Moderate Spectral Resolution Research Articles

VLTI-MATISSE L- and N-band aperture-synthesis imaging of the unclassified B[e] star FS Canis Majoris

Context. FS Canis Majoris (FS CMa, HD 45677) is an unclassified B[e] star surrounded by an inclined dust disk. The evolutionary stage of FS CMa is still debated. Perpendicular to the circumstellar disk, a bipolar outflow was detected. Infrared aperture-synthesis imaging provides us with a unique opportunity to study the disk structure. Aims. Our aim is to study the intensity distribution of the disk of FS CMa in the mid-infrared L and N bands. Methods. We performed aperture-synthesis imaging of FS CMa with the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) in the low spectral resolution mode to obtain images in the L and N bands. We computed radiative transfer models that reproduce the L- and N-band intensity distributions of the resolved disks. Results. We present L- and N-band aperture-synthesis images of FS CMa reconstructed in the wavelength bands of 3.4–3.8 and 8.6–9.0 μm. In the L-band image, the inner rim region of an inclined circumstellar disk and the central object can be seen with a spatial resolution of 2.7 milliarcsec (mas). An inner disk cavity with an angular diameter of ~6 × 12 mas is resolved. The L-band disk consists of a bright northwestern (NW) disk region and a much fainter southeastern (SE) region. The images suggest that we are looking at the bright inner wall of the NW disk rim, which is on the far side of the disk. In the N band, only the bright NW disk region is seen. In addition to deriving the inclination and the inner disk radius, fitting the reconstructed brightness distributions via radiative transfer modelling allows one to constrain the innermost disk structure, in particular the shape of theinner disk rim.

Deep Exploration of the Planets HR 8799 b, c, and d with Moderate-resolution Spectroscopy

Abstract The four directly imaged planets orbiting the star HR 8799 are an ideal laboratory to probe atmospheric physics and formation models. We present more than a decade’s worth of Keck/OSIRIS observations of these planets, which represent the most detailed look at their atmospheres to date by its resolution and signal-to-noise ratio. We present the first direct detection of HR 8799 d, the second-closest known planet to the star, at moderate spectral resolution with Keck/OSIRIS (K band; R ≈ 4000). Additionally, we uniformly analyze new and archival OSIRIS data (H and K band) of HR 8799 b, c, and d. First, we show detections of water (H2O) and carbon monoxide (CO) in the three planets and discuss the ambiguous case of methane (CH4) in the atmosphere of HR 8799 b. Then, we report radial-velocity (RV) measurements for each of the three planets. The RV measurement of HR 8799 d is consistent with predictions made assuming coplanarity and orbital stability of the HR 8799 planetary system. Finally, we perform a uniform atmospheric analysis on the OSIRIS data, published photometric points, and low-resolution spectra. We do not infer any significant deviation from the stellar value of the carbon-to-oxygen ratio (C/O) of the three planets, which therefore does not yet yield definitive information about the location or method of formation. However, constraining the C/O for all the HR 8799 planets is a milestone for any multiplanet system, and particularly important for large, widely separated gas giants with uncertain formation processes.

Improving the diameters of interferometric calibrators with MATISSE

ABSTRACT A good knowledge of the angular diameters of stars used to calibrate the observables in stellar interferometry is fundamental. As the available precision for giant stars is worse than the required per cent level, we aim to improve the knowledge of many diameters using MATISSE (Multiple AperTure mid-Infrared SpectroScopic Experiment) data in its different instrumental configurations. Using the squared visibility MATISSE observable, we compute the angular diameter value, which ensures the best-fitting curves, assuming an intensity distribution of a uniform disc. We take into account that the transfer function varies over the wavelength and is different from one instrumental configuration to another. The uncertainties on the diameters are estimated using the residual bootstrap method. Using the low spectral resolution mode in the Lband, we observed a set of 35 potential calibrators selected in the Mid-infrared stellar Diameter and Flux Compilation Catalogue with diameters ranging from about 1 to 3 mas. We reach a precision on the diameter estimates in the range 0.6 per cent to 4.1 per cent. The study of the stability of the transfer function in visibility over two nights makes us confident in our results. In addition, we identify one star, 75 Vir initially present in the calibrator lists, for which our method does not converge, and prove to be a binary star. This leads us to the conclusion that our method is actually necessary to improve the quality of the astrophysical results obtained with MATISSE, and that it can be used as a useful tool for ‘bad calibrator’ detection.

