Observational characteristics of high-latitude ionization trough seen by Swarm

This study investigates the distribution and formation mechanisms of ionization troughs inside an auroral oval (we call them high-latitude troughs) by analyzing Swarm observations from May to August 2014. This is the period during which simultaneous measurements of the plasma density, ion velocity, ionospheric radial current (IRC), and electron temperature are available. Because high-latitude troughs occur inside an auroral oval and mid-latitude troughs occur at the equatorward edge of an auroral oval, trough locations relative to the equatorward auroral boundary are a critical factor in distinguishing the two troughs. Using the IRC data derived from the magnetic field measurements, we determine the auroral boundary and the direction of field-aligned currents. The main features of high-latitude troughs identified from Swarm data are: (1) enhancements in the ion velocity and electron temperature, (2) downward field-aligned current (FAC), and (3) more frequent occurrence in the Northern (summer) Hemisphere than in the Southern (winter) Hemisphere and in the dawn and dusk sectors than in the noon and midnight sectors. The coincidence of the density minimum and velocity maximum indicates the role of high-speed plasma convection in the formation of high-latitude troughs; the frictional heating of the atmosphere promotes the O+ loss by dissociative recombination. The preferential occurrence of high-latitude troughs at dawn and dusk sectors under downward field-aligned currents indicates the contribution of electron evacuation to the formation of the trough.