. We develop a statistical method to characterize the auroral oval morphology by using 20 years of electron energy flux measurements from the Defense Meteorological Satellite Program/Special Sensor J (DMSP/SSJ); instead of relying on auroral oval boundaries, we derive the probability of observing aurora from a threshold of 2.109 eV/cm2/s/sr above which the total energy flux of electrons (in the energy range 1–30 keV) is defined as aurora. We then investigate the auroral occurrence probability (AOP) in the magnetic latitude‐magnetic local time (MLat‐MLT) sectors covered by DMSP for various conditions related to geomagnetic activity. Regardless of those conditions, the AOP distributions reveal a width asymmetry with a wider dawn‐to‐noon sector (06–12 MLT) compared to the dusk‐to‐midnight sector (18–24 MLT), the dawn preference getting even more pronounced as the geomagnetic activity decreases. In the context of an open magnetosphere, we investigate the relation between the observed extent asymmetry in the auroral oval and the magnetospheric plasma convection. Representing the plasma sheet magnetic flux as a one‐dimensional fluid subject to production on the nightside (closing of flux via reconnection) and loss on the dayside (opening of flux), we highlight similarities with the AOP in terms of MLT asymmetries. Finally, making use of this fluid model, we demonstrate that the corotation influence on the plasma convection pattern is consistent with the dawn‐dusk asymmetry observed in the AOP distributions.

<p>The aurora is a visible manifestation of Earth’s coupling to near-Earth space. The emitted light is produced by charged particles that precipitate into the upper atmosphere. These particles are usually located on closed magnetic field lines that connect directly between the northern and southern hemisphere. As a result, Earth’s aurora often appears in an oval shape surrounding the magnetic pole. Inside the oval, at high magnetic latitudes, is a region with open magnetic field lines that extend into the solar wind. This region of open magnetic flux is the polar cap and is a consequence of the Dungey cycle: Reconnection between the solar wind magnetic field and closed terrestrial field lines at the dayside magnetopause produces open field lines which are transported to the nightside where they are again closed by reconnection. A fundamental property of the magnetosphere-ionosphere system is that changes in the amount of open magnetic flux is equal to the net difference between the dayside and nightside reconnection rates. That is, the polar cap expands when dayside reconnection dominates and contracts when nightside reconnection dominates. This is known as the expanding/contracting polar cap paradigm, and has been studied extensively in the last few decades. The expansion and contraction of the aurora itself has received less attention. In this work, we use global auroral images to study the spatiotemporal evolution of the auroral oval. We investigate how the solar wind, open flux and auroral flux covary. Furthermore, we attempt to determine how well a pure fluid description of the auroral zone can explain the observed evolution.</p>

<p>Following the work done with the DMSP spectrometer data to derive the auroral occurrence probability in all covered MLat-MLT sectors above 50 degrees MLat (Decotte et al. 2023), here we use the Swarm magnetometer data to extract the probability to detect magnetic field perturbations in the East--West direction. We derive the integrated spectral density from the magnetic field data in a given frequency band, and we define a minimum power threshold above which fluctuations would indicate field-aligned currents. We obtain MLat-MLT distributions of magnetic field fluctuations for various geomagnetic conditions. We find strong similarities between the preferred region of magnetic perturbations and the Xiong and Lühr auroral boundaries (2014), suggesting that the auroral oval morphology could be investigated through magnetic field spectral power estimates. We compare the magnetic field fluctuation probability with the auroral occurrence probability (DMSP particle data) and we find a recurrent dawn-dusk asymmetric pattern in both distributions.  </p>