Terahertz Band-Pass Filters for Wideband Superconducting On-Chip Filter-Bank Spectrometers

A superconducting microstrip half-wavelength resonator is proposed as a suitable band-pass filter for broadband moderate spectral resolution spectroscopy for terahertz (THz) astronomy. The proposed filter geometry has a free spectral range of an octave of bandwidth without introducing spurious resonances, reaches a high coupling efficiency in the pass-band and shows very high rejection in the stop-band to minimize reflections and cross-talk with other filters. A spectrally sparse prototype filter-bank in the band 300-400 GHz has been developed employing these filters as well as an equivalent circuit model to anticipate systematic errors. The fabricated chip has been characterized in terms of frequency response, reporting an average peak coupling efficiency of 27% with an average spectral resolution of 940.

Estimate of the D/H Ratio in the Martian Upper Atmosphere from the Low Spectral Resolution Mode of MAVEN/IUVS

AbstractRecent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown unexpectedly large deuterium Lyman‐α emissions near the Martian southern summer solstice. The deuterium Lyman‐α brightness can reach ∼10% of the hydrogen Lyman‐α brightness below 200 km. In this study, we propose a method to estimate the D/H ratio in the Martian upper atmosphere through analysis of the Lyman‐α vertical profiles without the ability to spectrally separate the two atomic emission lines. Lyman‐α vertical profiles measured by MAVEN/IUVS at four periods are analyzed. During southern summer, the derived D/H ratios at 200 km are larger than the HDO/H2O ratio measured in the lower atmosphere, yet the extrapolated D/H ratio from 200 to 80 km agrees with the HDO/H2O ratio. This larger D/H ratio at higher altitude is attributed to the more efficient escape of hydrogen atoms compared to deuterium atoms. The method we describe accounts for uncertainties related to constraining the temperature and density profiles of H and D in the lower thermosphere. Because spectrally resolved D and H Lyman‐α measurements are not always available, this method provides a way to estimate D/H variations in their absence. This method will be useful for analysis of relevant data sets from past and future Mars missions.

The spatially resolved gas and dust connection in neutral inflows and outflows in nearby AGN

ABSTRACT Dusty, neutral outflows and inflows are a common feature of nearby star-forming galaxies. We characterize these flows in eight galaxies – mostly active galactic nuclei (AGN) – selected for their widespread Na i D signatures from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7). This survey employs deep, wide field-of-view (FOV) integral field spectroscopy at moderate spectral resolution (R = 7000 at Na i D). We significantly expand the sample of sightlines in external galaxies in which the spatially resolved relationship has been studied between cool, neutral gas properties – N(Na i), Weq(Na i D) – and dust – E(B − V) from both stars and gas. Our sample shows strong, significant correlations of total Weq with E(B − V)⋆ and g − i colour within individual galaxies; correlations with E(B − V)gas are present but weaker. Regressions yield slope variations from galaxy to galaxy and intrinsic scatter ∼1 Å. The sample occupies regions in the space of N(Na i) and $W_\mathrm{eq}^\mathrm{abs}$ versus E(B − V)gas that are consistent with extrapolations from other studies to higher colour excess [E(B − V)gas ∼ 1]. For perhaps the first time in external galaxies, we detect inverse P Cygni profiles in the Na i D line, presumably due to inflowing gas. Via Doppler-shifted Na i D absorption and emission lines, we find ubiquitous flows that differ from stellar rotation by $\gtrsim$100 km s−1 or have $|v_{\mathrm{ abs}} - v_{\mathrm{ em}}|\gtrsim 100$ km s−1. Inflows and outflows extend towards the edge of the detected stellar disc/FOV, together subtend 10–40 per cent of the projected disc, and have similar mean N(Na i) and Weq(Na i D). Outflows are consistent with minor axis or jet-driven flows, while inflows tend towards the projected major axis. The inflows may result from non-axisymmetric potentials, tidal motions, or halo infall.

