POLAR-CAP FIELD-ALIGNED CURRENTS FOR SOUTHWARD INTERPLANETARY MAGNETIC-FIELDS

It has been common to suppose that polar region field-aligned currents for southward interplanetary magnetic fields (IMF) consist of two par ts: 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 exam ine region 1 currents with the objective of establishing whether or no t they can exist partially on open field lines (i.e., inside the polar caps) for southward IMF. Magnetic field perturbations were used to id entify the field-aligned currents (FACs). Particle measurements from b oth the energetic particle detector and the soft particle detector on board the ISIS 2 satellite were used to distinguish between what we su ggest are open and closed field lines. Although the identification is not unambiguous, greater-than-or-equal-tokeV electrons are found princ ipally on closed field lines. The absence of greater-than-or-equal-tok eV electrons but the presence of less-than-or-equal-to 200 eV electron s in the polar cap or background polar rain is considered as the signa ture of open field lines. On some passes, region 1 sense FACs appear t o be composed of two parts. The poleward part of the current signature is accompanied by electron fluxes at energies less-than-or-equal-to 2 00 eV or occasionally by fluxes at background levels while the equator ward part of the interval is accompanied by electron fluxes at energie s both less-than-or-equal-to 200 eV and greater-than-or-equal-tokeV. O n other passes, region 1 sense currents are accompanied by both less-t han-or-equal-to 200 eV and greater-than-or-equal-tokeV electron fluxes during the entire pass. We propose that region 1 sense FACs flow on b oth closed and open field lines for the first situation and on closed field lines for the second situation. In seeking to understand why reg ion 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 t he IMF B(y). The IMF B(y) may also shift the region 1 currents on open field lines to one side (dawn or dusk) of the polar cap like the conv ection cells. Such a shift provides a consistent model of the data tak en on the dayside and we discuss why night side observations may be di fferent. We suggest that in the presence of southward IMF, region 1 cu rrents can be composed of two parts, one flowing on closed field lines and one flowing on open field lines. Because the portion of the regio n 1 currents inside the polar caps may be generated by different proce sses 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.