Nighttime Thermospheric Winds Research Articles

A climatology of the nighttime thermospheric winds over Sutherland, South Africa

We present a climatology of nighttime thermospheric neutral winds between February 2018 and January 2019 measured by a Fabry-Perot interferometer (FPI) located in Sutherland, South Africa (32.2°S, 20.48°E; geomagnetic latitude: 40.7°S). This FPI measures the nighttime oxygen airglow emission at 630.0 nm, which has a peak intensity at an altitude of roughly 250 km. The annual meridional and zonal winds at this location vary between −100 and 120 m/s and show typical midlatitude nocturnal and seasonal variations. During local summer months (December-February), the meridional wind is predominantly equatorward from dusk to predawn. However, during the winter months, the meridional wind is poleward from dusk, turns equatorward around midnight, and either remains in this direction for the rest of the night (June) or turns poleward again just before dawn (July and August). The zonal wind velocity is generally eastward during the evening until just before midnight, changing westward post-midnight. The zonal wind peaks at higher velocities during the winter months compared to the summer months. The eastward-to-westward transition occurs later during the winter months compared to the summer months. We compare results from HWM14 with the FPI measurements and find a better agreement between FPI measured winds and HWM14 modeled winds for the meridional component compared to the zonal component. In addition, the HWM14 zonal wind consistently peaked several hours ( ∼ 3 h) prior to the measured wind. Furthermore, the HWM14-modeled eastward-to-westward transitions times were a couple of hours ( ∼ 2 h) earlier during the months of November to January creating what looks like a phase shift compared to the measured wind. This phase shift was apparent in all months and we suggest this can be attributed to a phase shift of the terdiurnal tide between the model and measurements.

Validation of ICON-MIGHTI Thermospheric Wind Observations: 1. Nighttime Red-Line Ground-Based Fabry-Perot Interferometers.

Observations of the nighttime thermospheric wind from two ground-based Fabry-Perot Interferometers are compared to the level 2.1 and 2.2 data products from the Michelson Interferometer Global High-resolution Thermospheric Imaging (MIGHTI) onboard National Aeronautics and Space Administration's Ionospheric Connection Explorer to assess and validate the methodology used to generate measurements of neutral thermospheric winds observed by MIGHTI. We find generally good agreement between observations approximately coincident in space and time with mean differences less than 11 m/s in magnitude and standard deviations of about 20-35 m/s. These results indicate that the independent calculations of the zero-wind reference used by the different instruments do not contain strong systematic or physical biases, even though the observations were acquired during solar minimum conditions when the measured airglow intensity is weak. We argue that the slight differences in the estimated wind quantities between the two instrument types can be attributed to gradients in the airglow and thermospheric wind fields and the differing viewing geometries used by the instruments.

Storm‐Time Coupling of Equatorial Nighttime F Region Neutral Winds and Plasma Drifts

AbstractWe used observations from the Peruvian Fabry‐Perot Interferometer network and from the Jicamarca radar to study the coupling of equatorial nighttime thermospheric winds and ionospheric drifts under moderate solar flux conditions. We show that the coupling of the extended quiet time zonal winds and drifts increases from dusk to midnight and is stronger during equinox than during June solstice. After midnight, they are strongly coupled, except during December solstice when the drifts are stronger. The nighttime disturbance zonal winds and drifts, derived by removing the corresponding quiet time values, are westward with peak magnitudes around midnight. They are in close agreement, except at early night when the winds are stronger, and have strongest (weakest) magnitudes during equinox (June solstice). We also present observations showing the strong neutral wind‐plasma drift coupling during the September 2017 and August 2015 large geomagnetic storms. We show that during the early phase of the September 2017 storm there were large and short‐lived, prompt penetration electric field‐driven, correlated oscillations (~1 hr) in the vertical and zonal plasma drifts, and in the zonal and meridional winds. These are the first observations of prompt penetration‐driven equatorial zonal and meridional wind disturbances. In this event, the vertical and zonal drift oscillations were anticorrelated, and the zonal winds followed the zonal drift oscillations with a delay of ~15 min. Our results illustrate the strong coupling of equatorial thermospheric winds and plasma drifts during geomagnetically quiet as well as during short‐lived prompt penetration and long‐lasting disturbance dynamo events.

