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

• Published in: Geophysical research letters Vol. 50; no. 14
• Main Authors: Oyama, S., Hosokawa, K., Vanhamäki, H., Aikio, A., Sakanoi, T., Cai, L., Virtanen, I. I., Shiokawa, K., Nishitani, N., Shinbori, A., Ogawa, Y.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.07.2023; Wiley
• Subjects: Advection; Bifurcations; Collision dynamics; Convection; Convection patterns; Direction; Flow distortion; Flow distribution; high latitude; IMF dependence; Interferometers; Interplanetary magnetic field; Ion velocity; Ionosphere; Ionospheric ions; Ionospheric plasma; Ions; Latitude; Magnetic field; Magnetic fields; Momentum; Momentum transfer; Neutral particles; Particle collisions; Plasma; Plasma convection; Polar caps; Polar regions; Radio waves; Response time; Rivers; Streams; Thermosphere; Thermospheric dynamics; Velocity; Wind; Wind data; Wind effects; Wind variations; Winter; Zonal winds; Norway
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104334

A thermospheric wind data set from a Fabry‐Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the thermosphere and F‐region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti‐sunward plasma flow distorted in the polar cap. Plain Language Summary The ionosphere is partially ionized plasma, but the particle minority of ions plays an important role in controlling dynamics of the thermosphere. Particle collision is the fundamental process for momentum transfer from ionospheric ions to thermospheric neutral particles. The ionospheric plasma flow pattern at high latitudes depends on the direction of the interplanetary magnetic field (IMF), and the pattern may be projected on the thermospheric wind. However, the dependence is not yet well understood. This study derived statistical experimental features regarding the dependence of the thermospheric wind, analyzing data from an optical interferometer (Fabry‐Perot interferometer) and a radio wave technique (Dynasonde) in Tromsø, Norway. The wind pattern around midnight is different from the ionospheric plasma convection, in accordance with the IMF direction. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may cause the dependence because of anti‐sunward plasma flow distortion in the polar cap. In summary, this study concludes that the zonal wind bifurcation at auroral latitudes is caused by the ion velocity bifurcation, and that advection from the polar cap region affects the wind response time to the ion velocity bifurcation. Key Points The thermospheric wind from a Fabry‐Perot interferometer (630 nm) and the ionospheric plasma velocity from a Dynasonde were compared The zonal wind bifurcates immediately before midnight for interplanetary magnetic field By < 0, but for By > 0, it inverts gradually into the postmidnight sector The wind bifurcation signature is different from the ion velocity bifurcation, probably due to advection from the polar cap region