Engineering topics

Hedin, A. E.

Publications and source records attributed to Hedin, A. E..

At least 37 records · Page 2

NASA NTRS1988

Empirical global model of upper thermosphere winds based on atmosphere and dynamics explorer satellite data

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been used to generate an empirical wind model for the upper thermosphere, analogous to the MSIS model for temperature and density, using a limited set of vector spherical harmonics. The model is limited to above approximately 220 km where the data coverage is best and wind variations with height are reduced by viscosity. The data base is not adequate to detect solar cycle (F10.7) effects at this time but does include magnetic activity effects. Mid- and low-latitude data are reproduced quite well by the model and compare favorably with published ground-based results. The polar vortices are present, but not to full detail.

Hedin, A. E.↗

NASA NTRS1988

Atomic oxygen modeling in the upper thermosphere

Empirical models of atomic oxygen in the earth's thermosphere are discussed, and calibration problems in satellite drag and in situ mass spectrometer measurements are reviewed. Models based on drag data and mass spectrometer data are found to agree on average to within 15 percent, suggesting that the absolute values are reasonably well known in the upper thermosphere. Comparison of different models with various data sources show residuals of at least 15 percent which are the results of unmodeled magnetic storm, EUV, and geographical variations and smaller scale variations caused by gravity waves.

Hedin, A. E.↗

NASA NTRS1988

Comparison of OGO 6 measured thermospheric temperatures with the MSIS-86 empirical model

The thermospheric temperatures measured by the Fabry-Perod interferometer on the OGO 6 satellite are found to be reasonably represented by the mass spectrometer/incoherent scatter 1986 (MSIS-86) empirical model except for two anomalies, one in the South Atlantic and the other near noon local time. These anomalies are likely due to measurement problems. The OGO 6 temperature data were not used in the generation the MSIS models, so this is an independent comparison of measured and model temperatures. The measurements were made primarily during daytime at mid to low latitudes and throughout the day at high latitudes. On average, the measured temperatures are 16 K below the MSIS-86 model temperatures. Latitude gradients during solstices as well as for the yearly average are well represented by the model, as are high-latitude longitudinal and magnetic activity variations.

Hedin, A. E.↗

NASA NTRS1988

The atmospheric model in the region 90 to 2000 km

A comprehensive revision of the MSIS-83 earth atmosphere model was undertaken to extend predictions to the mesopause, on the basis of temperature, density, and composition data obtained by sounding rockets, satellites, and incoherent scatter radars. Attention is presently given to a semiempirical model similar to MSIS-83, which incorporates additional terms in order to represent seasonal differences in the morphology of composition variations under both quiet and disturbed polar region conditions. The data base is expanded to include both composition and temperature data from the Dynamics Explorer-2 satellite.

Hedin, A. E.↗

NASA NTRS1987

Characteristics of wavelike fluctuations in Dynamics Explorer neutral composition data

Wavelike fluctuations in neutral composition data obtained with the neutral atmospheric composition system quadrupole mass spectrometer carried aboard the Dynamics Explorer (DE 2) satellite are at a maximum in the vicinity of the magnetic poles. Typical rms amplitudes near the poles for N2, O, He, and Ar fluctuations in the 400- to 4000-km-wavelength band are found to be 11, 6, 6, and 20 percent, respectively. Amplitudes near the equator are roughly a third of the polar amplitudes, and activity in the 50- to 400-km-wavlength band is roughly 20 percent of the longer-wavelength activity.

Hedin, A. E.↗

NASA NTRS1987

Solar EUV induced variations in the thermosphere

The effects of solar activity on the thermosphere are reviewed. The neutral total density, temperature, and electron density increase with solar activity, and their variations are fairly well correlated with variations in solar 10.7-cm flux. The electron temperature, by contrast, has a complex behavior with solar activity, depending on altitude and season, as increased heat input is balanced by increased cooling provided by the enhanced electron density. Also, individual neutral species do not all increase in step but are influenced by dissociation and dynamical processes. The increased ion drag at higher solar activities inhibits the neutral wind from increasing in the same proportion as the neutral temperatures and densities.

