Rick Niciejewski

Validation of mesosphere and lower thermosphere winds from the high resolution Doppler imager on UARS

Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind-induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI-derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDII), also flown on UARS.

Upper thermosphere winds and temperatures in the geomagnetic polar cap : solar cycle, geomagnetic activity, and interplanetary magnetic field dependencies

Ground-based Fabry-Perot interferometers located at Thule, Greenland (76.5°N, 69.0°W, Λ=86°) and at Sondre Stromfjord, Greenland (67.0°N, 50.9°W, Λ=74°) have monitored the upper thermospheric (∼240-km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observations are obtained by determining the Doppler characteristics of the (O I) 15,867-K (630.0-nm) emission of atomic oxygen. The instruments operate on a routine, automatic, (mostly) untended basis during the winter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagnetic polar cap measurements now extends over the complete range of solar activity. We present an analysis of the measurements made between 1985 (near solar minimum) and 1991 (near solar maximum), as part of a long-term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared with the predictions of two semiempirical models: the vector spherical harmonic (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The results are also analyzed using calculations of thermospheric momentum forcing terms from the thermosphere-ionosphere general circulation model (TIGCM) of the National Center for Atmospheric Research (NCAR). The experimental results show that upper thermospheric winds in the geomagnetic polar cap have a fundamental diurnal character, with typical wind speeds of about 200 m s−1 at solar minimum, rising to up to about 800 m s−1 at solar maximum, depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modified wind velocity and altered direction often associated with changes in the orientation of the interplanetary magnetic field (IMF). The central polar cap (>∼80 magnetic latitude) antisunward wind speed is found to be a strong function of both solar and geomagnetic activity. The polar cap temperatures show variations in both solar and geomagnetic activity, with temperatures near 800 K for low Kp and F10.7 and greater than about 2000 K for high Kp and F10.7. The observed temperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. Theoretical analysis based on the NCAR TIGCM indicates that the antisunward upper thermospheric winds, driven by upstream ion drag, basically “coast” across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combination of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force term. The polar cap thermospheric thermal balance is dominated by horizontal advection, and adiabatic and thermal conduction terms.

Validation of O(1S) wind measurements by WINDII: the WIND Imaging Interferometer on UARS

This paper describes the current state of the validation of wind measurements by the wind imaging interferometer (WINDII) in the O(1S) emission. Most data refer to the 90-to-110-km region. Measurements from orbit are compared with winds derived from ground-based observations using optical interferometers, MF radars and the European Incoherent-Scatter radar (EISCAT) during overpasses of the WINDII fields of view. Although the data from individual passes do not always agree well, the averages indicate good agreement for the zero reference between the winds measured on the ground and those obtained from orbit. A comparison with winds measured by the high resolution Doppler imager (HRDI) instrument on UARS has also been made, with excellent results. With one exception the WINDII zero wind reference agrees with all external measurement methods to within 10 m s−1 at the present time. The exception is the MF radar winds, which show large station-to-station differences. The subject of WINDII comparisons with MF radar winds requires further study. The thermospheric O(1S) emission region is less amenable to validation, but comparisons with EISCAT radar data give excellent agreement at 170 km. A zero wind calibration has been obtained for the O(1D) emission by comparing its averaged phase with that for O(1S) on several days when alternating 1D/1S measurements were made. Several other aspects of the WINDII performance have been studied using data from on-orbit measurements. These concern the instruments phase stability, its pointing, its responsivity, the phase distribution in the fields of view, and the behavior of two of the interference filters. In some cases, small adjustments have been made to the characterization database used to analyze the atmospheric data. In general, the WINDII characteristics have remained very stable during the mission to date. A discussion of measurement errors is included in the paper. Further study of the instrument performance may bring improvement, but the utimate limitation for wind validation appears to be atmospheric variability and this needs to be better understood.