Estimate of the D/H ratio in the Martian upper atmosphere from the low spectral resolution mode of MAVEN/IUVS

Estimate of the D/H ratio in the Martian upper atmosphere from the low spectral resolution mode of MAVEN/IUVS

Total X-Ray Emission from the LMC Observed with HaloSat

Abstract We constrain the global parameters of the Large Magellanic Cloud (LMC) by modeling the first soft X-ray (0.4–6.8 keV) observations of the entire LMC produced in a single pointing with moderate spectral resolution by the HaloSat CubeSat. These data are best fit with the sum of two thermal plasma components in collisional ionization equilibrium and a power law. We find cool (0.210 ± 0.014 keV) and hot (0.89 ± 0.14 keV) components. The total X-ray luminosity of the LMC is (1.08 ± 0.14) × 1039 erg s−1. X-ray binaries make up a large fraction of the emission with a luminosity of (6.0 ± 0.8) × 1038 erg s−1, followed by cool gas from superbubbles, supernovae, and diffuse emission with a luminosity of (3.0 ± 0.3) × 1038 erg s−1. The hot gas from star formation contributes the smallest fraction, with a luminosity of (1.9 ± 0.5) × 1038 erg s−1. We estimate the total volume of the cool gas to be between (0.2–1.2) × 1010 pc3 and the hot gas to be between (1.0–5) × 107 pc3 for filling factors of f = 1 and 0.2. These volumes result in a total thermal energy for the cool gas between (1.4–3) × 1054 erg for electron densities of 0.017–0.04 cm−3, and a thermal energy for the hot gas between (1.7–4) × 1053 erg for electron densities of 0.12–0.3 cm−3. This yields cooling timescales for the cool and hot gas of (1.5–3) × 108 yr and (1.8–6) × 107 yr, respectively.

Retrieving Land Surface Temperature From Chinese FY-3D MERSI-2 Data Using an Operational Split Window Algorithm

Medium resolution spectral imager II (MERSI-2) is a payload for the Chinese meteorological satellite Feng Yun 3 (FY-3). China's satellite remote sensing observation capabilities such as climate change research can be improved during MERSI-2's operation in orbit, and the sensor is the world's first imaging instrument that can obtain a global infrared split-window data with a spatial resolution of 250 m. We developed an operational spilt-window (SW) algorithm to retrieve land surface temperature (LST) accurately from the MERSI-2 data. The SW algorithm coefficients were derived from a simulation dataset that was established with the Moderate spectral resolution atmospheric Transmittance model version 5.2 and the thermodynamic initial guess retrieval dataset. In the practical retrieval, the precise algorithm coefficients were determined by view zenith angle and atmospheric water vapor content (WVC), the atmospheric WVC were obtained from the ERA5 dataset, and the land surface emissivity was dynamically estimated using the advanced spaceborne thermal emission and reflection radiometer global emissivity dataset, considering the fractional of vegetation cover and snow cover. The retrieved LST compared with in situ LST, which was highly consistent with the in situ LST and that the root-mean-square error of the two is within 3 K. The retrieved LST was compared with the MYD11_L2 and MYD21_L2 LST products, and the results indicated that MERSI-2 LST was more consistent with the MYD21 LST. The operational SW algorithm for FY-3D MERSI-2 developed in this study could retrieve LST accurately and has a wide range of popularization and application values.

Vicarious Radiometric Calibration of Ocean Color Bands for FY-3D/MERSI-II at Lake Qinghai, China.

