An event study of the M-I-T coupling at a pseudo breakup during geomagnetically quiet conditions

A geomagnetically quiet-time (Kp=0+) pseudo breakup event on 20 February 2018 was investigated to examine the Magnetosphere-Ionosphere-Thermosphere coupling at auroral to subauroral latitudes in the Scandinavian sector using multiple ground-based instruments and spacecraft. Coinciding with appearance of the pseudo breakup at 71-73 MLat at approximately 21 MLT, a red arc emerged at the ionospheric trough minimum located at 68 MLat (L≈7.1). The ionospheric trough was scanned meridionally by the Swarm A and C spacecraft, and the measurements clearly showed an electron temperature peak (~15,000 K) at the trough minimum. We concluded that causality to produce the red arc was the heat flux transferred from the plasmapause similar to generation of the stable auroral red (SAR) arc. Different from the SAR arc, the red arc was found at the onset time. The red arc may represent a moment of SAR arc birth, which is generally masked by bright dynamic aurorae. The new feature is named the ephemeral auroral red (EAR) arc. Ion velocity measured by the Swarm A showed westward flow of about 1700 m/s in the trough but at the equator side slope and the flow speed at the trough minimum was almost 0 m/s. This suggests that the polarization electric field may not have been spatially uniform throughout the trough. A Dynasonde deployed at Tromsø (67 MLat, which is located near the equatorward trough edge) measured westward turning of the F-region ion velocity at the pseudo breakup. The collocated Fabry-Perot interferometer (FPI, 630.0 nm) also detected westward wind acceleration following the ion velocity change with almost no delay but with some relative speed. For an approximately 20-min interval after the pseudo breakup, Un∙Vi was negative, which suggests that the mechanical energy of the neutral particles was transferred to the plasma in the ionosphere, contributing to the Joule heating rate at a moment of sudden magnetospheric electric field change at the pseudo breakup. Here Un and Vi are the neutral wind and ion velocity, respectively. However, after this interval, Un∙Vi turned to be positive. The Dynasonde-FPI comparison suggests that the thermospheric wind was prompt to respond to the substorm-induced ion velocity change even in the low plasma density trough, and that inertia of the neutral particle plays a partial role to generate the thermal energy.