Solar Occultation Experiments Research Articles

First Results for Solar Soft X-Ray Irradiance Measurements from the Third-generation Miniature X-Ray Solar Spectrometer

Three generations of the Miniature X-ray Solar Spectrometer (MinXSS) have flown on small satellites with the goal “to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares, and to model the impact on Earth’s ionosphere and thermosphere.” The primary science instrument is the Amptek X123 X-ray spectrometer that has improved with each generation of the MinXSS experiment. This third-generation MinXSS-3 has a higher energy resolution and larger effective area than its predecessors and is also known as the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS). It was launched on the INSPIRESat-1 satellite on 2022 February 14, and INSPIRESat-1 has successfully completed its 6 month prime mission. The INSPIRESat-1 is in a dawn–dusk, Sun-synchronous orbit and therefore has had 24 hr coverage of the Sun during most of its mission so far. The rise of Solar Cycle 25 has been observed by DAXSS. This paper introduces the INSPIRESat-1 DAXSS solar SXR observations, and we focus the science results here on a solar occultation experiment and multiple flares on 2022 April 24. One key flare result is that the reduction of elemental abundances appears greatest during the flare impulsive phase, thus highlighting the important role of chromospheric evaporation during flares to inject warmer plasma into the coronal loops. Furthermore, these results are suggestive that the amount of chromospheric evaporation is related to flare temperature and intensity.

Solar Occultation Experiments (SOE) in the Venusian Atmosphere: effect of orbital parameters on the spatiotemporal distribution of measurements

AbstractSolar occultation is a satellite-based technique for high-resolution vertical profiling of planetary atmospheres. Owing to the distinctive observational geometry, the deduction of the spatiotemporal coverage of solar occultation measurements as a function of the spacecraft orbit is non-trivial. In this work, we have implemented python-based 3D simulations of the occultation-viewing geometry for a hypothetical Solar Occultation Experiment (SOE) to study the atmosphere of Venus. The simulations incorporate planetary motions and orbital propagation using the astropy and poliastro packages, and compute the instantaneous line-of-sight (LoS) tangent point using 3D vector algebra. SPICAV/SOIR data from Venus Express was used to validate the simulations, showing excellent agreement. Using the simulations, we conducted a first-of-its-kind theoretical study on the effect of varying different spacecraft orbital elements on the spatiotemporal distribution of solar occultation measurements in the Venusian atmosphere, confirming a highly sensitive dependence. The semimajor axis (a) and inclination (i) of the spacecraft orbit are found to influence the latitudinal extent of observations and the nature/duration of occultation seasons, while the eccentricity (e) and argument of periapsis (ω) determine the distinct regions of sparse observations. The spatiotemporal spread of individual SOE profiles is found to depend on the orbital parameters as well as the solar beta angle. Our results show that spacecraft orbits can be designed with appropriate parameters to optimize the coverage of SOE measurements in view of achieving specific science goals, providing valuable inputs for future missions to Venus that aim to implement the solar occultation technique.

Impact of gradients at the martian terminator on the retrieval of ozone from SPICAM/MEx

Rapid variations of pressure, temperature and illumination at the day–night terminator have the potential to cause asymmetries in the abundance distribution of the atmosphere constituents along the line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep density gradients across the terminator of Mars due to photolysis. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval technique in order to prevent inaccuracies.We investigated the impact of gradients along the LOS of the solar occultation experiment SPICAM/Mars Express for the retrieval of ozone under sunrise/sunset conditions. In order to test the impact of different gradients, we selected four occultations at sunrise and at sunset each. Sunset occultations are located near the equator, while sunrise observations are situated at high latitudes in the South, because of the geometry of the orbit.We used the diurnal variations in the ozone concentration obtained from a three-dimensional General Circulation Model (GEM-Mars) together with an adapted radiative transfer code (ASIMUT). The General Circulation Model (GCM) suggests that ozone variations strongly depend on latitude, altitude, and season. As shown by the model, near the equator and below 25 km, ozone changes only slightly with local time. Around 45 km, the density changes by several orders of magnitude across the terminator. At high latitudes in the South, during northern winter time, ozone variations at the terminator are negligible.The impact of gradients on ozone retrievals is strongly related to the local atmospheric structure as predicted by the GCM. Sunset ozone retrievals are smaller than retrievals obtained assuming a spherically symmetrical atmosphere, with a maximum change of about 20%. At sunrise, the impact of gradients on the retrievals is negligible. This behavior can be explained by the specific geometry of sunrise observations, all situated at high latitudes in the South.

