Search NASA⌕ Search

Engineering topics

Walterscheid, R. L.

Publications and source records attributed to Walterscheid, R. L..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Comparison of theories for gravity wave induced fluctuations in airglow emissions

A comparison is undertaken of theories for the gravity wave induced fluctuations in the intensity of airglow emissions and the associated temperature of the source region. The comparison is made in terms of Krassovsky's ratio eta(sub E) for a vertically extended emission region (eta(sub E) is the ratio of the vertically integrated normalized intensity perturbation to the vertically integrated intensity-weighted temperature perturbation). It is shown that the formulas for eta(sub E) in the works by Tarasick and Hines (1990) and Schubert et al. (1991) are in agreement for the case of an inviscid atmosphere. The calculation of eta(sub E) using the theory of Tarasick and Hines (1990) requires determination of their function chi; we show that chi is simply related to the 'single-level' Krassovsky's ratio eta of Schubert et al. (1991). The general relationship between chi and eta is applied to a simple chemical-dynamical model of the O2 atmospheric airglow and the altitude dependence of these quantities is evaluated for nonsteady state chemistry. Though the Tarasick and Hines (1990) formula for eta(sub E) does not explicitly depend on the scale heights of the minor constituents involved in airglow chemistry, eta(sub E) implicitly depends upon these scale heights through its dependences on chemical production and loss contained in chi. We demonstrate this dependence of eta(sub E) for the OH nightglow on atomic oxygen scale height by direct numerical evaluation of eta(sub E) in this case the dependence originates in the chemical production of perturbed ozone.

Walterscheid, R. L.↗
NASA NTRS1993

Gravity wave-driven fluctuations in the O2 atmospheric (0-1) nightglow from an extended, dissipative emission region

The wave-driven fluctuations in the O2(0-1) atmospheric nightglow is modeled and the parameter (eta) is calculated using a model that accounts for either three-body recombination of atomic oxygen atoms alone to form the O2(b exp 1 Sigma(g)(+)) state directly, or by the further inclusion of the process that allows the formation of the O2(c exp 1 Sigma(u)(-)) intermediate state. The calculations are performed for a latitude of 18 deg N and for the months of March and June. The general results, which display how (eta) varies with wave period, horizontal wavelength, season, and chemical scheme, show that for given values of wave period and horizontal wavelength it is not possible to discriminate between seasonal effects and between the effects of different chemical schemes at evanescent and short gravity wave periods. It is shown that, when quenching by atomic oxygen is ignored, the resulting values of (eta) calculated with the complete chemistry are similar to those obtained from the three-body recombination scheme alone.

Hickey, Michael P.↗
NASA NTRS1992

The neutral circulation in the vicinity of a stable auroral arc

The effects of asymmetric potential distributions, arc-neutral feedback, and cross-arc winds on the neutral circulation near auroral arc were examined using a sophisticated two-dimensional high-resolution model of the neutral atmosphere which was preconditioned by allowing weak diffuse aurora to exist prior to the formation of an arc. Simulations were performed with asymmetric potential structures and with an initial state which was warmed and accelerated due to a preexisting diffuse aurora, as well as with arc-zonal wind feedback and with large-scale cross-arc flow. It is shown that, with a preexisting zonal flow and asymmetrical arc structures, strong zonal winds are generated within the arc. Simulations with the arc-zonal feedback indicate that the effect of feedback is not large because of the limited duration of an arc. Simulations with a strong large-scale cross-arc wind indicate that the zonal wind is not strongly sensitive to the large-scale wind.

Walterscheid, R. L.↗
NASA NTRS1992

Seasonal and latitudinal variations of gravity wave-driven fluctuations in OH nightglow

A model which incorporates extended OH-layer emission and gravity-wave dynamics with eddy diffusivities is set forth to analyze seasonal and latitudinal variations in the OH nightglow. The 2D analysis is aimed at determining the nature of the relationship between the oscillation in intensity about a mean intensity and the oscillation in temperature of the emission region about a mean temperature. Competing eddy diffusivities modify the local values and altitudes of maximum wave amplitude, so long-period characterization of the relationship is imprecise. Seasonal trends in the relationship are noted for periods of several hrs or less that are primarily related to seasonal changes in the mesopause undisturbed temperature. Latitudinal trends are shown to be less pronounced, and trends that exist at intermediate periods are difficult to isolate due to interference effects.

