J. T. Emmert

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

Climatology of middle- and low-latitude daytime F region disturbance neutral winds measured by Wind Imaging Interferometer (WINDII)

We have modeled the global climatology of middle- and low-latitude F region daytime disturbance neutral winds using extensive measurements by the Wind Imaging Interferometer (WINDII) instrument on board the UARS. The perturbation winds were obtained by subtracting the quiet time values from the disturbed winds along the satellite orbit, which effectively removes average measurement bias. The zonal disturbance winds are mostly westward (except in the early morning sector), increase with latitude, and have largest values in the late afternoon sector. In general, the meridional perturbation winds are equatorward, increase linearly with latitude, and decrease from early morning to afternoon hours. The zonal and meridional perturbations increase roughly linearly with Kp and expand to lower latitudes with increasing magnetic activity. The meridional disturbance winds are largest for low solar flux conditions. We present empirical analytical models for longitudinally averaged disturbance winds from 60° to the equator. Our model winds are in poor agreement with results from the empirical wind model Horizontal Wind Model-93 during the entire daytime period. There are also important discrepancies between the average perturbations winds from WINDII and the National Center for Atmospheric Research thermosphere-ionosphere electrodynamic general circulation model, particularly at midlatitudes. These differences could be explained in part by the storm time dependence of the disturbance winds and by the variability of the high-latitude electric fields.

Climatologies of nighttime upper thermospheric winds measured by ground-based Fabry-Perot interferometers during geomagnetically quiet conditions: 2. High-latitude circulation and interplanetary magnetic field dependence

We analyze upper thermospheric (∼250 km) nighttime horizontal neutral wind patterns, during geomagnetically quiet (Kp < 3) conditions, over the following locations: South Pole (90°S), Halley (76°S, 27°W), Millstone Hill (43°N, 72°W), Sondre Stromfjord (67°N, 51°W), and Thule (77°N, 68°W). We examine the wind patterns as a function of magnetic local time and latitude, solar cycle, day of year, and the dawn-dusk and north-south components of the interplanetary magnetic field (IMF B y and B z ). In magnetic coordinates, the quiet time high-latitude wind patterns are dominated by antisunward flow over the polar cap, with wind speeds that generally increase with increasing solar extreme ultraviolet (EUV) irradiation. The winds are generally stronger during equinox than during winter, particularly over the South Pole in the direction of eastern longitudes. IMF B y exerts a strong influence on the wind patterns, particularly in the midnight sector. During winter, B y positive winds around midnight in the northern (southern) hemisphere are directed more toward the dusk (dawn) sector, compared to corresponding B y negative winds; this behavior is consistent with the B y -dependence of statistical ionospheric convection patterns. The strength of the wind response to B y tends to increase with increasing solar EUV irradiation, roughly in proportion to the increased wind speeds. Quiet time B y effects are detectable at latitudes as low as that of Millstone Hill (magnetic latitude 53°N). Quiet time B z effects are negligible except over the magnetic polar cap station of Thule.

Attribution of interminima changes in the global thermosphere and ionosphere

We present a statistical attribution analysis of the changes in global annual average thermospheric mass density and ionospheric total electron content (TEC) between the cycle 22/23 solar minimum (which occurred at epoch 1996.4) and the prolonged cycle 23/24 minimum (2008.8). The mass density data are derived from orbital drag, and the TEC data are derived from ground-based GPS receivers. The interminima change in mass density was −36% relative to the 1996.4 yearly average. Considering each multiplicative forcing independently, lower average geomagnetic activity during the cycle 23/24 minimum produced an interminima density change of at least −14%, solar extreme ultraviolet (EUV) irradiance forcing produced a density change of −1% to −13%, and changes in thermospheric CO2 concentration produced a density change of −5%. There was essentially no interminima change in global TEC derived from ground-based GPS receivers or space-based altimeters, even though past behavior suggests that it should have changed −3% (0.2 TEC units (1 TECU = 1016 el m−2)) in response to lower geomagnetic activity and −1% to −9% (0.1–0.8 TECU) in response to lower EUV irradiance. There is large uncertainty in the interminima change of solar EUV irradiance; the mass density and TEC data suggest a plausible range of 0% to −6%.

Altitude and solar activity dependence of 1967–2005 thermospheric density trends derived from orbital drag

We examine 1967–2005 thermospheric mass density trends (as well as 1967–2013 trends) derived from satellite orbit data, as a function of altitude, solar flux, and geomagnetic activity. At 400 km altitude, the estimated 1967–2005 trend is −2.0 ± 0.5% per decade. The estimated trends become increasingly negative with increasing height between 250 and 575 km, suggesting an exospheric temperature trend of −1 to −2 K per decade, which is much smaller than temperature trends that have been inferred from ground-based incoherent scatter radar measurements. The orbit-derived trend height profiles are in good agreement with model simulations of the enhanced cooling that results from increasing concentration of CO2 in the mesosphere and lower thermosphere. In contrast to earlier results, the solar flux dependence of the estimated trends is weak, relative to the trend uncertainty. There is some indication that the trends may be stronger during very low geomagnetic activity conditions. Estimation of the solar flux and geomagnetic activity dependence of the trends is complicated by monotonic decreases in these drivers over the past four solar minima together with the CO2 increase, all of which drive interminima decreases in density.