To calibrate the low signal response of the ocean color (OC) bands and test the stability of the Fengyun-3D (FY-3D)/Medium Resolution Spectral Imager II (MERSI-II), an absolute radiometric calibration field test of FY-3D/MERSI-II at the Lake Qinghai Radiometric Calibration Site (RCS) was carried out in August 2018. The lake surface and atmospheric parameters were mainly measured by advanced observation instruments, and the MODerate spectral resolution atmospheric TRANsmittance algorithm and computer model (MODTRAN4.0) was used to simulate the multiple scattering radiance value at the altitude of the sensor. The results showed that the relative deviations between bands 9 and 12 are within 5.0%, while the relative deviations of bands 8, and 13 are 17.1%, and 12.0%, respectively. The precision of the calibration method was verified by calibrating the Aqua/Moderate-resolution Imaging Spectroradiometer (MODIS) and National Polar-orbiting Partnership (NPP)/Visible Infrared Imaging Radiometer (VIIRS), and the deviation of the calibration results was evaluated with the results of the Dunhuang RCS calibration and lunar calibration. The results showed that the relative deviations of NPP/VIIRS were within 7.0%, and the relative deviations of Aqua/MODIS were within 4.1% from 400 nm to 600 nm. The comparisons of three on-orbit calibration methods indicated that band 8 exhibited a large attenuation after launch and the calibration results had good consistency at the other bands except for band 13. The uncertainty value of the whole calibration system was approximately 6.3%, and the uncertainty brought by the field surface measurement reached 5.4%, which might be the main reason for the relatively large deviation of band 13. This study verifies the feasibility of the vicarious calibration method at the Lake Qinghai RCS and provides the basis and reference for the subsequent on-orbit calibration of FY-3D/MERSI-II.

Internetwork Magnetic Fields Seen in Fe i 1564.8 nm Full-disk Images

Abstract We studied the properties of internetwork magnetic fields in the solar photosphere, taking advantage of full-disk Stokes V/I maps of the Fe i 1564.8 nm line, which were obtained during 2010–2019. In contrast to most previous studies, we used data with moderate spatial and spectral resolutions. Nonetheless, we were able to distinguish the internetwork field components and the active region/network boundary components using large Zeeman splitting of the Fe i 1564.8 nm line. Thus, our analysis provides a point of view quite different from that of previous studies. We analyzed the data statistically without ordinary inversions, yet we successfully derived some properties of internetwork fields; the internetwork is filled with small-scale magnetic fields, their strength is within the weak field regime of the Fe i 1564.8 nm line (300–400 G or less), and the internetwork fields are highly inclined. Although the results were obtained from the analysis performed from a different perspective, they are consistent with the majority of previous findings. In addition, no notable variation in the properties of the internetwork fields was found during the period covering most of solar cycle 24.

Observations of Radial Velocity Variability in Stars from the Spectra Obtained with the BTA Fiber-Fed Spectrograph in High Spectral Resolution Mode

Abstract—We present the results of observations of radial velocity variability in stars with exoplanets. Observations of a number of stars with characteristic stellar magnitudes from 8.54m to 10.5m were carried out using a new high-resolution (R = 35 000–120 000) fiber-fed spectrograph mounted at the foundation of the 6-m BTA telescope, in the mode of resolutions from R = 45 000 to R = 65 000 with a simultaneous registration of the calibration spectrum of a thorium–argon hollow cathode lamp. The achieved characteristic measurement accuracies ranged from a few to several tens of m s−1. In the future, the thorium-argon lamp will be supplemented with a Fabri–Perot etalon to reach the radial velocity measurement accuracy in stars up to 1 m s−1.

