Large Optical Thickness Research Articles

STRUCTURE OF PROMINENCE LEGS: PLASMA AND MAGNETIC FIELD

ABSTRACT We investigate the properties of a “solar tornado” observed on 2014 July 15, and aim to link the behavior of the plasma to the internal magnetic field structure of the associated prominence. We made multi-wavelength observations with high spatial resolution and high cadence using SDO/AIA, the Interface Region Imaging Spectrograph (IRIS) spectrograph, and the Hinode/Solar Optical Telescope (SOT) instrument. Along with spectropolarimetry provided by the Télescope Héliographique pour l’Etude du Magnétisme et des Instabilités Solaires telescope we have coverage of both optically thick emission lines and magnetic field information. AIA reveals that the two legs of the prominence are strongly absorbing structures which look like they are rotating, or oscillating in the plane of the sky. The two prominence legs, which are both very bright in Ca ii (SOT), are not visible in the IRIS Mg ii slit-jaw images. This is explained by the large optical thickness of the structures in Mg ii, which leads to reversed profiles, and hence to lower integrated intensities at these locations than in the surroundings. Using lines formed at temperatures lower than 1 MK, we measure relatively low Doppler shifts on the order of ±10 km s−1 in the tornado-like structure. Between the two legs we see loops in Mg ii, with material flowing from one leg to the other, as well as counterstreaming. It is difficult to interpret our data as showing two rotating, vertical structures that are unrelated to the loops. This kind of “tornado” scenario does not fit with our observations. The magnetic field in the two legs of the prominence is found to be preferentially horizontal.

Intensity correlations between reflected and transmitted speckle patterns

We study theoretically the spatial correlations between the intensities measured at the input and output planes of a disordered scattering medium. We show that at large optical thicknesses, a long-range spatial correlation persists and takes negative values. For small optical thicknesses, short-range and long-range correlations coexist, with relative weights that depend on the optical thickness. These results may have direct implications for the control of wave transmission through complex media by wavefront shaping, thus finding applications in sensing, imaging and information transfer.

Influence of aspect ratio on the light scattering properties of spherical aerosol particles

The influence of aspect ratio on the light scattering properties of ellipsoidal particles is studied by using T-matrix method and discrete ordinate method in this paper. The light scattering characteristic quantities including extinction efficiency, asymmetrical parameter, single scattering albedo, scattering phase matrix, and bidirectional reflectance distribution function (BRDF) are computed. It is found that light scattering properties of ellipsoidal particles are sensitive to aspect ratio. The aspect ratio can influence the oscillation frequencies and amplitudes of the scattering parameters except some special size parameters. The value of asymmetrical factor could be as large as 0.3 in wave crest value of size parameter while it is no more than 0.1 at the balance location. As for the multiple scattering, the characteristics of BRDF for different aspect ratios in different incident angles and optical thickness values are analyzed, and for a further study, the relative differences of BRDF influenced by aspect ratio, optical thickness, and incident angle are analyzed. The results show that the variation trends of BRDF for the ellipsoidal particles with various aspect ratios are basically the same. However, the curve of BRDF of spherical particles (i.e., with their aspect ratios being 1) is more variable. With the increasing of aerosol optical thickness and aspect ratio, the curves of BRDF at different aspect ratios for ellipsoidal particles tend to a steady and similar value and the relative difference of BRDF decreases. But with the increasing of aerosol optical depth, the relative difference of BRDF increases with the increasing of incident angles, especially in large optical thickness, the value of the relative difference of BRDF can be as large as 15%.

