Infrared Heterodyne Spectroscopy Research Articles

Intense Zonal Wind in the Martian Mesosphere During the 2018 Planet‐Encircling Dust Event Observed by Ground‐Based Infrared Heterodyne Spectroscopy

AbstractWe report on the direct measurements of zonal winds around 80 km altitude during the 2018 planet‐encircling dust event (PEDE) by infrared (IR) heterodyne spectroscopy. The observed Doppler shifts assume intense retrograde (easterly) winds (208 ± 17 m s−1, 159 ± 20 m s−1, 211 ± 20 m s−1on June 21, June 27, August 31, 2018, respectively) in the equatorial region during the 2018 PEDE. This is significantly stronger than those during non‐storm conditions reported by the previous study (Sonnabend et al., 2012,https://doi.org/10.1016/j.icarus.2011.11.009). The substantial retrograde wind during the PEDE is qualitatively consistent with the predictions by the Mars general circulation models (MGCMs), however, the observed wind on 31, August, are of a larger magnitude. We evaluated the mechanism of acceleration using the output from a high‐resolution MGCM. We find out that the stronger winds are related to strengthening the meridional circulation across the equator and forcing by gravity waves.

近红外外差光谱温室气体柱浓度的探测方法

近红外外差光谱温室气体柱浓度的探测方法

Long term evolution of temperature in the venus upper atmosphere at the evening and morning terminators

This paper contains a comprehensive dataset of long-term observations between 2009 and 2015 at the upper mesosphere/lower thermosphere providing temperature values at different locations of the morning and evening side of the terminator of Venus. Temperature information is obtained by line-resolved spectroscopy of Doppler broadened CO2 transitions features. Results are restricted to a pressure level of 1 µbar, ∼110 km altitude due the nature of the addressed non-LTE CO2 emission line at 10 µm. The required high spectral resolution of the instrumentation is provided by the ground-based spectrometers THIS (University of Cologne) and HIPWAC (NASA GSFC).For the first time upper mesosphere/lower thermosphere temperatures at the Venusian terminator derived from IR-het spectroscopy between 2009 and 2015 are investigated in order to clarify the local-time dependences, latitudinal dependences and the long-term trend. Measured temperatures were distributed in the range between 140 K and 240 K, with mean values equal to 199 K ± 17 K for the morning side of the terminator and 195 K ± 19 K for the evening side of the terminator. Within the uncertainty no difference between the averaged morning and evening terminator side temperature is found. In addition, no strong latitudinal dependency is observed at these near terminator local times. In contrast IR-het data from 2009 show a strong latitudinal dependency at noon, with a temperature difference of around 60 K between the equatorial and polar region (Sonnabend et al., 2012). Accord with the instruments of the Venus Express mission a northern-southern hemispherical symmetry is observed (Mahieux et al., 2012; Piccialli et al., 2015). The data shows no consistent long-term temperature trend throughout the six years of observation, but a variability in the order of tens of Kelvin for the different observing runs representing a time step of few month to two years. This is about the same order of magnitude as the variability within a single run with a typically time range of 2–10 days. This variation is not connected to the solar cycle. Sub-millimeter observations by Clancy et al. found a relation between temperatures and long-term variation in mesospheric water vapor, SO2, and sulfate aerosols (Clancy and Muhleman, 1991; Clancy et al., 2012). SO2 column densities observed by SOIR at the terminator are fairly stable over the time period of 2006–2011 (Mahieux et al., 2015), supporting the hypothesis of a relation between SO2 and temperature variations. The temperatures derived from the infrared heterodyne spectroscopy (IR-het) are compared to results from the Venus Express space mission (VEx). A consistence with the temperatures from the VEx instruments SOIR, VIRTIS and SPICAV is found. As the instruments probe different local time, SPICAV probes the pure nightside, SOIR across the terminator and IR-het the pure dayside atmosphere it is not surprising that the IR-het temperatures are mostly on the warmer side compared to results from SPICAV and SOIR.

