Fraunhofer Lines Research Articles

Modeling of inelastically scattered radiation: Rotational Raman scattering in the spherical Earth’s atmosphere

This study presents an approximative approach to solve the radiative transfer equation accounting for the rotational Raman scattering in a spherical atmosphere. The solution is obtained employing the method of characteristics in combination with the discrete-ordinates technique and the forward-adjoint perturbation theory. For a non-limb observational geometry, impact of the atmospheric sphericity on the filling-in of solar Fraunhofer lines is discussed. For a limb viewing geometry, the dependence of the filling-in on the surface elevation, surface albedo, and cloudiness below the instrument line of sight as well as the effect of the multiple scattering in the spatial domain is analyzed. A comparison of modeled and measured Ring spectra for a limb-viewing geometry is shown. The considered solution technique is implemented in the radiative transfer model SCIATRAN and is ready to use for atmospheric applications.

Sodium tracer measurements of an expanded dense aluminum plasma from e-beam isochoric heating

Spatially and temporally resolved visible absorption spectroscopy is performed on sodium D-lines present as surface contaminants on an expanded dense aluminum plasma plume. An 80-ns FWHM, intense, relativistic electron beam deposits 5.4 J into a 100-μm-thick Al foil, which isochorically heats and subsequently hydrodynamically expands the material through the warm dense matter state and into a classical-like plasma state, with a coupling parameter of approximately 0.2 and a degeneracy parameter of approximately 270. The Na contamination, carried along with the expanding plume, shows saturated absorption features in the dense Al continuum for λ> 450 nm. X-ray photoelectron spectroscopy and laser-induced breakdown spectroscopy confirm Na is a surface contaminant with an atomic concentration of ∼0.1% when interrogating identical foil samples. A spectroscopic-quality radiation transport model is used to post-process 2D hydrodynamic simulations to interpret the plasma conditions based on the measured Na 3p-3s doublet line profiles. A sodium number density of 3×1015 cm−3 best matches the experimental spectra, which originate from a dense surface plasma with ne=3.0±0.8×1018 cm−3.

Retrieval of mesospheric sodium from OSIRIS nightglow measurements and comparison to ground-based Lidar measurements

The understanding of the excitation mechanism of the sodium D-line nightglow emission is important for many investigations of atomic sodium in the upper mesosphere region. In 1939, Sidney Chapman proposed a first scheme of the chemical reactions involving sodium and ozone to explain the yellow sodium line doublet in the terrestrial nightglow. Later, it became clear that this mechanism could not explain the observed variability in the Na D-line ratio. Hence, Slanger et al. (2005) suggested a modified Chapman mechanism. However, it is well accepted that there is still a parameter that is not well quantified, i.e., the branching ratio fA that defines how many sodium atoms are produced (via the reaction Na + O3) in the excited Na(P2J) state. Here, we retrieve sodium profiles in the mesosphere region from limb measurements carried out with the OSIRIS instrument on the Odin satellite between 2002 and until recently. These profiles are compared to profiles obtained from ground-based Lidar measurements. Until 2010, the Lidar was located in Fort Collins, Colorado at 40.5° N and 105.0° W and was then moved to Logan, Utah at 41.8° N and 111.2° W. In this study, we found 20 nights with measurements suitable for comparison. By adjusting the branching ratio in the retrieval algorithm until the Lidar Na-profile and the satellite Na-profile agree best and combining the three different approaches for the comparison we propose a value of the branching ratio of: 0.064±0.028.

Development of a passive optical heterodyne radiometer for near and mid-infrared spectroscopy.

We have developed a novel laser heterodyne radiometer using a fiber-coupled distributed feedback laser as the local oscillator to perform spectroscopic measurements of small molecules in the near-infrared (NIR) spectral region. Here, we demonstrate measurement of HCN and CO2 in the lab and CH4 and CO2 in the atmospheric column. In addition, we demonstrate detection of a neutral iron, Fe(I), Fraunhofer line in the spectrum of the sun, at a vacuum wavelength of 1559.252 nm, that can be used to calibrate the wavelength scale of the instrument and enable verification of proper system operation for field applications.

