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NASA NTRS1985

The penetration of mountain waves into the middle atmosphere

A linear nonhydrostatic model of gravity waves forced by a bell-shaped ridge is used to investigate the penetration of mountain waves into the stratosphere and mesosphere during winter and fall. Gravity waves with horizontal scales less than 30 km are found to be trapped near the tropopause and the stratopause in regions of strong winds. The effect of trapping these modes produces a disturbance whose structure broadens with height. In the mesosphere the disturbance appears 20-40 km downstream from the forcing depending on the strength of the intervening winds. Wavebreaking associated with the mountain wave is predicted in the lower stratosphere as a result of wave superposition; no individual harmonic reaches breaking amplitude. In the mesosphere, wave breakdown is more prevalent, and the disturbance spectrum is relatively more monochromatic as a result of the filtering of the shorter scale modes by the lower atmosphere.

Schoeberl, M. R.↗
NASA NTRS1985

Wave observations in outer planet magnetospheres

The first measurements of plasma waves and wave-particle interactions in the magnetospheres of the outer planets were provided by instruments on Voyager 1 and 2. At Jupiter, the observations yielded new information on upstream electrons and ions, bow shock dissipation processes, trapped radio waves in the magnetospheres and extended Jovian magnetotail, pitch angle diffusion mechanisms and whistlers from atmospheric lightning. Many of these same emissions were detected at Saturn. In addition, the Voyager plasma wave instruments detected dust particles associated with the tenuous outer rings of Saturn as they impacted the spacecraft. Most of the plasma wave activity at Jupiter and Saturn is in the audio range, and recordings of the wave observations have been useful for analysis.

Scarf, F. L.↗
NASA NTRS1986

Effect of capillary waves on surface tension

The present study is concerned with the effect which a cutting off of the capillary waves has on surface tension, taking into account a calculation based on capillary-wave theory. For simplicity, three-dimensional systems are considered, and capillary-wave theory is used to calculate sigma-k, the surface tension of an interface where only those modes with at least one wave-vector component greater than k are allowed. Attention is given to a review of capillary-wave theory, the calculation of surface tensions, a determination of the range of validity of capillary-wave theory, and some numerical examples. The quantitative behavior of sigma-k and its relation to the surface tension of a finite-size system are considered. The most surprising result is that sigma-k can be significantly larger than the unconstrained surface tension.

Kayser, R. F.↗
NASA NTRS1986

Planetary waves near the mesospheric easterly jet

Analysis of temperatures retrieved from satellite limb radiance measurements of the stratosphere and mesosphere during January-February 1979 reveals 3 prominent waves: wave number 3 with period 2.1 days, wave number 4 with period 1.8 days, and wave number 1 with period approximately 9 days. Each of these has maximum amplitude in the equatorward shear zone of the summer mesospheric easterly jet and propagates westward. Characteristics of the 1.8 day wave number 4 mode indicate that it arises from instability of the jet. The set of three waves may comprise an interacting triad.

Burks, D.↗
NASA NTRS1986

Plasma wave observations at comet Giacobini-Zinner

The plasma wave instrument on the International Cometary Explorer (ICE) detected strong ion acoustic waves together with electromagnetic whistlers and low-level electron plasma oscillations when the spacecraft was within two million km of the nucleus of comet Giacobini-Zinner. As ICE approached the anticipated bow-shock location, electromagnetic and electrostatic wave levels increased significantly, but even amidst this turbulence, the wave instrument detected structures with familiar bow shock characteristics that were correlated with observations of localized electron heating phenomena. Just beyond the visible coma, high-amplitude broadband waves were detected accounting for the significant electron heating observed in this region. Near closest approach, broadband electrostatic noise was detected together with a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis, the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.

