Solar particles and the dayside limit of closed field lines

Low-altitude dayside profiles of solar-electron latitudes are examined as a function of magnetic activity. The solar-electron measurements are compared both with outer-zone electron measurements and with solar-proton measurements in an attempt to determine the dayside limit of closed geomagnetic-field lines. It is suggested that at magnetically quiet times the limit of closed field lines occurs at the position of the electron knee latitude (Λ ∼ 78°) and that, as magnetic activity increases, solar electrons scatter or diffuse across closed field lines. Thus the electron knee latitude at these times lies equatorward of the high-latitude limit of closed lines.