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The origin of thermal ion outflows exceeding 1km/s in the high-latitude F-regionhas been a subject of considerable debate. For cases with strongconvection electric fields, the "evaporation" of the ions due to frictionalheating below 400-500km has been shown to provide some satisfactoryanswers. By contrast, in the more frequent subclass of outflow eventsobserved over auroral arcs, called type-2, there is no observationalevidence for ion frictional heating. Instead, an electron temperatureincrease of up to 6000° K is observed over the outflow region. In thiscase, field-aligned electric fields have long been suspected to be involved,but this explanation did not seem to agree with expectations from the ionmomentum balance. In the present work we provide a consistent scenario forthe type-2 ion upflows based on our case study of an event that occurred on20 February 1990. We introduce, for the first time, the electron energybalance in the analysis. We couple this equation with the ion momentumbalance to study the salient features of the observations and conclude thattype-2 ion outflows and the accompanying electron heating events are indeedconsistent with the existence of a field-aligned electric field. However,for our explanation to work, we have to require that an allowance be made forelectron scattering by high frequency turbulence. This turbulence could begenerated at first by the very fast response of the electrons themselves toa newly imposed electric field that would be partly aligned with thegeomagnetic field. The high frequencies of the waves would make itimpossible for the ions to react to the waves. We have found the electroncollision frequency associated with scattering from the waves to be rathermodest, i.e. comparable to the ambient electron-ion collision frequency.The field-aligned electric field inferred from the observations is likewiseof the same order of magnitude as the normal ambipolar field, at least forthe case that we have studied in detail. We propose that the field-alignedelectric field is maintained by the north-south motion of an east-west arc.The magnetic perturbation associated with the arc itself converts a smallfraction of the perpendicular electric field into a field parallel to thetotal magnetic field, while the north-south motion ensures that theconversion never stops

Origin of type-2 thermal-ion upflows in the auroral ionosphere