Thermospheric winds over Abuja during solar minimum period

Monthly variations of averaged nighttime thermospheric winds have been investigated over Abuja, Nigeria (Geographic: 9.06°N, 7.5°E; Geomagnetic: 1.60°S). The reports are based on Fabry-Perot interferometer measurements of Doppler shifts and Doppler broadening of the 630.0 nm spectral emission. The results were obtained during a period of weak solar activity with the solar flux (F10.7) typically below 70 solar flux units. Inspection of the average monthly thermospheric winds from October 2017 to December 2017 found December meridional winds to be more equatorward than the October and November winds. Zonal winds are eastward with pre-midnight maximum speeds going above 100 m/s. Compared to Jicamarca zonal winds in the Peruvian sector for the same month of October, the magnitude of maximum Abuja zonal wind speed is weaker. We compare the observed diurnal variation with the recently updated Horizontal wind model (HWM 14). Most of the observational features of thermospheric wind diurnal variation are captured in the model variation. The HWM14 generally showed good agreement with the Abuja October and November zonal wind observations but overestimates the December meridional winds. Expected longer period analysis of the results from Abuja will stimulate a better understanding of wind climatology over the West African sector.

Climatologies of nighttime thermospheric winds and temperatures from Fabry‐Perot interferometer measurements: From solar minimum to solar maximum

AbstractWe present a climatology of quiet time thermospheric winds and temperatures estimated from high‐resolution Fabry‐Perot interferometer measurements of the 630.0 nm airglow emission spectral line shape. Three locations are examined in this long‐term study: northeastern Brazil (August 2009 to August 2014), a midlatitude site in North Carolina, USA (June 2011 to December 2014), and a midlatitude site in Morocco (November 2013 to December 2014). We discuss the day‐to‐day, seasonal, and solar cycle trends and variations of thermospheric meridional winds, zonal winds, neutral temperatures, and for the first time vertical winds. Observations made from solar minimum to solar maximum (with F10.7 values ranging from ∼70 to ∼159 solar flux units) confirm that neutral temperatures have a strong solar cycle dependence. However, this data set shows that the neutral winds are more closely tied to the seasonal variation, rather than the solar cycle. We also present comparisons between the two midlatitude sites and include neutral wind comparisons to the updated Horizontal Wind Model 14.

Solar activity dependency of multiday oscillations in the nighttime thermospheric winds observed by Fabry‐Perot interferometer

AbstractAn 11 year (1989–1995 and 2010–2013) nighttime thermospheric wind (at ~250 km) data set from Fabry‐Perot interferometer at Millstone Hill (42.6°N, 71.5°W) is used to investigate multiday oscillations (6–30 days) in the thermospheric nighttime winds. Dependencies of multiday oscillations on solar radiation (F10.7), solar wind speed (SW), and geomagnetic activity (Kp) are determined. We found that (1) there exist prominent quasi 27 day oscillations in the thermospheric zonal (meridional) wind, which appears to be more correlated to the same periodic oscillations in Kp and SW than in F10.7 during the solar maximum (solar declining phase and minimum); (2) quasi 13.5 day oscillations in the zonal wind occur during the solar maximum and increasing phases and are also more correlated to the same periodic oscillations in Kp and SW; and (3) in general, the correlations of the multiday oscillations in the thermospheric winds are more correlated with Kp and SW than F10.7. Our analyses also show that the zonal wind is more sensitive to SW than the meridional wind. We further find that (4) for high F10.7, the premidnight zonal wind is more westward toward increasing SW; for low F10.7, the postmidnight zonal wind is also more westward toward increasing SW; and (5) meridional winds tend to be more southward with increasing SW for medium and high F10.7.