16.9 MW, efficient 486.1 nm blue optical parametric oscillator using single BBO crystal

A single-resonant beta barium borate crystal optical parametric oscillator with high pulse energy output at the H-β Fraunhofer line of 486.1 nm was demonstrated. Pumped by a single-frequency 355 nm ultraviolet pulsed laser at 10 Hz repetition rate with maximum pulse energy of 411 mJ, maximum output energy of 162 mJ blue laser pulse at 486.1 nm was successfully obtained from a plano-concave cavity with a 40% output coupler, the corresponding optical-to-optical conversion efficiency was up to 39.4%, and the pulse width was around 9.6 ns. The blue laser beam size was 4.4 mm × 5.1 mm with the beam divergence angles of θx = 2.2 mrad and θy = 2.5 mrad. The peak power of output blue laser was 16.9 MW, and the signal linewidth was around 0.34 nm.

Compact wavelength tunable output around 440 nm pulsed laser for oceanic lidar application

A single-resonant BBO-based optical parametric oscillator (OPO) with 27 nm tuning range in deep-blue spectral band around 440 nm was developed. Pumped by a 355 nm ultraviolet laser with 4.9 mJ pulse energy at 100 Hz repetition rate, maximum pulse energy of 1.32 mJ at 441.4 nm wavelength, corresponding to more than 0.26 MW peak power, was successfully obtained from a plano-concave OPO cavity with an about 35% output coupler, the optical–optical​ conversion efficiency was up to 26.9%. The pulse width was around 5.0 ns, and the spectral linewidth was about 0.09 nm. When the output wavelength was tuned to the Fraunhofer lines of 440.5 nm and 438.4 nm, the pulse energy of 1.27 mJ and 1.15 mJ was obtained respectively with linewidth of both about 0.07 nm, well-matched to the corresponding Fraunhofer linewidth.

Diurnal variation of sun-induced chlorophyll fluorescence of agricultural crops observed from a point-based spectrometer on a UAV

Unmanned Aerial Vehicle (UAV)-based measurements allow studying sun-induced chlorophyll fluorescence (SIF) at the field scale and can potentially upscale results from ground to airborne/satellite level. The objective of this paper is to present the FluorSpec system providing SIF measurements at the field level onboard a UAV, and to evaluate the potential of this system for understanding diurnal SIF patterns for different arable crops. The core components of FluorSpec are a point spectrometer configured to measure in the O2 absorption bands at sub-nanometer resolution, bifurcated fibre optics to switch between the downwelling irradiance and upwelling radiance measurements, and a laser range finder allowing accurate atmospheric correction. The processing chain is explained and the capability of the novel Spectral Shape Assumption Fraunhofer Line Discrimination (SSA-FLD) method to retrieve SIF was tested. To test the reliability of FluorSpec diurnal SIF measurements, near-canopy diurnal SIF was monitored during the growing season over potato and sugar beet plants with a ground-based setup. The two crops exhibited a clear diurnal SIF pattern, which positively correlated with the photosynthetically active radiation (PAR). The divergence in diurnal patterns between SIF and PAR indicated that the crops might be suffering from heat stress. A significant correlation between SIF and the Photosystem II Quantum Yield was obtained. By mounting the FluorSpec on a UAV, SIF measurements were obtained over the same crops during a clear day. UAV-based SIF also exhibited a pronounced diurnal pattern similar to the ground-based measurements and it showed clear spatial variation within different crop fields. The obtained results demonstrate the ability of the FluorSpec system to reliably measure plant fluorescence at ground and field level, and the possibility of the UAV-based FluorSpec to bridge the scale gap between different levels of SIF observations.

Density, speed of sound, refractive index and relative permittivity of methanol, propan-1-ol or pentan-1-ol + aniline liquid mixtures. Application of the Kirkwood-Fröhlich model

