Characterization of the Thermospheric Mean Winds and Circulation during Solstice using ICON/MIGHTI Observations

Abstract: Using the horizontal neutral wind observations from the MIGHTI instrument onboard NASA's ICON (Ionospheric Connection Explorer) spacecraft with continuous coverage, we determine the climatology of the mean zonal and meridional winds and the associated mean circulation at low- to middle latitudes ($10^\circ$S-40$^{\circ}$N) for Northern Hemisphere {summer} solstice conditions between 90 km and 200 km altitudes, specifically on 20 June 2020 solstice as well as for a one-month period from 8 June-7 July 2020 {and for Northern winter season from 16 December 2019-31 January 2020, which spans a 47-day period, providing full local time coverage}. The data are averaged within appropriate altitude, longitude, latitude, solar zenith angle, and local time bins to produce mean wind distributions. The geographical distributions and local time variations of the mean horizontal circulation are evaluated. The instantaneous horizontal winds exhibit a significant degree of spatiotemporal variability often exceeding $\pm 150 $ m s$^{-1}$. The daily averaged zonal mean winds demonstrate day-to-day variability. Eastward zonal winds and northward (winter-to-summer) meridional winds are prevalent in the lower thermosphere, which provides indirect observational evidence of the eastward momentum deposition by small-scale gravity waves. The mean neutral winds and circulation exhibit smaller scale structures in the lower thermosphere (90-120 km), while they are more homogeneous in the upper thermosphere, indicating the increasingly dissipative nature of the thermosphere. The mean wind and circulation patterns inferred from ICON/MIGHTI measurements can be used to constrain and validate general circulation models, as well as input for numerical wave models.

• The paper demonstrates significant spatiotemporal variability with wind reversals in both zonal and meridional flows.
• It employs continuous ICON/MIGHTI observations and systematic data averaging to map wind distributions across altitude, local time, and latitude.
• The study’s findings validate existing wind climatologies and aid in refining general circulation models for atmospheric dynamics.

The paper leverages horizontal neutral wind observations from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument aboard NASA's Ionospheric Connection Explorer (ICON) to characterize thermospheric mean zonal and meridional winds, and circulation during solstice conditions at low- to mid-latitudes ($10^\circ$S--40$^{\circ}$N). The analysis focuses on Northern Hemisphere summer solstice conditions during 2020, specifically the 20th of June, a one-month period from June 8 to July 7, and a Northern winter season spanning from December 16, 2019, to January 31, 2020.

Key findings include:

• Horizontal winds exhibit significant spatiotemporal variability, often exceeding $\pm 150 \text{ m s}^{-1}$.
• Zonal and meridional mean winds demonstrate reversals in the lower thermosphere.
• Distributions of mean winds and circulation are more homogeneous in the upper thermosphere compared to the lower thermosphere.

Methods

The study determines the climatology of mean zonal and meridional winds and the associated mean circulation using continuous coverage from the MIGHTI instrument. Data averaging is performed within specified altitude, longitude, latitude, solar zenith angle, and local time bins. The geographic distributions and local time variations of the mean horizontal circulation are then evaluated. Data quality flags are used to filter the data, removing measurements flagged as "bad" and outliers exceeding 300 m/s.

Results

The instantaneous horizontal winds exhibit substantial spatiotemporal variability. Daily averaged zonal mean winds demonstrate day-to-day variability. Eastward zonal winds and northward (winter-to-summer) meridional winds are prevalent in the lower thermosphere, providing observational evidence of eastward momentum deposition by small-scale gravity waves. The mean neutral winds and circulation display smaller-scale structures in the lower thermosphere (90-120 km), while they are more homogeneous in the upper thermosphere, indicative of the increasing dissipation with altitude. The occurrence rates of wind speeds are evaluated, binned in 5 m/s intervals, revealing a Gaussian distribution generally centered around slow speeds. Nighttime wind variabilities are found to be greater than daytime variabilities and increase with height.

