High-resolution Solar Spectra Research Articles

Observed IRIS Profiles of the h and k Doublet of Mg ii and Comparison with Profiles from Quiescent Prominence NLTE Models

With the launch of the Interface Region Imaging Spectrograph (IRIS) mission, it is now possible to obtain high-resolution solar prominence spectra and to begin to distinguish the contributions of the many (apparent or not) threads that structure prominences. We aim at comparing unique observations obtained in the Mg ii h and k lines of a polar crown prominence with the radiative outputs from one-dimensional models built with non-local-thermodynamic equilibrium codes (Heinzel et al. Astron. Astrophys. 564, A132, 2014). We characterize the profiles obtained through thorough calibration procedures, with attention paid to the absolute values, full-width at half-maximum, and the ratio of k to h intensities. We also show that at the top of some structures, line-of-sight velocities of about $9~\mbox{km}\,\mbox{s}^{-1}$ can be detected. We find a range of static, low-pressure, low-thickness, low-temperature models that could fit k or h observed values, but that cannot satisfy the low observed k/h ratio. We investigate whether these low values might be explained by the inclusion of horizontal flows in small-scale threads. These flows are also necessary in another class of models, where the pressure is kept low but thickness and temperature are increased up to the observed thickness and up to 15 000 K.

Neural network radiative transfer solvers for the generation of high resolution solar irradiance spectra parameterized by cloud and aerosol parameters

This paper reports on the development of a neural network (NN) model for instantaneous and accurate estimation of solar radiation spectra and budgets geared toward satellite cloud data using a ≈2.4M record, high-spectral resolution look up table (LUT) generated with the radiative transfer model libRadtran. Two NN solvers, one for clear sky conditions dominated by aerosol and one for cloudy skies, were trained on a normally-distributed and multiparametric subset of the LUT that spans a very broad class of atmospheric and meteorological conditions as inputs with corresponding high resolution solar irradiance target spectra as outputs. The NN solvers were tested by feeding them with a large (10K record) “off-grid” random subset of the LUT spanning the training data space, and then comparing simulated outputs with target values provided by the LUT. The NN solvers demonstrated a capability to interpolate accurately over the entire multiparametric space. Once trained, the NN solvers allow for high-speed estimation of solar radiation spectra with high spectral resolution (1nm) and for a quantification of the effect of aerosol and cloud optical parameters on the solar radiation budget without the need for a massive database. The cloudy sky NN solver was applied to high spatial resolution (54K pixel) cloud data extracted from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the geostationary Meteosat Second Generation 3 (MSG3) satellite and demonstrated that coherent maps of spectrally-integrated global horizontal irradiance at this resolution can be produced on the order of 1min.

Characterization and verification of ACAM slit functions for trace-gas retrievals during the 2011 DISCOVER-AQ flight campaign

Abstract. The Airborne Compact Atmospheric Mapper (ACAM), an ultraviolet/visible/near-infrared spectrometer, has been flown on board the NASA UC-12 aircraft during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) campaigns to provide remote sensing observations of tropospheric and boundary-layer pollutants from its radiance measurements. To assure the trace-gas retrieval from ACAM measurements we perform detailed characterization and verification of ACAM slit functions. The wavelengths and slit functions of ACAM measurements are characterized for the air-quality channel (~304–500 nm) through cross-correlation with a high-resolution solar irradiance reference spectrum after necessarily accounting for atmospheric gas absorption and the ring effect in the calibration process. The derived slit functions, assuming a hybrid combination of asymmetric Gaussian and top-hat slit functions, agree very well with the laboratory-measured slit functions. Comparisons of trace-gas retrievals between using derived and measured slit functions demonstrate that the cross-correlation technique can be reliably used to characterize slit functions for trace-gas retrievals.

