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NASA NTRS2012

The Aquarius Salinity Retrieval Algorithm: Early Results

The Aquarius L-band radiometer/scatterometer system is designed to provide monthly salinity maps at 150 km spatial scale to a 0.2 psu accuracy. The sensor was launched on June 10, 2011, aboard the Argentine CONAE SAC-D spacecraft. The L-band radiometers and the scatterometer have been taking science data observations since August 25, 2011. The first part of this presentation gives an overview over the Aquarius salinity retrieval algorithm. The instrument calibration converts Aquarius radiometer counts into antenna temperatures (TA). The salinity retrieval algorithm converts those TA into brightness temperatures (TB) at a flat ocean surface. As a first step, contributions arising from the intrusion of solar, lunar and galactic radiation are subtracted. The antenna pattern correction (APC) removes the effects of cross-polarization contamination and spillover. The Aquarius radiometer measures the 3rd Stokes parameter in addition to vertical (v) and horizontal (h) polarizations, which allows for an easy removal of ionospheric Faraday rotation. The atmospheric absorption at L-band is almost entirely due to O2, which can be calculated based on auxiliary input fields from numerical weather prediction models and then successively removed from the TB. The final step in the TA to TB conversion is the correction for the roughness of the sea surface due to wind. This is based on the radar backscatter measurements by the scatterometer. The TB of the flat ocean surface can now be matched to a salinity value using a surface emission model that is based on a model for the dielectric constant of sea water and an auxiliary field for the sea surface temperature. In the current processing (as of writing this abstract) only v-pol TB are used for this last process and NCEP winds are used for the roughness correction. Before the salinity algorithm can be operationally implemented and its accuracy assessed by comparing versus in situ measurements, an extensive calibration and validation (cal/val) activity needs to be completed. This is necessary in order to tune the inputs to the algorithm and remove biases that arise due to the instrument calibration, foremost the values of the noise diode injection temperatures and the losses that occur in the feedhorns. This is the subject of the second part of our presentation. The basic tool is to analyze the observed difference between the Aquarius measured TA and an expected TA that is computed from a reference salinity field. It is also necessary to derive a relation between the scatterometer backscatter measurements and the radiometer emissivity that is induced by surface winds. In order to do this we collocate Aquarius radiometer and scatterometer measurements with wind speed retrievals from the WindSat and SSMIS F17 microwave radiometers. Both of these satellites fly in orbits that have the same equatorial ascending crossing time (6 pm) as the Aquarius/SAC-D observatory. Rain retrievals from WindSat and SSMIS F 17 can be used to remove Aquarius observations that are rain contaminated. A byproduct of this analysis is a prediction for the wind-induced sea surface emissivity at L-band.

Meissner, Thomas↗
NASA NTRS2012

The Aquarius Salinity Retrieval Algorithm

The first part of this presentation gives an overview over the Aquarius salinity retrieval algorithm. The instrument calibration [2] converts Aquarius radiometer counts into antenna temperatures (TA). The salinity retrieval algorithm converts those TA into brightness temperatures (TB) at a flat ocean surface. As a first step, contributions arising from the intrusion of solar, lunar and galactic radiation are subtracted. The antenna pattern correction (APC) removes the effects of cross-polarization contamination and spillover. The Aquarius radiometer measures the 3rd Stokes parameter in addition to vertical (v) and horizontal (h) polarizations, which allows for an easy removal of ionospheric Faraday rotation. The atmospheric absorption at L-band is almost entirely due to molecular oxygen, which can be calculated based on auxiliary input fields from numerical weather prediction models and then successively removed from the TB. The final step in the TA to TB conversion is the correction for the roughness of the sea surface due to wind, which is addressed in more detail in section 3. The TB of the flat ocean surface can now be matched to a salinity value using a surface emission model that is based on a model for the dielectric constant of sea water [3], [4] and an auxiliary field for the sea surface temperature. In the current processing only v-pol TB are used for this last step.

Meissner, Thomas↗
NASA NTRS2020

Atomic Data for Plasma Spectroscopy: The CHIANTI Database, Improvements and Challenges

CHIANTI is an atomic database and software package for modeling emission lines and continua from hot astrophysical plasmas. It is freely available to all researchers and has been widely used in the Heliophysics and Astrophysics communities for almost 25 years. In this review, we summarize the properties of the current version of the database and give an overview of the relevant atomic processes. We also discuss progress towards a complete implementation of collisional-radiative modeling, simultaneously solving for atomic level and ion populations for individual elements.