On the observed changes in upper stratospheric and mesospheric temperatures from UARS HALOE

Abstract. Temperature versus pressure or T(p) time series from the Halogen Occultation Experiment (HALOE) of the Upper Atmosphere Research Satellite (UARS) have been extended and re-analyzed for the period of 1991–2005 and for the upper stratosphere and mesosphere in 10-degree wide latitude zones from 60 S to 60 N. Even though sampling from a solar occultation experiment is somewhat limited, it is shown to be quite adequate for developing both the seasonal and longer-term variations in T(p). Multiple linear regression (MLR) techniques were used in the re-analyses for the seasonal and the significant interannual, solar cycle (SC-like or decadal-scale), and linear trend terms. Plots of the amplitudes and phases for the interannual (QBO and subbiennial) terms are provided. A simple SC-like term of 11-yr period was fitted to the time series residuals after accounting for the seasonal and interannual terms. Highly significant SC-like responses were found for both the upper mesosphere and the upper stratosphere. The phases of these SC-like terms were checked for their continuity with latitude and pressure-altitude; the larger amplitude responses are directly in-phase with that of standard proxies for the solar flux variations. The analyzed, max minus min, responses at low latitudes are of order 0.5 to 1 K, while at middle latitudes they are as large as 3 K in the upper mesosphere. Highly significant, linear cooling trends were found at middle latitudes of the middle to upper mesosphere (−1.5 to −2.0 K/decade), at tropical latitudes of the lower mesosphere (about −0.5 K/decade), and at 2 hPa (of order −1 K/decade). Both the diagnosed solar cycle responses and trends from HALOE for the mid to upper mesosphere at middle latitudes are larger than simulated with most models, perhaps an indication of decadal-scale dynamical forcings that are not being simulated so well.

Line parameters for the 01111–00001 band of 12C 16O 18O from SOIR measurements of the Venus atmosphere

CO 2 is the major constituent of the atmosphere of Venus. Absorption lines due to its 12C 16O 18O isotopologue have been observed for the first time in Venus spectra in the 2930–3015 cm −1 spectral region, where the HITRAN database does not contain any line from this isotopologue. The measurements were performed by the SOIR instrument, which is part of the SPICAV/SOIR instrument on board the Venus Express mission of ESA. SOIR measured the atmospheric transmission of the upper atmosphere of Venus ( z>70 km) by performing a solar occultation experiment using the atmosphere as a gigantic absorption cell. The identification of this newly observed band was first made recently from Mars atmosphere observations by US colleagues. We have made independent theoretical calculations of the positions of the lines of this new 01111–00001 absorption band, which coincide perfectly with the positions of the observed lines. Assuming an oxygen isotopic ratio similar to the one measured previously in the lower atmosphere of Venus, the line strengths of each observed line are deduced and listed.

Polar stratospheric cloud observations by MIPAS on ENVISAT: detection method, validation and analysis of the northern hemisphere winter 2002/2003

