Altitude Dependence of Quasi-Static Parallel Electric Field Generation

Based on a kinetic model, we reveal the important role of modified electron acoustic waves (MEAWs) in auroral electron acceleration. In the transition region between the magnetosphere and ionosphere, parallel electric fields are generated through mode coupling between kinetic Alfvén waves (KAWs) and MEAWs. These fields are subsequently sustained by continuous energy input from Alfvén waves originating in the magnetosphere, along with the thermal pressure of hot electrons that replace colder populations. Under the incidence of long-period Alfvén waves carrying upward field-aligned currents, a parallel potential drop forms in the transition region, leading to quasi-static electron acceleration. This mechanism provides a plausible link between shear Alfvén waves and quasi-static auroral electron acceleration. Our results further demonstrate that the lower boundary of the auroral acceleration region (AAR) descends as the potential drop increases, the hot electron density rises, or the hot electron temperature decreases. Moreover, the altitude of the AAR is modulated by ionospheric plasma density and temperature, which define the structure of the transition region. Specifically, lower ionospheric plasma temperature and density lead to a decrease in the lower boundary of the AAR. These findings contribute to explaining the formation of aurorae on Jupiter.