Densities (ρ) and speeds of sound (c) at a temperature T = 298.15 K, relative permittivities at 1 MHz (εr) and refractive indices at the sodium D-line (nD) at T = (293.15 K to 303.15 K), all of them at a pressure p = 0.1 MPa, are reported for binary liquid mixtures alkan-1-ol + aniline. The alkan-1-ols considered are methanol, propan-1-ol and pentan-1-ol. Also, the values of the excess molar volume (VmE), excess isentropic compressibility (κSE), excess speed of sound (cE), excess refractive index (nDE), excess relative permittivity (εrE) and its temperature derivative (∂εrE/∂T)p are calculated and fitted to Redlich-Kister polynomials. The agreement among the reported data and other literature sources is analysed by comparing VmE, nDE, εrE and the deviation of c from mole-fraction linearity (Δc). The positive excess molar internal energies at constant volume (Um, VE) show the dominance of the breaking of interactions between like molecules in the energy balance on mixing, particularly the breaking of strong dipolar interactions between aniline molecules. This contribution is also dominant for the εrE values, as they are negative and decrease with the length of the alkan-1-ol chain. Calculations on the concentration-concentration structure factor are consistent with these statements, revealing homocoordination in the studied systems. The VmE are negative, which together with the positive Um, VE indicate the existence of important structural effects in the studied mixtures. The application of the Kirkwood-Fröhlich model shows that the average relative orientation of neighbouring dipoles is similar in the mixtures methanol + aniline or + pyridine, in spite of the different character of the predominant interactions in the latter mixture (heterocoordination).

Passive optical heterodyne spectroscopy measurement of the Doppler shift of Fe(I) lines induced by the rotational velocity of the Sun

We have developed a sensitive, novel laser heterodyne radiometer using a fiber-coupled distributed feedback laser as the local oscillator to measure Fe(I) Fraunhofer lines Doppler-shifted by the rotational motion of the Sun. The radiometer repetitively measures the Doppler shift of an Fe(I) Fraunhofer line at a vacuum wavelength of 1559.252 nm induced by the Sun’s rotation on its eastern and western limbs. The measured shift is slightly smaller than expected as a result of the simple solar tracking system and collection optics used for demonstration. We show through calculation that a multichannel instrument of identical design in conjunction with a Keck-class telescope can be used to search for exoplanets in nearby star systems using the star-wobble radial velocity method with 1 − m e t e r s e c velocity resolution.

NDVI and Fluorescence Indicators of Seasonal and Structural Changes in a Tropical Forest Succession

In this research, the Sun-induced Chlorophyll Fluorescence (SIF) signal was derived from field-based spectrometers and Hyperion satellite imagery of Hong Kong’s Country Parks by applying the Fraunhofer Line Difference (FLD) method. Due to high levels of atmospheric aerosols and water vapour in the study area, a modified FLD method (FLD-M) was tested for its ability to deliver improved SIF estimates by cancelling out the effects of atmospheric scattering. The SIF results were compared with the Normalised Difference Vegetation Index (NDVI), and analysed according to seasonality, five successional age groups of forest and monoculture exotic plantations. Results indicate that SIF methods are more sensitive to seasonal phenology and diurnal fluctuations than the NDVI, indicating its greater sensitivity to photosynthetic activity. The SIF responds earlier and stronger, to senescence in winter and green-up in summer. Field spectrometer data showed NDVI to be unresponsive to time of day, whereas fluorescence responds to changes in sunlight intensity from 10:15 a.m. to 02:00 p.m. Both seasonal and diurnal results indicate that SIF is better than NDVI in representing the subtle changes in vegetation conditions. Although both NDVI and SIF distinguish between the four woody structural stages of vegetation, only SIF can distinguish between forest and exotic plantations, the difference being greater in the dry season. The study provides an improved operational remote sensing methodology for investigating the health status of tropical secondary forest, where atmospheric turbidity is high.

Systematic Assessment of Retrieval Methods for Canopy Far‐Red Solar‐Induced Chlorophyll Fluorescence Using High‐Frequency Automated Field Spectroscopy

