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.

The nightside poleward boundary of the auroral oval as seen by DMSP and the Ultraviolet Imager

“…However, it is an indirect method which is highly dependent on the spatial distribution of stations used. Another more accurate but less routine method uses solar energetic particle events to identify the polar cap boundary [Sergeev, 1990]. These events are extremely rare (< 10 times per year) but can be used to calibrate other methods.…”

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

“…The cusp/cleft aurora is perhaps the most unambiguous ionospheric signature of the open/closed field line separatrix surface in the dayside magnetosphere [Sergeev, 1990 In this study we concentrate attention on two characteristic dynamical features of the cusp/cleft aurora and their relations to major changes in the IMF Bz component and solar wind dynamic pressure, respectively. One aspect of solar wind-magnetosphere coupling under investigation is the latitudinal movements and jumps of the persistent cusp/cleft aurora, i.e.…”

Polar particle flux distribution and its spatial extent

“…Polar Cap (PC): In the PC, the magnetic field lines are generally assumed to be open and connected to the solar wind, whereas the field lines outside the PC are closed (with the exception of the cusp) and map to the plasma sheet and the plasma sheet boundary layer in the magnetosphere. These two substantially different magnetic and plasma domains are separated by a polar cap boundary (e.g., Sergeev, 1990;Newell et al, 2009). Early spacecraft observations revealed a very simple precipitation pattern of solar energetic electrons in the PC: a plateau pattern with intense and nearly homogeneous flux with a sharp drop-off to lower latitudes (for high energies e.g.…”