SOLAR SCANS

,

According to theory [e.g., Solomon, 1990], and as observations suggest [e.g., Mankin et al., 1991], this ratio is significantly reduced when aerosols are present at low temperatures due to HCl-destroying reactions that do not normally occur in the gas phase.This paper describes the HALOE experiment, the results of ground testing, and initial performance in orbit.Section 2 describes the measurement approach in more detail and discusses measurement requirements.Section 3 describes the geographic coverage of the HALOE measurements.Sections 4 and 5 discuss the instrument and performance, data processing, and the inversion approach.Section 6 describes data validation studies, and section 7 presents initial measurement results.

Measurement Approach and Requirements
The objectives of the HALOE experiment are met by measuring the absorption of solar energy due to stratospheric gases during spacecraft sunrise and sunset.The experiment geometry is illustrated in Figure 2 [see e.g., Russell, 1980].Absorption is measured as a function of tangent height pressure defined as the pressure at the altitude (Ho) of the closest approach of a ray path to the Earth surface.A vertical scan of the atmosphere is obtained by tracking the Sun position during occultation.Measurements made in the Earth limb absorption mode have the advantage of higher sensitivity than is possible when viewing in the nadir since the limb path contains from 30 to 60 times more absorber.High vertical resolution can be achieved because of the limb geometry and the exponential decrease of density with altitude.This results in most of the absorption occurring at or near the tangent point.Another advantage of solar occultation is that a relative measurement is made.Each gas mixing ratio vertical profile is determined by ratioing the solar energy reduced by atmospheric attenuation with the unattenuated solar energy measured outside the atmosphere.This makes the instrument virtually self-calibrating and especially useful in the study of long-term trends since errors due to unaccounted for drifts are greatly reduced.Some disadvantages of the occultation approach are that measurements are made only at the sunrise and sunset times, geographic coverage is constrained by the Earth-Sun geometry, and the measurement sampling rate is limited to 15 sunrises and 15 sunsets each day.

Basic Equations
A schematic of the broadband HALOE measurement approach is shown in Figure 3 The use of the broadband radiometer approach is possible when a dominant absorbing gas can be spectrally isolated.This is not always possible, and more sophisticated instrument techniques must be used.An example of this is the HC1 measurement.The strongest HC1 absorption lines occur at 3.4 gm, which is in the middle of the strongest CH4 band in nature.Measurement of HC1 is made more difficult by the approximately 1000 times greater CH4 concentration.The nature of the problem is shown by the calculated limb absorption spectrum at the 30-km tangent height in the HALOE HC1 band (Figure 4).As can be seen, CH4 is the dominant absorber, and the effect of HC1 on the broadband absorption is very small.Under these circumstances, a high spectral resolution measurement is required.The HALOE measurement approach used for HC1, HF, CH4, and NO is gas filter radiometry.
The HALOE gas filter instrument concept is illustrated schematically in Figure 5. Solar energy NS passing through the HALOE filter with transmission 'cf is split into two paths; one includes a gas cell containing the gas of interest (e.g., HC1 in Figure 5) with transmission 'cg, and the second is a vacuum path with transmission 'cv.After detection by detectors D1 and D2, the signals are sent to a differencing amplifier where voltage AV is developed.The instrument is balanced to a zero AV value (to within the system noise) using the electronic gain adjustment G in the gas path when HALOE views the Sun outside the atmosphere (% = 1.0).When the target (e.g., HC1) gas is encountered in the atmosphere during sunrise or sunset, an atmospheric transmission 'Ca arises which causes AV and V to change.These changes can then be related to the tangent point mixing ratio.The required sensitivity in order to measure tenuous gases like HC1 and HF is that the instrument be capable of measuring radiance differences of 2 parts in 10 5.
This value was determined based on retrievals using simulated noisy signals derived from the expected vertical profiles of HC1 and HF.
The gas filter channel modulation signal, M, can be formulated with considerable algebra into a succinct function; we state only the result here.
where The gas filter radiometer measurement approach can be particularly effective when sounding in the presence of heavy aerosol loading.If we assume to first order that the aerosol transmittance over a narrow spectral band (44 to 110 cm-1 for the gas filter channels) is spectrally flat, then equation (3) shows that the measured modulation will be unaltered when aerosols are encountered since the spectrally constant aerosol transmittance appears in every term.Our signal simulations using actual aerosol spectral slopes across the gas filter band show that spectral effects due to the aerosol are negligible.Another important factor which can cause signal changes when the aerosol layer is encountered on the limb is field-of-view mismatch.This has been well characterized in the laboratory and verified in orbit for all gas filter channels.Also, a correction function to remove the mismatch has been developed.Under these assumptions then, the effect of aerosols on a gas filter measurement should be small.Our experience in orbit shows that only HC1 and HF are affected by the mismatch effect, and it appears to cause no more than = 0.3 parts per billion by volume (ppbv) changes in retrieved mixing ratios.Sometimes, early in the mission the aerosol extinction was so large at low altitudes that the signal was lost or extremely small and a measurement was not possible.As the Mount Pinatubo aerosol layer has subsided with time, however, this has now become less of a problem.