Plasma drifts inferred from thermospheric neutral parameters during geomagnetic storms at 23°S

Abstract Nighttime thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the OI 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), Brazil. The observed nighttime East–West and North–South temperature gradients and neutral wind velocity variations are presented for two geomagnetic storm periods 7–10 July 1991 and 4–9 August 1991. Zonal plasma drifts inferred during an intense geomagnetic storm, from the nighttime neutral wind velocities and temporal temperature gradient variations, suggested that a strong zonal plasma drift (−240 m⧹s) on 9–10 July 1991 changed the direction of the zonal winds to westward at 01:30. Also it was noticed that, during the geomagnetic storm, the zonal temperature gradients were not well defined like those observed during quiet time. The meridional wind was towards the pole for the whole night during the geomagnetic storm of 9–10 July, a behaviour which is unusual during winter time (southern hemisphere) at Cachoeira Paulista. However, the meridional plasma drifts inferred did not show significant differences during the quiet and disturbed periods. The method presented to infer plasma drifts using the observed neutral parameters showed results comparable with those observed by incoherent scatter radar measurements at Arecibo.

An equinoctial asymmetry in the high‐latitude thermosphere and ionosphere

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Throughout the solar cycle, average nighttime thermospheric meridional winds are larger in spring than autumn despite similar levels of solar insolation. The average ion velocities are also larger in spring than autumn at solar maximum, but at solar minimum this position is reversed. Numerical simulations of the thermosphere and ionosphere have not predicted such asymmetries because they generally assume forcing functions that are symmetric about the solstices. The proposed explanation lies in the annual and diurnal variation in solar wind‐magnetosphere coupling caused by changes in the orientation of the geomagnetic pole, and hence the magnetosphere, with respect to the average orientation of the IMF (the Russell‐McPherron effect). This causes a 12‐hour phase difference between the times of maximum solar wind‐magnetosphere coupling at the two equinoxes. In addition, the orientation of the geomagnetic axis with respect to the average IMF is such that <By*Bz> > 0 for the March equinox and By*Bz> < 0 for September. This results in a further source of asymmetry of forcing of the high‐latitude ionosphere as the result of electric fields associated with the four sign combinations of By and Bz. Several predictions arise from the explanation given: for example, a high‐latitude station measuring thermospheric neutral winds in Alaska, 180° in longitude from Kiruna, might be expected to see nighttime thermospheric winds that are larger in the autumn than in the spring.

The seasonal behavior of high‐latitude thermospheric winds and ion velocities observed over one solar cycle

The seasonal variation of nighttime thermospheric winds observed at Kiruna has been found to be significantly dependent on solar activity. Of particular interest is the observation that there is a large difference between the March and September equinox winds, despite similar levels of solar insolation. The September equinox winds are more December solstice‐like. The average March equinox meridional winds are up to 70% larger than for September. The existence of an equinoctial asymmetry has not been predicted by either thermospheric or ionospheric model simulations, which assume that the equinoxes are fundamentally the same, and use forcing functions which are symmetric about the solstices. The average ion velocities measured at EISCAT are larger during the March equinox than the September at solar maximum, while the converse is true at solar minimum. In contrast, the March equinox nighttime thermospheric winds are larger for both solar maximum and solar minimum. Furthermore, the asymmetry is greater at solar maximum.

Thermospheric nighttime neutral temperature and winds over Fritz Peak Observatory: Observed and calculated solar cycle variation

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer for nearly 17 years between 1968 and 1985. We use data for two specific November periods, (1) 1984, near solar cycle minimum, and (2) 1979, near the peak of solar cycle 21, to illustrate the measured solar cycle variation of nighttime neutral gas temperature and winds over the station. In particular, we present data for November 11, 1979, the day following the day when the solar F10.7 radio flux emission reached its greatest daily value of 367 J (1 J = 1.0×10−22 W m−2 Hz−1). The nighttime measurements, all selected for geomagnetic quiet conditions, show a considerable variation of the thermospheric temperature between solar minimum and solar maximum of nearly 500 K but a relatively minor variation in the thermospheric winds. The recently developed National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) is used to simulate the geomagnetic quiet time variation of global thermospheric properties for the two periods and also to perform a time‐dependent simulation to calculate the thermospheric variation during November 9–12, 1979, when the solar F10.7 flux varied from 314 J, 367 J, 325 J, and 294 J, respectively. The TIE‐GCM histories are used to construct the diurnal variation of thermospheric temperatures and winds as a function of altitude over Fritz Peak Observatory. The aeronomy calculated by the TIE‐GCM is also used to predict the diurnal variation and height of the 630‐nm volume emission rate. The station processor calculates the emission‐weighted height‐integrated Doppler line profiles and Doppler shift profiles as would be observed from a ground station. These profiles are reduced into winds and temperatures in a manner similar to the experimental measurements and can be compared with the actual observations. The results show that the TIE‐GCM calculated Doppler temperature and winds are in reasonable agreement with the observations for the two periods representing solar minimum and extreme maximum conditions, suggesting that the solar flux model and aeronomic processes in the TIE‐GCM can simulate the main thermospheric variations throughout the solar cycle. Furthermore, the time‐dependent calculated variations of Doppler temperature and winds also remain in good agreement with the observations of November 11, 1979, which is the day following the maximum solar EUV perturbation of the Earth's upper atmosphere during the entire solar cycle 21. These results show the need for time‐dependent calculations when geophysical parameters have large changes during the course of the period of simulation. The station processor results indicate that the altitude of the 630‐nm emission varies during the solar cycle by keeping at a near‐constant pressure surface, whose geometrical height changes with the solar cycle.

