Decrease In Optical Depth Research Articles

Thermal emission and scattering by aligned grains: Plane-parallel model and application to multiwavelength polarization of the HL Tau disc

ABSTRACT Telescopes are now able to resolve dust polarization across circumstellar discs at multiple wavelengths, allowing the study of the polarization spectrum. Most discs show clear evidence of dust scattering through their unidirectional polarization pattern typically at the shorter wavelength of $\sim 870 \, \mu$m. However, certain discs show an elliptical pattern at ∼3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at ∼1.3 mm shows a transition between the two patterns making it the first example to reveal such transition. We use the T-matrix method to model elongated dust grains and properly treat scattering of aligned non-spherical grains with a plane-parallel slab model. We demonstrate that a change in optical depth can naturally explain the polarization transition of HL Tau. At low optical depths, the thermal polarization dominates, while at high optical depths, dichroic extinction effectively takes out the thermal polarization and scattering polarization dominates. Motivated by results from the plane-parallel slab, we develop a simple technique to disentangle thermal polarization of the aligned grains T0 and polarization due to scattering S using the azimuthal variation of the polarization fraction. We find that, with increasing wavelength, the fractional polarization spectrum of the scattering component S decreases, while the thermal component T0 increases, which is expected since the optical depth decreases. We find several other sources similar to HL Tau that can be explained by azimuthally aligned scattering prolate grains when including optical depth effects. In addition, we explore how spirally aligned grains with scattering can appear in polarization images.

Mechanisms Behind the Extratropical Stratiform Low‐Cloud Optical Depth Response to Temperature in ARM Site Observations

AbstractGround‐based observations from three middle‐ and high‐latitude sites managed by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program are used to determine the sensitivity of the low‐cloud optical depth to temperature and to test whether observations support mechanisms previously proposed to affect the optical depth feedback. Analysis of cloud optical depth retrievals support previous satellite findings that the optical depth decreases or stays constant with increases in temperature when the cloud is warm but increases when the cloud is cold. The cloud liquid water path sensitivity to warming largely explains the optical depth sensitivity at all sites. Mechanisms examined in this study involve the temperature dependence of (a) the moist‐adiabatic lapse rate, (b) cloud phase partitioning, (c) drying efficiency of cloud top mixing, (d) cloud top inversion strength, and (e) boundary layer decoupling. Mechanism (a) is present across all clouds and explains 30% to 50% of the increase in liquid water path with warming at temperatures below 0 °C. However, the cloud's adiabaticity, the ratio between the liquid water path and its theoretical maximum, is at least as important and determines how the liquid water path sensitivity to temperature varies with temperature. At temperatures below 0 °C, the adiabaticity increases with warming, and the data support mechanism (b). At warmer temperatures, the adiabaticity decreases with warming, overwhelming mechanism (a) and resulting in the liquid water path decreasing with warming. This adiabaticity decrease arises primarily because of mechanism (d), and to a lesser degree because of mechanism (e). No evidence is found supporting mechanism (c).

A connection between γ-ray and parsec-scale radio flares in the blazar 3C 273

We present a comprehensive 5-43 GHz VLBA study of the blazar 3C 273 initiated after an onset of a strong $\gamma$-ray flare in this source. We have analyzed the kinematics of new-born components, light curves, and position of the apparent core to pinpoint the location of the $\gamma$-ray emission. Estimated location of the $\gamma$-ray emission zone is close to the jet apex, 2 pc to 7 pc upstream from the observed 7 mm core. This is supported by ejection of a new component. The apparent core position was found to be inversely proportional to frequency. The brightness temperature in the 7 mm core reached values up to at least $10^{13}$ K during the flare. This supports the dominance of particle energy density over that of magnetic field in the 7 mm core. Particle density increased during the radio flare at the apparent jet base, affecting synchrotron opacity. This manifested itself as an apparent core shuttle along the jet during the 7 mm flare. It is also shown that a region where optical depth decreases from $\tau\sim1$ to $\tau<<1$ spans over several parsecs along the jet. The jet bulk flow speed estimated at the level of 12c on the basis of time lags between 7 mm light curves of stationary jet features is 1.5 times higher than that derived from VLBI apparent kinematics analysis.