A&M databases↗
NASA NTRS2019

Detection of Nonthermal Hard X-Ray Emission from the “Fermi Bubble” in an External Galaxy

We report new Chandra hard X-ray (>2 keV) and Jansky Very Large Array C-band observations of the nuclear super bubble of NGC 3079, an analog of the “Fermi bubble” in our Milky Way. We detect extended hard X-ray emission on the southwest (SW)side of the galactic nucleus with coherent multiwavelength features in radio, Hα, and soft X-ray. The hard X-ray feature has a cone shape with possibly a weak cap, forming a bubble-like structure with a diameter of∼1.1 kpc. A similar extended feature, however, is not detected on the northeast (NE)side, which is brighter in all other wavelengths such as radio, Hα, and soft X-ray. Scattered photons from the nuclear region or other nearby point-like X-ray bright sources, inverse Compton emission from cosmic-ray(CR)electrons via interaction with the cosmic microwave background, or any individually faint stellar X-ray source populations, cannot explain the extended hard X-ray emission on the SW side and the strongly NE/SW asymmetry. A synchrotron emission model, plus a thermal component accounting for the excess at∼1 keV, can well characterize the broadband radio/hard X-ray spectra. The broadband synchrotron spectra do not show any significant cutoff, and even possibly slightly flatten at higher energy. This rule out a loss-limited scenario in the acceleration of the CR electrons in or around this superbubble. As the first detection of kiloparsec-scale extended hard X-ray emission associated with a galactic nuclear superbubble, the spatial and spectral properties of the multiwavelength emissions indicate that the CR leptons responsible for the broadband synchrotron emission from the SW bubble must be accelerated in situ, instead of transported from the nuclear region of the galaxy.

Jiang-Tao Li↗
NASA NTRS1973

Lunar polarization studies at 3.1 mm wavelength

Observations of the distribution of linearly polarized lunar thermal emission were made at a wavelength of 3.1 mm with The University of Texas 4.88 m parabolic reflector (0.042 deg HPBW). A shadow corrected, rough surface, thermal emission model for a homogeneous moon was least-squares-fitted to the polarization data. Results indicate an effective lunar dielectric constant of 1.34 plus or minus 0.04 with surface roughness characterized by a standard deviation of 17 (plus or minus 5) deg for surface slopes with a normal probability density, independent of lunar phase. A comparison of these results with published values at other wavelengths suggests that the effective lunar dielectric constant, as obtained by lunar emission measurements, decreases with decreasing wavelength of observation. This wavelength dependence may be interpreted in terms of an inhomogeneous surface and/or a surface that possesses intermediate scale surface roughness.

White, T. L.↗
NASA NTRS1990

The illusive Geminga: What is it

The first unassociated gamma-ray source was discovered by SAS-2 in 1973 (Kniffen, et al., 1975) and later confirmed by COS-B (Bennett, et al., 1977). Followed the announcement, there were numerous attempts to find a counterpart, and many models were developed to explain the source. Now over fifteen years later this illusive source still remains as one of the major riddles of astrophysics. The question of why an object, which is able to emit such energetic photons is so well concealed at other wavelengths, still remains to be answered. The association with the Einstein source 1E 0630+178 is the most favored (Bignami, Caraveo, and Lamb, 1983), but this cannot be considered proven. The pulsar emission model of Ruderman and Cheng (1988) is appealing in its broad applicability, but awaits observational confirmation. The EGRET (Energetic Gamma-Ray Experimental Telescope) instrument on the Gamma-Ray Observatory provides a major improvement in observational capability to better define the location and spectrum of this source, and hopefully leads to a confident identification.

