Systematics of the equatorward diffuse auroral boundary

DMSP/F2 and DMSP/F4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with Kp as a function of local time. This work extends a previous study of dawn and dusk boundaries to the noon and midnight regions. The boundaries are well‐ordered by Kp in the night sector but show increasingly greater scatter from dawn to postnoon. In the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity. The equatorward diffuse auroral boundary is well fit by a circle at each activity level. The center of the circle is offset from the geomagnetic pole, and the radius of the circle increases with increasing magnetic activity. The circular fits are projected to the ecliptic plane by using the Mead‐Fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with Volland‐Stern type convection electric field predictions. Previously found variations of the electric field with Kp are confirmed, including a rotation of the axis of symmetry, away from the dawn‐dusk meridian. All equatorward auroral boundaries from DMSP for 1978 were compiled. Using the equations for boundary variations with Kp , each evening sector boundary was projected to a midnight boundary. The projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large‐scale magnetospheric convection electric field. The auroral boundary index is given in monthly plots for 1978.