Geographic Coverage
The Pointing errors determined in ground testing are 15 arc sec in azimuth and 12 arc sec in elevation.Atmospheric temperature random and systematic errors assumed in studying effects on the experiment are 0.5 K and 3.5 K, respectively.Errors in interfering gas mixing ratios vary from channel to channel but, in general, are of the order of 8% and 20% for random and systematic effects, respectively.The actual values for each channel have been determined by doing retrieval simulations.Absorption line strength and half width errors vary greatly but generally range from about 5 to 10%.The remaining errors which have been estimated based on detailed analysis of laboratory test data are summarized in Table 1.
Spectral response.The HALOE instrument end-to-end spectral response was determined using a specially designed grating monochromator, in combination with a 2800 K solar simulator source.The relative spectral response was measured in every HALOE channel with a spectral resolution ranging from 2 to 4 cm -1.In addition, the spectral properties of the optical filters, characterized as functions of temperature (10øC to 30øC) and angle of incidence using a 0.06 cm -1 resolution Nicolet interferometer, were included in the data processing software.Normalized plots of the end-to-end spectral response function for each HALOE channel (vacuum path for the gas filter channels) are shown in Figure 14.
Another spectral element for the gas filter channels is the gas filter cell.The spectra of flight cells installed in HALOE were measured extensively with the Nicolet interferometer, and a nonlinear least squares spectral line fitting program was developed to allow inference of the optical mass path that gives the best fit to the spectral line structure observed with the interferometer.This approach provided an essentially infinite resolution characterization of the gas filter portion of the spectral response.expression, a knife edge test was done after the instrument was integrated on the spacecraft, and the resulting end-to-end "impulse" response was compared with the predicted response using equation ( 9).The comparison verified that (9) provides an excellent representation of the time response; consequently, any distortion due to uncorrected error from this source will be very small in the total experiment.Optoelectronic linearitv.Optoelectronic linearity is of particular interest for the gas filter channels since the detector response as a function of solar signal on each detector must be closely matched.The electronics response must be matched as well.The end-to-end linearity of the HC1, HF, and CI-I4 channels was determined during the ground "Sun look test" by placing wire grid attenuators of known transmission in front of the HALOE telescope while viewing the Sun.Attenuators of approximately 10%, 20%, and 50% were used.Insufficient signal existed to determine NO linearity in this test.The grids were rotated in place through angles of 60 ø and 120 ø to test for polarization effects and none were found.The requirement is that the deviation in AV, due to nonlinearity, be a small fraction (< 1%) of AV itself.Tests showed that this requirement was met or exceeded in all gas filter channels.Ground tests show that this nonlinearity is a very small error source for the radiometer channels and can be neglected.