Hedin, A. E.↗

NASA NTRS1986

A dynamo theory prediction for solar cycle 22: Sunspot number, radio flux, exospheric temperature, and total density at 400 km

Using the dynamo theory method to predict solar activity, a value for the smoothed sunspot number of 109 + or - 20 is obtained for solar cycle 22. The predicted cycle is expected to peak near December, 1990 + or - 1 year. Concommitantly, F(10.7) radio flux is expected to reach a smoothed value of 158 + or - 18 flux units. Global mean exospheric temperature is expected to reach 1060 + or - 50 K and global total average total thermospheric density at 400 km is expected to reach 4.3 x 10 to the -15th gm/cu cm + or - 25 percent.

Schatten, K. H.↗

NASA NTRS1986

Hemispheric asymmetries of the thermospheric semiannual oscillation

An analysis of the data on the asymmetries of the terrestrial thermospheric semiannual oscillation (SAO), recorded by satellites, radar stations, and rocket soundings (Hedin, 1983), is presented. The possible sources for the thermospheric SAO are discussed. An interpretation is presented of the hemispheric asymmetries of the thermospheric SAO in terms of gravity-wave activities, in which the proposed mechanism is associated with the hemispheric asymmetries in the atmospheric circulation due to the orographic differences between the two hemispheres. Consideration is given to the energy requirements for the observed hemispheric difference in the thermospheric SAO. The overall evidence supporting the proposed interpretation is discussed.

Maeda, K.↗

NASA NTRS1986

DE-2 mass spectrometer observations relevant to the Shuttle glow

A comparison of satellite mass spectrometer data from Dynamics Explorer-2 and Atmosphere Explorer-C and -D provides new evidence supporting the hypothesis that NO2 is the source of the observed continuum glow near ram-facing surfaces of the Space Shuttle. Surface reactions of thermospheric N and O in mass spectrometer ion sources produce NO and NO2 in amounts highly dependent on surface temperature and composition, with direct exposure of ion source surfaces to rammed gas a necessary condition for the production of large amounts of NO2. Initial orbit data indicate that a period of surface conditioning is necessary before these odd nitrogen molecules can be produced efficiently.

Engebretson, M. J.↗

NASA NTRS1985

Interaction of zonal winds with the equatorial midnight pressure bulge in the earth's thermosphere - Empirical check of momentum balance

The paper is concerned with the effect of the equatorial midnight pressure on the nighttime zonal winds in the altitude range 300-400 km. The analysis is based on zonal momentum balance of measured quantities at the specified altitude combined with the nighttime average-pressure variations given by the Atmosphere Explorer-E (AE-E) satellite and the ion density given by the model of Chiu (1975). It is found that the nighttime pressure variation obtained from temperatures and densities measured on AE-E is consistent with the observed variations in the zonal wind and that the zonal wind decay time due to ion drag and viscosity reasonably accounts for the observed decay in velocity leading to the midnight minimum.

Herrero, F. A.↗

NASA NTRS1985

Morphology of thermospheric composition variations in the quiet polar thermosphere from Dynamics Explorer measurements

Composition data obtained by the Neutral Atmosphere Composition Spectrometer carried aboard the Dynamics Explorer 2 satellite have been examined for variations in the polar regions during magnetically quiet conditions. A persistent enhancement of N2 density and a depletion of He density are present in the vicinity of the magnetic poles with maximum density response in the morning hours (magnetic) on average. A universal time (UT) variation in average density levels is evident near both the geographic and magnetic poles. There are systematic morphology changes with UT and between summer and winter which are qualitatively consistent with the shifting and spreading of thermospheric heating effects by the global circulation systems driven by EUV and magnetospheric sources. The magnitude of the UT variations is larger in the southern hemisphere on average, and the UT variations are larger in local winter than summer.

Hedin, A. E.↗

NASA NTRS1985

On the structure and dynamics of the thermosphere

Thermospheric temperature, composition and wind measurements from the Dynamics Explorer satellite (DE-2) are interpreted using a three dimensional, multiconstituent spectral model. The analysis accounts for tides driven by the absorbed solar radiation as well as energy and momentum coupling involving the magnetosphere and lower atmosphere. Phenomena associated with the annual tide, polar circulation, magnetic storms and substorms are discussed.

Mayr, H. G.↗

NASA NTRS1985

Models of Venus neutral upper atmosphere - Structure and composition

Models of the Venus neutral upper atmosphere, based on both in situ and remote sensing measurements, are provided for the height interval from 100 to 3500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100-150 km, 150-250 km, and 250-3500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed.