Global distribution and interannual variations of mesospheric and lower thermospheric neutral wind diurnal tide: 2. Nonmigrating tide

On the basis of the TIDI mesospheric and lower thermospheric neutral wind observations from 2002 (March) to 2007 (June), we analyze the interannual variations of nonmigrating diurnal tides from eastward zonal wave number 3 (E3) to westward zonal wave number 3 (W3). We focus on possible QBO-related variations in these nonmigrating diurnal tide components. We found: (1) a strong reverse QBO effect on the W2 meridional diurnal tide in the September equinox and the December solstice, which suggests a W2 source of nonlinear interaction between planetary wave 1 and the migrating diurnal tide; (2) the QBO effect on the peak height during the June solstice on the E3 zonal diurnal tide; (3) several nonmigrating tide components (E3, E2, E1, W3 meridional, and W3 zonal) with similar eastward phase QBO enhancement during the March equinox to the migrating diurnal tide, although to a lesser degree from 2002 to 2005; (4) the QBO effects, in some cases, during 2006 and 2007 are either less or opposite those observed between 2002 and 2005.

TIME DEPENDENT THERMOSPHERIC NEUTRAL RESPONSE TO THE 2-11 NOVEMBER 1993 STORM PERIOD

Abstract Many satellite and ground-based observations from 2–11 November 1993 werecombined in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure toderive realistic time dependent global distributions of the auroral precipitation and ionosphericconvection. These were then used as inputs to the Thermosphere–Ionosphere–ElectrodynamicsGeneral Circulation Model (TIEGCM) to simulate the thermospheric and ionospheric responseduring the storm period. The November 1993 storm was an unusually strong storm associatedwith a recurring high speed stream of solar plasma velocity in the declining phase of the solarcycle. Significant gravity waves with phase speeds of about 700 m/s caused by Joule heating werepresent in the upper thermosphere as perturbations to the neutral temperature and wind fields,especially on 4 November. The observed gravity waves in the meridional wind and in the height ofthe electron density peak at several southern hemisphere stations were generally reproduced bythe model using the AMIE high latitude inputs. Both model and observed equatorward windswere enhanced during the peak of the storm at Millstone Hill and at Australian ionosondestations. The observed neutral temperature at Millstone Hill increased about 400 K during thenight on 4 November, returning to normal on 9 November, while the model increased 300 K thefirst night at that location but was still elevated on 11 November. Enhanced westward windsduring the storm were evident in the UARS WIND Imaging Interferometer (WINDII) data. Theenhanced westward winds in the model were largest around 40–45° magnetic latitude at night,and also tended to be largest in the longitudes containing the magnetic poles. The peak westwardwind enhancements at 0 LT reached about 250 m/s at 300 km, and about 100 m/s at 125 km thefirst day of the storm at 40° magnetic latitude. At 20° magnetic latitude, the maximum westwardwind enhancements at 125 km at 0 LT appeared 2–4 days after the major part of the storm,indicating very long time constants in the lower thermosphere. The model showed global averageneutral temperature enhancements of 188 K after the peak of the storm that decayed with time,and which correlated with variations 8 h earlier in the Dst index and in the electric potential dropinput from AMIE. The global average temperature enhancement of 188 K corresponded to apotential drop increase of only about 105 kV. The results showed that the TIEGCM usingrealistic AMIE auroral forcings were able to reproduce many of the observed time dependentfeatures of this long-lived geomagnetic storm. The overall global average exospheric temperaturevariation correlated well with the time variation of the cross-tail potential drop and the Dst indexduring the storm period. However, the enhanced westward winds at mid-latitudes were stronglyrelated to the corrected Joule heating defined by the time dependent AMIE inputs.

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.

Observations of high‐latitude lower thermospheric winds from Thule Air Base and Søndre Strømfjord, Greenland