Scarf, F. L.↗
NASA NTRS1985

Recent research on magnetospheric wave-particle interactions

Highlights of recent Stanford University VLF research in the Antarctic include new observations of wave-induced particle precipitation and controlled experiments on nonlinear wave growth phenomena. Higher-than-expected levels of burst precipitation have been discovered inside the plasmasphere, near L = 2, using subionospheric signal perturbations called 'Trimpi events'. Studies of burst precipitation have been extended to the region poleward of the plasmapause using the Siple transmitter signal as a waveguide probe. Experiments on the 'coherent wave instability', using the amplitude and frequency modulation capability of the new Siple transmitter, have produced exciting new results. Examples are: (1) better definition of the power threshold for the stimulation of temporal wave growth, (2) generation of strong sidebands by unamplified 'beat' waves and (3) generation of chorus-like elements within a band of simulated hiss. Using a new digital processing technique developed at Stanford, new features of the phase behavior of growing waves have been found. Opportunities for extending these experiments are discussed.

Helliwell, R. A.↗
NASA NTRS1986

An analysis of the role of drift waves in equatorial spread F

An account is given of results of rocket measurements of the wave number spectrum of equatorial spread F irregularities, with emphasis on wavelengths less than 100 m. The measurements were made from two sounding rockets launched from Peru as part of Project Condor. The Condor density fluctuation spectra display a break at a wavelength near 100 m, identical to that found in the PLUMEX experiment (Kelley et al., 1982). The Condor data also confirm a subrange in which the density and the wave potential obey the Boltzmann relation - a strong indication of the presence of low-frequency electrostatic waves with finite wavelength parallel to the magnetic field, perhaps low-frequency drift waves as proposed by Kelley et al. The Condor data are also consistent with the previous conjecture that drift waves only exist above 300 km altitude. To investigate the difference in spectra observed over two altitude ranges, the data must be fitted to a form for the power spectrum taken from Keskinen and Ossakow (1981). The fitted spectrum, along with empirically determined growth and dissipation rates, is used to calculate the energy pumped into the spectrum at long wavelengths as well as the energy dissipated at shorter wavelengths. It is found that the energy is balanced by classical collisional effects in the low-altitude case, but energy balance in the high-altitude case requires an enhanced dissipation of about 500 times that due to classical diffusion. The model is consistent with, but does not uniquely imply, an inverse cascade of drift wave turbulence in equatorial spread F.

Labelle, J.↗
NASA NTRS1986

A comparison of SIR-B directional ocean wave spectra with aircraft scanning radar spectra

Directional ocean wave spectra derived from Shuttle Imaging Radar-B (SIR-B) L-band imagery collected off the coast of Southern Chile on 11 and 12 October 1984 were compared with independent spectral estimates from two airborne scanning radars. In sea states with significant wave heights ranging from 3 to 5 meters, the SIR-B-derived sspectra at 18 deg and 25 deg off nadir yielded reasonable estimates of wavelengths, directions, and spectral shapes for all wave systems encountered, including a purely azimuth-traveling system. A SIR-B image intensity variance spectrum containing predominantly range-traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum. The prediction of a U.S. Navy global spectral ocean wave model on 11 October 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30 deg with respect to the mean of the aircraft and spacecraft estimates.

Beal, R. C.↗
NASA NTRS1986

Gravity waves in the middle atmosphere: Recent progress and needed studies

The recent recognition of the important role played by gravity waves in the large-scale circulation and thermal structure of the mesosphere and lower thermosphere has stimulated considerable research on their properties and their middle atmosphere effects. For example, these studies have begun to provide important information on gravity wave scales, propagation, filtering, and the processes responsible for saturation and turbulent diffusion. There remain, however, many areas in which our current understanding of middle atmosphere gravity waves is deficient. The purpose here is to review the progress that has been made to date and to suggest areas in which additional studies are most needed. Gravity wave scales, gravity wave saturation mechanisms, turbulence production and turbulent diffusion, and distribution of gravity wave energies and momentum fluxes with height and time are discussed.