Kniffen, Donald A.↗
NASA NTRS1998

Single photon infrared emission spectroscopy: a study of IR emission from UV laser excited PAHs between 3 and 15 micrometers

Single-photon infrared emission spectroscopy (SPIRES) has been used to measure emission spectra from polycyclic aromatic hydrocarbons (PAHs). A supersonic free-jet expansion has been used to provide emission spectra of rotationally cold and vibrationally excited naphthalene and benzene. Under these conditions, the observed width of the 3.3-micrometers (C-H stretch) band resembles the bandwidths observed in experiments in which emission is observed from naphthalene with higher rotational energy. To obtain complete coverage of IR wavelengths relevant to the unidentified infrared bands (UIRs), UV laser-induced desorption was used to generate gas-phase highly excited PAHs. Lorentzian band shapes were convoluted with the monochromator-slit function in order to determine the widths of PAH emission bands under astrophysically relevant conditions. Bandwidths were also extracted from bands consisting of multiple normal modes blended together. These parameters are grouped according to the functional groups mostly involved in the vibration, and mean bandwidths are obtained. These bandwidths are larger than the widths of the corresponding UIR bands. However, when the comparison is limited to the largest PAHs studied, the bandwidths are slightly smaller than the corresponding UIR bands. These parameters can be used to model emission spectra from PAH cations and cations of larger PAHs, which are better candidate carriers of the UIRs.

Non-NASA Center↗
NASA NTRS2008

Passive and Active L-Band System and Observations During the 2007 CLASIC Campaign

This article describes the upgraded PALS instrument and the characteristics of data acquired from the Cloud Land Atmospheric Interaction Campaign (CLASIC) 2007. The data acquired over lake passes were used to remove the radiometer calibration bias. The calibrated radiometer data showed significant consistency with the L-band land emission model for soil surfaces published in the literature. We observed significant temporal (days) changes of a few dB in the radar data. The change of radar backscatter appeared to correlate well with the change of in-situ soil moisture or the soil moisture data derived from the PALS dual-polarized brightness temperatures. The radar vegetation index also correlated well with the vegetation opacity estimated from the radiometer data. The preliminary analyses suggest complementary information contained in the surface emissivity and backscatter signatures for the retrieval of soil moisture and vegetation water content.

Soil moisture↗
NASA NTRS2014

Three-Dimensional Trajectory Design for Reducing Climate Impact of Trans-Atlantic Flights

The impact of aircraft emissions and contrails on the environment adds an additional aspect to aircraft trajectory optimization. This study developed a three-dimensional trajectory optimization algorithm for trans-Atlantic flights in cruise to generate aircraft trajectories that minimize environmental impacts due to CO2 emissions and contrails in the presence of winds. The climate-optimal trajectory is developed using dynamic programming that adjusts a wind-optimal aircraft heading while determining the optimal locations, altitudes and times for en-route step climbs. Flying wind-optimal routes minimize aircraft travel time, fuel burn and associated emissions during cruise while adjusting aircraft heading and en-route step climbs at the optimal locations and times minimize climate impact of contrails. This capability integrates an air traffic management simulation with aircraft fuel burn and emissions models, contrail formation and dispersion models, simplified climate response models, and a common climate metric. A study was conducted to evaluate the potential cost and benefits of flying climate-optimal routes in North Atlantic Airspace and their impacts to the Organized Track System design based on the trans-Atlantic air traffic during a day, July 12, 2012. Results show eastbound flights achieved a larger environmental benefit with less additional fuel burn than westbound flights that operated in strong headwinds that caused more additional fuel burn and aircraft emissions to avoid traversing contrails favorable regions.

climate impact of aviation↗
NASA NTRS1979

Analysis of the helium component of the local interstellar medium

Observations of the extraterrestrial He I 584-A emission made with an EUV experiment aboard OSO 8 are analyzed and compared with previous data from a similar experiment aboard the STP 72 1 satellite. The observations are also compared with an exhaustive set of emission models, and an envelope of possible velocities and temperatures is defined for the local interstellar medium. This envelope begins near 9 km/s with temperatures of 6000 to 8000 K, continues upward through 15 km/s with temperatures of 6000 to 15,000 K, and finally terminates near 27 km/s with temperatures of 11,000 to 14,000 K; the densities corresponding to these velocities and temperatures cover the range from 0.008 to 0.02 per cu cm. Previous observations of hydrogen L-alpha emission from the interstellar wind are reviewed and compared with the 584-A observations. With a typical value of 12% assumed for the cosmic helium abundance, the present observations imply a hydrogen density of 0.067 to 0.17 per cu cm, in good agreement with the values obtained from the L-alpha measurements.

