THE INTERPLANETARY MAGNETIC-FIELD B-Y EFFECTS ON LARGE-SCALE FIELD-ALIGNED CURRENTS NEAR LOCAL NOON - CONTRIBUTIONS FROM CUSP PART AND NONCUSP PART

The effect of the interplanetary magnetic field (IMF) B(y) component on the distribution of dayside large-scale field-aligned currents (FACs) is modeled. The study is done by dividing these FACs into a ''cusp part' and a ''noncusp part'' according to the coexisting plasma. We propose that the location of the cusp part FACs shifts in the longitudinal direction whereas the location of the noncusp part FACs shifts in both longitudinal and latitudinal directions in response to the IMF B(y). If combined, the noncusp part region 1 FAC can be found poleward of the cusp part FAC systems when the IMF B(y) is strong. Since these two FACs (the cusp region FAC and the poleward-leaped region 1 FAC) flow in the same direction poleward of the cusp, they reinforce each other in building a strong FAC which is called DPY-FAC. The strong IMF B(y) effect on the dayside FACs near local noon is thus explained by this model. Another important result is that the polewardmost part of the DPY-FAC (or NBZ current under strong IMF B(y)) flows on closed field lines even though it is located where the polar cap conventionally exists. Relative distortions of noncusp part FAC compared to the polar cap size are expected to be less significant when the IMF is southward as compared to IMF northward, yet the distortion itself is expected even for southward IMF as long as /B(y)/ > /B(z)/. The present model is supported by Viking particle and magnetic field data.