Updated Smithsonian Astrophysical Observatory Ozone Monitoring Instrument (SAO OMI) formaldehyde retrieval

Abstract. We present and discuss the Smithsonian Astrophysical Observatory (SAO) formaldehyde (H2CO) retrieval algorithm for the Ozone Monitoring Instrument (OMI) which is the operational retrieval for NASA OMI H2CO. The version of the algorithm described here includes relevant changes with respect to the operational one, including differences in the reference spectra for H2CO, the fit of O2–O2 collisional complex, updates in the high-resolution solar reference spectrum, the use of a model reference sector over the remote Pacific Ocean to normalize the retrievals, an updated air mass factor (AMF) calculation scheme, and the inclusion of scattering weights and vertical H2CO profile in the level 2 products. The setup of the retrieval is discussed in detail. We compare the results of the updated retrieval with the results from the previous SAO H2CO retrieval. The improvement in the slant column fit increases the temporal stability of the retrieval and slightly reduces the noise. The change in the AMF calculation has increased the AMFs by 20%, mainly due to the consideration of the radiative cloud fraction. Typical values for retrieved vertical columns are between 4 × 1015 and 4 × 1016 molecules cm−2, with typical fitting uncertainties ranging between 45 and 100%. In high-concentration regions the errors are usually reduced to 30%. The detection limit is estimated at 1 × 1016 molecules cm−2.

Measurement of the Earth’s rotational speed via Doppler shift of solar absorption lines

This paper describes an experiment regularly presented to advanced undergraduate Physics students at the Universidad de los Andes in Bogotá, Colombia. The purpose of the experiment is to use high-resolution solar spectra to measure the horizontal speed of the laboratory caused by terrestrial rotation. Using this result, the radius of the Earth can be deduced. It is also possible to observe the Earth’s motion towards or away from the Sun, and hence compute our planet’s orbital eccentricity.

The effects of different solar flare characteristics on the global thermosphere

Given the ability of global models of the upper atmosphere to utilize high-resolution solar spectra to specify the solar soft X-ray and EUV irradiance, researchers now have the ability to perform detailed theoretical analysis of the response of the upper atmosphere to dynamic solar forcing more completely than ever before. Therefore, it is possible to develop a more quantitative understanding of the response of the thermosphere to solar flares. In this study, the effect of different characteristics of solar flares on the thermosphere is investigated. This is done in a theoretical manner, using synthetic solar irradiance data that is based on observations as input to the global ionosphere-thermosphere model (GITM). Specifically, the neutral response to the total incident energy, peak flare magnitude, background irradiance magnitude, duration of the impulsive phase, and decay time is investigated. It is found that the density response at 400 km altitude is linearly dependent on the total integrated energy above the background level being deposited into the atmosphere, and that the day-side response is strongly dependent on both the total incident energy into the system and the peak flare magnitude. Also, the decay time of the flare is important in determining the time at which the maximum global response occurs. Finally, the duration of the impulsive phase is found to have little effect on the global response of the system.

ULIRS, an optimal estimation retrieval scheme for carbon monoxide using IASI spectral radiances: sensitivity analysis, error budget and simulations

Abstract. This paper presents a new retrieval scheme for tropospheric carbon monoxide (CO), using measured radiances from the Infrared Atmospheric Sounding Interferometer (IASI) onboard the MetOp-A satellite. The University of Leicester IASI Retrieval Scheme (ULIRS) is an optimal estimation retrieval scheme, which utilises equidistant pressure levels and a floating pressure grid based on topography. It makes use of explicit digital elevation and emissivity information, and incorporates a correction for solar surface reflection in the daytime with a high resolution solar spectrum. The retrieval scheme has been assessed through a formal error analysis, via the simulation of surface effects and by an application to real IASI data over a region in Southern Africa. The ULIRS enables the retrieval of between 1 and 2 pieces of information about the tropospheric CO vertical profiles, with peaks in the sensitivity at approximately 5 and 12 km. Typical errors for the African region relating to the profiles are found to be ~20% at 5 and 12 km, and on the total columns to range from 18 to 34%. Finally the performance of the ULIRS is shown for a range of simulated geophysical conditions.