Fritts, D. C.↗
NASA NTRS1986

Mesospheric gravity-wave climatology at Adelaide

The MF Adelaide partial-reflection radar has been operating continuously since November 1983. This has enabled a climatology of gravity-wave activity to be constructed for the mesosphere. The data have been analyzed for a medium-period range of 1 to 8 hr. and a longer period range between 8 and 24 hr. covering the inertio-period waves. The tidal motions have been filtered out prior to analysis. For the data analyses so far (Nov. 1983 to Dec. 1984), a number of interesting features emerged. Firstly, the wave activity at heights above 80 km shows a small seimannual variation with season with the activity being strongest in summer and winter. At heights below 80 km however, there is a similar but more marked variation with the weakest amplitudes occurring at the time of the changeovers in the prevailing circulation. If breaking gravity waves are responsible for much of the turbulence in the mesosphere, then the periods March to April and September to October might also be expected to be periods of weak turbulence. The wave field appears to be partially polarized. The meridional amplitudes are larger than the zonal amplitudes, especially in water. It is found that the degree of polarization is about 15% in summer and 30% in winter. The polarized component is found to propagate in the opposite direction to the background flow in the stratosphere, which suggests that the polarization arises through directional filtering of the waves as they propagate up from below.

Vincent, R. A.↗
NASA NTRS1986

Alfven-wave dissipation: A support mechanism for quiescent prominences

High resolution filtergrams or spectrograms of the main body of quiescent prominences often show a very vivid dynamical picture that cannot be reconciled with static models. Even if large differences exist between individual prominences in this respect, at least parts of the prominence are usually found to be in a 'choppy', turbulent state. Evidence for systematic flows are found in local regions in the prominence and also in the transition zone in the surroundings. These two regions are probably decoupled magnetically. Alfven waves are generally believed to be responsible for the heating in the upper chromosphere and corona (Hollweg 1986). Since evidence for the presence of Alfven-waves has also been found in the solar wind field, it is highly probable that such waves are generated in the convection zone of the sun and propagated outwards in the solar atmosphere wherever a proper magnetic field is present to carry the waves. The most basic magnetic formations in the solar atmosphere are simple loops. They occur all over the solar surface and cover a large range of magnetic field strengths. Loops with the strongest magnetic fields are found in active regions. It is to be expected that the Alfven-wave flux which is channelled into the loops from below, could show considerable variation both with heliocentric latitude, with time and locally between neighbouring loops. What happens when a magnetic loop is exposed to the appropriate Alfven-wave flux required to heat the upper solar atmosphere is examined.

Jensen, Eberhart↗
NASA NTRS1986

Analysis of the Giacobini-Zinner bow wave

The cometary bow wave of P/Giacobini-Zinner has been analyzed using the complete set of ICE field and particle observations to determine if it is a shock. Changes in the magnetic field and plasma flow velocities from upstream to downstream have been analyzed to determine the direction of the normal and the propagation velocity of the bow wave. The velocity has then been compared with the fast magnetosonic wave speed upstream to derive the Mach number and establish whether it is supersonic, i.e., a shock, or subsonic, i.e., a large amplitude wave. The various measurements have also been compared with values derived from a Rankine-Hugoniot analysis. The results indicate that, inbound, the bow wave is a shock with M = 1.5. Outbound, a subsonic Mach number is obtained, however, arguments are presented that the bow wave is also likely to be a shock at this location.

Smith, E. J.↗
NASA NTRS1986

Analysis of the Giacobini-Zinner bow wave

The cometary bow wave of P/Giacobini-Zinner was analyzed using the ICE field and particle observations to determine if it is a shock. Changes in the magnetic field and plasma flow velocities from upstream to downstream were analyzed to determine the direction of the normal and the propagation velocity of the bow wave. The velocity was compared with the fast magnetosonic wave speed upstream to derive the Mach number and establish whether it is supersonic, i.e., a shock, or subsonic, i.e., a large amplitude wave. The measurements were compared with values derived from a Rankine-Hugoniot analysis. The results indicate that inbound the bow wave is a shock with M = 1.5. Outbound, a subsonic Mach number is obtained. However, arguments that the bow wave is also likely to be a shock at this location are presented.

