PULSATING CUSP AURORA FOR NORTHWARD INTERPLANETARY MAGNETIC-FIELD

We report a characteristic bifurcation of the cusp aurora in associati on with northward transitions of the interplanetary magnetic field (IM F), leading to the coexistence of latitudinally separated forms, calle d types I and 2. In the northernmost form (type 2), which is located a t similar to 77 - 79 degrees MLAT, there appears a sequence of auroral brightenings at the poleward boundary of the preexisting luminosity. Each brightening is followed by equatorward motion and fading of the i ndividual forms. Furthermore, coherent intensity variations of the two latitudinally separated forms are often observed. This is documented by three case examples. The most active phase of the type 2 aurora is observed during intervals of 15 - 20 min duration following northward turnings of the IMF and with nonzero IMF B-y component, corresponding to the transition state before a new equilibrium state of solar wind-m agnetosphere coupling is established. The auroral observations are dis cussed in relation to theoretical models of ionospheric convection, fi eld-aligned currents and solar wind-magnetosphere coupling in the pola r cusp region. The type 2 auroral activity in the north is interpreted as an ionospheric signature which is directly related to reconnection between the IMF and the terrestrial lobe field at the high-latitude m agnetopause, whereas the type 1 aurora is coupled to the low latitude magnetopause. The coherent activations of the two auroral forms indica te that the corresponding current systems are strongly coupled at the ionospheric level.