Engineering topics

Ismail, S.

Publications and source records attributed to Ismail, S..

30 records · Page 2

NASA NTRS1990

Airborne lidar observations in the wintertime Arctic stratosphere - Ozone

Large-scale distributions of ozone (O3) were measured with an airborne lidar system as part of the 1989 Airborne Arctic Stratospheric Expedition. Measurements of O3 distributions were obtained between January 6 and February 15, 1989, on 15 long-range flights into the polar vortex from the Solar Air Station, Norway. The observed O3 distribution was found to clearly indicate the edge of the polar vortex and to be an effective tracer of dynamical processes in the lower stratosphere. On the last two flights of the expedition, large regions with reduced O3 levels were observed by the lidar inside the polar vortex. Ozone had decreased by as much as 17 percent in the center of these areas, and using the in situ measurements made on the ER-2 aircraft, it was concluded that this decline was due to chemical O3 destruction.

Browell, E. V.↗

NASA NTRS1990

Airborne lidar observations in the wintertime Arctic stratosphere - Polar stratospheric clouds

Polar stratospheric cloud (PSC) distributions in the wintertime Arctic stratosphere and their optical characteristics were measured with a multiwavelength airborne lidar system as part of the 1989 Airborne Arctic Stratospheric Expedition. PSCs were observed on 10 flights between January 6 and February 2, 1989, into the polar vortex. The PSCs were found in the 14-27 km altitude range in regions where the temperatures were less than 195 K. Two types of aerosols with different optical characteristics (Types 1a and 1b) were observed in PSCs thought to be composed of nitric acid trihydrate. Water ice PSCs (Type 2) were observed to have high scattering ratios (greater than 10) and high aerosol depolarizations (greater than 10 percent) at temperatures less than 190 K.

Browell, E. V.↗

NASA NTRS1989

Logistics resupply and emergency crew return system for Space Station Freedom

Sometime in the late 1990's, if all goes according to plan, Space Station Freedom will allow the United States and its cooperating partners to maintain a permanent presence in space. Acting as a scientific base of operations, it will also serve as a way station for future explorations of the Moon and perhaps even Mars. Systems onboard the station will have longer lifetimes, higher reliability, and lower maintenance requirements than seen on any previous space flight vehicle. Accordingly, the station will have to be resupplied with consumables (air, water, food, etc.) and other equipment changeouts (experiments, etc.) on a periodic basis. Waste materials and other products will also be removed from the station for return to Earth. The availability of a Logistics Resupply Module (LRM), akin to the Soviet's Progress vehicle, would help to accomplish these tasks. Riding into orbit on an expendable launch vehicle, the LRM would be configured to rendezvous autonomously and dock with the space station. After the module is emptied of its cargo, waste material from the space station would be loaded back into it. The module would then begin its descent to a recovery point on Earth. Logistics Resupply Modules could be configured in a variety of forms depending on the type of cargo being transferred. If the LRM's were cycled to the space station in such a way that at least one vehicle remained parked at the station at all times, the modules could serve double duty as crew emergency return capsules. A pressurized LRM could then bring two or more crew-persons requiring immediate return (because of health problems, system failure, or unavoidable catastrophes) back to Earth. Large cost savings would be accrued by combining the crew return function with a logistics resupply system.

Ahne, D.↗

NASA NTRS1987

Tropopause fold structure determined from airborne lidar and in situ measurements

Airborne differential absorption lidar and in situ data obtained during the April 20, 1984 flight experiment conducted over Nevada and California are analyzed. The O3 and aerosols profiles and in situ measurements reveal a 2.0-km-deep layer (with high O3 concentrations and enhanced aerosol backscattering) and a correlation of 0.8 between O3 and aerosol backscatter (with both values decreasing about 25 percent along the central axis of the fold). It is observed that the cold boundary of the fold has weaker gradients, larger-scale undulations, and more irregularity than the warm boundary. The potential vorticity distribution along the flight path was derived from radiosonde data. A positive correlation between the O3 mixing ratio and the potential vorticity values in the fold is detected; the average ratio between O3 and potential vorticity is 50.2 ppbv/10 to the -5th sq cm deg per g s.

Browell, E. V.↗

NASA NTRS1986

Influence of Rayleigh-Doppler broadening on the selection of H2O dial system parameters

Computer simulations have enabled the performance of a H2O Differential Absorption Lidar (DIAL) system to be studied by spectrally analyzing the forward propagating and backscattered laser energy. The simulations were done for a high altitude (21 km) DIAL system operating in a nadir-viewing mode. The influence of Rayleigh Doppler broadening on DIAL measurement accuracies were evaluated and show that the Rayleigh broadening influence, which can be corrected to first order in regions free of large aerosol gradients, reduces the sensitivity of DIAL H2O measurement errors in the upper tropospheric region. The ability to correct the Rayleigh broadening and the selection of H2O DIAL parameters when all the systematic effects are combined, were discussed.