Thermal structure of Venus׳ nightside mesosphere as observed by infrared heterodyne spectroscopy at 10 μm

Ground-based heterodyne spectroscopy is used to observe the night side of Venus by probing single pressure broadened CO2 absorption lines. From the pressure induced line broadening, the predominant temperature at different altitude layers can be deduced. It is found, that heterodyne spectroscopy is sensitive to probe the mesosphere between ~60km and 90km with an altitude resolution of ~4.5km.During two observing campaigns in March and May 2012, four different locations on the planet were investigated. Herein, we report on the retrieval of vertical temperature profiles in the nightside atmosphere of Venus. Retrieval of atmospheric parameters is based on a Levenberg–Marquard χ2 optimization that iteratively compares observed data to telluric transmittance corrected Venus׳ top-of-atmosphere spectra calculated using a radiative transfer algorithm. The deduced profiles are compared to the Venus International Reference Atmosphere and some found to be in satisfactory agreement. Sub-Doppler resolution Infrared heterodyne observations can provide temperature measurements that complement existing sub-mm and space based observations.

A search for methane in the atmosphere of Mars using ground-based mid infrared heterodyne spectroscopy

We report our search for methane in the atmosphere of Mars using high-spectral resolution heterodyne spectroscopy in the 7.8μm wavelength region. Resolving power and frequency precision of >106 of the technique enable identification and full resolution of a targeted spectral line in the terrestrial-Mars spectrum observed from the ground. Observations were carried out on two occasions, in April 2010 and May 2012 at the McMath-Pierce Solar Telescope and the NASA Infrared Telescope Facility, respectively. A single line in the ν4 band of methane at 1282.62448cm−1 was targeted in both cases. No absorption due to methane was detected and only upper limits of ∼100ppb for the martian atmospheric methane concentration were retrieved. Lack of observing time (due to weather) and telluric opacity greater than anticipated led to reduced signal-to-noise ratios (SNR). Based on current measurements and calculations, under proper viewing conditions, we estimate an achievable detection limit of ∼10ppb using the infrared heterodyne technique – adequate for confirming reported detections of methane based on other techniques.

Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO 2

We present direct observations of Mars zonal wind velocities around northern spring equinox (L S = 336°, L S = 355°, L S = 42°) during martian year 27 and 29. Data was acquired by means of infrared heterodyne spectroscopy of CO 2 features at 959.3917 cm −1 (10.4232 μm) and 957.8005 cm −1 (10.4405 μm) using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) at the McMath–Pierce telescope of the National Solar Observatory on Kitt Peak in Arizona and the NASA Infrared Telescope Facility on Mauna Kea, Hawaii between 2005 and 2008. Winds were measured on the dayside of Mars with an unprecedented spatial resolution allowing sampling of up to nine independent latitudes over the martian disk. Retrieved wind velocities depend strongly on latitude and season with values ranging from 180 m/s prograde to −94 m/s retrograde. A comparison of the observational results to predicted values from the Mars Climate Database yield a reasonable agreement between modeling and observation.

Beam integrated high-resolution infrared spectra: Accurate modeling of thermal emission from extended clear atmospheres

We describe a technique for analysis of high resolution thermal IR spectra measured with a finite beam (field of view) on the target planetary atmosphere. The model developed allows a beam integrated radiative transfer calculation over the varying line-of-sight velocity, temperature, and species abundance distributed over the field of view, thus recovering information on atmospheric composition, dynamics, and thermal structure. The modeling procedure is comprises four primary aspects: segmentation of the projected observational beam to a grid of beam elements; determining the geometrical properties of the planetary atmospheric region associated with each beam element; calculating the spectrum emergent from the top of the atmosphere of each element; and weighting the ensemble of spectra with the characteristic observational beam response profile and accumulating the contributions. This prescription results in a beam integrated spectrum—a more accurate representation of fully resolved IR rotation–vibration molecular spectra of planetary atmospheres observed with finite aperture telescopes. The method is implemented as a numerical code appropriate for modeling the spectroscopic observations. This paper describes the basic formalism for beam integrated radiation transfer of planetary atmospheres, discusses the salient points of the modeling process, and quantifies limitations. Application of this technique will be illustrated using high resolution atmospheric molecular spectral line measurements of Titan obtained by infrared heterodyne spectroscopy to extract zonal wind field, temperature and C 2H 6 abundance.