Optical observations of thermospheric neutral winds at Arecibo between 1980 and 1987

Since 1980, optical observations of nighttime thermospheric winds have been made using a Fabry‐Perot interferometer at the Arecibo Observatory, Puerto Rico. High‐resolution spectra of the O(¹D) airglow emission at 630.0 nm are obtained by observing at eight equally spaced azimuth positions and a fixed zenith angle of 60°. The neutral wind field is inferred by assuming that each component of the wind velocity vector may be expanded in a linear Taylor expansion about a point directly above the observatory. Both the zonal and meridional components of the thermospheric wind field are observed to have well‐defined seasonal and nocturnal variations. For each year between 1980 and 1987, eastward flow was observed in the evening hours, with an average peak velocity of about 100 m s−1 near 2200 AST. In the winter months, the zonal wind generally remains eastward throughout the night. However, in summer, a reversal to westward flow is usually observed after local midnight. In the meridional direction, the largest equatorward velocities are observed in summer. A reduction (or reversal) in the meridional wind velocity is most often observed after midnight in the summer and equinoctial months. We find that the nocturnal and seasonal variations in the neutral wind field at Arecibo are remarkably unaffected by changes in the solar cycle.

Nighttime variation of thermospheric winds and temperatures over fritz peak observatory during the geomagnetic storm of March 2, 1983

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer during the March 2, 1983, geomagnetic storm. The winds and temperatures are obtained from the Doppler shifts and line profiles of the [O I] 15,867 K (630 nm) line emission. The planetary geomagnetic index, Kp, increased abruptly from 4 to 7 during the beginning of the measuring period near 0000 UT on March 2 and remained at nearly that level throughout the night. During the early evening hours near 1900 LT (0200 UT) the zonal winds measured in both the east and west directions from Fritz Peak Observatory were westward at a speed of about 100 m s−1. The westward winds increased in magnitude during the next 2 hours, with the wind speed east of the station reaching 400 m s−1 and the winds to the west of the station reaching 300 m s−1. The westward winds remained at roughly these speeds until about local midnight (0700 UT), when they gradually decreased to about 50 m s−1 in the east, and the winds to the west of the station shifted to eastward at about 50 m s−1. The meridional winds measured in the early evening hours both to the north and south of the station were poleward at speeds ranging between 50 and 10 m s−1. Near local midnight the wind measured north of the station gradually shifted to equatorward, reaching a maximum speed of about 300 m s−1 near 0200 LT (0900 UT) and then decreasing to about 100 m s−1 near dawn. The wind measured south of the station remained poleward throughout the night at speeds ranging between 25 and 100 m s−1. The neutral gas temperature over Fritz Peak Observatory increased throughout the night from about 1300° to 1600°K. Calculations made with the National Center for Atmospheric Research thermospheric general circulation model suggest that the observed winds over Fritz Peak Observatory (L ≅ 3) are under the influence of an expanded magnetospheric convection pattern during the storm. Enhanced magnetospheric convection at mid‐latitudes characteristically produces westward winds during the early evening hours and a strong equatorward surge in the postmidnight winds.