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT has made extensive measurements of polar stratospheric clouds (PSCs) in the northern hemisphere winter 2002/2003. A PSC detection method based on a ratio of radiances (the cloud index) has been implemented for MIPAS and is validated in this study with respect to ground-based lidar and space borne occultation measurements. A very good correspondence in PSC sighting and cloud altitude between MIPAS detections and those of other instruments is found for cloud index values of less than four. Comparisons with data from the Stratospheric Aerosol and Gas Experiment (SAGE) III are used to further show that the sensitivity of the MIPAS detection method for this threshold value of cloud index is approximately equivalent to an extinction limit of 10-3km-1 at 1022nm, a wavelength used by solar occultation experiments. The MIPAS cloud index data are subsequently used to examine, for the first time with any technique, the evolution of PSCs throughout the Arctic polar vortex up to a latitude close to 90° north on a near-daily basis. We find that the winter of 2002/2003 is characterised by three phases of very different PSC activity. First, an unusual, extremely cold phase in the first three weeks of December resulted in high PSC occurrence rates. This was followed by a second phase of only moderate PSC activity from 5-13 January, separated from the first phase by a minor warming event. Finally there was a third phase from February to the end of March where only sporadic and mostly weak PSC events took place. The composition of PSCs during the winter period has also been examined, exploiting in particular an infra-red spectral signature which is probably characteristic of NAT. The MIPAS observations show the presence of these particles on a number of occasions in December but very rarely in January. The PSC type differentiation from MIPAS indicates that future comparisons of PSC observations with microphysical and denitrification models might be revealing about aspects of solid particle existence and location.

Spectra calculations in central and wing regions of CO 2 IR bands between 10 and [formula omitted]. III: atmospheric emission spectra

A theoretical model for the prediction of CO 2 absorption in both central and wing regions of infrared absorption bands was presented in the companion paper I. It correctly accounts for line-mixing effects and was validated by comparisons with laboratory spectra in the 600– 1000 cm −1 region. This quality was confirmed using atmospheric transmissions measured by solar occultation experiments in the second paper. The present work completes these studies by now considering atmospheric emission in the 10– 20 μm range. Comparisons are made between computed atmospheric radiances and measurements obtained using four different Fourier transform experiments collecting spectra for nadir, up-looking, as well as limb (from balloon and satellite) geometries. Our results confirm that using a Voigt model can lead to very large errors that affect the spectrum more than 300 cm −1 away from the center of the CO 2 ν 2 band. They also demonstrate the capability of our model to represent accurately the radiances in the entire region for a variety of atmospheric paths. This success opens interesting perspectives for the sounding of pressure and temperature profiles, particularly at low altitudes. Another benefit of the quality of the model should be an increased accuracy in the retrieval of atmospheric state parameters from broad features in the measured spectra (clouds, aerosols, heavy trace gases).

Spectra calculations in central and wing regions of CO 2 IR bands between 10 and [formula omitted]. II. Atmospheric solar occultation spectra

The theoretical approach based on the Energy Corrected Sudden Approximation presented in the previous companion paper is used in order to account for line-mixing effects in infrared bands of CO 2. Its performance, which was demonstrated using laboratory spectra is confirmed here by considering atmospheric transmission in the 10– 14 μm region. Comparisons are made between forward calculations of atmospheric transmission spectra and values measured using two different solar occultation experiments based on high resolution Fourier transform instruments. The results demonstrate that neglecting line-mixing and using a Voigt model can lead to a very large overestimation of absorption that may extend over more than 300 cm −1 in the wing of the CO 2 ν 2 band. They also demonstrate the capability of our model to represent accurately the absorption in the entire region for a variety of atmospheric paths. Among positive consequences of the quality of the model, the possibility of retrieving amounts of (heavy) trace gases with weak and broad absorption features is demonstrated.

Spectra calculations in central and wing regions of CO2 IR bands between 10 and 20 $mu;m. III: atmospheric emission spectra

A theoretical model for the prediction of CO2 absorption in both central and wing regions of infrared absorption bands was presented in the companion paper I. It correctly accounts for line-mixing effects and was validated by comparisons with laboratory spectra in the 600–1000cm−1 region. This quality was confirmed using atmospheric transmissions measured by solar occultation experiments in the second paper. The present work completes these studies by now considering atmospheric emission in the 10–20μm range. Comparisons are made between computed atmospheric radiances and measurements obtained using four different Fourier transform experiments collecting spectra for nadir, up-looking, as well as limb (from balloon and satellite) geometries. Our results confirm that using a Voigt model can lead to very large errors that affect the spectrum more than 300cm−1 away from the center of the CO2ν2 band. They also demonstrate the capability of our model to represent accurately the radiances in the entire region for a variety of atmospheric paths. This success opens interesting perspectives for the sounding of pressure and temperature profiles, particularly at low altitudes. Another benefit of the quality of the model should be an increased accuracy in the retrieval of atmospheric state parameters from broad features in the measured spectra (clouds, aerosols, heavy trace gases).