High‐resolution infrared spectroscopy of ethane in Titan's stratosphere in the Huygens epoch

High‐resolution infrared spectroscopy of ethane (C2H6) emission features formed in the stratosphere of Titan was collected on disc center at 11.74 μm wavelength (851 cm−1) on 15 January 2005 UT. The observations were obtained at the Subaru 8.2 m telescope of the National Astronomical Observatory of Japan on Mauna Kea, Hawaii, using the NASA Goddard Space Flight Center Heterodyne Instrument for Planetary Winds and Composition (HIPWAC). Fully resolved rotational‐vibrational transitions of C2H6 were measured with resolving power λ/Δλ ≥ 106 by infrared heterodyne spectroscopy (IRHS). The spectrum is reproduced most effectively by vertical profiles of ethane abundance that are uniform through the stratosphere and enhanced within the mesosphere. Profiles in which there is a significant gradient within the stratosphere are not favored. The retrieved stratospheric ethane mole fraction depends weakly on the form invoked for the mesospheric enhancement. Two forms of the ethane mole fraction profile are found to reproduce the observed spectrum effectively: the best fitting results are obtained with a profile in which the mesospheric ethane concentration increases logarithmically versus decreasing pressure, retrieving a stratospheric ethane concentration of 8.2 ± 2.1 × 10−6 (1σ), increasing proportional to p−1.2 from the stratopause through the mesosphere (p is pressure). A second form of profile, in which the mesospheric ethane concentration is enhanced uniformly by a factor of 9.5, retrieves a stratospheric concentration of 9.7 ± 4.9 × 10−6 (1σ), with the enhancement discontinuity at about one scale height above the stratopause. The retrieved stratospheric mole fraction is consistent with earlier retrievals from IRHS and is somewhat less than contemporaneous retrievals from infrared spectroscopy at lower resolution by the Cassini spacecraft. The retrieved mesospheric concentration is consistent with in situ measurements in Titan's thermosphere made by the Cassini Ion Neutral Mass Spectrometer instrument during the Titan flyby (Waite et al., 2005).

High spatial resolution mapping of Mars mesospheric zonal winds by infrared heterodyne spectroscopy of CO2

We present Mars zonal wind measurements by means of infrared heterodyne spectroscopy of CO2 features at 959.3917 cm−1 (10.423 μm). Observations were carried out using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) from 5–8 December 2005 shortly after Mars opposition at the McMath‐Pierce Solar Telescope of the National Solar Observatory on Kitt Peak in Arizona with an unprecedented spatial resolution of ∼1.7 arcsec on a ∼16 arcsec Martian disk. Mars was observed at six different latitudes close to the west (evening) limb and zonal winds were retrieved from Doppler shifts between CO2 non‐thermal emission from the mesosphere and absorption features from low atmospheric regions. The season on Mars was late northern winter (LS ≈ 337°). We retrieved retrograde winds at latitude 45°N (−27 ± 17 m/s) that were particularly strong at the equator (up to −80 ± 13 m/s) and prograde winds at high southern latitudes (75°S) up to 51 ± 29 m/s.