The geomagnetic quiet nighttime thermospheric wind pattern over Fritz Peak Observatory during Solar Cycle Minimum and Maximum

Nighttime thermospheric winds and temperatures at F region heights have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer from about November 1972 to the present. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867 K (630 nm) line emission. The data obtained during geomagnetic quiet conditions (Ap ≤ 20) have been separated into two groups representing solar cycle minimum conditions (1972–1977) and solar cycle maximum conditions (1978–1979). The monthly variation of the nighttime zonal and meridional winds are presented for both groups. The results show that during solar cycle minimum the zonal winds are predominantly eastward during the winter months at speeds of 50–75 m s−1 and westward during the summer months at speeds of 50–100 m s−1. The meridional winds measured to the north and south of Fritz Peak Observatory are equatorward throughout most of the night at nearly the same speed. The equatorward winds in the summer peak near midnight at about 100–150 m s−1. The equatorward winds in the winter also peak near midnight but with smaller speeds, 50–75 m s−1. During solar cycle maximum the zonal winds are again predominantly eastward during the winter months, reaching peak speeds of 60–75 m s−1 in the early evening hours and decreasing afterward. In the summer the winds are eastward during the early evening hours, shifting to westward near midnight and increasing to a maximum speed of 75–100 m s−1 in the early morning hours. The meridional winds are poleward at ∼20 m s−1 in the early evening hours, shifting to equatorward near 2100 LT and then increasing in magnitude with maximum speeds near or after midnight. There is a considerable difference in the speed of the winds measured to the north and south of Fritz Peak Observatory. During the summer the equatorward winds measured to the north are 150–175 m s−1, whereas the equatorward winds measured to the south are 50–75 m s−1. During the winter the speeds are smaller, 50–75 m s−1 and 25–30 m s−1 to the north and south of the station, respectively. Calculations made with the National Center for Atmospheric Research thermospheric general circulation model for different seasons are in reasonable agreement with the observations during both solar minimum and solar maximum. During solar cycle maximum the observed and calculated meridional wind differences to the north and south of Fritz Peak Observatory are attributed to enhanced auroral heating and ion drag associated with magnetospheric convection north of the observatory. The equatorward driven winds slow south of Fritz Peak Observatory owing to the diminishing influence of high‐latitude forcings.

F-region neutral winds and temperatures at equatorial latitudes: Measured and predicted behaviour during geomagnetically quiet conditions

Nighttime thermospheric winds and temperatures were measured over Roi Namur Island, Kwajalein Atoll, Marshall Islands, with a Fabry-Perot interferometer; measurements were obtained during August and September 1977, August 1978, July 1979, and March and July 1980. The winds and temperatures were determined from the Doppler shifts and line profiles of the 630.0 nm twilight and nightglow line emission. All of the measured wind vectors lie in the South-East quadrant, however, there is considerable variation in magnitude and direction within that quadrant. During 1977 all of the data exhibit the same qualitative behaviour. At evening astronomical twilight the wind vector is directed eastward with velocities ranging between 50 and 120 m s −1. The wind vector turns clockwise, increasing in magnitude to 130–160 m s −1 at 23 L.T. in an East South-East direction and then decreasing to < 50 m s −1 at 04 L.T. in a southward direction. During 1978 the wind vector generally turns counterclockwise during the night, with considerable day-to-day variability. The measured wind vector variation during July 1979 and March and July 1980 is quite irregular, but the wind vector consistently remains directed toward the South-East quadrant. The measured nighttime variation of the neutral gas gemperature indicates a 200–300 K temperature drop between sunset and sunrise. The average temperature, however, increases by about 400 K from 1977 to 1980. The NCAR thermospheric general circulation model (TGCM) is used to calculate the nighttime variation of winds and temperatures over Kwajalein Atoll. Numerical experiments made with the TGCM indicate that F-region winds in the equatorial region are strongly influenced by variations of ion drag caused by E × B drifts in the equatorial ionosphere, upward-propagating tides from the lower atmosphere, and high-latitude magnetospheric convection. With a reasonable prescription of the three effects, agreement can be obtained between the calculated and observed variation of winds over Kwajalein. By altering the timing between the interaction of tides and E × B drifts it is possible to derive either a clockwise or counterclockwise wind vector rotation during the night. TGCM calculations suggest that much of the observed variability of winds can be attributed to variations of E × B drifts, tides and to a lesser extent high-latitude magnetospheric convection.