Hickey, M. P.↗
NASA NTRS1992

Finite amplitude gravity waves in the Venus atmosphere generated by surface topography

A two-dimensional, fully nonlinear, nonhydrostatic, gravity wave model is used to study the evolution of gravity waves generated near the surface of Venus. The model extends from near the surface to well above the cloud layers. Waves are forced by applying a vertical wind at the bottom boundary. The boundary vertical wind is determined by the product of the horizontal wind and the gradient of the surface height. When wave amplitudes are small, the near-surface horizontal wind is the zonally averaged basic-state zonal wind, and the length scales of the forcing that results are characteristic of the surface height variation. When the forcing becomes larger and wave amplitudes affect the near-surface horizontal wind field, the forcing spectrum becomes more complicated, and a spectrum of waves is generated that is not a direct reflection of the spectrum of the surface height variation. Model spatial resolution required depends on the amplitude of forcing; for very nonlinear cases considered, vertical resolution was 250 m, and horizontal resolution was slightly greater than 1 km. For smaller forcing amplitudes, spatial resolution was much coarser, being 1 km in the vertical and about 10 km in the horizontal. Background static stability and mean wind are typical of those observed in the Venus atmosphere.

Young, R. E.↗
NASA NTRS1991

Gravity wave-driven fluctuations in OH nightglow from an extended, dissipative emission region

The paper theoretically examines the modification of the characteristics of OH nightglow from an extended emission region by eddy momentum and eddy thermal diffusivities. The reactions that account for OH decay are presented with the additional modifications demonstrating the importance of the upper limit of vertical integration of the extended source. When the vertical wavelengths are smaller than the thickness of the main OH emission region, oscillations cause the effects, particularly at long wavelengths. Eddy diffusion increases the vertical wavelength, and therefore the interference effects related to a finite range of vertical integration can be reduced by accounting for eddy diffusion. It is determined that the effects of eddy momentum and thermal diffusivities are important elements of gravity wave dynamics and should be considered when calculating OH emission perturbations and related variables.

Schubert, G.↗
NASA NTRS1989

The neutral E region zonal winds during intense postmidnight diffuse aurora - Response to observed particle fluxes

Zonal winds associated with diffuse auroras were simulated using observed fluxes of electron precipitation and a high-resolution time-dependent numerical model described by Waltershcheid et al. (1985). A very strong dynamically unstable E region jet is simulated for an electric field of 50 mV/m, and the results suggest a connection with omega band. These zonal winds are not as strong as those reported by Lyons and Walterscheid (1985), but are much stronger (by a factor of 3 or greater) than the E region winds simulated by Fuller-Rowell (1985). The effects of large-scale cross-arc winds were simulated to evaluate their effect; it is shown that strong cross-arc winds are required to prevent the formation of a strong E zonal jet. The results agree qualitatively with radar observations of zonal winds by Johnson et al. (1987).

Walterscheid, R. L.↗
NASA NTRS1986

Feedback between neutral winds and auroral arc electrodynamics

The feedback between neutral atmospheric winds and the electrodynamics of a stable, discrete auroral arc is analyzed. The ionospheric current continuity equation and the equation for neutral gas acceleration by ion drag are solved simultaneously, as a function of time. The results show that, in general, the electric field in the ionosphere adjusts to neutral wind acceleration so as to keep auroral field-aligned currents and electron acceleration approximately independent of time. It is thus concluded that the neutral winds that develop as a result of the electrodynamical forcing associated with an arc do not significantly affect the intensity of the arc.

Lyons, L. R.↗
NASA NTRS1986

Viscous pumping and the spin-down of thermospheric gyres and jets

Strong gyres and jets can be generated at auroral latitudes in the thermosphere by enhanced electric fields during geomagnetic substorms. Typical height profiles of ion density suggest that the ion drag force should generate large curvature in the vertical profile of the winds in the highly viscous region of the thermosphere above about 200 km. It is proposed that the poststorm spin-down of these gyres and jets proceeds via Ekman circulations driven by the curvatures in the height profiles of the winds. Analytic and numerical calculations of the ageostrophic winds forced by curvature in model geostrophic wind profiles show that the ageostrophic wind speeds and directions depend mainly on the kinematic viscosity in the region of curvature and the total change in shear in the geostrophic wind. Ageostrophic wind speeds for typical thermospheric jets can exceed 200 m/s (about 50 percent of the jet winds). Spin-down times of thermospheric jets and cyclonic gyres by the Ekman pumping mechanism are estimated at less than about 6 hours.