Terminal speed of a gaseous stratus with finite optical depth over a luminous flat source

Abstract We reexamine the terminal speed of a moving stratus irradiated by an infinite flat source, considering relativistic radiative transfer in the stratus. For the case of a particle, V. Icke (1989, A&amp;A, 216, 294) analytically derived the terminal speed of $(4-\sqrt{7})c/3 \sim 0.45\,c$, whereas the terminal speed of a stratus with finite optical depth is calculated under the Eddington approximation (J. Fukue, 2014, PASJ, 66, 13), and becomes larger up to 0.7 c in the optically thin limit. In this paper, we numerically calculate radiative transfer in the stratus without the Eddington approximation, and obtain the terminal speed. In the optically thick limit the terminal speed approaches 0.47 c. In the optically thin limit, in contrast to the previous analytical study, it becomes small as the optical depth decreases, and approaches 0.26 c. This is due to the anisotropic effect of the radiation field in the optically thin regime.

Dust aerosol effects on cirrus and altocumulus clouds in Northwest China

Dust aerosol effects on the properties of cirrus and altocumulus cloud in Northwest China were studied for the period March–May 2007 by using the satellite data of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), Aqua, and CloudSat. Dusty clouds were defined as those mixed with dust aerosols or existing in dust aerosol conditions, while pure clouds were those in a dust-free environment. For dusty altocumulus clouds, the mean values of cloud optical depth (OPD), cloud liquid water path (LWP), cloud ice water path (IWP), cloud effective particle radius (Re), and cloud effective particle diameter (D e) were 6.40, 40.23 g m-2, 100.70 g m-2, 8.76 μm, and 40.72 μm, respectively. For pure altocumulus clouds, the corresponding mean values were 9.28, 76.70 g m-2, 128.75 g m-2, 14.03 μm, and 48.92 μm, respectively. These results show a significant decrease of OPD, LWP, IWP, R e, and D e of approximately 31%, 48%, 22%, 38%, and 17% because of the effects of dust aerosols. Moreover, the effects of dust aerosols on liquid-phase altocumulus clouds were greater than on ice-phase altocumulus clouds. Regarding dusty cirrus clouds, the mean values of OPD, IWP, and D e were 5.11, 137.53 g m-2, and 60.44 μm, respectively. In contrast, the mean values were 6.69, 156.17 g m-2, and 66.63 μm, respectively, for pure cirrus clouds, with a 24% decrease in OPD, a 12% decrease in IWP, and a 9% decrease in D e. These results indicate that dust aerosols can significantly change cloud properties, leading to a reduction of OPD, LWP, and effective particle size for both altocumulus and cirrus clouds in Northwest China.

Interannual variation of cloud optical properties at ACRF Manus and Nauru sites from MFRSR measurements

The long-term measurements of Multi-Filter Rotating Shadowband Radiometers and other instruments at two Atmospheric Radiation Measurements Climate Research Facility sites of Manus and Nauru have been processed to develop the climatology of cloud properties in the tropical warm pool region. Due to their unique geolocations and associated large-scale circulation patterns, cloud properties at these two sites exhibit distinctive characteristics. At the Nauru site, cloud properties are statistically significantly correlated with Pacific Decadal Oscillation index; the monthly anomaly values of cloud fraction, overcast cloud occurrence and optical depth decrease with Southern Oscillation Index (SOI). At the Manus site, monthly anomaly values of cloud fraction, overcast cloud occurrence and optical depth, to some extent, are bifurcately correlated with SOI, depending on the phase of El Niño/Southern Oscillation. The correlation of SOI and the MFRSR retrieved cloud optical properties can be explained by the drifting of tropical convection center in the equatorial area.