Three-dimensional Monte Carlo calculation of atmospheric thermal heating rates

We present a fast Monte Carlo method for thermal heating and cooling rates in three-dimensional atmospheres. These heating/cooling rates are relevant particularly in broken cloud fields. We compare forward and backward photon tracing methods and present new variance reduction methods to speed up the calculations. For this application it turns out that backward tracing is in most cases superior to forward tracing. Since heating rates may be either calculated as the difference between emitted and absorbed power per volume or alternatively from the divergence of the net flux, both approaches have been tested. We found that the absorption/emission method is superior (with respect to computational time for a given uncertainty) if the optical thickness of the grid box under consideration is smaller than about 5 while the net flux divergence may be considerably faster for larger optical thickness. In particular, we describe the following three backward tracing methods: the first and most simple method (EMABS) is based on a random emission of photons in the grid box of interest and a simple backward tracing. Since only those photons which cross the grid box boundaries contribute to the heating rate, this approach behaves poorly for large optical thicknesses which are common in the thermal spectral range. For this reason, the second method (EMABS_OPT) uses a variance reduction technique to improve the distribution of the photons in a way that more photons are started close to the grid box edges and thus contribute to the result which reduces the uncertainty. The third method (DENET) uses the flux divergence approach where – in backward Monte Carlo – all photons contribute to the result, but in particular for small optical thickness the noise becomes large. The three methods have been implemented in MYSTIC (Monte Carlo code for the phYSically correct Tracing of photons In Cloudy atmospheres). All methods are shown to agree within the photon noise with each other and with a discrete ordinate code for a one-dimensional case. Finally a hybrid method is built using a combination of EMABS_OPT and DENET, and application examples are shown. It should be noted that for this application, only little improvement is gained by EMABS_OPT compared to EMABS.

One-Dimensional Nonequilibrium Radiation-Transport Equation under Diffusion Approximation and Its Discrete Scheme

Based on the nonlocal thermodynamic equilibrium state and large optical thickness of plasma, we establish one-dimensional nonequilibrium radiation-transport equation from diffusion approximation. Through finite volume method, the discrete scheme of radiation-transport equation and the conditions for its definite solution are proposed. The reliability of radiation-transport equation and its discrete scheme is validated.

Soot temperature and volume fraction retrieval from spectrally resolved flame emission measurement in laminar axisymmetric coflow diffusion flames: Effect of self-absorption

The effects of self-absorption on the inverted soot temperature and volume fraction distributions in coflow laminar diffusion flames were investigated using two correction methods. Self-absorption was found unimportant when the optical thickness of the flame is less than 0.3, but can be very important in flames of large optical thicknesses. Due to the uncertainty in soot absorption function E(m), different assumptions of the variation of E(m) with wavelength were investigated. Decreasing E(m) with wavelength leads to largest errors in the recovered soot temperature and volume fraction when self-absorption correction is not accounted for. The assumption of constant E(m) in general results in smallest errors in the inverted soot volume fraction and the increasing E(m) with wavelength gives rise to smallest errors in the recovered soot temperature. Neglect of self-absorption correction in general leads to underestimate of the inverted soot temperature, while the inverted soot volume fraction can be higher or lower than the true value, depending on how E(m) varies with wavelength. The self-absorption correction method of Snelling et al. [D.R. Snelling, K.A. Thomson, G.J. Smallwood, Ö.L. Gülder, E.J. Weckman, R.A. Fraser, AIAA J. 40(9) (2002) 1789–1795] is effective in recovering accurate soot temperature and volume fraction distributions from noise-free emission data even for flames with optical thickness as large as 5. For inversion of emission data containing noise it is still advantageous to account for self-absorption. Multi-wavelength emission data from a laminar coflow ethylene/air diffusion flame at atmospheric pressure and a methane/air diffusion flame at 40atm were reanalyzed to demonstrate how self-absorption affects the reconstructed temperature and soot volume fraction distributions. It is preferred to detect flame emissions at longer wavelengths in the visible spectrum to minimize the self-absorption effect. The effect of self-absorption should in general be taken into account in the reconstruction of soot temperature and volume fraction from the line-of-sight spectral flame emission data using the method of Snelling et al.

Effective Bandgap Lowering of CdS Deposited by Successive Ionic Layer Adsorption and Reaction

SILAR (successive ionic layer adsorption and reaction) is a solution process used to deposit semiconductors that has become very common in the past few years as a method to deposit light absorbers in nanoporous solar cells. Films of CdS (possibly the most commonly deposited semiconductor using this method) often show an anomalously low apparent bandgap (lowered by as much as 10%) for sufficiently thick films, although this effect has been ignored in most cases. Here, we study this bandgap lowering and show that it is due not so much to a lower bandgap but rather to a particularly long absorption tail that extends far into the red, and that is amplified by a large optical thickness in the high surface area nanoporous films. The tail is presumably due to as yet unidentified, but probably bulk defects in the CdS. Additionally, the absorption coefficient of the SILAR CdS was nearly twice as high as normal values.