Walterscheid, R. L.↗
NASA NTRS1985

The perturbed neutral circulation in the vicinity of a symmetric stable auroral arc

In the present investigation, a sophisticated high-resolution time-dependent numerical model is used to study the neutral response to changes in the E region electrodynamical forcing and particle heating associated with the appearance of a stable auroral arc. The initial condition is a nighttime ionosphere in the presence of a background precipitation of electrons associated with diffuse auroras. A simulation study is conducted of the perturbation in the neutral circulation. The simulations were performed on the basis of a model which describes nonlinear, nonhydrostatic viscous flow on a rotating atmosphere.

Walterscheid, R. L.↗
NASA NTRS1985

Generation of auroral Omega bands by shear instability of the neutral winds

Thermospheric neutral wind acceleration via ion drag in the conducting E-region of the ionosphere is greatly increased by electron precipitation associated with auroras. This increased acceleration can lead to the development of significant horizontal wind shears, which were found to be unstable to the Kelvin-Helmholtz shear instability. Numerical simulation of the neutral response to an intense, postmidnight, diffuse aurora shows tne formation of an E-region 'jet stream' within the aurora, with peak winds speeds greather than 700 m/s after one hour. It is proposed that this jet stream produces unstable Kelvin-Helmholtz waves, which can drive waves of discrete aurora along the poleward boundary of the preexisting diffuse aurora. It is suggested that such auroral waves, driven by the neutral winds, form eastward propagating waves (omega bands) occasionally observed along the poleward boundary of postmidnight diffuse auroras. It was found that neutral wind shears that develop in response to discrete auroral arcs are unstable; however, the resulting wind waves are not expected to drive significant auroral waves along discrete arcs.

Lyons, L. R.↗
NASA NTRS1985

Zonal winds and the angular momentum balance of Venus' atmosphere within and above the clouds

Temperatures and pressures inferred from radio occultation data acquired by the Pioneer Venus orbiter between September 1982 and November 1983 are used to derive cyclostrophic zonal winds in the middle atmosphere of Venus (1350 to 2.1 mb, 10 deg to 70 deg latitude). The main feature of the wind field is a jet positioned just above the cloud tops at 70 km and approximately 48 deg latitude. The maximum speed of the jet is about 130 m/s. A comparison with results of similar analyses on Pioneer Venus radio occultation data obtained between December 1978 and October 1981 suggests an equatorward shift of the jet and a decrease in jet speed during this five-year time interval. It is proposed that the poleward transport of westward zonal momentum by the upper branch of the cloud level Hadley cell supplies the excess momentum of the jet and maintains it against dissipation. The location of the jet thereby provides a minimum estimate of the latitudinal extent of the Hadley cell. Cyclostrophic zonal wind velocities decrease with height above about 70-75 km. It is suggested that this deceleration of the superrotation in equatorial latitudes is due to the dissipation of vertically propagating thermal tides forced primarily at altitudes around 65 km.

Walterscheid, R. L.↗
NASA NTRS1985

The neutral wind 'flywheel' as a source of quiet-time, polar-cap currents

The neutral wind pattern over the summer polar cap can be driven by plasma convection to resemble the convection pattern. For a north-south component of the interplanetary magnetic field Bz directed southward, the wind speeds in the conducting E-region can become approximately 25 percent of the electric field drift speeds. If convection ceases, this neutral wind distribution can drive a significant polar cap current system for approximately 6 hours. The currents are reversed from those driven by the electric fields for southward Bz, and the Hall and field-aligned components of the current system resemble those observed during periods of northward Bz. The current magnitudes are similar to those observed during periods of small, northward Bz; however, observations indicate that electric fields often contribute to the currents as much as, or more than, the neutral winds.

Lyons, L. R.↗
NASA NTRS1982

Modelling of auroral electrodynamical processes: Magnetosphere to mesosphere

Research conducted on auroral electrodynamic coupling between the magnetosphere and ionosphere-atmosphere in support of the development of a global scale kinetic plasma theory is reviewed. Topics covered include electric potential structure in the evening sector; morning and dayside auroras; auroral plasma formation; electrodynamic coupling with the thermosphere; and auroral electron interaction with the atmosphere.

Chiu, Y. T.↗