Weller, C. S.↗
NASA NTRS1980

Physical conditions in the solar atmosphere above an active region

From a series of EUV spectra obtained at several heights above the limb in a solar active region, the volume emission measure is derived as a function of the electron temperature in the temperature range 70,000-1,500,000 K and the electron density at two locations. The emission measure from the coronal material (temperature greater than 700,000 K) is nearly the same everywhere and represents most of the material in the line of sight, while the emission measure from the transition region material (temperature between 70,000 and 250,000 K) fluctuates by two orders of magnitude from position to position above the active region. This is in agreement with the picture of this active region as consisting of a number of well-defined loops or lower portions of loops at transition region temperatures that are inhomogeneously distributed in much larger and more diffuse loop structures at coronal temperatures. The coronal data are in reasonable agreement with simple coronal models. Emission measures near 1,000,000 K evaluated using different ions differ by a factor of 4, suggesting difficulties with the atomic physics data.

Mariska, J. T.↗
NASA NTRS2000

The X-Ray Lightcurve of Eta Carinae: Refinement of the Orbit and Evidence for Phase Dependent Mass Loss

We solve the RXTE X-ray lightcurve of the extremely luminous and massive star eta Carinae with a colliding wind emission model to refine the ground-based orbital elements. The sharp decline to X-ray minimum at the end of 1997 fixes the date of the last periastron passage at 1997.95 +/- 0.05, not 1998.13 as derived from ground-based radial velocities. This helps resolve a discrepancy between the ground-based radial velocities and spatially-resolved velocity measures obtained by STIS. The X-ray data are consistent with a mass function f(M) approx. = 1.5, lower than the value f(M) approx. = 7.5 previously reported, so that the masses of eta Carinae and the companion are M(sub eta) greater than or = 80 solar mass and M(sub c) approx. 30 solar mass respectively. In addition the X-ray data suggest that the mass loss rate from eta Carinae is generally less than 3 x 10(exp -4) solar mass/yr, about a factor of 5 lower than that derived from some observations in other wavebands. We could not match the duration of the X-ray minimum with any standard colliding wind model in which the wind is spherically symmetric and the mass loss rate is constant. However we show that we can match the variations around X-ray minimum if we include an increase of a factor of approx. 20 in the mass loss rate from eta Carinae for approximately 80 days following periastron. If real, this excess in M would be the first evidence of enhanced mass flow off the primary when the two stars are close (presumably driven by tidal interactions). Our interpretation of the X-ray data suggest that the ASCA and RXTE X-ray spectra near the X-ray minimum are significantly contaminated by unresolved hard emission (E greater than or = 2 keV) from sonic other nearby source, probably associated with scattering of tile colliding wind emission by circumstellar dust. Based on the X-ray fluxes the distance to n Carinae is 2300 pc with formal uncertainties of only approx. 10%.

Corcoran, M. F.↗
NASA NTRS2011

GRB 091127: The Cooling Break Race on Magnetic Fuel

Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts, and infer physical parameters of the ultra-relativistic outflow. Methods. We used multi-wavelength (NIR to X-ray) follow-up observations obtained with GROND simultaneously in the g' r' t' i' z' JH filters and the XRT onboard the Swift satellite in the 0.3 to 10 keY energy range. The resulting afterglow light curve is of excellent accuracy with relative photometric errors as low as 1 %, and the spectral energy distribution (SED) is well-sampled over 5 decades in energy. These data present one of the most comprehensive observing campaigns for a single GRB afterglow and allow us to test several proposed emission models and outflow characteristics in unprecedented detail. Results. Both the multi-color light curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a cooling break moving from high to lower energies. The early light curve is well described by a broken power-law, where the initial decay in the optical/NlR wavelength range is considerably flatter than at X-rays. Detailed fitting of the time-resolved SED shows that the break is very smooth with a sharpness index of 2.2 +/- 0.2, and evolves towards lower frequencies as a power-law with index -1.23 +/- 0.06. These are the first accurate and contemporaneous measurements of both the sharpness of the spectral break and its time evolution. Conclusions. The measured evolution of the cooling break (V(sub c) varies as t(sup -1.2) is not consistent with the predictions of the standard model, wherein V(sub c) varies as t(sup -05) is expected. A possible explanation for the observed behavior is a time dependence of the microphysical parameters, in particular the fraction of the total energy in the magnetic field epsilon(sub Beta). This conclusion provides further evidence that the standard fireball model is too simplistic, and time-dependent micro-physical parameters may be required to model the growing number of well-sampled afterglow light curves.