Smith, E. J.↗
NASA NTRS1987

Experimental Study of Shock Wave Interference Heating on a Cylindrical Leading Edge

An experimental study of shock wave interference heating on a cylindrical leading edge representative of the cowl of a rectangular hypersonic engine inlet at Mach numbers of 6.3, 6.5, and 8.0 is presented. Stream Reynolds numbers ranged from 0.5 x 106 to 4.9 x 106 per ft. and stream total temperature ranged from 2100 to 3400 R. The model consisted of a 3" dia. cylinder and a shock generation wedge articulated to angles of 10, 12.5, and 15 deg. A fundamental understanding was obtained of the fluid mechanics of shock wave interference induced flow impingement on a cylindrical leading edge and the attendant surface pressure and heat flux distributions. The first detailed heat transfer rate and pressure distributions for two dimensional shock wave interference on a cylinder was provided along with insight into the effects of specific heat variation with temperature on the phenomena. Results show that the flow around a body in hypersonic flow is altered significantly by the shock wave interference pattern that is created by an oblique shock wave from an external source intersecting the bow shock wave produced in front of the body.

Wieting, Allan R.↗
NASA NTRS1986

Electric and magnetic observations of the structure of standing waves in the magnetosphere

Electric and magnetic field instruments on the DE 1 spacecraft provided observations of toroidal standing wave oscillations of magnetic shells in 1981 and 1982. The amplitude and nodal structure of these waves, as a function of increasing magnetic latitude, was studied and compared with model calculations of these waves. Eight pulsation events were selected for study. The pulsations in these events ranged in period from 20 to 300 s. They were observed at L values between 2.8 and 9.7 and at magnetic latitudes from the equator to 48 deg. The duration of these pulsation events was 20 to 30 min, and they usually consisted of a 5- to 10-min growth in amplitude, a 5- to 10-min interval of fluctuating amplitude, and a 5- to 20-min interval of decreasing amplitude. The relative E and B amplitudes and the phase relations between the E and B waves could be qualitatively explained by the nodal structure of the waves predicted by model calculations. Detailed quantitative comparisons of E and B amplitudes were in agreement with more recent model calculations. In a few cases, electron densities were also available as derived from plasma frequency observations; in these cases the standing wave periods calculated were reasonably close to, but somewhat less than, the observed periods.

Cahill, L. J., Jr.↗
NASA NTRS1986

Plasma wave turbulence in the strong coupling region at comet Giacobini-Zinner

Within 100,000 km of comet Giacobini-Zinner's nucleus, strong plasma wave turbulence was detected by the ICE electric and magnetic field wave instruments. The spatial profiles of the wave amplitudes are compared with measurements of the heavy ion fluxes of cometary origin, the plasma electron density, and the magnetic field strength. The general similarity of the wave and heavy ion profiles suggest that the waves might be generated by free energy in the pick-up ion distribution function. However, the expected parallel streaming instability of electrostatic modes generates waves with frequencies that are too low to explain the observations. The observed low frequency magnetic turbulence is plausibly explained by the lower hybrid loss-cone instability of heavy ions.

Coroniti, F. V.↗
NASA NTRS1986

Electron acceleration via kinetic Alfven waves

Obliquely propagating shear Alfven waves are effected by kinetic effects. The polarization and dispersion of these waves are discussed and it is shown that electrons can be effectively accelerated along field lines by these waves. Several nonlinear effects such as solitary waves, nonlinear damping due to current-driven instabilities, and anomalous resistivity are also discussed. Recent observations of electric and magnetic field perturbations on auroral field lines which seem to be related to these waves are described. However, the evidence for the existence of these kinetic Alfven waves is not firm.

Goertz, C. K.↗
NASA NTRS1985

Plasma waves and instabilities

An account is given of the waves and instabilities occurring at collisionless shocks, with attention to the mechanisms responsible for the generation of these waves. The transition region of the shock usually involves an abrupt broadband burst of electrostatic noise that extends from below the lower hybrid resonance to near the electron plasma frequency, and by a broadband burst of whistler mode EM noise below the electron cyclotron frequency. Electrostatic lower hybrid waves are also noted. Upstream of the shock, electron plasma oscillations, ion acoustic waves, and intense ULF MHD waves are often observed. The region downstream of the shock is usually very chaotic; electrostatic waves often extend long distances into the downstream region, together with whistler mode emissions.

Gurnett, Donald A.↗