Ceres’ sunlight atlas

Context. Astronomical research dealing with accurate radial velocity measurements need reliable astronomical standards to calibrate the spectrographs and to assess possible systematics. Stellar radial velocity standards offer a reference at the level of a few hundred m s −1 and are not adequate for most present needs. Aims. We aim to show that sunlight reflected by asteroids is a fairly accessible way to record a high-resolution solar spectrum from the whole disk, which can therefore be used as a radial velocity standard and can improve the uncertainties of solar line positions. Methods. We used solar light reflected by the asteroid Ceres observed with HARPS to measure solar lines’ wavelengths. Results. We provide a new solar atlas with 491 line wavelengths in the range 540−690 nm and 222 lines in the range 400−410 nm obtained from reflected solar spectrum of Ceres. These measurements are consistent with those of Allende Prieto & Garcia Lopez (1998b) based on FTS solar atlases but with a factor 3 higher precision. Conclusions. This atlas provides a benchmark for wavelength calibration to check radial velocity accuracy down to 44 m s −1 locally and a few m s −1 globally. The asteroid-based technique could provide a new way to track radial velocity shifts with solar activity cycle, as well as to derive convective shifts suitable for comparison with theoretical atmospheric models. It could also be used to study radial velocity deviations in spectrographs such as those recently detected in HIRES and UVES. Dedicated HARPS observations of other asteroids could improve present results substantially and these investigations have been solicited.

Contamination of the 5394 Å spectral region by telluric lines

The spectral region in the vicinity of 5394 Å contains three prominent photospheric spectral lines, which can be used as a solar plasma diagnostic tool. The occurrence of telluric lines in this region is a potential source of systematic and random errors in these solar spectral lines. The goal of our investigation was to determine the telluric line contamination of this interesting spectral region. Several series of high-resolution solar spectra within an interval of about 4 Å around the 5394 Å wavelength were observed at different zenith distances of the Sun. Comparison of these spectra has permitted identification of telluric lines in this spectral interval. The observations were carried out with the horizontal solar spectrograph of the Heliophysical Observatory in Debrecen. Telluric feature blending was identified in the blue and red wings of the Fe I 5393.2 Å line, and in the local continuum of the Mn I 5394.7 Å line. The blue wing of the Fe I 5395.2 Å line is contaminated by a weak telluric feature too. The red continuum of this line has a more prominent telluric contamination. A dozen of water vapor telluric lines that determined the observed telluric features were identified in this spectral interval. The profiles of three telluric lines that have a significant influence on both the profiles of solar spectral lines and the level of local continuum were derived, and the variation of their parameters (equivalent width and central depth) with air mass were analyzed.

An improved high-resolution solar reference spectrum for earth's atmosphere measurements in the ultraviolet, visible, and near infrared

We have developed an improved solar reference spectrum for use in the analysis of atmospheric spectra from vacuum wavelengths of 200.07 through 1000.99 nm. The spectrum is developed by combining high spectral resolution ground-based and balloon-based solar measurements with lower spectral resolution but higher accuracy irradiance information. The new reference spectrum replaces our previous reference spectrum, and its derivatives, for use in a number of physical applications for analysis of atmospheric spectra, including: wavelength calibration; determination of instrument transfer (slit) functions; Ring effect (Raman scattering) correction; and correction for spectral undersampling of atmospheric spectra, particularly those that are dilute in absorbers. The applicability includes measurements from the GOME, SCIAMACHY, OMI, and OMPS satellite instruments as well as aircraft-, balloon-, and ground-based measurements.

