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

Oyama, Shin‐ichiro; Hosokawa, K.; Vanhamäki, Heikki; Aikio, Anita; Sakanoi, Takeshi; Cai, Lei; Virtanen, Ilkka; Shiokawa, Kazuo; Watanabe, Midori; Shinbori, Atsuki; Ogawa, Y.

doi:10.1029/2023gl104334

Cited by 3 publications

(3 citation statements)

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“…The stronger westward wind in the dawn and much stronger southward wind in the postmidnight indicate the existence of dawndusk asymmetry of neutral wind in both zonal and meridional directions. This dawn-dusk asymmetry of zonal wind above 110 km is generally in agreement with previous modeling and observational studies of the E− and F-region winds (i.e., Richmond et al, 2003;Oyama et al, 2023a;Oyama et al, 2023b). The dawn-dusk asymmetry of zonal wind in the auroral oval E-region was investigated in more detail through theoretical analysis of solutions of the gradient wind equation by taking account of TMA measurements in a most recent study by Larsen et al (2022).…”

Section: Discussionsupporting

confidence: 86%

“…In the postmidnight sector, at altitudes above 110 km, there are strong southward winds. This wind feature has been demonstrated in previous modeling investigations (Fuller-Rowell, 1984; and recent observational studies using Fabry-Perot interferometers in the F-region (Oyama et al, 2023a;Oyama et al, 2023b). Figure 5 shows a quiver figure of the median winds as a function of magnitude local time and geomagnetic activity level, with red, black, and green corresponding to weak, moderate, and strong geomagnetic activity, respectively.…”

Section: Discussionsupporting

confidence: 77%

“…This hypothesis has been principally established by numerical simulations (Richmond et al, 2003; and theoretic analysis (Larsen et al, 2022). An investigation of F-region neutral winds measurements from Fabry Perot interferometers also confirms the existence of dawndusk asymmetry of the winds (Oyama et al, 2023a;Oyama et al, 2023b). Larsen et al (2022) also showed that the zonal wind velocity limit is set by the inertial wind value in the dusk sector and by twice the geostrophic wind speed in the dawn sector, which is much smaller than the initial wind value.…”

Section: Introductionmentioning

confidence: 79%

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Responses of the wintertime auroral E-region neutral wind to varying levels of geomagnetic activity

Zhan,

Keappler

2023

Front. Astron. Space Sci.

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Introduction: The auroral E-region is an important interface where forces from the magnetosphere and the lower atmosphere converge and have a significant effect on the vertical structure of the neutral winds. The resulting vertical neutral wind structure has been reported to be associated with altitude-dependent and nonlinear effects from different forces. We conduct a statistical examination of the reactions of wintertime neutral winds to four various degrees of geomagnetic activity, with a focus on the impacts of ion drag.Methods: We derive neutral winds using the PFISR measurements covering 2010–2019 and will give a statistical view of the auroral E-region neutral wind with a focus on the winter nighttime during different disturbed conditions. We investigate the effects of the geomagnetic activity on the neutral winds and the tidal components by dividing the dataset into 4 subsets. Tidal decomposition is conducted by least square fitting of the seasonal median winds to obtain the mean, diurnal amplitude, diurnal phase, semidiurnal amplitude, and semidiurnal phase.Results and discussion: We find that 1) when geomagnetic activity increases, dawn-dusk asymmetry exists in both zonal and meridional winds in the upper E-region with stronger zonal wind in the dusk sector than in the dawn sector and much stronger meridional wind in the dawn sector than in the dusk sector. 2) Tidal decomposition results reveal that geomagnetic activity has more significant effects on the meridional diurnal amplitude than zonal diurnal amplitude while the zonal and meridional semidiurnal amplitudes show similar changes when the geomagnetic activity increases. In addition, the maximum semidiurnal amplitude, particularly in the zonal direction, appears at a higher altitude with larger values as geomagnetic activity increases, indicating an ascending transition altitude for the semidiurnal oscillations. The ascending trend of maximum semidiurnal amplitude appearing at higher altitudes during more disturbed conditions has not been reported before. 3) Zonal wind over 110 km demonstrates increasing ion drag effects in the evening sector and the effects of coupled ion drag and other factors after midnight.

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Section: Discussionsupporting

confidence: 86%

Section: Discussionsupporting

confidence: 77%

Section: Introductionmentioning

confidence: 79%

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Responses of the wintertime auroral E-region neutral wind to varying levels of geomagnetic activity

Zhan,

Keappler

2023

Front. Astron. Space Sci.

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

Oyama,

Vanhamäki,

Cai

et al. 2024

Space Weather

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Solar cycles 24–25 were quiet until a geomagnetic storm with a Sym‐H index of −170 nT occurred in late March 2023. On March 23–24, a Fabry‐Perot interferometer (FPI; 630 nm) in Tromsø, Norway, recorded the highest thermospheric wind speed of over 500 m/s since 2009. Comparisons with magnetometer readings in Scandinavia showed that a large amount of electromagnetic energy was transferred to the ionosphere‐thermosphere system. Total electron content maps suggested an enlarged auroral oval and revealed that the FPI observed winds near the polar cap instead of inside the oval for a long period during the storm main phase. The FPI wind had a strong equatorward component during the storm, likely because of the powerful anti‐sunward ionospheric plasma flow in the polar cap. The positive Y‐component of the IMF for 6 days before the storm caused a successive westward component of the FPI‐measured wind during the storm main phase. On March 24, the first day of the storm recovery phase, thermospheric wind disturbed and the ionospheric density decreased significantly at high latitudes. This density depression lasted for several days, and a large amount of electromagnetic energy during the storm modified the thermospheric dynamics and ionospheric plasma density.

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EISCAT Observations of Depleted High‐Latitude F‐Region During an HSS/SIR‐Driven Magnetic Storm

Ellahouny,

Aikio,

Vanhamäki

et al. 2024

JGR Space Physics

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The effect of storms driven by solar wind high‐speed streams (HSSs) on the high‐latitude ionosphere is inadequately understood. We study the ionospheric F‐region during a moderate magnetic storm on 14 March 2016 using the EISCAT Tromsø and Svalbard radar latitude scans. AMPERE field‐aligned current (FAC) measurements are also utilized. Long‐duration 5‐day electron density depletions (20%–80%) are the dominant feature outside of precipitation‐dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12–21 magnetic local time, MLT) the depleted region is 10–18 magnetic latitude (MLAT) in width, with the largest latitudinal extent 62–80 MLAT in the afternoon. The second region is in the morning to pre‐noon sector (04–10 MLT), where the depletion region occurs at 72–80 MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion‐frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion‐neutral frictional heating transported by neutral winds. Even though depleted F‐regions may occur within any of the large‐scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.

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

Cited by 3 publications

References 36 publications

Responses of the wintertime auroral E-region neutral wind to varying levels of geomagnetic activity

Responses of the wintertime auroral E-region neutral wind to varying levels of geomagnetic activity

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

EISCAT Observations of Depleted High‐Latitude F‐Region During an HSS/SIR‐Driven Magnetic Storm

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