Optical depth measurements of fireballs from aluminized high explosives

Spatially and temporally resolved attenuation of 532 nm light by fireballs from aluminized high explosives is determined using photometric and imaging methods. For all measurements, the absorbance is found to peak during the early period of high reactivity of post-detonation gases. The characteristic attenuation length is less than 1 cm during the peak absorption period, and remains on the order of centimeters until a significant decrease in optical depth occurs about 160 μs after the detonation. These results suggest that, for gram-scale and larger aluminized charges, the optically thick assumption will most likely be valid for the period in which fireball luminosity is significant.

Effects of aerosol optical properties on deep convective clouds and radiative forcing

The aerosol radiative effects (ARE) on the deep convective clouds are investigated by using a spectral‐bin cloud‐resolving model coupled with a radiation scheme and an explicit land surface model. The sensitivity of cloud properties and the associated radiative forcing to aerosol single‐scattering albedo (SSA) are examined. The ARE on cloud properties is pronounced for mid‐visible SSA of 0.85. Relative to the case without ARE, the cloud fraction and optical depth decrease by about 18% and 20%, respectively. Ice particle number concentrations, liquid water path, ice water path, and droplet size decrease by more than 15% when the ARE is introduced. The ARE causes a surface cooling of about 0.35 K and significantly high heating rates in the lower troposphere (about 0.6 K day−1 higher at 2 km), both of which lead to a more stable atmosphere and hence weaker convection. The weaker convection explains the less cloudiness, lower cloud optical depth, less LWP and IWP, smaller droplet size, and less precipitation resulting from the ARE. The daytime‐mean direct forcing induced by black carbon is about 2.2 W m−2 at the top of atmosphere (TOA) and −17.4 W m−2 at the surface for SSA of 0.85. The semi‐direct forcing is positive, about 10 and 11.2 W m−2 at the TOA and surface, respectively. Both the TOA and surface total radiative forcing values are strongly negative for the deep convective clouds, attributed mostly to aerosol indirect forcing. Aerosol direct and semi‐direct effects are very sensitive to SSA when aerosol optical depth is high. Because the positive semi‐direct forcing compensates the negative direct forcing at the surface, the surface temperature and heat fluxes decrease less significantly with the increase of aerosol absorption (decreasing SSA). The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.

Relationships between Marine Stratus Cloud Optical Depth and Temperature: Inferences from AVHRR Observations

AbstractStudies using International Satellite Cloud Climatology Project (ISCCP) data have reported decreases in cloud optical depth with increasing temperature, thereby suggesting a positive feedback in cloud optical depth as climate warms. The negative cloud optical depth and temperature relationships are questioned because ISCCP employs threshold assumptions to identify cloudy pixels that have included partly cloudy pixels. This study applies the spatial coherence technique to one month of Advanced Very High Resolution Radiometer (AVHRR) data over the Pacific Ocean to differentiate overcast pixels from the partly cloudy pixels and to reexamine the cloud optical depth–temperature relationships. For low-level marine stratus clouds studied here, retrievals from partly cloudy pixels showed 30%–50% smaller optical depths, 1°–4°C higher cloud temperatures, and slightly larger droplet effective radii, when they were compared to retrievals from the overcast pixels. Despite these biases, retrievals for the overcast and partly cloudy pixels show similar negative cloud optical depth–temperature relationships and their magnitudes agree with the ISCCP results for the midlatitude and subtropical regions. There were slightly negative droplet effective radius–temperature relationships, and considerable positive cloud liquid water content–temperature relationships indicated by aircraft measurements. However, cloud thickness decreases appear to be the main reason why cloud optical depth decreases with increasing temperature. Overall, cloud thickness thinning may explain why similar negative cloud optical depth–temperature relationships are found in both overcast and partly cloudy pixels. In addition, comparing the cloud-top temperature to the air temperature at 740 hPa indicates that cloud-top height generally rises with warming. This suggests that the cloud thinning is mainly due to the ascending of cloud base. The results presented in this study are confined to the midlatitude and subtropical Pacific and may not be applicable to the Tropics or other regions.