The study computes the zonal mean by averaging a field variable $\psi$ over time $t$ and longitude $x$:

$$\bar{\psi}(z,t) = \frac{1}{\Delta x\,\Delta \tau}\int_t^{t+\Delta \tau}\int_x^{x+\Delta x} \psi(x,y,z,t) \, dx dt$$

where:

• $\bar{\psi}(z,t)$ is the zonal mean of the field variable $\psi$ at altitude $z$ and time $^{\circ}$0
• $^{\circ}$1 is the longitudinal interval over which the average is taken
• $^{\circ}$2 is the time interval over which the average is taken
• $^{\circ}$3 is the value of the field variable at longitude $^{\circ}$4, latitude $^{\circ}$5, altitude $^{\circ}$6, and time $^{\circ}$7

The horizontal wind climatology for the Northern Hemisphere summer solstice is determined by averaging over measurements from June 8 to July 7, 2020. In the lower thermosphere, eastward winds reach up to 40 m/s, while daytime westward mean zonal winds in the upper thermosphere can exceed -60 m/s. Meridional winds are weakly southward between $^{\circ}$890-105 km and northward between $^{\circ}$9105-120 km. Above 120 km, winds are directed southward. Latitude-local time cross-sections of the mean zonal and meridional winds at representative thermospheric altitudes reveal semidiurnal variations in zonal winds within the 90-105 km layer, and diurnal variations in meridional winds.

Latitude-longitude cross-sections demonstrate that the monthly-mean morphology of horizontal winds significantly changes in the lower thermosphere between 90 and 120 km. The upper thermosphere at 185-200 km is dominated by westward and southward winds, indicative of the pole-to-pole solstitial meridional circulation. The associated latitude-local time and latitude-longitude distributions of the mean horizontal circulation are evaluated.

The analysis also includes winds and circulation patterns during the Northern Hemisphere winter solstice, using data from December 16, 2019, to January 31, 2020. The zonal winds are found to be westward at low latitudes and eastward at midlatitudes, with slower speeds than during the summer.

Discussion

The momentum balance for neutral winds is described by:

$\pm 150 \text{ m s}^{-1}$0

where:

• $\pm 150 \text{ m s}^{-1}$1 is the neutral wind vector
• $\pm 150 \text{ m s}^{-1}$2 is the thermodynamic pressure
• $\pm 150 \text{ m s}^{-1}$3 is the mass density
• $\pm 150 \text{ m s}^{-1}$4 is the gravitational acceleration
• $\pm 150 \text{ m s}^{-1}$5 is the rotation rate of Earth
• $\pm 150 \text{ m s}^{-1}$6 is the viscous shear stress
• $\pm 150 \text{ m s}^{-1}$7 is the collision frequency between neutrals and species k
• $\pm 150 \text{ m s}^{-1}$8 is the velocity of species k
• $\pm 150 \text{ m s}^{-1}$9 is the momentum deposition by eddies or small-scale waves

The large-scale behavior of thermospheric mean winds is controlled by the pressure gradient force, inertia, ion drag, and Coriolis force. The eastward mean zonal winds observed in the mesosphere and lower thermosphere (MLT) between 90-110 km are associated with eastward gravity wave momentum deposition. ICON/MIGHTI measurements reveal a reversal of mean meridional winds in the MLT. Above 120 km, radiative processes and ion-neutral coupling become increasingly important in driving the horizontal circulation. The observed westward zonal winds and southward meridional winds in the upper thermosphere are maintained by the pressure gradient force, modulated by ion drag and the Coriolis effect.

Conclusion

The study presents the mean behavior of thermospheric zonal and meridional winds at 90-200 km, as observed by the MIGHTI instrument. It provides a comprehensive picture of mean winds and horizontal circulation for solstice conditions. The observed eastward mean flow and northward meridional flows in the lower thermosphere are attributed to eastward gravity wave momentum forcing. In the upper thermosphere, the morphology of ICON/MIGHTI mean winds is generally in good agreement with previous wind climatologies. The mean wind and circulation patterns can be used to constrain and validate general circulation models.