Abundances from integrated spectra of 47 Tucanae (NGC 104)

ABSTRACT 47 Tucanae is among the most studied and observed globular clusters, given its proximity. The aim of this work is to study in detail the integrated spectrum of 47 Tucanae, as a template, in order to have a list of reliable lines that are validated for a moderate spectral resolution case. The spectrum of 47 Tucanae is reproduced by computing synthetic spectra, taking into account individual element abundances. The results are compared with other methods. We reproduce the integrated spectrum in the range 4500–9000 Å and derive abundances from individual lines of Na, Mg, Al, Si, Ca, Ti, Ba, and Eu. We report a list of lines that are suitable for abundance derivation. Adopting these abundances, we are able to fit the well-known triplets of Mg i and Ca ii. Finally, the effect of multiple stellar populations through enhanced Na abundances are tested. Element abundances derived are compatible with previous literature abundances, from different methods. The method applied here to build integrated synthetic spectra can be used to derive abundances from observed integrated spectra of distant and faint clusters, that are usually observed at low or moderate spectral resolution.

Spectral Deconvolution With Deep Learning: Removing the Effects of Spectral PSF Broadening

We explore novel methods of recovering the original spectral line profiles from data obtained by instruments that sample those profiles with an extended or multipeaked spectral transmission profile. The techniques are tested on data obtained at high spatial resolution from the Fast Imaging Solar Spectrograph (FISS) grating spectrograph at the Big Bear Solar Observatory and from the Interferometric Bidimensional Spectrometer (IBIS) instrument at the Dunn Solar Telescope. The method robustly deconvolves wide spectral transmission profiles for fields of view sampling a variety of solar structures (granulation, plage and pores) with a photometrical precision of less than 1%. The results and fidelity of the method are tested on data from IBIS obtained using several different spectral resolution modes. The method, based on convolutional neural networks (CNN), is extremely fast, performing about $10^5$ deconvolutions per second on a CPU and $10^6$ deconvolutions per second on NVIDIA TITAN RTX GPU for a spectrum with 40 wavelength samples. This approach is applicable for deconvolving large amounts of data from instruments with wide spectral transmission profiles, such as the Visible Tunable Filter (VTF) on the DKI Solar Telescope (DKIST). We also investigate its application to future instruments by recovering spectral line profiles obtained with a theoretical multi-peaked spectral transmission profile. We further discuss the limitations of this deconvolutional approach through the analysis of the dimensionality of the original and multiplexed data.

Global X-Ray Properties of the Vela and Puppis A Supernova Remnants

Abstract The Vela and Puppis A supernova remnants (SNRs) comprise a large emission region of ∼8° diameter in the soft X-ray sky. The HaloSat CubeSat mission provides the first soft X-ray (0.4–7 keV) observation of the entire Vela SNR and Puppis A SNR region with a single pointing and moderate spectral resolution. HaloSat observations of the Vela SNR are best fit with a two-temperature thermal plasma model consisting of a cooler component with keV in collisional ionization equilibrium and a hotter component with keV in nonequilibrium ionization. Observations of the Puppis A SNR are best fit with a single-component plane-parallel shocked plasma model with keV in nonequilibrium ionization. For the first time, we find the total X-ray luminosities of both components of the Vela SNR spectrum in the 0.5–7 keV energy band to be erg s−1 for the cooler component and erg s−1 for the hotter component. We find the total X-ray luminosities of the Vela and Puppis A SNRs to be erg s−1 and erg s−1.

Fabrication of Bismuth Absorber Arrays for NTD-Ge Hard X-ray Microcalorimeters

The high-spectral-resolution detection of hard X-rays (E > 20 keV) is a challenging and nearly unexplored area in space astrophysics. Traditionally hard X-ray detectors present moderate spectral resolutions, although few tens of eV one could open new frontiers in the study of nuclear processes and high-temperature plasma dynamics in energetic processes. This can be achieved by using cryogenic microcalorimeters. Within a research activity aimed at developing arrays of neutron transmutation-doped germanium (NTD-Ge) microcalorimeters for the high-spectral-resolution detection (about 50 eV@60 keV) of hard X-rays (20 keV 60 μm) bismuth absorber arrays. The adopted technological process and the study of related process parameters are discussed; preliminary results on produced arrays are given.