Ozone abundance on Mars from infrared heterodyne spectra: II. Validating photochemical models

Ozone is an important observable tracer of martian photochemistry, including odd hydrogen (HO x ) species important to the chemistry and stability of the martian atmosphere. Infrared heterodyne spectroscopy with spectral resolution ⩾ 10 6 provides the only ground-based direct access to ozone absorption features in the martian atmosphere. Ozone abundances were measured with the Goddard Infrared Heterodyne Spectrometer and the Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. Retrieved total ozone column abundances from various latitudes and orbital positions ( L S = 40 ° , 74°, 102°, 115°, 202°, 208°, 291°) are compared to those predicted by the first three-dimensional gas phase photochemical model of the martian atmosphere [Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. J. Geophys. Res. 109, doi:10.1029/2004JE002268. E07004]. Observed and modeled ozone abundances show good agreement at all latitudes at perihelion orbital positions ( L S = 202 ° , 208°, 291°). Observed low-latitude ozone abundances are significantly higher than those predicted by the model at aphelion orbital positions ( L S = 40 ° , 74°, 115°). Heterogeneous loss of odd hydrogen onto water ice cloud particles would explain the discrepancy, as clouds are observed at low latitudes around aphelion on Mars.

Ozone abundance on Mars from infrared heterodyne spectra: I. Acquisition, retrieval, and anticorrelation with water vapor

Observations of ozone on Mars were made using the Goddard Space Flight Center's Infrared Heterodyne Spectrometer and Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility. Ozone is an important observable tracer of martian photochemistry. Infrared heterodyne spectroscopy with spectral resolution ⩾ 10 6 is the only technique that directly measures ozone in the martian atmosphere from the surface of the Earth. Ozone column abundances down to the martian surface were acquired in seven data sets taken between 1988 and 2003 at various orbital positions ( L S = 40 ° , 74°, 102°, 115°, 202°, 208°, 291°). Ozone abundances are compared with those retrieved using ultraviolet techniques, showing good agreement. Odd hydrogen (HO X ) chemistry predicts anticorrelation of ozone and water vapor abundances. Retrieved ozone abundances consistently show anticorrelation with corresponding water vapor abundances, providing strong confirmation of odd hydrogen activity. Deviation from strict anticorrelation between the observed total column densities of ozone and water vapor suggests that constituent vertical distribution is an additional, significant factor.

Evaluation of quantum-cascade lasers as local oscillators for infrared heterodyne spectroscopy

We report experiments evaluating the feasibility of quantum-cascade lasers (QCLs) at mid-infrared wavelengths for use as local oscillators (LOs) in a heterodyne receiver. Performance tests with continuous-wave (cw) lasers around 9.6 and 9.2 microm were carried out investigating optical output power, laser linewidth, and tunability. A direct comparison with a CO2 gas laser LO is presented as well. The achieved system sensitivity in a heterodyne spectrometer of only a factor of 2 above the quantum limit together with the measured linewidth of less than 1.5 MHz shows that QCLs are suitable laser sources for heterodyne spectroscopy with sufficient output power to replace gas lasers as LOs even in high-sensitivity astronomical heterodyne receivers. In addition, our experiments show that the tunability of the lasers can be greatly enhanced by use of an external cavity.

Self- and nitrogen broadening of the ν1020 11,10←19 10,9 ethylene transition at [formula omitted

Self- and nitrogen (N 2) broadening of ethylene (C 2H 4) have been measured for the ν 10 20 11,10←19 10,9 ethylene transition at 927.01879 cm −1 . The measurements were made using infrared heterodyne spectroscopy with a spectral resolution of ∼1.7×10 −4cm −1. At 296 K , the retrieved self-broadening coefficient for the observed transition is 0.1244(92) cm −1/ atm ; the nitrogen-broadening coefficient is 0.0757(99) cm −1/ atm . These results represent the most conservative treatment of experimental and statistical errors. A different purely statistical approach provides consistent values with uncertainties of only a few percent.