Thermospheric response observed over Fritz Peak, Colorado, during two large geomagnetic storms near Solar Cycle Maximum

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high resolution Fabry‐Perot spectrometer. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867K (630 nm) line emission. Measurements made during two large geomagnetic storm periods near solar cycle maximum reveal a thermospheric response to the heat and momentum sources associated with these storms that is more complex than the ones measured near solar cycle minimum. In the earlier measurements made during solar cycle minimum, the winds to the north of Fritz Peak Observatory had an enhanced equatorward component and the winds to the south were also equatorward, usually with smaller velocities. The winds measured to the east and west of the observatory both had an enhanced westward wind component. For the two large storms near the present solar cycle maximum period converging winds are observed in each of the cardinal directions from Fritz Peak Observatory. These converging winds with speeds of hundreds of meters per second last for several hours. The measured neutral gas temperature in each of the directions also increases several hundred degrees Kelvin. Numerical experiments done with the NCAR thermospheric general circulation model (TGCM) suggest that the winds to the east and north of the station are driven by high‐latitude heating and enhanced westward ion drag associated with magnetospheric convection. The cause of the enhanced poleward and eastward winds measured to the south and west of Fritz Peak Observatory, respectively, is not known. During geomagnetic quiet conditions the circulation is typically from the southwest toward the northeast in the evening hours. We speculate that the enhanced flow to the south and west of Fritz Peak Observatory may be due to either (1) a hemispheric difference in the thermospheric energy input rate, (2) equatorial heating by neutral hydrogen precipitation, or (3) an increased midnight equatorial temperature bulge or some combination of these sources. Since the measurements indicate that the winds in the 630‐nm airglow layer converge from all directions at the observatory, we speculate that Fritz Peak is at the boundary of two circulation patterns during part of the storm.

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.

Nighttime thermospheric winds at high latitudes

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

Observations of large‐scale thermospheric waves during geomagnetic storms

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867 K (6300 Å) line emission. Measurements made during three large geomagnetic storms, when Kp exceeded 8, have shown large‐scale thermospheric waves that are probably generated by impulsive heating at high latitudes. These waves are observed to propagate equatorward over the observatory, and on one occasion a poleward traveling disturbance was observed several hours later. The characteristic features of the observed waves during the three storm periods are described, and the global properties of these waves are examined by using a numerical model of the zonally symmetric thermosphere. The model calculations show reasonable agreement with the observations over Fritz Peak Observatory, thus enabling an estimate to be made of the high‐latitude energy input that launched the waves.

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.

Direct measurements of nighttime thermospheric winds and temperatures, 3. Monthly variations during solar minimum

We used a high-resolution Fabry-Perot spectrometer at Fritz Peak Observatory (39.9°N, 105.5°W) to determine the nighttime variation of thermospheric temperatures and winds from the line profiles and Doppler shifts of the O I 15,867-K (6300 A) line emission. With data obtained during the period April 1975 through March 1976 we examined the variation of the nighttime neutral gas temperature, zonal winds, and meridional winds at F layer heights throughout the year during solar cycle minimum. We compare our observed temperatures to predictions made by the MSIS (mass spectrometer and incoherent scatter) empirical model of neutral temperature and composition in the thermosphere. The calculated nighttime variation of thermospheric temperature is about 100°–150°K lower than measurements for summer and equinox months and about 50°–75°K lower than measurements for winter months. We compare the observed winds to the zonal and meridional winds, which we calculate from a three-dimensional semiempirical dynamic model of the thermosphere by using the pressure gradients specified by the MSIS model. The calculated nighttime winds are in general agreement with observed winds; both reveal a systematic monthly variation during geomagnetic quiet times. The nighttime equatorward meridional winds are greater in summer than in winter. The zonal winds are eastward throughout most of the night during winter; however, as the season progresses toward summer, a transition develops in the early morning hours from eastward to westward winds. This transition occurs earlier in the night as summer approaches. We use model predictions of the diurnal variations of winds and temperatures to calculate diurnal averages of the zonal and meridional wind components over Fritz Peak Observatory. The seasonal variation of these zonal mean values is presented.