Polar Cap and Cusp Boundaries at Day and Night

Abstract

We discuss the practical ways to determine the polar cap boundary (PCB, i. e. the boundary between open and closed flux tubes) from observations available at low altitudes. A theory generally predicts a simple topology of PCB (one bundle of the open flux tubes in each polar cap) and the character of its shape and size dependence on IMF but the accurate predictions require to model self-consistently the processes both at the magnetopause and in the magnetotail. The practical ways of PCB indication based on transparent physics and supported by the observations are very few. Among them we discuss: the equatorward boundary of auroral particle precipitation in the cusp proper (in the sector of dayside convection throat); the trapping boundary of the magnetospheric energetic particles; the sharp boundary of the solar electron plateau. The physics of the latter boundary is discussed in more detail. Recent observations indicate that the whole plasma sheet is filled by the solar electrons and that their sharp precipitation boundary originates inside of the closed plasma sheet tubes, indeed, due to the particle scattering (weak non-adiabatic process) in the tail current sheet. The nightside portion of this solar electron plateau boundary generated in this way can be distinguished from its dayside part (which coincides with PCB) by its energy (rigidity) dependent behaviour. In cases of significant north-south hemispherical difference of the solar electron flux the PCB can be accurately determined everywhere. The morphology and physics of the boundaries of the solar electron precipitation as well as their relationship with the patterns of other phenomena (precipitation, convection, FAC etc.) deserve to be studied in much more detail.