Patchy He II reionization and the physical state of the intergalactic medium

We present a Monte Carlo model of He II reionization by quasi-stellar objects (QSOs, quasars) and its effect on the thermal state of the clumpy intergalactic medium (IGM). The model assumes that patchy reionization develops as a result of the discrete distribution of QSOs. It includes various recipes for the propagation of the ionizing photons, and treats photoheating self-consistently. The model predicts the fraction of He III, the mean temperature in the IGM, and the He II mean optical depth - all as a function of redshift. It also predicts the evolution of the local temperature versus density relation during reionization. Our findings are as follows. The fraction of Hem increases gradually until it becomes close to unity at z ∼ 2.8-3.0. The He II mean optical depth decreases from τ ∼ 10 at z ≥ 3.5 to τ ≤ 0.5 at z ≤ 2.5. The mean temperature rises gradually between z ∼ 4 and z ∼ 3 and declines slowly at lower redshifts. The model predicts a flattening of the temperature-density relation, with significant increase in the scatter during reionization at z ∼ 3. Towards the end of reionization, the scatter is reduced and a tight relation is re-established. This scatter should be incorporated in the analysis of the Lya forest at z ≤ 3. Comparison with observational results of the optical depth and the mean temperature at moderate redshifts constrains several key physical parameters.

Optical coherency of sunphotometry, sky radiometry and lidar measurements during the early phase of Pacific 2001

Passive sunphotometry and sky radiometry data at sites in the Lower Fraser Valley (Langley-Lochiel) and Saturna Island were synchronously acquired with ground-based scanning lidar (Rapid Acquisition Scanning Aerosol Lidar, RASCAL) and airborne lidar (AERosol Imaging Airborne Lidar, AERIAL) during the Pacific 2001 Air Quality Study. The temporal and spatial behavior of these optical measurements is investigated during a pollution event which occurred from 13 to 16 August 2001. A mid-day minimum in lidar-derived extinction to backscatter ratios (Sa) values, was attributed, at least in part, to the relative humidity (RH) induced optical influence of the column integrated fine mode and/or coarse mode particles in the PBL. Systematically larger Sa values predicted by the sunphotometer and sky radiance inversions were hypothesized to be due to differences between the retrieved refractive index and the actual refractive index of the coarse mode. Aerosol optical depth differences were within maximum error bounds (0.02) when comparing the sunphotometry with spatial maps derived from AERIAL transects. Daily temporal trends of optical and microphysical parameters derived from sunphotometry and sky radiance data at this site were consistent with information deduced from the lidar and meteorological data; while the daily aerosol optical depth decrease was clearly associated with particle removal induced by daytime sea-breeze advection, a significant-to-dominant part of this decrease was associated with decreasing RH growth effects on at least one day.

Fog Luminance Evaluation in Daytime

In daytime, fog creates a luminous veil that impairs the visual indices required by motorists to coordinate their task of driving. The level of fog luminance depends on many parameters. A method is described to compute the luminance of a fog layer on the basis of its optical depth, the droplet size distribution, and the relative position of both the sun and the motorist. The results are then analyzed and indicate that fog luminance depends to a great extent on the direction of observation with respect to the sun. Except when the motorist is traveling toward the sun, the fog luminance follows the variations of horizontal illuminance, which increases with the altitude of the sun yet decreases with droplet size distribution. In most instances, the influence of the optical depth calculated over the height of the fog layer remains small. Only when facing the sun in a fog with droplet diameters less than or equal to 1 μm does the luminance increase significantly as optical depth decreases. The impact on the visibility parameters is presented.

The Lyα forest around high-redshift galaxies

Motivated by the relative lack of neutral hydrogen around Lyman-break galaxies deduced from recent observations, we investigate the properties of the Lya forest around high-redshift galaxies. The study is based on improved numerical SPH simulations implementing, in addition to standard processes, a new scheme for multiphase and outflow physics description. Although on large scales our simulations reproduce a number of statistical properties of the intergalactic medium (because of the small filling factor of shock-heated gas), they underpredict the Lyα optical depth decrease inside 1 Mpc h - 1 of the galaxies by a factor of 15. We interpret this result as arising from the combined effect of infal occurring along the filaments, which prevents efficient halo gas clearing by the outflow, and the insufficient increase of (collisional) hydrogen ionization produced by the temperature increase inside the hot, outflow-carved bubble. Unless either feedback is not properly modelled in cosmological simulations or an observational selection bias is present, we speculate that local photoionization could be a viable explanation to solve the puzzle.