Spectroscopic measurements of halocarbons and hydrohalocarbons by satellite‐borne remote sensors

Infrared spectra recorded by the Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) and the Interferometric Monitor for Greenhouse Gases (IMG) remote sensors have been analyzed by means of line‐by‐line radiative transfer calculations in order to evaluate the possibilities offered by solar occultation and by nadir instruments to monitor the cholorofluorocarbons (CFCs) and their substitutes. The reliability of the existing spectroscopic parameters has been examined, and it was found that only laboratory parameters measured at high resolution reproduce the satellite observations well. It is shown that solar occultation spectra can give information regarding the atmospheric abundance of CFC‐113, in addition to the usual retrievals of CFC‐11, CFC‐12 and HCFC‐22. Also, relying on existing emission scenarios, it is foreseen that future solar occultation experiments, such as the Atmospheric Chemistry Experiment (ACE), will be able to detect HCFC‐142b and HFC‐134a, from the year 2005 onward and at low tangent heights. As for the nadir‐looking missions, it is found that CFC‐11, CFC‐12 and HCFC‐22 can be retrieved from IMG spectra, provided that numerous measurements are averaged over space or time, thereby mitigating the usefulness of such measurements for determining surface sources. The improved geometrical scanning performances of the future nadir‐looking Infrared Atmospheric Sounding Interferometer (IASI) and the Tropospheric Emission Spectrometer (TES), however, are shown to be more promising in this respect.

Measurements of temperature‐dependent absorption cross sections of C2H2 in the VUV‐UV region

Using a synchrotron radiation as a continuum light source, we have carried out laboratory measurements of the temperature‐dependent photoabsorption cross sections of C2H2 from 120 to 230 nm region at 295 K and from 120 to 215 nm region at 150 K with a spectral resolution of 0.007 nm. In the presently studied spectral region the cross section values vary by 6 orders of magnitude, and the absorption features vary from sharp rovibronic structure, shaded bands, to diffuse bands. High‐temperature cross sections of C2H2 in the 120–140 nm region has also been measured for the purpose of identifying possible hot bands. The previous assignments of several hot bands in the 130–140 nm region were crucial in the determination of the electronic structure of a valence E state. However, we find that the previously assigned hot bands at 135.09 and 136.24 nm are not hot bands at all because they are insensitive to temperature change from 150 to 370 K. The Galileo spacecraft has made incredibly successful observations of the Jovian system including the prominent C2H2 auroral features, and the Jupiter flyby of the Cassini spacecraft is yet to come. Equipped with higher‐resolution spectrometers, the Cassini mission promises to provide higher scientific returns regarding the Jovian auroral observations. Solar and stellar occultation experiments at Saturn by the Cassini spacecraft will provide more detailed observations of the Saturnian upper atmosphere than ever before. The data presented in the present work are an extension of our effort to provide the required data to the scientific community and will have a significant impact on our understanding of the atmospheres of Jupiter and Saturn, in particular, and other outer planets and satellites, in general.

Refractive sounding by use of satellite solar occultation measurements including an assessment of its usefulness to the Stratospheric Aerosol and Gas Experiment Program

Vertical profiles of atmospheric density and temperature obtained with the technique of solar refractive sounding can potentially be used to improve satellite solar occultation trace species retrievals by reducing the uncertainties associated with Rayleigh scattering and the temperature dependence of absorption bands. The required refraction measurements and the algorithm utilized to recover density and temperature are described. Simulations are performed to estimate the measurement accuracy that is necessary to retrieve useful meteorological soundings at stratospheric altitudes. The method is applied to data measured by the Stratospheric Aerosol and Gas Experiment (SAGE) II. Unfortunately, because of poor vertical sampling and measurement uncertainties, the meteorological profiles derived from the SAGE II data are not consistently accurate enough to improve the SAGE II estimates for the concentrations of trace species. However, the qualitatively decent results provide optimism for future development and implementation of visible refractive sounding as a tool to help improve the accuracy of trace species retrievals within solar or stellar occultation experiments, including the SAGE III program.

