A missing variable in solar wind-magnetosphere-ionosphere coupling studies

The auroral electrojet (AL) index is commonly used to characterize the magnetospheric activity. The ground magnetic perturbations that comprise AL are formed predominantly by Hall currents, however, and the Hall conductivity is sensitive to plasma sheet Te and Ne parameters, which control the production of energetic electrons via their field‐aligned acceleration. Using AL index and nightside‐dawn sector plasma sheet electron data, we demonstrate that a hotter, lower density plasma sheet is associated with stronger ground magnetic variations in the nightside auroral zone than a colder, denser plasma sheet under similar solar wind driving conditions. The Te1/2/Ne‐dependent difference may exceed a factor of 4 during the growth phase events, whereas the acceleration during substorm expansions smears it out. Background plasma sheet state is mainly controlled by the solar wind velocity and density. We suggest that plasma sheet Te1/2/Ne variable constitutes an important (and currently ignored) variable in studies of solar wind‐magnetosphere‐ionosphere coupling. Key PointsPlasma sheet Te/Ne controls electron acceleration and Hall conductanceAssociated AL difference up to an order of magnitude in the absence of substormsSW density and velocity dictate plasma sheet Te/Ne in the absence of substorms