Semiautonomous FTS Observation System for Remote Sensing of Stratospheric and Tropospheric Gases

Abstract A solar-viewing Fourier transform spectrometer (FTS) at Thule, Greenland (76.5°N, 68.8°W, 225 m MSL), has been in operation as part of the Network for the Detection of Atmospheric Composition Change [NDACC; formerly the Network for the Detection of Stratospheric Change (NDSC)] since 1999. Observations have been made, on average, 77 days yr−1 during the 8 months, excluding polar night. The semiautonomous operation of the instrument, including its associated optical, cryogenic, and control systems, is of primary importance to acquiring long-term data records efficiently and is herein described. Discussed in this paper are the data processing and spectra analysis methodology that are used to convert the measured interferograms into geophysical data products. Vertical profile retrievals derived from the high-resolution solar absorption spectra use the optimal estimation method. Total column amounts then represent the integration of these vertical profiles. As an example of this process, results are presented for daily average total column amounts of HF, HCl, ClONO2, and CCl2F2 from 2001 through 2007. The means of unperturbed summertime observations are used in a preliminary study of their annual trends.

Clarifications on exoplanets

On page 47 of the article in the May 2009 issue of Physics Today, figure 2b is mistakenly credited to Geoff Marcy rather than the National Solar Observatory. The high-resolution solar spectrum was produced with the Fourier-transform spectrometer at the McMath-Pierce Solar Facility in 1984 and serves as a proxy for a high-resolution spectrum of a Sun-like star. Credit for the image should go to NSO/AURA/NSF.Also, on page 51, the first complete sentence in the second column should read as follows: “The plot assumes a 2.5-m telescope operating at a wavelength of 800 nm with an advanced coronagraph capable of imaging a planet separated from its glaring star by as little as 160 milliarcseconds.”© 2009 American Institute of Physics.

UV spectral measurements at moderately high resolution and of OH resonance scattering resolved by polarization during the MANTRA 2002–2004 stratospheric balloon flights

A moderately high-resolution (<0.1 nm) grating spectrometer designed to measure the solar radiation in the spectral range 295–315 nm was flown on the MANTRA stratospheric balloon payloads of 2002 and 2004. The instrument measures both the direct sunlight and the radiation scattered by the atmosphere. The latter can be observed in two orthogonal polarization directions, at 90° from the solar azimuth and at several elevations above the horizon. As the OH molecule is the principal resonant scatterer in this spectral region, this permits the inference of both ozone and OH column amounts as well as limited profile information. This paper describes the instrument and its in-flight characterization, the basic data processing and the influence of several aspects of the flight profile. The direct sun measurements are analyzed both to characterize the spectrometer responsivity to scattered radiation and to estimate the ozone abundance at the flight altitude and above. An example of a high-resolution solar spectrum at 37 km altitude is presented and compared with others in the literature. The measured OH and Rayleigh-scattered spectra are used to derive OH radiation intensity measurements (the OH airglow), which are compared with others in the literature.

On the accuracy of oscillator strengths in the near-infrared for the Gaia space mission

We have tested the accuracy of oscillator strengths and wavelengths of some spectral lines (FeI, Si I, Ca II) of interest for the Gaia space mission in the near infrared (848-875 nm). The method used in this work consists of fitting synthetic lines calculated using a 3D hydrodynamical simulation of the solar surface with a high-resolution solar spectrum. We found good agreement when atomic data measured in the laboratory were used.

The High-Resolution Solar Reference Spectrum between 250 and 550 nm and its Application to Measurements with the Ozone Monitoring Instrument

We have constructed a new high resolution solar reference spectrum in the spec- tral range between 250 and 550 nm. The primary use of this spectrum is for the calibration of the Dutch - Finnish Ozone Monitoring Instrument (OMI), but other applications are men- tioned. The incentive for deriving a new high resolution solar reference spectrum is that available spectra do not meet our requirements on radiometric accuracy or spectral resolu- tion. In this paper we explain the steps involved in constructing the new spectrum, based on available low and high resolution spectra and discuss the main sources of uncertainty. We compare the result with solar measurements obtained with the OMI as well as with other UV-VIS space-borne spectrometers with a similar spectral resolution. We obtain excellent agreement with the OMI measurements, which indicates that both the newly derived solar reference spectrum and our characterization of the OMI instrument are well understood. We also find good agreement with previously published low resolution spectra. The absolute intensity scale, wavelength calibration and representation of the strength of the Fraunhofer lines have been investigated and optimized to obtain the resulting high resolution solar ref- erence spectrum.