Filgas, R.↗
NASA NTRS1988

Extremely low nonalloyed and alloyed contact resistance using an InAs cap layer on InGaAs by molecular-beam epitaxy

Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n-type InAs/In(0.53)Ga(0.47)As/In(0.52)Al(0.48)As structures grown on InP(Fe) by molecular-beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 microns. For a 150-A-thick InAs layer doped to 2 x 10 to the 18th/cu cm and a 0.1-micron-thick InGaAs layer doped to 1 x 10 to the 18th/cu cm, a specific contact resistance of 2.6 x 10 to the -8th ohm-asterisk sq cm was measured for the nonalloyed contact, while a resistance less than 1.7 x 10 to the -8th ohm-asterisk sq cm is reported for the alloyed contact. Conventional Au-Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.

Peng, C. K.↗
NASA NTRS1992

A toy model for estimating N2O emissions from natural soils

A model of N2O emissions from natural soils, whose ultimate objective is to evaluate what contribution natural ecosystems make to the global N2O budget and how the contribution would change with global change, is presented. Topics covered include carbon and nitrogen available in the soil, delivery of nitrifiable N, soil water and oxygen status, soil water budget model, effects of drainage, nitrification and denitrification potentials, soil fertility, N2O production, and a model evaluation. A major implication of the toy model is that the tropics account for more than 80 percent of global emission.

Fung, Inez↗
NASA NTRS1991

Phasing the Very Large Array on Galileo in the presence of Jupiter's strong radio emission

Work is in progress to determine the feasibility of using the Very Large Array (VLA) radio telescope to receive telemetry from Galileo during its close encounter with Io on 7 Dec. 1995. The VLA was used previously to receive telemetry from Voyager 2 at Neptune. However, Jupiter's strong radio emission is an additional complication in the case of the Galileo encounter. This article analyzes the effect of Jupiter's radio emission on the phase-adjustment procedure ('autophasing') used to maintain coherence among the 27 VLA antennas. Results of an experiment designed to mimic the Io encounter are presented. As expected, Jupiter's strong radio emission has a considerable effect on the autophasing procedure. A simple emission model is found to give a good approximation to the fringe-visibility plots derived from the VLA data, and that successful model is used to estimate the VLA's ability to autophase on Galileo during the Io encounter. The effect of Jupiter should be small for projected baselines longer than approximately 800 m and completely negligible for projected baselines longer than approximately 1.1 km. The most extended configuration of the VLA (the A configuration) probably can be used successfully for telemetry reception during the Io encounter. Further analysis and testing of the effect of correlated noise from Jupiter is necessary before a final decision can be made about the feasibility of using the second largest (B) configuration of the VLA for reception of Galileo telemetry. Use of the B configuration could simplify the upgrades needed to support the Io encounter. Tests to help choose the preferred VLA configuration could be performed by using the VLA to observe the Magellan spacecraft at Venus during Jul. and Oct. 1991. Examination of the effects of planet noise on the VLA have implications beyond the use of that telescope for supporting the Io encounter. The effects of planet radio emission on spacecraft data received by antenna arrays are relevant to choosing the exact locations of antennas that might be built by the Deep Space Network in coming years.

Ulvestad, J. S.↗
NASA NTRS1999

A Process-based, Climate-Sensitive Model to Derive Methane Emissions from Natural Wetlands: Application to 5 Wetland Sites, Sensitivity to Model Parameters and Climate

Methane emissions from natural wetlands constitutes the largest methane source at present and depends highly on the climate. In order to investigate the response of methane emissions from natural wetlands to climate variations, a 1-dimensional process-based climate-sensitive model to derive methane emissions from natural wetlands is developed. In the model the processes leading to methane emission are simulated within a 1-dimensional soil column and the three different transport mechanisms diffusion, plant-mediated transport and ebullition are modeled explicitly. The model forcing consists of daily values of soil temperature, water table and Net Primary Productivity, and at permafrost sites the thaw depth is included. The methane model is tested using observational data obtained at 5 wetland sites located in North America, Europe and Central America, representing a large variety of environmental conditions. It can be shown that in most cases seasonal variations in methane emissions can be explained by the combined effect of changes in soil temperature and the position of the water table. Our results also show that a process-based approach is needed, because there is no simple relationship between these controlling factors and methane emissions that applies to a variety of wetland sites. The sensitivity of the model to the choice of key model parameters is tested and further sensitivity tests are performed to demonstrate how methane emissions from wetlands respond to climate variations.

Walter, Bernadette P.↗