XCO<sub>2</sub> observations using satellite measurements with moderate spectral resolution: investigation using GOSAT and OCO-2 measurements

Abstract. In light of the proposed space segment of Europe's future CO2 monitoring system, we investigate the spectral resolution of the CO2 spectrometer, which measures earthshine radiance in the three relevant spectral bands at 0.76, 1.61, and 2.06 µm. The Orbiting Carbon Observatory-2 (OCO-2) mission covers these bands with fine spectral resolution but limited spatial coverage, which hampers the monitoring of localized anthropogenic CO2 emission. The future European CO2 monitoring constellation, currently undergoing feasibility studies at the European Space Agency (ESA), is targeting a moderate spectral resolution of 0.1, 0.3, and 0.3–0.55 nm in the three spectral bands with a high signal-to-noise ratio (SNR) as well as a spatial resolution of 4 km2 and an across-track swath width >250 km. This spectral and radiometric sizing is deemed to be favorable for large-swath imaging of point sources of CO2 emission. To assess this choice, we use real and synthetic OCO-2 satellite observations, which we spectrally degrade to the envisaged lower spectral resolution. We evaluate the corresponding CO2 retrieval accuracy by taking the Total Carbon Column Observing Network (TCCON) observations as reference. Here, a lower spectral resolution enhances the scatter error of the retrieved CO2 column mixing ratio (XCO2) but has little effect on the station-to-station variation in the biases. We show that the scatter error gradually increases with decreasing spectral resolution. Part of the scatter error increase can be attributed to the retrieval noise error which can be compensated for by a future instrument with improved SNR. Moreover, we consider the effect of the reduced spectral resolution on the capability to capture regional XCO2 variations and XCO2 plumes from selected OCO-2 orbits. The investigation using measurements from the Greenhouse gases Observing SATellite (GOSAT) and synthetic measurements confirms our finding and indicates that one major source of uncertainties regarding CO2 retrieval is the insufficient information on aerosol properties that can be inferred from the observations. We hence recommend the implementation of simultaneous, co-located measurements that have a larger information content on aerosols with an auxiliary instrument in the future European observing system.

Inversions of synthetic umbral flashes: a selection of wavelength sampling

Context. Imaging spectrographs are popular instruments used to obtain solar data. They record quasi-monochromatic images at selected wavelength positions. By scanning the spectral range of the line, it is possible to obtain bidimensional maps of the field-of-view with a moderate spectral resolution. Aims. In this work, we evaluate the quality of spectropolarimetric inversions obtained from various wavelength samplings during umbral flashes. Methods. We computed numerical simulations of nonlinear wave propagation in a sunspot and constructed synthetic Stokes profiles in the Ca II 8542 Å line during an umbral flash using the NLTE code NICOLE. The spectral resolution of the Stokes profiles was downgraded to various cases with differences in the wavelength coverage. A large set of wavelength samplings was analyzed and the performance of the inversions was evaluated by comparing the inferred chromospheric temperature, velocity, and magnetic field with the actual values at the chromosphere of the numerical simulation. Results. The errors in the inverted results depend to a large extent on the location of the wavelength points across the profile of the line. The inferred magnetic field improves with the increase of the spectral resolution. In the case of velocity and temperature, low spectral resolution data produce a match of the inverted atmospheres with the actual values comparable to wavelength samplings with finer resolution, while providing a higher temporal cadence in the data acquisition. Conclusions. We validated the NLTE inversions of spectropolarimetric data from the Ca II 8542 Å during umbral flashes, during which the atmosphere undergoes sudden dramatic changes due to the propagation of a shock wave. Our results favor the use of fine spectral resolution for analyses that focus on the inference of the magnetic field, whereas the estimation of temperature and velocity fluctuations can be performed with lower spectral resolution.