Spatial response. The optical field of view (FOV) for each HALOE channel was measured by
Thermal sensitivity.Thermal sensitivity of signals in HALOE can arise from two main sources.The optical broadband interference filter characteristics, for example, are temperature dependent, as already noted.Also, thermal changes will induce small optical misalignments due to thermomechanical stresses on the optical mainframe.The former is the main source of thermally induced signals in the bolometer channels, and the latter is the principal factor in the gas filter channels.In addition, the gas filter detectors have temperature dependent responsivities, but this effect is small since they are held at a constant temperature (to < 0.01 K) by thermoelectric coolers.The gas filter channels are the greatest concern because of the small radiance differences being observed.As already noted in section 2, an empirical fit approach which employs data obtained just prior to sunset or after sunrise is being used to model drift rate and to correct the orbital data.
Estimated errors.The effect of all experiment errors on estimated uncertainties in retrieved parameters was studied using a Monte Carlo simulation in combination with the onion peeling retrieval approach.The study was based on the errors in Table 1 and other sources of uncertainty discussed previously.An alternate error estimation approach has also been studied using the analysis technique developed by Rodgers [1990].Our preliminary estimate of the single-profile precision over the 20-to 35-km range is 3 to 8%, depending on channel, and the zonal mean profile precision is estimated to be 2 to 5%.Accuracies (combination of random and systematic errors) are estimated to be 2 to 3 times the precision values.A detailed description of error studies performed and data limitations is provided in a series of validation papers in preparation.
Gas response test.The culminating HALOE ground test was the end-to-end gas response test (GRT).The purpose of this test was to create a simulated atmosphere in front of the HALOE instrument using gas cells to vary the type, mixing ratio, and pressure of gas in the cells; then to record the HALOE signals; and, finally, to compare calculated signals with the measurements.The calculated signals were determined from first principles using the flight software and key properties of the instrument characterized in laboratory testing.No empirical adjustments or parameterized calibration fits were used in the calculations.The instrument was placed in a thermal/vacuum chamber so that its temperature could be controlled.A test apparatus was constructed containing the simulated atmosphere gas cells that could be moved in place outside the chamber, optically aligned, and the apparatus purged with dry nitrogen to remove or reduce residual laboratory air so that absorption (especially due to water vapor and CO2) was at a minimum.The test apparatus contained a 2800 K solar simulator source, optics, and two gas cells, one of which could be used to create a target gas "atmosphere" and the other to provide the simultaneous presence of interfering gases.Mixing valves were built into the system so that various N2-gas mixing ratios and total pressures could be achieved.Ranges of mixing ratio and pressure were preselected to simulate, to the extent possible, limb path signal levels that would be measured by HALOE in orbit.In some cases, manufacturer-supplied premixed gases were used.The mass paths of gas in the simulated atmosphere cells were determined both by pressure gauge readings and by spectral analysis using high-resolution Fourier transform spectroscopy.The greatest problem encountered was in handling and determining how much HF was in the cell at any given time.After a significant amount of study, satisfactory results were obtained.Typical results for HCI and HF are shown in Figure 16  for HF.The reasons for this are under investigation, but this is not so important since the orbital instrument temperature never drops below 20øC.The most reliable radiometer channel for study of agreement between measurements and calculations is CO2.In this case, calculated transmittances agreed with observed values to within 2-3%.Comparisons were also made for the NO2, H20, and 03 transmittances.In these cases, agreement was much worse, 4-40% (the largest difference was for 03), and we attribute this to gas-handling problems.Since the radiometer measurements are simple in concept and CO2 agrees well, no further effort was expended to reduce differences.
An initial GRT was done in 1989, then the instrument was shipped to the spacecraft manufacturer for the first series of integration activities, and in 1990, HALOE was shipped back to the Langley Research Center for final testing, including a repeat GRT.Similar results to the first GRT were obtained.Also, a comparison was made between 1989 and 1990 GRT measurements for the same target cell pressures to assess the repeatability of the instmmentIGRT system.This would be an upperlimit determination of instrument repeatability since the repeatability of the test setup is also included.The agreement for 20 ø instrument temperature between the 1989 and 1990 measurements for the same mixing ratio and pressure test points was of the order of 1% for CH4 and 2-4% for HC1 and NO.This could not be done for HF because HF gas-handling problems which were encountered in the 1989 test improved in 1990.
The GRT results attest to the quality and degree of characterization of the HALOE instrument, and they lend confidence that the instrument is well understood.The precision indicated by the GRT measurements suggests that the HALOE measurement repeatability when viewing the same atmosphere is better than 1-4%.The comparison between measurements and calculations show that when the atmospheric pressure, temperature, and gas mixing ratio parameters are known, the measured HALOE signal can be predicted to within a few percent using the flight software.

