Thermospheric wind response to a sudden ionospheric variation in the trough: event at a pseudo-breakup during geomagnetically quiet conditions

Abstract

The thermospheric wind response to a sudden westward turning of the ion velocity at a high latitude was studied by analyzing data obtained with a Fabry–Perot interferometer (FPI; 630 nm), Dynasonde, and Swarm A & C satellites during a conjunction event. The event occurred during a geomagnetically quiet period (Kp = 0 +) through the night, but some auroral activity occurred in the north. The collocated FPI and Dynasonde measured the thermospheric wind (U) and ionospheric plasma velocity (V), respectively, in the F region at the equatorward trough edge. A notable scientific message from this study is the possible role of thermospheric wind in the energy dissipation process at F-region altitude. The FPI thermospheric wind did not instantly follow a sudden V change due to thermospheric inertia in the F region. At a pseudo-breakup during the event, V suddenly changed direction from eastward to westward within 10 min. U was concurrently accelerated westward, but its development was more gradual than that of V, with U remaining eastward for a while after the pseudo-breakup. The delay of U is attributed to the thermospheric inertia. During this transition interval, U∙V was negative, which would result in more efficient generation of frictional heating than the positive U∙V case. The sign of U∙V, which is related to the relative directions of the neutral wind and plasma drift, is important because of its direct impact on ion-neutral energy exchange during collisions. This becomes especially important during substorm events, where rapid plasma velocity changes are common. The sign of U∙V may be used as an indicator to find the times and locations where thermospheric inertia plays a role in the energy dissipation process. Graphical Abstract

IMF Dependence of Midnight Bifurcation in the Thermospheric Wind at an Auroral Latitude Based on Nine Winter Measurements in Tromsø, Norway

“…Figure 1 shows the hourly mean thermospheric wind pattern sorted by the IMF clock angle in the GSM Y ‐ Z plane, which is based on the spacecraft‐interspersed, 1‐min averaged near‐Earth solar wind (OMNI) magnetic field and plasma parameters (Papitashvili & King, 2020). A quiet‐time wind pattern was made of measurements under a SuperMAG index, SME (Gjerloev, 2012), less than 40 nT but not sorted by the IMF clock angle (Oyama et al., 2023), which is overlaid in blue. Data number for each time bin is generally smaller at early evening and late morning and larger at midnight.…”

“…The hourly mean winds were sorted by the interplanetary magnetic field (IMF) clock angle in the geocentric solar magnetospheric Y ‐ Z plane. The quiet‐time wind pattern, which was derived from the FPI measurements under SME < 40 nT but with no separation by the IMF clock angle (Oyama et al., 2023), is overlaid in blue. A reference magnitude of the wind vector is illustrated at the top.…”

“…(2003, 2012) and Figure A1 of Oyama et al. (2023). This method is designed to cancel out the vertical wind and the etalon gap drift in deriving the meridional and zonal components of the wind from a pair of north‐south and east‐west LoS speeds, respectively.…”

“…Consequently, at high latitudes, traces of ionospheric plasma convection are seen everywhere in the thermosphere. For example, westward thermospheric wind can often be observed in the dusk sector at auroral latitudes, where westward ion drag acts against the background eastward thermospheric wind (Barreto‐Schuler et al., 2021; Cai et al., 2019; Conde et al., 2001; Oyama et al., 2022, 2023; Wang, Lühr, & Ma, 2012b; Wang, Lühr, Ritter, & Kervalishvili, 2012; Wang et al., 2018; Xu et al., 2019).…”

Thermospheric Wind Response to March 2023 Storm: Largest Wind Ever Observed With a Fabry‐Perot Interferometer in Tromsø, Norway Since 2009

“…In contrast to the thermospheric dynamics at mid/low‐latitudes, more observational studies on thermospheric winds at high latitudes are needed to draw a comprehensive picture during geomagnetic storms. This is because the high‐latitude thermosphere responds to solar wind and magnetospheric variations more directly (e.g., Oyama, Aikio, et al., 2023; Oyama, Hosokawa, et al., 2023) and may dissipate more energies than that we might expect in association with auroral fine structures (e.g., Deng & Ridley, 2007; Kavanagh et al., 2022). For example, the interplanetary magnetic field (IMF) B y dependence of the ionospheric plasma convection induces a maximum anti‐sunward thermospheric wind at the dusk (dawn) side in the polar cap region for the IMF B y negative (positive) conditions (McCormac et al., 1985; Thayer et al., 1987).…”