Electrodynamic structure of the morning high‐latitude trough region

Abstract

We describe the electrodynamics of a postmidnight, high‐latitude ionospheric trough, observed with the European Incoherent Scatter radar in northern Scandinavia on 24–25 June 2003 around 22:00–02:30 UT during quiet conditions. The UHF radar made meridian scans with a 30 min cadence resulting in nine cross sections of ionospheric parameters. The F region electric field was also determined with the tristatic system. Ionospheric equivalent currents, calculated from ground magnetometer data, mostly show an electrojet‐like current that is reasonably uniform in the longitudinal direction. Combined analysis of the conductances and equivalent current with a local Kamide‐Richmond‐Matsushita (KRM) method yields the ionospheric electric field and field‐aligned current (FAC) in a 2‐D (latitude‐longitude) area around the radar. We conclude that the most likely scenario is one where the trough is initially created poleward of the auroral oval by downward FAC that evacuates the F region, but as the trough moves to lower latitudes during the early morning hours, it becomes colocated with the westward electrojet. There the electron density further decreases due to increased recombination caused by enhanced ion temperature, which in turn is brought about by a larger convection speed. Later in the morning the convection speed decreases and the trough is filled by increasing photoionization.

Postmidnight ionospheric troughs in summer at high latitudes

“…The reason for ion temperature increase cannot be readily associated with high ion velocities in all cases because measurements of ion velocities are not reliable for all four events. Vanhamäki et al [2016] observe that for the trough on 24-25 June 2003, vertical currents are upward at the poleward side and downward in the center, where the density has the minimum, as well as on the equatorward side, which may contribute to shaping the trough. Also the deepest regions are colocated with large eastward ion velocities (more than 600 m/s in the Earth fixed frame), for at least three of the four cases (Figures 2, 4, and 8).…”

“…Vanhamäki et al [2016] have shown that the electron density in areas of large plasma flow can be reduced by approximately a quarter, compared to the surrounding environment. Vanhamäki et al [2016] have shown that the electron density in areas of large plasma flow can be reduced by approximately a quarter, compared to the surrounding environment.…”

“…The latter event is analyzed in detail in the companion paper [Vanhamäki et al, 2016]. A relatively low number of CP3 experiments during summer are available; thus, the number of events is also very small.…”

“…For the trough in 24-25 June 2003, analyzed in detail byVanhamäki et al [2016], the flow is also eastward. Similar to Figure 3.4.…”

Ionospheric conductances and currents of a morning sector auroral arc from Swarm‐A electric and magnetic field measurements

“…The European Incoherent Scatter Scientific Association (EISCAT) radars can be used to derive the conductances and electric fields either at one point [e.g., Davies and Lester , ; Aikio et al , , ; Aikio and Selkälä , ] or along meridian scans [e.g., Vanhamäki et al , ]. The method of characteristics [ Amm et al , ] has been used to derive mesoscale (∼100 km) spatial distributions of FACs and conductances by combining ground magnetic measurements with electric field data from the Scandinavian Twin Auroral Radar Experiment (STARE) radar (decommissioned in 2005), with additional assumptions about the ionospheric Hall/Pedersen conductance ratio.…”