Infrared heterodyne spectroscopic measurements of transition frequencies and intensities of ethylene and isotopic ethylene ( 13C 12CH 4) between 840 and 980 cm −1

Infrared heterodyne spectroscopy ( λ/Δ λ⩾10 6) enabled the precision measurement of lineshapes of 168 ethylene absorption transitions for 12C 2H 4 and 88 absorption transitions for 13 C 12 CH 4 . Of the 168 12C 2H 4 transitions, 96 are assigned to ν 7, 17 to ν 10, 10 to ν 4, and 45 to hotband transitions. The 88 13 C 12 CH 4 transitions have not been assigned. Measurements were made at gas temperatures in the range of 293– 297 K and at pressures in the range of 0.05– 0.5 Torr . Absolute line frequencies and line intensities were retrieved for each of the transitions. Frequencies and absolute intensities of unblended strong lines were determined to better than 5×10 −5 cm −1 and 10%, respectively.

Direct measurement of winds on Titan

We report the first direct measurement of wind velocity in the atmosphere of Titan, one of only two examples in our solar system of a slowly‐rotating body with a dense atmosphere and a prime target of the Cassini mission. Zonal wind velocity was determined from Doppler shift of ethane lines emitted from Titan's stratosphere (∼0.1–7 mbar) measured by infrared heterodyne spectroscopy near 12 µm (λ/Δ λ ≥ 106). Prograde zonal circulation, in the direction of global rotation, is established with 94% statistical confidence. Results provide information regarding Titan meteorology constraining dynamical theories for slowly‐rotating bodies, provide otherwise unobtainable data to optimize the Cassini Huygens Probe investigations, and demonstrate the capability for remotely measuring winds on a small, distant object.

Ethane abundance on Titan

Ethane (C 2H 6) abundance in Titan's stratosphere is determined from recent ground based high spectral resolution measurements of individual ethane emission line spectra. Lines near 12 μm in the ν 9 band of C 2H 6 were measured at a resolving power of λ/Δλ ∼ 10 6 with infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility (IRTF). Globally averaged constant-with-height C 2H 6 mole fractions are retrieved for various possible thermal profiles on Titan. A range of possible stratospheric temperatures is investigated with respect to the data and a corresponding range of acceptable globally averaged ethane mole fractions is retrieved. The data and physical constraints imposed by the observations limit the temperatures in Titan's upper stratosphere to 160–180 K. Corresponding acceptable mole fractions can range from 4 × 10 −6 to 1.6 × 10 −5 depending on the thermal profile used. For a currently “recommended” thermal profile a mole fraction of 9.4(−4.7, +9.4) × 10 −6 is retrieved, some-what lower than previous results, but centrally placed in the acceptable temperature-abundance parameter space.

Physics and chemistry of upper atmospheres of planets from infrared observations

An important goal of the study of atmospheres of planets is to provide an understanding of the relationship and interaction among phenomena occurring at different altitude and pressure regions and the effects of external influences on these phenomena. To probe multiple pressure regimes in general requires measurements in different spectral regions and with a wide range of spectral resolutions. This presents formidable technical challenges. To directly probe phenomena in the upper neutral atmosphere (pressures ⩽30 mbar) is particularly difficult. Much of the information on local chemistry and physics is contained in the near Doppler profiles of infrared molecular lines formed in these regions and spectral resolving powers λ Δλ > 10 6 are required to adequately measure the lineshapes. The very high spectral resolution technique of infrared heterodyne spectroscopy capable of making such line measurements is briefly described. Several investigations are described to illustrate the range of phenomena that can be studied and the unique physical and chemical parameters that can be retrieved. These investigations include direct measurement of global thermospheric circulation on Venus using Doppler shifts of nonthermal CO 2 emission lines near 10 μm; the study of Jupiter's deep interior utilizing spectroscopy of H 2 quadrupole lines (17 and 28.2 μm>) formed in the stratosphere; the evaluation of photochemical processes which determine the composition of Mars and the outer planets by retrieving constituent abundances (O 3 near 9.5 μm on Mars and hydrocarbons on Jupiter and Neptune); and the determination of the compositional and thermal structure in Jupiter's auroral region using line profiles of auroral ethylene (C 2H 4) and ethane (C 2H 6) emission near 10.5 and 12 μm. The value of multispectral and temporal observations for more comprehensive studies of atmospheric phenomena and the internal and external processes driving them are also discussed.