AbstractRemote sensing of solar‐induced chlorophyll fluorescence (SIF) offers potential to infer photosynthesis across scales and biomes. Many retrieval methods have been developed to estimate top‐of‐canopy SIF using ground‐based spectroscopy. However, inconsistencies among methods may confound interpretation of SIF dynamics, eco‐physiological/environmental drivers, and its relationship with photosynthesis. Using high temporal‐ and spectral resolution ground‐based spectroscopy, we aimed to (1) evaluate performance of SIF retrieval methods under diverse sky conditions using continuous field measurements; (2) assess method sensitivity to fluctuating light, reflectance, and fluorescence emission spectra; and (3) inform users for optimal ground‐based SIF retrieval. Analysis included field measurements from bi‐hemispherical and hemispherical‐conical systems and synthetic upwelling radiance constructed from measured downwelling radiance, simulated reflectance, and simulated fluorescence for benchmarking. Fraunhofer‐based differential optical absorption spectroscopy (DOAS) and singular vector decomposition (SVD) retrievals exhibit convergent SIF‐PAR relationships and diurnal consistency across different sky conditions, while O2A‐based spectral fitting method (SFM), SVD, and modified Fraunhofer line discrimination (3FLD) exhibit divergent SIF‐PAR relationships across sky conditions. Such behavior holds across system configurations, though hemispherical‐conical systems diverge less across sky conditions. O2A retrieval accuracy, influenced by atmospheric distortion, improves with a narrower fitting window and when training SVD with temporally local spectra. This may impact SIF‐photosynthesis relationships interpreted by previous studies using O2A‐based retrievals with standard (759–767.76 nm) fitting windows. Fraunhofer‐based retrievals resist atmospheric impacts but are noisier and more sensitive to assumed SIF spectral shape than O2A‐based retrievals. We recommend SVD or SFM using reduced fitting window (759.5–761.5 nm) for robust far‐red SIF retrievals across sky conditions.

Novel Remote Sensing Index of Electron Transport Rate Predicts Primary Production and Crop Health in L. sativa and Z. mays

Photosynthesis performance can be assessed quantitatively with light response curves. These curves record the Electron Transport Rate (ETR) as a function of light intensity. Then, statistical fit on these curves parameterize light use efficiency, maximum photosynthetic activity and the reaction of the apparatus to stress. While this technique is performed with portable fluorometers in field conditions, it is difficult to scale it to the canopy level. The Fraunhofer line discrimination technique, which detects fluorescence signals emitted during photosynthesis, is a promising method to assess photosynthetic performance of canopies. In this study, we define a remote sensing ETR index based on a combination of three parameters: sun-induced fluorescence, normalized differential vegetation index and light intensity. Two representatives of C3 and C4 photosynthesis, L. sativa and Z. mays, experienced a fertilization concentrations gradient. ETR increased with light intensity in both crops. In L. sativa, ETR assumed a linear relationship between the photosynthetic activity and light intensity, with a correlation of R2 = 0.99 to the portable fluorometer. Additional parametrization revealed a resilience of its reaction centers to photoinhibition in maximum light intensities. When Z. mays experienced open field conditions, ETR correlated with the plant’s status. While the results of this study are promising, the index still requires validation in terms of temporal track and spatial variability.

First detection of two superoutbursts during the rebrightening phase of a WZ Sge-type dwarf nova: TCP J21040470+4631129

Abstract We report on photometric and spectroscopic observations and analysis of the 2019 superoutburst of TCP J21040470+4631129. This object showed a 9 mag superoutburst with early superhumps and ordinary superhumps, which are the features of WZ Sge-type dwarf novae. Five rebrightenings were observed after the main superoutburst. The spectra during the post-superoutburst stage showed Balmer, He i, and possible sodium doublet features. The mass ratio is derived as 0.0880(9) from the period of the superhump. During the third and fifth rebrightenings, growing superhumps and superoutbursts were observed, which have never been detected during a rebrightening phase among WZ Sge-type dwarf novae with multiple rebrightenings. To induce a superoutburst during the brightening phase, the accretion disk needs to have expanded beyond the 3 : 1 resonance radius of the system again after the main superoutburst. These peculiar phenomena can be explained by the enhanced viscosity and large radius of the accretion disk suggested by the higher luminosity and the presence of late-stage superhumps during the post-superoutburst stage, plus by more mass supply from the cool mass reservoir and/or from the secondary because of the enhanced mass transfer than those of other WZ Sge-type dwarf novae.