Statistical equilibrium of silicon in the solar atmosphere

The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. High-resolution solar spectra were used to determine solar $\log gf\epsilon_{\rm Si}$ values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metal-poor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. The statistical equilibrium of \ion{Si}{i} turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental $f$-values of Garz corrected for the Becker et al.'s measurement is $7.52 \pm 0.05$. Combined with an extended line sample with selected NIST $f$-values, the solar abundance is $7.52 \pm 0.06$, with a nearly perfect ionisation equilibrium of $\Delta\log\epsilon_\odot(\ion{Si}{ii}/\ion{Si}{i}) = -0.01$.

Ground-based solar absorption studies for the Carbon Cycle science by Fourier Transform Spectroscopy (CC-FTS) mission

Carbon cycle science by Fourier transform spectroscopy (CC-FTS) is an advanced study for a future satellite mission. The goal of the mission is to obtain a better understanding of the carbon cycle in the Earth's atmosphere by monitoring total and partial columns of CO 2, CH 4, N 2O, and CO in the near infrared. CO 2, CH 4, and N 2O are important greenhouse gases, and CO is produced by incomplete combustion. The molecular O 2 column is also needed to obtain the effective optical path of the reflected sunlight and is used to normalize the column densities of the other gases. As part of this advanced study, ground-based Fourier transform spectra are used to evaluate the spectral region and resolution needed. Spectra in the 3950–7140 cm −1 region with a spectral resolution of 0.0042 cm −1 recorded at Kiruna (67.84°N, 20.41°E, and 419 m above sea level), Sweden, on 1 April 1998, were degraded to the resolutions of 0.01, 0.1, and 0.3 cm −1. The effect of spectral resolution on the retrievals has been investigated with these four Kiruna spectra. To obtain further information on the spectral resolution, optical components and spectroscopic parameters required by the future mission, high-resolution solar absorption spectra between 2000 and 15000 cm −1 were recorded using Fourier transform spectrometers at Kitt Peak (31.9°N, 111.6°W, and 2.1 km above sea level), Arizona, on 25 July 2005 and Waterloo (43.5°N, 80.6°W, and 0.3 km above sea level), Ontario, on 22 November 2006 with spectral resolutions of 0.01 and 0.1 cm −1, respectively. Dry air volume mixing ratios (VMRs) of CO 2 and CH 4 were retrieved from these ground-based observations. The HITRAN 2004 spectroscopic parameters are used with the SFIT2 package for the spectral analysis. The measurement precisions for CO 2 and CH 4 total columns are better than 1.07% and 1.13%, respectively, for our observations. Based on these results, a Fourier transform spectrometer (maximum spectral resolution of 0.1 cm −1 or 5 cm maximum optical path difference (MOPD)) operating between 2000 and 15000 cm −1 is suggested as the primary instrument for the mission. Further progress in improving the atmospheric retrievals for CO 2, CH 4, and O 2 requires new laboratory measurements of the spectroscopic line parameters.

Synthetic high-resolution near-IR spectra of the Sun for planetary data reductions made from ATMOS/Spacelab-3 and Atlas-3 data