In-Orbit Instrument Performance
The HALOE instrument has been operating continuously since science observations began on October 11,1991, except for periods when two grazing events occurred: one shortly after operations started and the other near the end of December.A grazing event is a time when either the Sun does not set at the spacecraft or the dark period is very short and the lowest measurement tangent point is very high, e.g., > 90 km.The instrument performance has been essentially flawless.In all cases the orbital performance meets or exceeds

Level 0-1 Processine
This stage of processing unpacks the HALOE level 0 or raw data, calibrates the data, removes instrument effects, develops source functions from the solar scan data, and registers the data with pressure and altitude.Registration is done by using an NMC temperature versus pressure profile to do a CO2 channel signal simulation and then by comparing the simulation to measurement.The altitude range above 30 km is used to avoid any serious effects due to aerosol contamination.The unpacking separates the data into files that correspond to various instrument modes.These data are then processed on the UARS Central Data Handling Facility (CDHF) at the Goddard Space Flight Center and transmitted to the HALOE remote access computer (RAC) for detailed analysis.The level 0-1 processing therefore provides a set of corrected and calibrated signals that are ready for retrieval calculations.

Level 1-2 Processins
This step uses transmission profiles, difference signal profiles from the gas filter channels, and solar source functions from the solar scans to retrieve temperature, pressure, and mixing ratios of HC1, HF, CH4, H20, 03, NO, NO2, aerosol extinction, and temperature versus pressure.We are retrieving temperature only above --30 km at present because of effects due to aerosol contamination.
The retrieval method incorporates a simple "onion peel" procedure stabilized at the top and bottom of the profile with a scalar optimal estimation formulation [Connor and Rodgers, 1989].The forward model for the gas filter channels (HF, HC1, CH4, NO) is a rigorous line-by-line code which is necessary for the effective high spectral resolution of these channels.All spectral dependence, including thermal and Doppler shift effects, is explicitly modeled.Along-path mixing ratio gradients are also included in the forward model for the diurnally active gases NO, NO2, and 03.The radiometer channels are modeled using the emissivity growth and Curtis-Godson approximations using correction tables.These models have been validated against a line-byline transmission code to better than 99% accuracy, and they are extremely fast, allowing a vector implementation of the optimal estimation equations.Again, full thermal and spectral dependence of the instrument is rigorously modeled, in this case, through a large set of transmission tables.Most major interfering gases are retrieved as primary gases in other channels.Nonretrieved interference is minor (such as N20 in HC1) contributing <1% error.However, interference from the Mount Pinatubo aerosol layer causes a major effect on the radiometer channel retrievals below the top of the aerosol layer.We have devised a correction approach which is based on retrieval using the gas filter channels, coupled with a Mie-scattering model to determine the aerosol extinction at the radiometer channel wavelengths [Hervig et. al. 1993].This approach works very well based on comparison with correlative measurements.
After correction for aerosol interference, HALOE 03 and H20 data agree well with correlative measurements to within about 10-15%.No correlative data are yet available for NO2.

