Alan R. Marshall

High-Resolution Doppler Imager on the Upper Atmosphere Research Satellite

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite has been providing measurements of the wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. Examination of various calibration data indicates the instrument has remained remarkably stable since launch. The instrument has a thermal drift of about 30 m/s/ degree(s)C (slightly dependent on wavelength) and a long-term temporal drift that has amounted to about 80 m/s since launch. These effects are removed in the data processing leaving an uncertainty in the instrument stability of approximately 2 m/s. The temperature control of the instrument has improved significantly since launch as a new method was implemented. The initial temperature control held the instrument temperature at about +/- 1 degree(s)C. The improved method, which holds constant the temperature of the optical bench instead of the radiator, keeps the instrument temperature at about 0.2 degree(s)C. The calibrations indicate very little change in the sensitivity of the instrument. The detector response has shown no degradation and the optics have not changed their transmittance.

Observations of the quasi 2‐day wave from the High Resolution Doppler Imager on Uars

A strong westward traveling oscillation, with a period of 2 days and zonal wave number 3, is observed in the mesospheric and lower thermospheric winds from the High Resolution Doppler Imager on the Upper Atmosphere Research Satellite. The important events happen in January, July, and September/October, of which the occurrence in January is the strongest with an amplitude over 60ms−1. Detailed analyses for the periods of January 1992 and January 1993 reveal a cause-and-effect relationship in the wave developing process at 95km. The global structures of the wave amplitude and phase are also presented.

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.

Measurements of stratospheric winds by the high resolution Doppler imager

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) determines winds in the stratosphere (10–40 km) by measuring the Doppler shift of the rotational lines in the O2 atmospheric B and γ bands. These lines are observed as absorption features in scattered sunlight. The Doppler shifts are found by fitting the observed high-resolution spectrum to a single-scattering model. The model includes effects due to scattering from clouds and the ground. Results are compared to radiosonde measurements and the standard deviation between the two measurement systems is found to be between 8 m/s and 12 m/s. This standard deviation includes the errors in both measurement systems as well as geophysical noise due to the different sampling times, sampling locations, and sampling volumes. Monthly averages of HRDI stratospheric winds near the equator are examined from December 1991 to April 1995. These averages reveal the three-dimensional structure of the quasi-biennial oscillation (QBO) and its evolution through one full cycle. In particular, we note that the QBO extends over a wide latitude and altitude range and that the easterly and westerly descent rates are similar above 25 km. HRDI measurements show that there is strong meridional shear in the zonal winds above 35 km during the solstices, indicating that inertial instability may play a role in the dynamics of the upper stratosphere. The HRDI wind measurements also show that there is a significant annual cycle in the tropics and that there is substantial interannual variability in the semiannual oscillation in the upper stratosphere.

GLOBAL MESOSPHERIC TIDAL WINDS OBSERVED BY THE HIGH RESOLUTION DOPPLER IMAGER ON BOARD THE UPPER ATMOSPHERE RESEARCH SATELLITE

This paper presents preliminary results of a study of mesospheric and lower thermosphedc diurnal tidal winds obtained with the High Resolution Doppler Imager (HI) on the Upper Atmosphere Research Satellite (UARS). Zonal averages of the meddional and zonal wind fields are performed at fixed local times. These analyses reveal tidal structures in the mean meridional wind field during the months of February and March 1992. Results are presented for a typical day in February 1992. The observed maximum amplitude of meridional wind is approximately 75 m s -1. The observed vertical wavelength is about 20 km, with amplitudes increasing linearly with height. In addition to the tides, the zonal wind field reveals many features of the mesospheric

Zonal mean winds in the equatorial mesosphere and lower thermosphere observed by the High Resolution Doppler Imager

This paper presents analyses of mesospheric and lower thermospheric zonal mean winds observed by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). Monthly averages of the equatorial zonal mean zonal winds are presented for January 1992 through June 1993. Equatorial zonal winds in the 70–90 km region are dominated by a semiannual oscillation (SAO), ranging from 30 m s−1 (westerly) to −100 m s−1 (easterly). At high latitudes the zonal wind variations are predominantly annual. Above 90 km, the low-latitude flow is easterly at all times, punctuated by a small semiannual variation. This behavior may be related to the deposition of momentum by the diurnal tides.