Validation of the ORA spatial inversion algorithm with respect to the Stratospheric Aerosol and Gas Experiment II data

We present the results of a comparison of the total extinction altitude profiles measured at the same time and at same location by the ORA (Occultation Radiometer) and Stratospheric Aerosol and Gas Experiment II solar occultation experiments at three different wavelengths. A series of 25 events for which the grazing points of both experiments lie within a 2 degrees window has been analyzed. The mean relative differences observed over the altitude range 15-45 km are -8.4%, 1.6%, and 3% for the three channels (0.385, 0.6, and 1.02 microm). Some systematic degradation occurs below 20 km (as the result of signal saturation and possible cloud interference) and above 40 km (low absorption). The fair general agreement between the extinction profiles obtained by two different instruments enhances our confidence in the results of the ORA experiment and of the recently developed vertical inversion algorithm applied to real data.

The solar Ly-alpha line profile

Solar Ly-alpha irradiance measurements from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) on the Upper Atmosphere Research Satellite (UARS) have been made since 1991 October with a spectral resolution of 0.1 nm. The uniqueness of the small molecular oxygen cross section near Ly-alpha permits the Ly-alpha radiation to penetrate much deeper into the atmosphere than the other emissions near Ly-alpha. We have taken advantage of this phenomenon by performing solar occultation experiments near the Ly-alpha to evaluate precisely the instrument scattered light contribution. After correcting for scattered light, the broad wings of the solar Ly-alpha line can be extracted out to 5 nm from line center with a typical accuracy of +/-20%. The variability in the Ly-alpha wings near 2 nm from line center is about one-half that of the Ly-alpha core emission, defined within 0.1 nm from line center. These Ly-alpha profile measurements are found to be consistent with the Skylab radiance measurements and theoretical models of the Ly-alpha line profiles computed using partial redistribution of photons in the source function.

Martian Aerosols: Near-Infrared Spectral Properties and Effects on the Observation of the Surface

Imaging spectroscopic measurements of Mars acquired by the ISM instrument on Phobos-2 are used to investigate the NIR spectral properties of aerosols and the effects of atmospheric scattering on inferred mineralogy of the surface. Estimates of aerosols spectra between 0.77 and 2.6 μm are derived above Tharsis and Ophir Planum. The spectral continua are consistent with the particle size distribution derived using data from the solar occultation experiment on-board the spacecraft (effective radius ≈1.2 μm, with an effective variance ≈0.2). The aerosols spectra contain water-ice absorption features and possibly absorptions due to clay and/or sulfates. For these particular observations, aerosol opacity is apparently uncorrelated with surface altimetry, suggesting a spatially uniform column density. The largest effect of the aerosols on surface spectra is in dark regions, where the continuum spectral slope becomes more negative and the 1-μm absorption due to Fe in pyroxene is shifted toward longer wavelengths. The effects of aerosols on spectra of bright regions are insufficiently large to change mineralogic interpretations based on ISM data, i.e., that bright regions in Tharsis are dominated spectrally by hematite, but that additional ferric minerals are probably present in other areas including Arabia.