Time resolved three-dimensional acousto-optic imaging of thick scattering media

Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound-modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate three-dimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom.

Parameters of shock waves with emission in gases

Parameters of emitting shock waves in gases are investigated in the limiting case when there is no screening of emission from the shock front by the precursory layer. The one-dimensional quasi-steady-state formulation of the problem with deceleration of high-speed gas flow against a plane fixed obstacle under conditions of strong emission is given. The case of the shock waves of large optical thickness is analytically considered over a wide range of variation of the obstacle reflectivity. The parameters of emitting shock waves generated in experiments in shock tubes in the inert argon gas are estimated using the methods developed and compared with the measurement results. The shock “adiabats” of optically thick shock waves are considered with allowance for the radiation energy losses. The calculations are carried out for aluminium plasma.

Combined retrievals of boreal forest fire aerosol properties with a polarimeter and lidar

Abstract. Absorbing aerosols play an important, but uncertain, role in the global climate. Much of this uncertainty is due to a lack of adequate aerosol measurements. While great strides have been made in observational capability in the previous years and decades, it has become increasingly apparent that this development must continue. Scanning polarimeters have been designed to help resolve this issue by making accurate, multi-spectral, multi-angle polarized observations. This work involves the use of the Research Scanning Polarimeter (RSP). The RSP was designed as the airborne prototype for the Aerosol Polarimetery Sensor (APS), which was due to be launched as part of the (ultimately failed) NASA Glory mission. Field observations with the RSP, however, have established that simultaneous retrievals of aerosol absorption and vertical distribution over bright land surfaces are quite uncertain. We test a merger of RSP and High Spectral Resolution Lidar (HSRL) data with observations of boreal forest fire smoke, collected during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS). During ARCTAS, the RSP and HSRL instruments were mounted on the same aircraft, and validation data were provided by instruments on an aircraft flying a coordinated flight pattern. We found that the lidar data did indeed improve aerosol retrievals using an optimal estimation method, although not primarily because of the contraints imposed on the aerosol vertical distribution. The more useful piece of information from the HSRL was the total column aerosol optical depth, which was used to select the initial value (optimization starting point) of the aerosol number concentration. When ground based sun photometer network climatologies of number concentration were used as an initial value, we found that roughly half of the retrievals had unrealistic sizes and imaginary indices, even though the retrieved spectral optical depths agreed within uncertainties to independent observations. The convergence to an unrealistic local minimum by the optimal estimator is related to the relatively low sensitivity to particles smaller than 0.1 (μm) at large optical thicknesses. Thus, optimization algorithms used for operational aerosol retrievals of the fine mode size distribution, when the total optical depth is large, will require initial values generated from table look-ups that exclude unrealistic size/complex index mixtures. External constraints from lidar on initial values used in the optimal estimation methods will also be valuable in reducing the likelihood of obtaining spurious retrievals.

AEROSOL PROPERTIES IN MOSCOW ACCORDING TO 10 YEARS OF AERONET MEASUREMENTS AT THE METEOROLOGICAL OBSERVATORY OF MOSCOW STATE UNIVERSITY

Different microphysical, optical and radiative properties of aerosol were analyzed according to the 10 years of measurements (2001–2010) at the Meteorological Observatory of Moscow State University within the framework of international AERONET program. Volume aerosol size distribution was shown to have a bimodal character with dominating the fine mode aerosol particles at effective radius of r eff-fine =0.15 µm. In smoke conditions reff-fine was shown to increase to 0.25 µm at extremely large aerosol optical thickness (AOT). Real and imaginary parts of refractive index are characterized by REFR=1.45, REFI=0.01 respectively, changing to REFR=1.49, REFI=0.006 for smoke aerosol. AOT seasonal changes are characterized by the increase towards warm period with a local minimum in June. The joint analysis of aerosol characteristics with the NOAA_NCEP_CPC_CAMS_OPI climatology shows the nature of these changes. For typical conditions aerosol single scattering albedo (SSA) is about 0.9 at 675 nm and is characterized by a distinct decrease with wavelength while in forest fires conditions it is significantly higher (SSA=0.95). The interaction between volume aerosol concentration of different aerosol fractions was obtained with a distinct decrease of variation towards large aerosol content.

