Pulsating cusp aurora for northward interplanetary magnetic field


We report a characteristic bifurcation of the cusp aurora in association with northward transitions of the interplanetary magnetic field (IMF), leading to the coexistence of latitudinally separated forms, called types 1 and 2. In the northernmost form (type 2), which is located at ∼77 – 79° 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 individual 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 By component, corresponding to the transition state before a new equilibrium state of solar wind-magnetosphere coupling is established. The auroral observations are discussed in relation to theoretical models of ionospheric convection, field-aligned currents and solar wind-magnetosphere coupling in the polar 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 magnetopause, whereas the type 1 aurora is coupled to the low - latitude magnetopause. The coherent activations of the two auroral forms indicate that the corresponding current systems are strongly coupled at the ionospheric level.

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JGR: Space Physics



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