We have constructed synthetic solar spectra for the 2302–4800 cm −1 (2.08–4.34 μm) range, a spectral range where planetary objects mainly emit reflected sunlight, using ATMOS (Atmospheric Trace Molecule Spectroscopy)/Spacelab-3 and Atlas-3 spectra, of which resolution is 0.01 cm −1. We adopted Voigt line profiles for the modeling of line shapes based on an atlas of line identifications compiled by Geller [Geller, M., 1992. Key to Identification of Solar Features. A High-Resolution Atlas of the Infrared Spectrum of the Sun and the Earth Atmosphere from Space. NASA Reference Publ. 1224, vol. III. NASA, Washington, DC, pp. 1–22], who derived solar line positions and intensities from contaminated high-resolution solar spectra obtained by ATMOS/Spacelab-3. Because the ATMOS spectra in these wavelength ranges are compromised by absorption lines of molecules existing in Earth's high-altitude atmosphere and in the compartment of the spacecraft, the direct use of these high-resolution solar spectra has been inconvenient for the data reductions of planetary spectra. We compared the synthetic solar spectra with the ATMOS spectra, and obtained satisfactory fits for the majority of the solar lines with the exception of abnormal lines, which do not fit with Voigt line profiles. From the model fits, we were able to determine Voigt line parameters for the majority of solar lines; and we made a list of the abnormal lines. We also constructed telluric-line-free solar spectra by manually eliminating telluric lines from the ATMOS spectra and filling the gaps with adjacent continua. These synthetic solar spectra will be useful to eliminate solar continua from spectra of planetary objects to extract their own intrinsic spectral features.

Accurate modeling of spectral fine-structure in Earth radiance spectra measured with the Global Ozone Monitoring Experiment

We present what we believe to be a novel approach to simulating the spectral fine structure (<1 nm) in measurements of spectrometers such as the Global Ozone Monitoring Experiment (GOME). GOME measures the Earth's radiance spectra and daily solar irradiance spectra from which a reflectivity spectrum is commonly extracted. The high-frequency structures contained in such a spectrum are, apart from atmospheric absorption, caused by Raman scattering and by a shift between the solar irradiance and the Earth's radiance spectrum. Normally, an a priori high-resolution solar spectrum is used to simulate these structures. We present an alternative method in which all the required information on the solar spectrum is retrieved from the GOME measurements. We investigate two approaches for the spectral range of 390-400 nm. First, a solar spectrum is reconstructed on a fine spectral grid from the GOME solar measurement. This approach leads to undersampling errors of up to 0.5% in the modeling of the Earth's radiance spectra. Second, a combination of the solar measurement and one of the Earth's radiance measurement is used to retrieve a solar spectrum. This approach effectively removes the undersampling error and results in residuals close to the GOME measurement noise of 0.1%.

A benchmark study for CHIANTI based on RESIK solar flare spectra

Aims. To perform a benchmark analysis for the recent version of the CHIANTI atomic database (v. 5.2) based on high-resolution solar flare X-ray spectra in the range 3.4-6.1 angstrom from the RESIK crystal spectrometer on the CORONAS-F spacecraft.Methods. A C5.8 flare occurring on 2003 February 22 was chosen for analysis. RESIK spectra of this flare include emission lines of He-like and H-like K, Ar, S, and Si, with some dielectronic lines. Initially, two independent plasma diagnostic techniques are employed: an emission measure (EM) loci analysis using the line flux and the line contribution function G(T-e, N-e), and a new method based on continuum fluxes and contribution functions. We further apply a differential emission measure (DEM) analysis, from which CHIANTI synthetic spectra are derived. The continuum from RESIK spectra is checked against simultaneous RHESSI and GOES observations. Comparisons of CHIANTI synthetic spectra with those from the MEKAL code in the 3.4-6.1 angstrom range are also presented.Results. The emitting plasma appears multi-thermal, having one dominant temperature component determined independently from the line and continuum EM loci and DEM analyses. Consistency between line and continuum emissions requires photospheric elemental abundances (Asplund et al. 2005), with a depleted sulphur abundance. With the exception of RESIK channel 4 (5.0-6.1 angstrom), we find overall very good agreement between the calculated and observed intensities. From comparisons with other instruments, RESIK's precision in the continuum level is confirmed to be within the estimated 20% uncertainties in the intensity calibration. We find general agreement between CHIANTI and MEKAL isothermal spectra, but we note that the atomic data for the Si XII and Si XIII ions contained in CHIANTI are more complete.Conclusions. RESIK observations of both lines and continua are suitable for characterising the properties of the flaring plasma such as temperature, emission measure and elemental abundance. These spectra can be used to evaluate any atomic database.