The Distribution Hydrocarbons in Neptune's Upper Atmosphere

Data from the Voyager UVS solar occultation experiment are analyzed to determine the distribution of hydrocarbons in the upper atmosphere of Neptune. Objective inversion techniques are used to infer densities from the transmission properties of the atmosphere measured by the UVS. Densities of H2, CH4, and C2H6 are determined over limited altitude ranges and constraints are placed on the abundances of C2H2 and C2H4. The inferred densities when used in conjunction with models for the transport of CH4 and C2H6 imply that the CH4 mole fraction in the stratosphere is between 6 × 10-4 and 5 × 10-3, i.e., more than one order of magnitude larger than expected if the tropopause cold trap is operating. This conclusion is independent of assumptions about the temperature profile in the stratosphere. C2H6 is produced from CH4 photolysis with an efficiency of 30-50%. The eddy diffusion coefficient has a value of 1.0 ± 0.3 × 105 cm2 sec-1 at and below 300 km, rising exponentially to a value of 2.4+0.7-0.5 × 106 cm2sec-1 at 550 km. The C2H6 densities determined from this study are consistent with results from groundbased and Voyager infrared observations. The H2 densities are in excellent agreement with results from the Voyager radio occultation.

Infrared absorption-coefficient data on SF 6 applicable to atmospheric remote sensing

Spectral absorption coefficients k v (cm -1 atm -1) of SF 6 have been measured in the central Q-branches of the v 3-fundamental at 947 cm -1 at various temperature-pressure combinations representing tangent heights in solar-occultation experiments or layers in the atmosphere. The data obtained with the Doppler-limited spectral resolution (∼ 10 -4 cm -1) of a tunable-diode laser spectrometer are useful in the atmospheric remote sensing of this trace gas.

Absorption coefficients of CFC-11 and CFC-12 needed for atmospheric remote sensing and global warming studies

Spectral absorption coefficients (or absorption cross-sections) k v (cm -1 atm -1) of CFC-11 and CFC-12 have been measured in the spectral region (8–12 μm) known as the atmospheric window. Data obtained with a grating spectrometer, which has wide spectral coverage and adequate spectral resolution (0.4 cm -1), are compared with the so-called NCAR cross-sections that have recently been introduced into the HITRAN Tape. We have also performed measurements, at various temperature-pressure combinations which are chosen to represent tangent heights (as in solar-occultation experiments) or layers in the atmosphere, of the k v of CFC-12 in the 921 and 923 cm -1 Q-branches and at 924.1, 927.6, 931.6 935.0 and 1106 cm -1 with the Doppler-limited spectral resolution (∼10 -4cm -1) of a tunable-diode laser spectrometer. The latter are especially well suited for atmospheric remote sensing, while the grating spectra are useful in global warming studies. The effects of temperature and pressure on the spectral absorption coefficients are discussed.

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985–1989 period. Annual as well as semi-annual waves are observed in the 50–70 km altitude region. In the latitude range of ±30 the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than ±15). There is a hemispheric asymmetry in the ozone annual wave in the 20 30 region, with northern hemispheric ozone having a larger amplitude than southern hemispheric ozone. In this region temperature is nearly in phase with ozone in both hemispheres and is reduced in amplitude in the northern hemisphere. The equatorial region is characterized by a strong semi-annual wave in addition to the annual variation, and temperature is nearly out of phase with ozone. At all latitudes there is a larger ozone concentration at sunrise than at sunset. The sunrise sunset difference increases with increasing altitude

Atmospheric constituent density profiles from full disk solar occultation experiments

An algorithm is presented for the retrieval of atmospheric number density profiles by inversion of full disk solar occultation data. A general derivation of the normalized absorption profile obtained from such measurements is first given, assuming an arbitrary solar intensity distribution. Using Fourier transform techniques, the resulting integral equation is then solved exactly to yield an expression for the number density in terms of the measured absorption profile. This exact mathematical solution of the integral equation eliminates the need for an iterative solution based on an assumed model density profile and makes the inversion algorithm presented here unique among the currently used algorithms known to the authors. The formal solution is implemented numerically by introducing a suitable representation of the data in terms of Laguerre polynomials, and model calculations are presented for the special case of a uniform solar intensity distribution. The results of these calculations are used to demonstrate the convergence properties of the algorithm and its suitability for accurate reproduction of several representative model density profiles with both constant and altitude-dependent scale heights. Finally, a simple numerical calculation is performed to estimate the effect of nonuniform solar intensity distributions on the retrieved density.