Comparison of HRDI wind measurements with radar and rocket observations

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 measured 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. These positions have been determined to an accuracy of approximately 5 ms−1 from the comparison of winds measured by HRDI with those obtained by MF radars. Excellent agreement is found between HRDI measured winds and winds observed with radars and rockets. In addition, the sensitivity of HRDI to migrating tides and other large scale waves is demonstrated.

Operational performance of the TIMED Doppler Interferometer (TIDI)

The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds in the mesosphere and thermosphere (60-180 km) as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm and rotational lines in the O2(0-0) Atmospheric band. Wind measurement accuracies approach 3 ms-1 in the mesosphere and 15 ms-1 in the thermosphere. The TIDI instrument’s performance during the first year and a half of operation is discussed in this paper. Many subsystems are working as designed. The thermal control system is holding the instrument temperatures at their desired set-points. The CCD detector is working as expected with no changes observed in the gain, bias or read noise. The instrument suffers from a light leak that causes the background to be elevated and increases the uncertainty in the wind measurement. Nothing can be done to eliminate this problem but modeling of the background has eliminated any systematic effect. Water outgassing from the spacecraft or instrument has deposited as ice on some part of the optics and reduced the instrument’s sensitivity. This problem has been reduced by two spacecraft rolls which pointed the TIDI radiator to view more of the earth causing the optics to warm up and sublimate much of the ice.

Stellar Alignment of the High Resolution Doppler Imager

A method is described for determining the angular misalignment between the High Resolution Doppler Imager instrument and its spacecraft, the Upper Atmospheric Research Satellite, using observations of stars by the instrument. Roll, pitch, and yaw misalignments have been measured to within 0.0025 deg (l<r), which is an order of magnitude better than the largest tolerable alignment error. The misalignment results are validated using observations of the nighttime 5577-A O^S) green line emission. In addition, the long-term behavior of the derived alignment offsets, which shows a clear dependence of the alignment on the solar (3 angle, is presented. Nomenclature px = vector in the instruments viewing direction expressed in the x coordinate frame Tx/y = transformation matrix from the y to the x coordinate frame t = measured time of the star crossing, s to = calculated star crossing time for the known star position and spacecraft position and attitude, s rjo = known angular misalignment vector, deg ?7i = angular misalignment between the instrument and spacecraft frame about the roll axis of the spacecraft, deg 772 = angular misalignment between the instrument and spacecraft frame about the pitch axis of the spacecraft, deg 773 = angular misalignment between the instrument and spacecraft frame about the yaw axis of the spacecraft, deg 3rj/31 = derivative of the misalignment with respect to crossing time expressed as a three-element column vector (in roll, pitch, and yaw), deg/s

High-resolution Doppler imager: instrument performance from late 1991 to mid-1999

The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite has been providing measurements of the wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. Mesospheric temperatures, ozone and O(1D) densities, and stratospheric aerosol extinctions coefficients, are also retrieved. The instrument characteristics have been carefully monitored by frequent calibrations during the nearly eight years of operation. The instrument sensitivity showed a significant decrease (close to 50% in some cases) during the first seven and a half years of operation which was caused by the piezoelectric-controlled etalons slowly drifting from a parallel state. A recalibration of the etalons in late 1998 resulted in close to a complete recovery of the instrument sensitivity. The loss of sensitivity was linear with time, with discrete changes occurring at times. Careful modeling of the data permits a determination of the sensitivity as a function of time, allowing the data to be corrected for this systematic effect.