Level 2-3 Processing
The primary level 3 or "mapped" products from HALOE include pressure versus longitude cross sections on selected days, pressure versus latitude for selected time periods, and polar orthographic projections.The latitude cross sections are generated on time scales of a couple of weeks or more and seasonally.The accuracy and precision of these products are high and therefore they provide very reliable means to study trends over time.,,,,I , , ,,,,,,I , , ,,,,,,I , , ,,,,, The NO mixing ratio has very low values in the lower stratosphere at all latitudes (< 0.25 ppbv).It increases to a stratospheric peak of --10 ppbv, decreases to a minimum in the lower mesosphere, and then increases sharply with altitude up to the limit of the observations.We also note that correlations with solar activity have been clearly seen, especially during the geomagnetic storm of November 8-10, 1991, a time period when enhanced charged-particle production occurred.The data show that during this period, NO levels in the 80-km to 130-km range increased by a factor of 3 and then decayed to previous levels after only a few days.The NO2 cross section shows a maximum sunset mixing ratio at = 8 mbar of = 10 ppbv and a hemispheric asymmetry (larger values to the south).Equatorward gradients are observed in both hemispheres.The NO2 level is low in the lower stratosphere (< 1 ppbv) at all latitudes.The mixing ratio levels also decrease and reach the HALOE noise limit near or just

Summary
The HALOE experiment was successfully launched and activated on the UARS spacecraft.The orbital operation has been virtually flawless since science data collection began on October 11, 1991, up to the present January 30, 1992.Measurements are made continuously except for periods when solar grazing events occur, resulting in no occultations.The latitude coverage over a 1-year period ranges from 80øS to 80øN.Initial sunrise observations began in the Antarctic and sunsets occurred in the tropics.As of this writing, many measurement sweeps over broad latitude ranges have occurred providing coverage of all seasons in both hemispheres.All instrument performance criteria either meet or exceed specifications.The occurrence of the Mount Pinatubo aerosol layer has complicated HALOE orbital operations and has restricted the lower altitude range where useful measurements can be made to about 20 km in especially dense aerosol regions in the tropics.This lower limit has been achieved following considerable flight testing and unanticipated fine adjustments that were made to the pointer/tracker system using ground-issued commands.
The retrieved mixing ratios all look reasonable and extend to higher than expected altitudes.We particularly note that H20 and 03 extend to 80 km-and 90 km respectively the CFCs) becomes an indicator of anthropogenic chlorine input to the middle atmosphere.Hydrogen chloride, on the other hand, is an indicator of total chlorine input from both natural and man-made sources.The relative importance of these two sources of stratospheric chlorine can be inferred by studying changes in HC1 and HF with time.The ratio of HC1/HF also is a useful indicator of perturbed chemistry in the presence of stratospheric aerosols.