Remote Sensing of Earth's Limb by TIMED/GUVI: Retrieval of thermospheric composition and temperature

The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N2, and O2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002–2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N2 densities are lower than the NRLMSISE-00 model, but O2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

Initial ground-based thermospheric wind measurements using Doppler asymmetric spatial heterodyne spectroscopy (DASH)

We present the first thermospheric wind measurements using a Doppler Asymmetric Spatial Heterodyne (DASH) spectrometer and the oxygen red-line nightglow emission. The ground-based observations were made from Washington, DC and include simultaneous calibration measurements to track and correct instrument drifts. Even though the measurements were made under challenging thermal and light pollution conditions, they are of good quality with photon statistics uncertainties between about three and twenty-nine meters per second, depending on the nightglow intensity. The wind data are commensurate with a representative set of Millstone Hill Fabry-Perot wind measurements selected for similar geomagnetic and solar cycle conditions.

Evidence for stratospheric sudden warming effects on the upper thermosphere derived from satellite orbital decay data during 1967–2013

We investigate possible impact of stratospheric sudden warmings (SSWs) on the thermosphere by using long-term data of the global average thermospheric total mass density derived from satellite orbital drag during 1967–2013. Residuals are analyzed between the data and empirical Global Average Mass Density Model (GAMDM) that takes into account density variability due to solar activity, season, geomagnetic activity, and long-term trend. A superposed epoch analysis of 37 SSW events reveals a density reduction of 3–7% at 250–575 km around the time of maximum polar vortex weakening. The relative density perturbation is found to be greater at higher altitudes. The temperature perturbation is estimated to be −7.0 K at 400 km. We show that the density reduction can arise from enhanced wave forcing from the lower atmosphere.

Traveling planetary‐scale waves in the lower thermosphere: Effects on neutral density and composition during solar minimum conditions

The effects of breaking of traveling, planetary scale Rossby waves (TPWs) in the lower thermosphere are investigated with respect to the mixing of neutral constituents. We use numerical simulations of the Whole Atmosphere Community Climate Model, eXtended version, whose meteorology below 92 km is constrained by atmospheric specifications obtained from operational weather forecast/data assimilation system. The Fourier spectra show that the amplitude of TPWs with periods between 3 and 10 days are statistically significant in some years; the amplitude and phase of the band-pass filtered behavior is consistent with the behavior of the 5 day wave. A wavelet analysis using the S-transform shows that large variations with periods between 3 and 10 days can occur in relatively narrow temporal windows (20–30 days) during boreal winter. The momentum flux entering the lower thermosphere during the times of TPW amplification is shown to be large, and the amplifications of the TPWs in the thermosphere are not always associated with stratospheric sudden warming. The subtropical zonal accelerations are consistent with Rossby wave encountering a surf zone at low latitudes, resulting in wave breaking. The zonal acceleration is shown to be associated with a meridional diffusion, which is largest in the lower thermosphere where the wave activity and the wave breaking are also large. The ultimate effect on neutral density and composition is a meridional, down-gradient mixing; although this horizontal diffusion is largest below 110 km, the effects on the composition are amplified with increasing altitude, due to the diffusive separation of the thermosphere.

Geospace variability during the 2008–2009 Whole Heliosphere Intervals

We simulate the ionosphere and thermosphere throughout the extended solar minimum epoch from 2008 to 2009 using geospace models, systematically validating the models with databases of observed geospace composition. We isolate and quantify observed changes of as much as 4 total electron content unit (TECU) (1 TECU = 1016 elections m−2) (~36%) in global (60°S−60°N) ionospheric total electron density and as much as 19 × 10−12 kg m−3 (~75%) in global thermospheric mass density at 250 km associated with fluctuating solar EUV radiation and geomagnetic activity during this nominally “quiet” period. Corresponding modeled responses to both solar EUV radiation and geomagnetic activity are about a factor of 2 smaller than is observed. We identify, as well, semiannual and annual oscillations that produce geospace variability comparable to that produced by external solar and geomagnetic influences, and which cause distinct differences among the three individual Whole Heliosphere Intervals. From the first Whole Heliosphere Interval (March–April 2008) to the third Whole Heliosphere Interval (June–July 2009) total electron content (60°S−60°N) decreased 3.6 TECU (~32%) and mass density at 250 km decreased 9 × 10−12 kg m−3 (~34%) due to these oscillations. Reliable attribution of the geospace base state during the 2008–2009 solar minimum epoch and geospace comparisons among the Whole Heliosphere Intervals thus requires that the semiannual and annual oscillations be properly distinguished in addition to the concurrent solar and heliospheric effects which have been the primary goal of the majority of Whole Heliosphere Interval (WHI) characterizations.