Midlatitude Thermospheric Winds Research Articles

Asymmetric behaviour and geomagnetic dependencies of zonal winds in the middle latitude upper thermosphere

In this study we use the CHAMP wind observations from 2002 to 2004 to investigate features of mid-latitude thermospheric winds at low and high geomagnetic activity levels. The winds are observed between the geomagnetic latitudes 40–50° north or south within an altitude range of 370–450 Km and averaged longitudinally. The low geomagnetic activity level wind generally leads eastward in both hemispheres for all the seasons. Maximum westward speeds going above 100 m/s are observed at all levels of geomagnetic activity. Most investigations on F-region parameters assume the equinoxes are fundamentally the same. A first time separation of Equinox season using CHAMP winds reveals equinoctial asymmetric behavior of the wind speed at the two levels considered. Interhemispheric asymmetry in neutral wind behaviour is clearly evident in the solstice seasons with local winds considered in each of the hemispheres. This is consistent with asymmetry in the amount of solar illumination received in each of the hemispheres and differences in the magnetic field strength at magnetically conjugate regions.

Mid-latitude thermospheric wind changes during the St. Patrick’s Day storm of 2015 observed by two Fabry-Perot interferometers in China

In this work, we utilize thermospheric wind observations by the Fabry-Perot interferometers (FPI) from the Kelan (KL) station (38.7°N, 111.6°E, Magnetic Latitude: 28.9°N) and the Xinglong (XL) station (40.2°N, 117.4°E, Magnetic Latitude: 30.5°N) in central China during the St. Patrick’s Day storm (from Mar. 17 to Mar. 19) of 2015 to analyze thermospheric wind disturbances and compare observations with the Horizontal Wind Model 2007 (HWM07). The results reveal that the wind measurements at KL show very similar trends to those at XL. Large enhancements are seen in both the westward and equatorward winds after the severe geomagnetic storm occurred. The westward wind speed increased to a peak value of 75 m/s and the equatorward wind enhanced to a peak value of over 100 m/s. There also exist obvious poleward disturbances in the meridional winds during Mar. 17 to Mar. 19. According to the comparison with HWM07, there exist evident wind speed and temporal differences between FPI-winds and the model outputs in this severe geomagnetic storm. The discrepancies between the observations and HWM07 imply that the empirical model should be used carefully in wind disturbance forecast during large geomagnetic storms and more investigations between measurements and numerical models are necessary in future studies.

Impact of the semidiurnal lunar tide on the midlatitude thermospheric wind and ionosphere during sudden stratosphere warmings

AbstractVariability of the midlatitude ionosphere and thermosphere during the 2009 and 2013 sudden stratosphere warmings (SSWs) is investigated in the present study using a combination of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations and thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulations. Both the COSMIC observations and TIME‐GCM simulations reveal perturbations in the F region peak height (hmF2) at Southern Hemisphere midlatitudes during SSW time periods. The perturbations are ∼20–30 km, which corresponds to 10–20% variability of the background mean hmF2. The TIME‐GCM simulations and COSMIC observations of the hmF2 variability are in overall good agreement, and the simulations can thus be used to understand the physical processes responsible for the hmF2 variability. Through comparison of simulations with and without the migrating semidiurnal lunar tide (M2), we conclude that the midlatitude hmF2 variability is primarily driven by the propagation of the M2 into the thermosphere where it modulates the field‐aligned neutral winds, which in turn raise and lower the F region peak height. Though there are subtle differences, the consistency of the behavior between the 2009 and 2013 SSWs suggests that variability in the Southern Hemisphere midlatitude ionosphere and thermosphere is a consistent feature of the SSW impact on the upper atmosphere.

Climatology and storm time dependence of nighttime thermospheric neutral winds over Millstone Hill

We use 630.0 nm nightglow Fabry‐Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late‐night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at ∼2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM‐93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time‐dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in ∼6 hours; the late‐night eastward winds are smaller and reach their peak values ∼3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward ∼6–10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.

The response of the mid-latitude thermospheric wind to magnetic activity

Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory.

Midlatitude thermospheric winds and temperatures and their relation to the auroral electrojet activity index

Nighttime thermospheric winds and temperatures over Fritz Peak Observatory (39.9°N, 105.5°W) have been determined from measurements of the Doppler shifts and line profiles of the OI 15867‐K (6300 Å) line emission. Measurements have been made during various geomagnetic storm periods from 1973 to 1979. The peak equatorward meridional wind and westward zonal wind during these storms are linearly correlated with the square of the auroral electrojet index, , with correlation coefficients of 0.815 and 0.754, respectively. The measured neutral gas temperature increase is related to the time integral of the index with a correlation coefficient of 0.938.

Midlatitude F region neutral winds and temperatures during the geomagnetic storm of March 26, 1976

A Fabry‐Perot interferometer has been used to determine midlatitude thermospheric wind velocities and temperatures during the large geomagnetic storm of March 26, 1976. These quantities were determined from the doppler shifts and broadening of the twilight and nightglow 630.0‐nm emission line observed from the Laurel Ridge Airglow Observatory (40°8′N, 790°10′W). Following the onset of the storm at ∼0300 hours UT, the wind vector increased from a twilight, quiet period value of ∼100 m/s (eastward) to a maximum of ∼600 m/s, directed generally southward. Large velocities (>200 m/s) persisted from ∼0400 hours UT to the end of the observing period (∼1100 hours UT) and were generally larger by a factor of ≥2 to the north of the observatory than to the south. A temperature rise of ≥500 K during the course of the storm was noted in all directions, with pronounced, relatively rapid fluctuations (of up to 600 K) to the north.

Relationship between midlatitude thermospheric winds and the time rate of change of DSt

Nighttime thermospheric winds and temperatures over Fritz Peak Observatory (39.9°N, 105.5°W) have been determined from the Doppler shifts and line profiles of the (OI) 15876‐K (6300 Å) line emission. The peak winds measured during several geomagnetic storms are shown to be linearly related to the time rate of change of the equatorial ring current parameter (d(DSt)/dt). During these geomagnetic storm periods the time variation of the measured meridional winds are also strongly correlated with the time rate of change of DSt although the observed winds at midlatitudes may lag variations in this parameter by several hours. The energy input derived from d(DSt)/dt is consistent with the high latitude energy input that is necessary in a thermospheric dynamic model to bring the calculated meridional winds into agreement with the observations. These results suggest that d(DSt)/dt is a useful index for determining the magnitude and time history of the high latitude energy input during both quiet and disturbed geomagnetic conditions for use in global thermospheric dynamic models.

Midlatitude thermospheric winds from incoherent scatter radar and theory

Ionospheric plasma vertical drifts measured by the Millstone Hill incoherent scatter radar are decomposed into estimates of meridional neutral particle motion by accounting for ion diffusion and semidiurnal electric fields. For two equinoctial days with relatively high F‐region electron densities the resulting neutral circulation at 300 km is composed of equatorward winds of 100 to 150 m sec−1 at night and poleward winds of 25 to 50 m sec−1 during the day. Harmonic analysis of the wind pattern yields a diurnal northward component of 80 to 100 m sec−1 with a maximum near noon, a semidiurnal component of 35 m sec−1 at 0600 LT and a prevailing southward wind of about 20 m sec−1. The observations are compared with tidal theory predictions and are interpreted by means of a simplified integration of the horizontal equations of motion through which effects of zonal advection, ion drag, viscosity, and pressure gradients are studied. Although the observed wind pattern is consistent in general with that derived from global models based on satellite drag observations, various dicrepancies concerning the amplitude and phase of the diurnal bulge are noted and discussed.