Aerosol direct radiative effect over clouds from a synergy of Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer (MODIS) reflectances

Abstract. The retrieval of geophysical parameters is increasingly dependent on synergistic use of satellite instruments. More sophisticated parameters can be retrieved and the accuracy of retrievals can be increased when more information is combined. In this paper, a synergistic application of Ozone Monitoring Instrument (OMI), on the Aura platform, and Moderate Resolution Imaging Spectroradiometer (MODIS), on the Aqua platform, Level 1B reflectances is described, enabling the retrieval of the aerosol direct radiative effect (DRE) over clouds using the differential aerosol absorption (DAA) technique. This technique was first developed for reflectances from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the Environmental Satellite (Envisat), which had the unique capability of measuring contiguous radiances from the ultraviolet (UV) at 240 to 1750 nm in the shortwave-infrared (SWIR), at a moderate spectral resolution of 0.2 to 1.5 nm. However, the spatial resolution and global coverage of SCIAMACHY was limited, and Envisat stopped delivering data in 2012. In order to continue the DRE data retrieval, reflectances from OMI and MODIS, flying in formation, were combined from the UV to the SWIR. This resulted in reflectances at a limited but sufficient spectral resolution, available at the OMI pixel grid, which have a much higher spatial resolution and coverage than SCIAMACHY. The combined reflectance spectra allow the retrieval of cloud microphysical parameters in the SWIR, and the subsequent retrieval of aerosol DRE over cloud scenes using the DAA technique. For liquid cloud scenes in the south-east Atlantic region with cloud fraction (CF) >0.3, the area-averaged instantaneous aerosol DRE over clouds in June to August 2006 was 25 Wm−2 with a standard deviation of 30 Wm−2. The maximum area-averaged instantaneous DRE from OMI–MODIS in August 2006 was 75.6±13 Wm−2. The new aerosol DRE over-cloud dataset from OMI–MODIS is compared to the SCIAMACHY dataset for the period 2006 to 2009, showing a very high correlation. The OMI–MODIS DRE dataset over the Atlantic Ocean is highly correlated to above-cloud AOT measurements from OMI and MODIS. It is related to AOT measurements over Ascension Island in 2016, showing the transport of smoke all the way from its source region in Africa over the Atlantic to Ascension and beyond.

Mapping impervious surface fractions using automated Fisher transformed unmixing

Spatial explicit monitoring of impervious surface fractions provides essential information for urban planning, urban disaster prevention and mitigation. Moderate spectral and spatial resolution satellite observation has become a tool to perform such monitoring across different spatial and temporal scales. Yet, one major challenge in urban remote sensing is the high spatial heterogeneity causing the presence of image pixels constituting a mixture of different land cover materials. Here we apply an automated unmixing procedure specifically designed to optimally account for the pronounced spectral variability (i.e. high within-class and low between-class variability) prevailing in urban settings. The approach is based on Fisher Discriminant Analysis (FDA), a data dimensionality reduction technique in which between-class variability is maximized and within-class variability is minimized while transforming spectra from the reflectance to the Fisher feature space. We integrated the FDA transformation in the widely-applied Multiple Endmember Spectral Mixture Analysis (MESMA) approach in an attempt to more effectively address the endmember variability problem. We made use of online spectral libraries to train the Fisher transformation parameters and steer the automated selection of image endmembers. Our Fisher transformed MESMA algorithm (F-MESMA) was tested on medium spatial resolution Landsat scenes and impervious fraction maps were generated for five major cities located in different regions of the world. The output of F-MESMA was compared to the output of state-of-the-art spectral mixture analysis approaches which were specifically designed to reduce endmember variability issues. We could demonstrate that compared to other spectral transformation approaches, the ratio of within- vs between-class variability of the endmembers was most strongly reduced after applying the Fisher transformation. As a direct consequence, across all five cities F-MESMA consistently provided the most accurate impervious surface fraction estimates (RMSEF-MESMA = 0.13 vs. RMSEalternative approaches = [0.16–0.17]).