Temperature and abundances in the Jovian auroral stratosphere: 2. Ethylene as a probe of the microbar region

Individual emission lines of ethylene (C2H4) near 10.5 μm were measured from the equatorial and north polar regions of Jupiter. Observations were made at a spectral resolution of 0.00083 cm−1 using infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The line shape information possible with this resolving power permitted the retrieval of quiescent ethylene abundances and the investigation of abundance and thermal structure variability in the polar auroral region. A rough distribution of the north polar emission as a function of longitude was obtained with an instantaneous field of view (full width at half maximum) of ∼1 arc sec on the planet. The greatest C2H4 emission was observed near the nominal north polar methane hot spot (60°N, 180° longitude, System III, 1965). It was found to be confined to <10° longitude on the planet. Using a Voyager‐derived thermal profile, retrieved ethylene mole fractions for equatorial and north polar quiescent (non‐hot spot) regions were consistent with results from existing photochemical models. At the hot spot an 18‐fold increase in abundance was required near the 10‐μbar level to reproduce the data. Alternatively varying the stratospheric thermal profile, a 67–137 K increase in temperature was required at the ∼10‐μbar level to satisfy the observed emission, if the C2H4 mole fraction is fixed to the quiescent value. These results provide the first direct probe of the upper stratosphere of Jupiter and give upper limits to the temperature increase near the source of the north polar thermal infrared aurora. Combined with results from similar measurements of auroral ethane emission (Livengood et al., this issue) probing the 1‐mbar region, altitude information on the thermal structure can be obtained for the first time. The ethylene line emission region extends to the few microbar pressure level and may overlap the region where the H2 ultraviolet aurora is formed; thus it can be used as a probe of the coupling between the ultraviolet and thermal infrared phenomena.

Temperature and abundances in the Jovian auroral stratosphere: 1. Ethane as a probe of the millibar region

We report infrared heterodyne spectroscopy (λ/Δλ ∼ 106) of C2H6 emission at 11.9 μm from the northern Jovian auroral region, in observations conducted over December 2–7, 1989. Accurately measured line shapes provide information on C2H6 abundance as well as temperature and permit retrieval of the source pressure region. Enhanced emission was observed in the longitude range ∼150°–180° at ∼60° north latitude, approximately corresponding to the CH4 7.8‐μm hot spot and the region of brightest UV aurora. Significant brightness variations were observed in the hot spot emissions on a time scale of ∼20 hours. Analysis of the brightest hot spot spectra indicates C2H6 mole fractions of ∼(6.3–6.8) × 10−6 at temperatures of ∼182–184 K at 1 mbar, compared to mole fractions of (3.8 ± 1.4) × 10−6 averaged over spectra outside the hot spot at a temperature of ∼172 K at the same pressure. Fixing the mole fraction to the lower limit retrieved in the quiescent (non‐hot spot) region allows the temperature at 1 mbar to be as high as ∼200 K within the hot spot. These results provide upper limits to the temperature increase near the source of the C2H6 thermal infrared emission. Combined with results from similar measurements of ethylene emission probing the ∼10‐μbar region (Kostiuk et al., this issue), altitude information on the thermal structure of the Jovian auroral stratosphere can be obtained for the first time.

Theoretical study of the electric dipole moment function of the ClO molecule

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2∏ using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. Remaining questions concerning the position of the maximum of the EDMF may be resolved through experimental measurement of dipole moments of higher vibrational levels. The band strength of the 1–0 fundamental transition is computed to be 12±2 cm−2 atm−1 in good agreement with three experimental values, but larger than a recent value of 5 cm−2 atm−1 determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.