Fig. 4 .Fig. 5 .
Fig. 4. Calculated 30-km absorption spectrum in the HALOE HC1 channel.Positions of the HC1 lines are shown by the tic marks at the top.
Fig. 8. •OE latitude versus time coverage for 1 ye•.extreme latitude positions.All nine HALOE parameters are measured for every occultation event.As an example, latitude versus longitude coverage for the month of November 1991 is shown in Figure 10.During this month, soundings were made from 60øS to 53øN. 4. Instrument Description and Performance 4.1.Instrument Descrir)tion _ General description of the main components of the HALOE instrument (Figure 11) include the sensor assembly containing the infrared telescope and optical mainframe, the biaxial gimbal assembly to provide solar tracking in the azimuth and elevation directions, the Sun sensor assembly, the on-gimbal electronics assembly (GEA), and the platform electronics assembly (PEA).Figure 11 shows the telescope dust cover door in place, radiator surfaces for cooling the optical mainframe and GEA, and the spacecraft adapter mounting plate.Thermal control of HALOE is maintained by (1) multilayer insulation which covers the entire instrument, (2) the radiator surfaces shown, and (3) strip heaters located on the primary and secondary mirrors of the telescope, on the Sun sensor assembly, on the gas channel and mainframe radiator surfaces, and on the spacecraft adapter.The instrument flies in an inverted position from that shown in the figure.The launch configuration included pyrotechnic actuators to cage the gimbals and hold the telescope door closed.The gimbals were released four days after launch on September 16, and the telescope door was opened October 2. The delay in door opening was to allow time for spacecraft outgassing to occur.Science data collection viewing the Sun was delayed another nine days until October 11 to allow more time for the instrument to outgas through the telescope.A special concern exists when an instrument is solar viewing since the Sun energy can polymerize foreign material, thereby fixing the contamination on the mirror surface and permanently changing the instrument spectral response in an unknown way.Hydrocarbons have absorption features that coincide with the spectral positions of the HC1 and CH4 channels.Hence, great caution was used in activating HALOE, and similar considerations have been used in operating the instrument inorbit so that contamination effects will be minimized.The GEA contains the power conditioning, thermal control, data acquisition, and data handling electronics which require scanning a knife edge and a narrow slit (10% of the FOV width) between HALOE and a solar simulator source and by scanning the IFOV across the surface of the actual Sun from the ground.The primary parameters characterized in these tests were the normalized Fig. 14a.HALOE broadband end-m-end spectral response for the CH4, HC1, CO2, and HF channels.
/min for CH4.The FOV mismatch observed in orbit is = 0.3 arc sec.The off-axis rejection properties of the instrument show < 0.1% of full Sun signal when the 1FOV center is 4 arc min off the solar edge.A major challenge to the operation of HALOE in orbit has been the effect of the Mount Pinatubo aerosol layer on the pointer/tracker system.This layer is very dense, and HALOE data show that it extends over a broad altitude range from 28 km down to 16 km in places.The extinction is so large that the Sun sensor magnitude threshold value used to signal the beginning or end of an event becomes nearly zero at 25 to 28 km altitude, depending on geographic location.After considerable study of the orbital data, fine adjustment was made to the Sun sensor threshold value to increase the useful altitude range.The value is now set at 0.1% of full Sun signal for sunset and 0.5% for sunrise.Changes were also made to the pointer/tracker sunrise acquisition algorithm to decrease the time required for solar acquisition.The IR telescope tracking lock point on the Sun is changed routinely depending on latitude from 4 arc min to 8 arc min down from the top edge.These changes provide altitude coverage in the worst case dense aerosol condition down to about 20 km.Most of the time the coverage is of the order of 5 km or more lower than this, and it should continue to improve with time as the aerosol extinction subsides.The pointer/tracker has the capability to track down to the solid Earth point on sunset and acquire and begin track above about 5 km on sunrise under background aerosol conditions.The performance of the pointer/tracker has met specifications throughout the mission.
HALOE HF pressure versus latitude cross section, sunset for September 21 to October 15, 1992.HALOE HC1 pressure versus latitude cross section, sunset for September 21 to October 15, 1992.HALOE NO pressure versus latitude cross section, sunset for September 21 to October 15, 1992.above the stratopause.The gradient at = 65øS is most apparent in the NO2 cross section, but it exists in the other cross sections as well and is associated with the boundary of the southern vortex.Studies are under way to assess the plot covers the same time period as the latitude cross sections.The presence of low CI-14 is seen in the polar region inside the Antarctic vortex which, from independent wind and potential vorticity data, was mostly south of 60øS during this period.There were times when the vortex elongated and extended as far north as 45øS.It is clear that the low CH4 characteristic of the vortex extends to at least 40øS.It is also evident that low CH4 mixing ratio air from the vortex is affecting methane levels at 25øS.Trajectory studies are under way to determine how much of the low methane is due to vortex movement over the September 21 to October 15 time period as opposed to transport out of the vortex itself.
, and NO extends to 130 km.An extensive data validation program is under way, and initial comparisons with correlative underflight data are encouraging.Several internal consistency checks, including sunrise/sunset differences, HF versus CH4 and H20 versus CH4 regressions, and comparison of pressure versus latitude cross-section features all give excellent results.Qualitative comparisons with 1985 Spacelab 3 ATMOS data show reasonable agreement.Therefore every indication from studies performed to date is that the data are of high quality and are suitable for application to scientific studies.HALDE CH4 31.6-mbarsurface orthographic projection for September 21 to October 15, 1992.air exists as far north as 40øS, and the effects of this air are seen in the tropics at 25øS.Once the validation effort is complete, detailed scientific investigations will be carried out using these and other features of the data.