First Year On-Orbit Calibration of the Chinese Environmental Trace Gas Monitoring Instrument Onboard GaoFen-5

Environmental trace gas monitoring instrument (EMI) onboard GaoFen-5 was launched in May 2018 and has successfully operated on-orbit for more than a year. EMI contains four grating spectrometers, covering wavelengths in the range 240–710 nm with a spectral resolution of 0.3–0.5 nm, and enables one-day global coverage. For EMI on-orbit calibration, two onboard solar diffusers (SD), one surface reflectance aluminum diffuser, and one quartz volume diffuser (QVD) are used to measure solar spectra. The solar spectra are used to perform accurate spectral and radiometric calibrations. EMI on-orbit spectral calibration contains wavelength calibration and instrument spectral response function (ISRF), both of which are the key quantities in trace gas retrievals based on differential optical absorption spectroscopy (DOAS) analysis. The wavelength calibration is performed using the Fraunhofer lines in the solar spectrum, and the ISRF parameters are obtained by fitting high-resolution and EMI-measured solar spectra. Based on the known solar irradiation and characteristic of SD, SD radiance can be calculated and is used for EMI on-orbit radiometric calibration. The radiometric calibration also determines the absolute Earth reflectance spectra that are used as inputs for atmospheric retrieval algorithms. For EMI on-orbit radiometric monitoring, aluminum diffuser is used as reference SD to monitor QVD degradation. An internal white light source is used to detect pixel performance and monitor radiometric throughput.

Obtención de espectros usando un smartphone en la clase de Física

This article presents the basics for converting a smartphone as a home spectrometer. Using a Compact Disc Digital Audio and a properly located slot, the emission spectrum of a compact fluorescent lamp and other sources can be observed. This work includes the methodological steps to measure the Fraunhofer's lines of the sun with the greatest spectral width. The proposal is designed to carry out a practical class of light spectra both from institutions of secondary education as well as in introductory courses in university physics.

Theoretical and experimental results of optically pumped sodium atoms

Optical pumping is a process that produces a nonthermal Zeeman sublevel population distribution. It plays an important role in alkali-metal magnetometers for two uses, constructing the sub-Doppler features to lock the laser frequency and providing the spin-polarized atoms to precess around the magnetic field. To comprehensively study the properties of sodium (Na) magnetometer, we present the detailed theoretical analysis of the saturated absorption spectroscopy (SAS) and the magneto-optical resonance signal in sodium D2 line. Accordingly, we have implemented two experimental schemes, the counter propagating pump-probe SAS of thermal Na vapor and the remote sensing Bell-Bloom magnetometry based on the Larmor precession frequency. In the first experiment, sub-Doppler features of D2 consist of two velocity selective optical pumping (VSOP) resonances located at the D2a and D2b transitions and a crossover (CO) resonance located between the VSOP resonances are observed. In the second experiment, an estimated intrinsic magnetometric sensitivity of δBint=286 pT in a 1 Hz bandwidth, is achieved by demodulating the recorded fluorescence signal of the resultant magneto-optical resonance near the 300 kHz Larmor precession frequency. Both kinds of experimental results are in good agreement with our theoretical calculations.

480-nm distributed-feedback InGaN laser diode for 10.5-Gbit/s visible-light communication.

In this Letter, we demonstrate a novel distributed-feedback (DFB) InGaN-based laser diode with narrow-linewidth emission at ∼480nm (sky blue) and its application to high-speed visible-light communication (VLC). A significant side-mode suppression ratio (SMSR) of 42.4 dB, an optical power of ∼14mW, and a resolution-limited linewidth of ∼34pm were obtained under continuous-wave operation. A 5-Gbit/s VLC link was realized using non-return-to-zero on-off keying modulation, whereas a high-speed 10.5-Gbit/s VLC data rate was achieved by using a spectral-efficient 16-quadrature-amplitude-modulation orthogonal frequency-division multiplexing scheme. The reported high-performance sky-blue DFB laser is promising in enabling unexplored dense wavelength-division multiplexing schemes in VLC, narrow-line filtered systems, and other applications where single-frequency lasers are essential such as atomic clocks, high-resolution sensors, and spectroscopy. Single-frequency emitters at the sky-blue wavelength range will further benefit applications in the low-path-loss window of underwater media as well as those operating at the H-beta Fraunhofer line at ∼486nm.