The Far‐Infrared Spectral Energy Distributions of X‐Ray–selected Active Galaxies

[Abridged] We present ISO far-infrared (IR) observations of 21 hard X-ray selected AGN from the HEAO-1 A2 sample. We compare the far-IR to X-ray spectral energy distributions (SEDs) of this sample with various radio and optically selected AGN samples. The hard-X-ray selected sample shows a wider range of optical/UV shapes extending to redder near-IR colors. The bluer objects are Seyfert 1s, while the redder AGN are mostly intermediate or type 2 Seyferts. This is consistent with a modified unification model in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wavebands and of the broad emission line region which results in a mixture of behaviors for AGN with similar optical emission line classifications. The resulting limits on the column density of obscuring material through which we are viewing the redder AGN are 100 times lower than for the standard optically thick torus models. The resulting decrease in optical depth of the obscuring material allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass 10^9 solar and size of few hundred pc is able to generate optical-far-IR SEDs which reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.

Modeling IR spectra of OH/IR stars at different phases

We investigate the spectral energy distributions (SEDs) of OH/IR stars (OH127.8+0.0 and OH26.5+0.6) having thick dust envelopes at different pulsation phases. Using new infrared observational data including the Infrared Space Observatory (ISO) data, we determine the new pulsation parameters. The deep silicate absorption features show significant variations depending on the pulsation phase. The variations are mainly due to changes in the properties of dust envelopes around the OH/IR stars. Comparing the results of detailed radiative model calculations with observations, we explore the changes of the relevant parameters of the envelopes and central stars depending on the pulsation phase. We find that when the central luminosity increases from the minimum to maximum phase, the inner radius of the dust shell increases with velocity faster than the outer shell expansion velocity and the dust shell optical depth decreases. During the phase change from the minimum to maximum, we find that dust formation ceases and about a half of the dust grains in the volume difference should have evaporated. During the phase change from the maximum to minimum, we find that the dust formation should be enhanced because the inner radius is decreasing. In the outer radii of the dust shell, the constant dust winds are easily maintained. We expect that the dust evaporation process driven by pulsation could be a mechanism for crystallizing the dust grains in inner regions of the dust shells around OH/IR stars.

The Effect of Luminous Lens Blending in Gravitational Microlensing Experiments

The most important uncertainty in the results of gravitational microlensing experiments comes from the difficulties of photometry caused by blending of source stars. Recently Nemiroff (1997) pointed out that the results of microlensing experiments can also be affected by the blending of light from the lens itself if a significant fraction of lenses are composed of stars. In this paper, we estimate the effects of lens blending on the optical depth determination and the derived matter distribution toward the Galactic bulge by using realistic models of the lens matter distribution and a well-constrained stellar luminosity function. We find that the effect of lens blending is largest for lenses located in the Galactic disk. However, lens blending does not seriously affect both the determination of the optical depth and the Galactic matter distribution. The decrease in optical depth is $\sim 10%$ even under the extreme assumption that lenses are totally composed of stars and disk matter distribution follows a maximal disk model, in which the lens blending effect is most severe.

The Evolution of Debris from Comet D/Shoemaker–Levy 9 on Jupiter

We present low-resolution photometric spectra in the wavelength range 407–907 μm of impact sites G, H, E, C, K, L, Q1, and W plus K together, arising from the collision of Comet D/Shoemaker–Levy 9 with Jupiter. The spectra were obtained from narrow band CCD images taken between July 15th and July 22nd using the 1-m Jacobus Kapteyn Telescope on La Palma. Our results indicate the impact debris to have low overall optical depths (τ < 1) which decrease with increasing wavelength for all individual sites observed. Our results also imply that mass absorption of sunlight is the dominant process occurring in the impact debris and that a decrease in optical depth with time is due to the material dissipating in the jovian atmosphere.