Keating, G. M.↗

NASA NTRS1984

Correlations between thermospheric density and temperature, solar EUV flux, and 10.7-cm flux variations

Simultaneously measured thermospheric N2 densities and solar EUV fluxes obtained by the AE-E satellite are compared with ground-based solar 10.7-cm fluxes and calcium plage measurements. Short-wavelength (coronal) EUV emissions correlate better with thermospheric density than the 10.7-cm flux, although the reduction in density residuals is small. Correlation of density with a calcium plage index was somewhat worse than with 10.7. Although the overall correlation of 10.7-cm flux with EUV fluxes is high, the best fit slopes are different for short- and long-term variations, and there are instances where the short-term behavior of various EUV emissions and that of the 10.7-cm flux are distinctly different. For both density and EUV, a two-factor formula based on daily 10.7-cm flux and a running mean of the 10.7-cm flux provides a better fit than using the daily 10.7-cm flux alone.

Hedin, A. E.↗

NASA NTRS1984

A dynamo theory prediction for solar cycle 22 - Sunspot number, radio flux, exospheric temperature, and total density at 400 km

Using the 'dynamo theory' method to predict solar activity, a value for the smoothed sunspot number of 109 + or - 20 is obtained for solar cycle 22. The predicted cycle is expected to peak near December, 1990 + or - 1 year. Concommitantly, F(10.7) radio flux is expected to reach a smoothed value of 158 + or - 18 flux units. Global mean exospheric temperature is expected to reach 1060 + or - 50 K and global total average total thermospheric density at 400 km is expected to reach 4.3 x 10 to the -15th gm/cu cm + or - 25 percent.

Schatten, K. H.↗

NASA NTRS1984

Thermospheric superrotation revisited

The characteristics of thermospheric superrotation and accompanying latitudinal variations in temperature and composition are examined theoretically. Superrotation was measured by the Dynamics Explorer 2 satellite, which provided data for the atmospheric rotational velocity with respect to that of the planet for all seasons for 300-350 km altitudes. Zonally averaged circulation is described with momentum equations. A spectral model covers the superrotation, and account is taken of symmetric radiative and auroral components, asymmetric ion drag, an asymmetric pressure gradient and a viscous component. The horizontal wind field of the diurnal tide and diurnal variations in ion density which produce a momentum source are the largest contributors to superrotation at low and midlatitudes.

Mayr, H. G.↗

NASA NTRS1983

A revised thermospheric model based on mass spectrometer and incoherent scatter data - MSIS-83

It is noted that the model presented here extends the previous description of neutral parameters to the base of the thermosphere in a continuous manner while maintaining the basic structure of the MSIS model at higher altitudes. As the altitude decreases, the composition approaches lower atmosphere values, whereas yearly, and to a lesser extent daily, variations in temperature and density are in reasonable agreement with earlier results for the lower thermosphere. An alternate description is given of magnetic storm variations on the basis of the three hour ap indices and an 8- to 10-hour exponential decay in thermospheric density and temperature response after a heating event. Additional coefficients are included for the time independent and magnetic activity terms, among them a longitudinally dependent seasonal magnetic activity effect. The description of molecular oxygen derives from mass spectrometer and EUV absorption measurements rather than ion chemistry.

Hedin, A. E.↗

NASA NTRS1983

Global empirical model of the Venus thermosphere

Direct measurements of neutral CO2, O, CO, N2, He, and N densities from the Pioneer Venus Orbiter Neutral Mass Spectrometer are described in terms of a spherical harmonic representation (latitude and local time coordinates) of exospheric temperature and number densities at 150 km, using modified Bates temperature profiles. The exospheric temperatures are determined from the altitude variations of atomic oxygen. A global average temperature of 228 K is derived with a first harmonic variation of 5%. The altitude profiles are extended downward to 100 km by using empirical formulas to provide a transition through the turbopause region (simulating the effect of eddy diffusion and vertical flows) and matching entry probe density data. The model reflects the observed variations of temperature and density with the 10.7 cm radio flux index. For a given change in flux at the planet, the exospheric temperature on Venus changes by only 10% of the change seen in the terrestrial thermosphere.

Hedin, A. E.↗