Estimating soot volume fraction and temperature in flames using stochastic particle swarm optimization algorithm

A simulation investigation for simultaneous reconstruction of distributions of temperature and soot volume fraction from multi-wavelength emission in a sooting flame using the stochastic particle swarm optimizer (PSO) algorithm is presented. The self-absorption of the flame is considered. The selection of parameters of the stochastic PSO algorithm and detection wavelengths is analyzed. The effects of measurement errors and optical thickness of the flame on the accuracy of the reconstruction are investigated. It proved that the stochastic PSO algorithm is robust and can obtain accurate distributions of temperature and soot volume fraction from line-of-sight intensities in only several wavelengths, especially in the flame with large optical thickness, while other methods neglecting self-attenuation of the flame will take some errors.

A new algorithm for the retrieval of atmospheric diffuse transmittance in Moderate Resolution Imaging Spectroradiometer (MODIS) atmospheric correction: a study in the Bohai Sea

Based on 183 groups of in situ data measured with a spectroradiometer during 2003–2007 in the Bohai Sea, we calculated the atmospheric direct transmittance T, diffuse transmittance t, Ångström exponent α and turbidity coefficient β of aerosol optical thickness, and aerosol multiple forward scattering factor B a. An empirical formula relating B a with the two Ångström parameters, α and β, is established. The determination coefficient between aerosol multiple forward scattering factors, B a, estimated by using the above empirical formula and those measured by using an instrument is 0.85. Based on the formula, a new algorithm for the retrieval of atmospheric diffuse transmittance in Moderate Resolution Imaging Spectroradiometer (MODIS) atmospheric correction, instead of the MODIS standard algorithm, is proposed. Tests of the algorithms with in situ observations show that the MODIS standard algorithm leads to evident overestimation of diffuse transmittance, while the new algorithm contributes to a notable improvement in accuracy, especially for cases where the value of diffuse transmittance is small. The relative error and rms. error of the new algorithm are merely about a third and a quarter of those obtained by the MODIS standard algorithm, respectively. The new algorithm gives an estimation of t that matches very well with in situ observations in the Bohai Sea. The proposed algorithm can be utilized in MODIS atmospheric correction as an alternative algorithm, which may improve the accuracy of MODIS atmospheric correction in the Bohai Sea and other near-shore areas with large aerosol optical thickness.

Contrails of Small and Very Large Optical Depth

Abstract This work deals with two kinds of contrails. The first comprises a large number of optically thin contrails near the tropopause. They are mapped geographically using a lidar to obtain their height and a camera to obtain azimuth and elevation. These high-resolution maps provide the local contrail geometry and the amount of optically clear atmosphere. The second kind is a single trail of unprecedentedly large optical thickness that occurs at a lower height. The latter was observed fortuitously when an aircraft moving along the wind direction passed over the lidar, thus providing measurements for more than 3 h and an equivalent distance of 620 km. It was also observed by Geostationary Operational Environmental Satellite (GOES) sensors. The lidar measured an optical depth of 2.3. The corresponding extinction coefficient of 0.023 km−1 and ice water content of 0.063 g m−3 are close to the maximum values found for midlatitude cirrus. The associated large radar reflectivity compares to that measured by ultrasensitive radar, thus providing support for the reality of the large optical depth.

Effects of radiative forcing by black carbon aerosol on spring rainfall decrease over Southeast Asia

The effects of black carbon (BC) aerosol radiative forcing on spring rainfall in Southeast Asia are studied using numerical simulations with the NASA finite-volume General Circulation Model (fvGCM) forced with monthly varying three-dimensional aerosol distributions from the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART). During the boreal spring, March–April–May (MAM), BC from local emissions accumulates over Southeast Asia. The BC aerosol layer, which extends from the surface to higher elevation above planetary boundary layer (PBL), absorbs solar radiation and heats the mid-troposphere through a semi-direct effect over regions of large aerosol optical thickness (AOT) and thereby significantly perturbs large-scale and meridional circulations. Results show that anomalous precipitation patterns and associated large-scale circulations induced by radiative forcing by BC aerosol can explain observed precipitation reductions, especially over Southeast Asia. Therefore, BC aerosol forcing may be one of the important factors affecting the spring rainfall trend over Southeast Asia.

UVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10 15 cm −2 in close agreement with the value provided by the VTS3 empirical atmospheric model.

Electron Dose Kernels to Account for Secondary Particle Transport in Deterministic Simulations

For low-energy photons, charged-particle equilibrium usually exists within the patient treatment volume, in which case the photon absorbed dose D is equal to the collisional kerma Kc; however, this is not true for the dose buildup region near the surface of the patient or at interfaces of dissimilar materials, such as tissue/lung, where corrections for secondary electron transport may be significant. This is readily treated in Monte Carlo codes, yet difficult to treat explicitly in deterministic codes due to the large optical thicknesses and added numerical complexities in reaching convergence in photon-electron transport problems. To properly treat three-dimensional electron transport physics deterministically, yet still achieve reasonably fast and accurate whole-body computation times using high-energy photons, angular-energy-dependent transport “electron dose kernels” (EDK-SN) have been developed. These kernels were derived via full physics Monte Carlo electron transport simulations and are applied using scaling based on rapid deterministic photon solutions over the problem phase-space, thereby accounting for the dose from charged-particle electron transport. As a result, accurate whole-body doses may be rapidly achieved for high-energy photon sources by performing a single deterministic SN multigroup photon calculation on a parallel cluster with PENTRAN, then linking the SN-derived photon fluxes and net currents to Monte Carlo–based EDKs to account for a full physics dose. Water phantom results using a uniform 0- to 8-MeV step uniform beam indicate that the dose can be accurately obtained within the uncertainty of a full physics Monte Carlo simulation. Followup work will implement this method on phantoms.

Radiation pressure and pulsation effects on the Roche lobe

Several observational pieces of evidence indicate that specific evolutionary channels which involve Roche lobe overflow are not correctly accounted for by the classical Roche model. We generalize the concept of Roche lobe in the presence of extra forces (caused by radiation pressure or pulsations). By computing the distortion of the equipotential surfaces, we are able to evaluate the impact of these perturbing forces on the stability of Roche-lobe overflow (RLOF). Radiative forces are parametrized through the constant reduction factor that they impose on the gravitational force from the radiating star (neglecting any shielding in case of large optical thickness). Forces imparted by pulsations are derived from the velocity profile of the wind that they trigger. We provide analytical expressions to compute the generalized Roche radius. Depending on the extra force, the Roche-lobe radius may either stay unchanged, become smaller, or even become meaningless (in the presence of a radiatively- or pulsation-driven wind). There is little impact on the RLOF stability.

Observations and Modeling of the Surface Aerosol Radiative Forcing during UAE2

Abstract Aerosol radiative forcing in the Persian Gulf region is derived from data collected during the United Arab Emirates (UAE) Unified Aerosol Experiment (UAE2). This campaign took place in August and September of 2004. The land–sea-breeze circulation modulates the diurnal variability of the aerosol properties and aerosol radiative forcing at the surface. Larger aerosol radiative forcing is observed during the land breeze in comparison to the sea breeze. The aerosol optical properties change as the onshore wind brings slightly cleaner air. The mean diurnal value of the surface aerosol forcing during the UAE2 campaign is about −20 W m−2, which corresponds to large aerosol optical thickness (0.45 at 500 nm). The aerosol forcing efficiency [i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m−2 (τ500)−1] is −53 W m−2 (τ500)−1 and the average single scattering albedo is 0.93 at 550 nm.

Simulation of Laser-Induced Incandescence Measurements in an Anisotropically Scattering Aerosol Through Backward Monte Carlo

Laser-induced incandescence (LII) measurements carried out in aerosols having a large particle volume fraction must be corrected to account for extinction between the energized aerosol particles and the detector, called signal trapping. While standard correction techniques have been developed for signal trapping by absorption, the effect of scattering on LII measurements requires further investigation, particularly the case of highly anisotropic scattering and along a path of relatively large optical thickness. This paper examines this phenomenon in an aerosol containing highly aggregated soot particles by simulating LII signals using a backward Monte Carlo analysis; these signals are then used to recover the soot particle temperature and soot volume fraction. The results show that inscattered radiation is a substantial component of the LII signal under high soot loading conditions, which can strongly influence properties derived from these measurements. Correction techniques based on Bouguer’s law are shown to be effective in mitigating the effect of scatter on the LII signals.