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 (referred to as high-latitude troughs) by analyzing Swarm observations from May–August 2014. Simultaneous measurements of plasma density, 3-dimensional ion velocity, ionospheric radial current (IRC), and electron temperature are available during this period. Because high-latitude troughs appear within an auroral oval while mid-latitude troughs appear at the equatorward edge of the auroral oval, the positioning of troughs relative to the equatorward auroral boundary becomes critical for distinguishing between the two types of troughs. We ascertain the auroral boundary and the orientation of field-aligned currents using IRC data derived from magnetic field measurements. The principal features of high-latitude troughs identified from Swarm data include: (a) enhancements in ion velocity and electron temperature, (b) the presence of downward or absent field-aligned current (FAC), and (c) a 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 alignment of the density minimum with the velocity maximum underscores the role of high-speed plasma convection in the formation of high-latitude troughs; atmospheric frictional heating promotes the O⁺ loss through dissociative recombination. The prevailing appearance of high-latitude troughs at dawn and dusk sectors, coupled with downward field-aligned currents, indicates the involvement of outward electron evacuation in trough formation.