Long-Term Evolution of Comet SL-9 Impact Features: July 1994–September 1996

We present a two year study of the evolution of SL9 impact aerosol debris we observed between 0.4 and 0.9 micrometers with continuous high temporal coverage from July 1994 through September 1996 and at 1.7 and 2.3 micrometers during three observing runs in July 1994 and March and August 1995. Temporal cylindrical map projections at red continuum wavelengths in the region covered by the impact debris show the contributions of different mechanisms in producing the complicated morphological evolution of the sites during the first month. Long-term horizontal aerosol transport was mainly due to the zonal jets in the upper troposphere with extreme measured velocities of −10 and 20 m s−1. A comparison of the zonal drift of the core sites in the red continuum and in the 890-nm methane band (sensitive to higher levels) during the first month do not show significant velocity differences between these filters, indicating a low vertical wind shear in the upper troposphere. The spread of the aerosols resulted from the meridional and vertical shears of the zonal winds. Rapid initial outward expansions (speeds of ∼30 to 60 m s−1) and interactions with nearby vortices (speeds of ∼10 to 25 m s−1) also contributed to the dispersion of particulates. Using methane band images we have measured a steady poleward and equatorward meridional transport of the particulates with velocities of ∼−6 and 40 cm s−1, respectively. Particulates were detected up to ∼−20° by August 1995. Limb brightening in the 890-nm methane band was observed up to ∼−30° during the last observation (September 1996) reported here, indicating that a small population of aerosols was still present two years after impact. Photometric observations in the 890-nm band, together with a radiative transfer model, allowed us to calculate the evolution of the aerosol optical depth in the main impact core areas and in the subsequent SL9 band. We found a rapid decrease in optical depth in the largest impacts during July and August 1994 (from ≈3.2 to 2.1), followed by a gradual decrease during the next two years to ≈0.3 (June 1996). This behavior can be explained by simple models of debris horizontal dispersion by the wind shear and by sedimentation. Calculations of the characteristic times related to the microphysical processes in the aerosols (sedimentation, coagulation, and coalescence) together with their observed residence times (≥2 years) indicates that this persistent population of particles had sizes ≤0.1 micrometers during 1995 and 1996.

Spurious changes in the ISCCP dataset

A long‐term decrease in mean cloud optical depth and mean cloud top temperature exists in the International Satellite Cloud Climatology Project (ISCCP) dataset for the July 1983 to December 1990 time period equatorward of 50° latitude. The decrease in optical depth of 2 is large compared to its mean of 4.6 for this period. The nature of this decrease suggests that it is not real, but rather an artifact of incomplete normalization of the visible channel on successive polar orbiters used as the calibration standard for ISCCP. This decrease has large impacts on the amounts of different cloud types defined by ISCCP, especially for cloud types whose mean amount is small. The decrease in mean cloud top temperature of 5 K has a smaller effect on the amounts of various cloud types. More accurate calibration techniques are needed in order to establish long‐term climate trends from ISCCP.

Variability of the broad absorption lines in the QSO UM 232

Low-resolution spectra of UM 232 taken in 1978, 1979, and 1988 at Lick Observatory are presented. Large changes in the Si IV lambda 1397, CIV lambda 1549, and Al III lambda 1857 broad absorption lines are apparent. The decrease in column density in all three ions and an observed brightening of the QSO suggests that these changes are due to an increase in the ionization level driven by an increase in the central source luminosity. This mechanism has been proposed by Smith and Penston to explain small changes in the absorption spectrum of the QSO 1246-057. The spectra of UM 232 show that the fractional decrease in optical depth is smaller at higher outflow velocies. The structure of the broad absorption-line region (BALR) is investigted by estimating an ionization parameter for each ion species as a function of velocity.