Lower thermospheric winds have been determined from Fabry-Perot interferometer (FPI) measurements of the Doppler shift of the 5577-A O(1S) emission over Thule Air Base (76.5°N, 69.0°W) and Sondre Stromfjord (67.0°N, 50.9°W), Greenland. These winds normally correspond to the altitude of the peak of the airglow O(1S) emission layer, near 97 km. The altitude ambiguity due to auroral contamination has been reduced by eliminating data when the intensity of the emission increases significantly. Contamination by airglow emission of 5577-A O(1S) originating from higher altitudes has been investigated by an FPI simulation code. The simulation results indicate that this latter emission may contribute an anomalous diurnal oscillation to ground-based 5577-A O(1S) FPI measurements of lower thermospheric wind. The agreement of diurnal phases between that deduced from the green-line measurements and that determined from simultaneous red-line observation supports this conclusion. The same simulation applied to observations from Sondre Stromfjord shows that the upper layer contamination is much weaker and is not serious. Significant day-to-day variation is evident in the lower thermospheric wind field. Average neutral winds are calculated, and a harmonic analysis is carried out to examine the major low-frequency wind components. The seasonal variations of these wind components are compared with radar data and model predictions. The observations are generally in good agreement with model results. The comparison between FPI and radar results also shows reasonable agreement. The semidiurnal amplitudes observed with the Sondre Stromfjord radar during the Lower Thermospheric Coupling Study (LTCS-I) and LTCS-2 periods are always greater than the climatological values obtained from averaging FPI and Chatanika radar observations. This result shows the variability that can be expected when comparing “instantaneous” estimates of tidal parameters with climatological results.

TIMED Doppler interferometer (TIDI)

The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds, temperatures, and constituents in the mesosphere and thermosphere (60 - 300 km) region of the atmosphere as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm, OI 630.0 nm, OII 732.0 nm, O2(0-0), O2(0-1), Na D, OI 844.6 nm, and OH in the spectral region 550 - 900 nm. Wind measurement accuracies will approach 3 ms-1 in the mesosphere and 15 ms-1 in the thermosphere. The TIDI instrument has several novel features that allow high measurement accuracies in a modest-sized instrument. These include: an optical system that simultaneously feeds the views from four scanning telescopes which are pointed at plus or minus 45 degrees and plus or minus 135 degrees to the spacecraft velocity vector into a high-resolution interferometer, the first spaceflight application of the circle-to-line imaging optic (CLIO), and a high quantum efficiency, low noise CCD.

Assimilative mapping of ionospheric electrodynamics in the thermosphere‐ionosphere general circulation model comparisons with global ionospheric and thermospheric observations during the GEM/SUNDIAL period of March 28–29, 1992

Satellite and ground-based observations from March 28 to 29, 1992, were combined in the assimilative mapping of ionospheric electrodynamics (AMIE) procedure to derive realistic global distributions of the auroral precipitation and ionospheric convection which were used as inputs to the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere general circulation model (TIGCM). Comparisons of neutral model winds were made with Fabry-Perot measurements and meridional winds derived from ionosondes. The peak equatorward winds occurred 1–2 hours later in the model. Gravity waves launched from high-latitude Joule heating sources reached the equator in about 2 hours and agreed with observed variations in the height of the maximum electron density (hmF2) and in the meridional winds. Joule heating events produced minima in the O/N2 ratio that moved equatorward and usually westward in longitudinal strips which lasted about a day. Changes in the O/N2 ratio and in the peak electron density (NmF2) were strongly correlated so the observed daytime NmF2 values for stations near 50° magnetic latitude were generally reproduced by AMIE-TIGCM on the second day of the simulation. The AMIE-TIGCM underestimated the electron density after midnight by up to a factor of 2 in midlatitudes, while the modeled F2 layer was about 35 km lower than the observations at midnight. Shifting the model winds 2 hours earlier at night could double the NmF2 at 0400 LT and increase hmF2 by 20 km. NmF2 could also be increased at night by realistically increasing the TIGCM nighttime downward fluxes of O+ at the upper boundary.

Formation characteristics of sporadic Na layers observed simultaneously by lidar and airglow instruments during ALOHA-90

Sporadic Na (Nas) layers were observed by the airborne lidar during ALOHA-90 on the 22, 25 and 27 March flight missions. Perturbations in the O2 and OH nighttime airglow emission intensities and temperatures were also observed by instruments on the aircraft and at Haleakala Crater (20.8°N, 156.2°W) during these events. The most striking correlation between the airglow and lidar measurements occurred during the northbound flight leg of the 25 March mission. When the Nas layer formed at 90.7 km, while the Electra aircraft was between 750 and 500 km south of Haleakala, the O2 temperatures near 95 km above the Electra and Haleakala increased by approximately 45 K. The data for this night suggest a connection between Nas and a large-scale wave, and suggest that the wave is tidal in nature. The data also suggest that some Nas layers can form very quickly over large geographic areas. Fast chemical processes are required to generate the large amounts of atomic Na involved in some of these events.