Polar cap field‐aligned currents for southward interplanetary magnetic fields

Abstract

It has been common to suppose that polar region field‐aligned currents for southward interplanetary magnetic fields (IMF) consist of two parts: region 1 and region 2 currents. It is often suggested that both of these current systems flow on closed field lines. In this pilot study the limited data available from the ISIS 2 satellite are used to examine region 1 currents with the objective of establishing whether or not they can exist partially on open field lines (i.e., inside the polar caps) for southward IMF. Magnetic field perturbations were used to identify the field‐aligned currents (FACs). Particle measurements from both the energetic particle detector and the soft particle detector on board the ISIS 2 satellite were used to distinguish between what we suggest are open and closed field lines. Although the identification is not unambiguous, ≳keV electrons are found principally on closed field lines. The absence of ≳keV electrons but the presence of ≲ 200 eV electrons in the polar cap or background polar rain is considered as the signature of open field lines. On some passes, region 1 sense FACs appear to be composed of two parts. The poleward part of the current signature is accompanied by electron fluxes at energies ≲ 200 eV or occasionally by fluxes at background levels while the equatorward part of the interval is accompanied by electron fluxes at energies both ≲ 200 eV and ≳keV. On other passes, region 1 sense currents are accompanied by both ≲ 200 eV and ≳keV electron fluxes during the entire pass. We propose that region 1 sense FACs flow on both closed and open field lines for the first situation and on closed field lines for the second situation. In seeking to understand why region 1 currents sometimes flow only on closed field lines and sometimes flow on open as well as closed field lines, we suggest a control by the IMF By. The IMF By may also shift the region 1 currents on open field lines to one side (dawn or dusk) of the polar cap like the convection cells. Such a shift provides a consistent model of the data taken on the dayside and we discuss why night side observations may be different. We suggest that in the presence of southward IMF, region 1 currents can be composed of two parts, one flowing on closed field lines and one flowing on open field lines. Because the portion of the region 1 currents inside the polar caps may be generated by different processes from the portion on closed field lines, we refer to the region 1 currents on open field lines as R1O currents. The region 1 currents on closed field lines may be called R1C currents.