Ismail, S.↗

NASA NTRS1985

Airborne Lidar measurements of aerosols, mixed layer heights, and ozone during the 1980 PEPE/NEROS summer field experiment

A detailed summary of the NASA Ultraviolet Differential Absorption Lidar (UV DIAL) data archive obtained during the EPA Persistent Elevated Pollution Episode/Northeast Regional Oxidant Study (PEPE/NEROS) Summer Field Experiment Program (July through August 1980) is presented. The UV dial data set consists of remote measurements of mixed layer heights, aerosol backscatter cross sections, and sequential ozone profiles taken during 14 long-range flights onboard the NASA Wallops Flight Center Electra aircraft. These data are presented in graphic and tabular form, and they have been submitted to the PEPE/NEROS data archive on digital magnetic tape. The derivation of mixing heights and ozone profiles from UV Dial signals is discussed, and detailed intercomparisons with measurements obtained by in situ sensors are presented.

Browell, E. V.↗

NASA NTRS1985

Error caused by using a constant extinction/backscattering ratio in the lidar solution

The Bernoulli solution of the lidar equation with the assumption of a constant extinction/backscattering ratio can lead to errors in the derived aerosol extinction and backscattering profiles. This paper presents a general theoretical analysis of the errors that result from differences between the assumed and actual extinction/backscattering ratio profiles. Examples of the influence of the constant extinction/backscattering ratio assumption on the lidar derived aerosol extinction profile are presented for various laser wavelengths.

Sasano, Y.↗

NASA NTRS1985

Spaceborne lidar system for measurements of atmospheric water vapor and aerosols

The inclusion of a differential absorption lidar (DIAL) system as part of the NASA Earth Observing System (EOS) is proposed. Functioning at 720 nm, the DIAL could provide atmospheric water vapor profiles in the troposphere and stratosphere, and provide data for characterizing the physical properties of clouds. The use of frequency doubling of the laser could also open a window on the 355 nm region, and thereby molecular density and temperature profiles. The date would be of use in studies of the global hydrological cycle, the global radiation balance, climate, meteorology, and atmospheric structure and transport phenomena.

Browell, E. V.↗

NASA NTRS1985

Ultraviolet DIAL measurements of O3 profiles in regions of spatially inhomogeneous aerosols

The differential absorption lidar (DIAL) technique generally assumes that atmospheric optical scattering is the same at the two laser wavelengths used in the DIAL measurement of a gas concentration profile. Erros can arise in this approach when the wavelengths are significantly separated, and there is a range dependence in the aerosol scattering distribution. This paper discusses the errors introduced by large DIAL wavelength separations and spatial inhomogeneity of aerosols in the atmosphere. A Bernoulli solution for determining the relative distribution of aerosol backscattering in the UV region is presented, and scattering ratio boundary values for these solutions are discussed. The results of this approach are used to derive a backscatter correction to the standard DIAL analysis method. It is shown that for the worst cases of severe range dependence in aerosol backscattering, the residual errors in the corrected DIAL O3 measurements were less than 10 ppbv for DIAL wavelengths at 286 and 300 nm.

Browell, E. V.↗

NASA NTRS1984

Spaceborne lidar investigations of the atmosphere

Atmospheric investigations with a spaceborne lidar system are discussed. Measurements of aerosols, O3, and H2O with the NASA/LaRC airborne DIAL system are presented as examples of data obtained from space. The NASA/CNES study of an autonomous differential absorption lidar system is described. This system is a precursor to a spaceborne lidar system. Simulations of spaceborne lidar experiments are reviewed, and laser requirements for a spaceborne lidar system are presented.

Browell, E. V.↗

NASA NTRS1983

Characteristics of optical emissions and particle precipitation in polar cap arcs

Auroral emission features at high geomagnetic latitudes are identified and characterized using simultaneous optical and particle data from the ISIS-2 satellite. Polar cap arcs are identified from two-dimensional geomagnetic transforms of the optical data along with precipitating electron data for the time at which the satellite is on the field line intersecting the arc. No precipitating protons were detected for any of the arc crossings. The precipitations particle characteristics include: (1) an electron energy spectrum with a peak in the range 350-750 eV superposed on a soft spectrum like that observed in polar rain; (2) a normally isotropic pitch angle dependence, with the exception that field-aligned fluxes are observed in association with an inverted 'v' event; and (3) an energy flux range of approximately 0.8-3 erg/(sq cm s). A possible explanation of the observed precipitating particle characteristics is that parallel electric fields are accelerating polar rain type spectra at an altitude of several thousand km.

Murphree, J. S.↗

NASA NTRS1981

Spatial distribution of energetic particles in the distant magnetotail

The spatial distribution of energetic particles in the distant magnetotail is analyzed by using identical sets of two complementary energetic particle experiments aboard the IMP 7 and 8 spacecraft. It is noted that the combined IMP 7 and 8 data set provides nearly 10 years of satellite observation. The quantitative distributions of energetic electrons in two energy ranges and energetic protons are given in terms of occurrence frequency and the average particle flux intensity over an 80 earth radii x 70 earth radii plane perpendicular to the sun-earth line. A particle distribution in the shape of an O with a crossbar over the solar magnetospheric YZ plane in the antisolar direction is clearly demonstrated at all energies and for both species. It is pointed out that the O part of the distribution corresponds to the magnetosheath region and that the crossbar is the average plasma sheet configuration. A distinct, species dependent, dawn-dusk asymmetry in particle distribution is observed in the plasma sheet and, to a lesser extent, in the magnetosheath.

Meng, C.-I.↗