Pressure and temperature induced red-shift of the sodium D-line during HMX deflagration

The sodium D-line is often present in optical spectra of combustion due to its high prevalence and emissivity. Collision theory predicts the spectral peak to have a red-shift dependent on pressure, P, and temperature, T. Here we show that the conditions reached during deflagration of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) permit the red-shift of the sodium D-line to be calibrated to 1.5 GPa. Deflagration at these pressures is achieved using a split Hopkinson pressure bar apparatus, with temperatures of circa 2900 K from the greybody continuum away from spectral features. Lower deflagration pressures, of 0.5 to 0.9 GPa, are achieved in a fallhammer test, with temperatures of circa 4000 K. The red-shift exhibits the predicted PT−0.7 dependence with a constant of proportionality of (950 ± 30) GPa-1 · K0.7 · nm. Using the serendipitous presence of sodium, this optical technique allows for fast measurements of both pressure and temperature from the same light source in one measurement.

Wind of change: retrieving exoplanet atmospheric winds from high-resolution spectroscopy

Context. The atmosphere of exoplanets has been studied extensively in recent years, making use of numerical models to retrieve chemical composition, dynamical circulation, or temperature from the data. One of the best observational probes in transmission is the sodium doublet thanks to its extensive cross-section. However, modelling the shape of planetary sodium lines has proven to be challenging. Models with different assumptions regarding the atmosphere have been employed to fit the lines in the literature, yet statistically-sound, direct comparisons of different models are needed to paint a clear picture. Aims. We aim to compare different wind and temperature patterns, as well as to provide a tool to distinguish them based on their best fit for the sodium transmission spectrum of the hot Jupiter HD 189733b. We parametrise different possible wind patterns that have already been tested the in literature and introduce the new option of an upwards-driven vertical wind. Methods. We construct a forward model where the wind speed, wind geometry, and temperature are injected into the calculation of the transmission spectrum. We embed this forward model in a nested sampling retrieval code to rank the models via their Bayesian evidence. Results. We retrieve a best-fit to the HD 189733b data for vertical upward winds |vver(mean)| = 40 ± 4 km s−1 at altitudes above 10−6 bar. With the current data from HARPS, we cannot distinguish wind patterns for higher-pressure atmospheric layers. Conclusions. We show that vertical upwards winds in the upper atmosphere provide a possible explanation for the broad sodium signature in hot Jupiters. We highlight other influences on the width of the doublet and we explore strong magnetic fields acting on the lower atmosphere as one possible origin of the retrieved wind speed.

New deep coronal spectra from the 2017 total solar eclipse

Context. The origin of the high temperature of the solar corona, in both the inner bright parts and the more outer parts showing flows toward the solar wind, is not understood well yet. Total eclipses permit a deep analysis of both the inner and the outer parts of the corona using the continuum white-light (W-L) radiations from electrons (K-corona), the superposed spectrum of forbidden emission lines from ions (E-corona), and the dust component with F-lines (F-corona). Aims. By sufficiently dispersing the W-L spectrum, the Fraunhofer (F) spectrum of the dust component of the corona appears and the continuum Thomson radiation can be evaluated. The superposed emission lines of ions with different degrees of ionization are studied to allow the measurement of temperatures, non-thermal velocities, Doppler shifts, and abundances to constrain the proposed heating mechanisms and understand the origin of flows that lead to solar wind. Methods. We describe a slit spectroscopic experiment of high spectral resolution to provide an analysis of the most typical parts of the quasi-minimum type corona observed during the total solar eclipse of Aug. 21, 2017 from Idaho, USA. Streamers, active region enhancements, and polar coronal holes (CHs) are measured well using deep spectra. Results. Sixty spectra are obtained during the totality with a long slit, covering ±3 solar radii in the range of 510 nm to 590 nm. The K+F continuum corona is exposed well up to two solar radii. The F-corona can be measured even at the solar limb. New weak emission lines were discovered or confirmed. The rarely observed Ar X line is detected almost everywhere; the Fe XIV and Ni XIII lines are clearly detected everywhere. For the first time hot lines are also measured inside the CH regions. The radial variations of the non-thermal turbulent velocities of the lines do not show a great departure from the average values. No significantly large Doppler shifts are seen anywhere in the inner or the middle corona. The wings of the Fe XIV line show some non-Gaussianity. Conclusions. Deep slit coronal spectra offered an opportunity for diagnosing several aspects of coronal physics during a well observed total eclipse without extended investments. The analysis of the ionic emission line profiles offers several powerful diagnostics of the coronal dynamics; the precise measurement of the F-continuum component provides insight into the ubiquitous dust corona at the solar limb.