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NASA NTRS1974

Whistler waves observed upstream from collisionless shocks

Waves in the frequency range 0.5-4. Hz have been studied in the region upstream of the earth's bow shock with data from the flux-gate magnetic field experiment on Imp 6. Such waves are invariably detected adjacent to the shock, persisting upstream for intervals often less than a minute but occasionally of the order of many hours. Analysis of 150 examples of these waves during a 3-month interval indicates that propagation directions generally make angles of between 20 and 40 deg with the field direction. The waves as measured in the spacecraft frame of reference are either left- or right-hand-polarized with respect to the average field direction. The left-handed waves generally have lower frequencies than the right-handed waves, and the left-handed frequencies never exceed 2.5 Hz. The measured sense of polarization is found to depend on the propagation direction (or alternatively, the field direction) relative to the solar wind direction.

Fairfield, D. H.↗

NASA NTRS1974

Waves in the Jovian upper atmosphere

We examine a propagating wave interpretation of the temperature profile features observed in the Jovian upper atmosphere by Veverka et al. (1974). Inertia-gravity waves with frequencies on the order of .003 per sec are consistent with the data. If the interpretation is correct, and if the waves carry energy upward, it implies (1) that there is excitation of such waves at lower levels, (2) that eddy diffusivities on the order of 1,000,000 sq cm/sec are probably generated by the waves, and (3) that the energy carried by waves is important to the upper atmospheric heat balance.

French, R. G.↗

NASA NTRS1974

A simulation of synthetic aperture radar imaging of ocean waves

A simulation of radar imaging of ocean waves with synthetic aperture techniques is presented. The modelling is simplistic from the oceanographic and electromagnetic viewpoint in order to minimize the computational problems, yet reveal some of the physical problems associated with the imaging of moving ocean waves. The model assumes: (1) The radar illuminates a one-dimensional, one harmonic ocean wave. (2) The scattering is assumed to be governed by geometrical optics. (3) The radar is assumed to be down-looking, with Doppler processing (range processing is suppressed due to the one-dimensional nature of the problem). (4) The beamwidth of the antenna (or integration time) is assumed to be sufficiently narrow to restrict the specular points of the peaks and troughs of the wave. The results show that conventional processing of the image gives familiar results if the ocean waves are stationary. When the ocean wave dispersion relationship is satisfied, the image is smeared due to the motion of the specular points over the integration time. In effect, the image of the ocean is transferred to the near field of the synthetic aperture.

Swift, C. T.↗

NASA NTRS1974

Wave climate model of the Mid-Atlantic shelf and shoreline (Virginian Sea): Model development, shelf geomorphology, and preliminary results

A computerized wave climate model is developed that applies linear wave theory and shelf depth information to predict wave behavior as they pass over the continental shelf as well as the resulting wave energy distributions along the coastline. Reviewed are also the geomorphology of the Mid-Atlantic Continental Shelf, wave computations resulting from 122 wave input conditions, and a preliminary analysis of these data.

Goldsmith, V.↗

NASA NTRS1974

Effects of interplanetary shock waves on energetic charged particles

Experimental data on the influence of interplanetary perpendicular and oblique shock waves on the ambient energetic protons are presented along with a theoretical analysis of the acceleration of particles in almost perpendicular shock waves. It was found that low-energy protons can be accelerated in perpendicular shock waves by repeated crossings of the shock front up to a maximum energy given by the product of their initial energy times the ratio of the magnetic fields. High-energy protons need to stay at the shock front for longer times than low-energy protons in order to reach the same relative energy gain. In the theoretical study of proton acceleration at almost perpendicular shock waves, it was found that protons reflected at shock waves with the angle between the upstream magnetic field and the shock normal greater than about 80 deg achieve large energy gains at the shock front. The larger this angle, the higher the energy gain. However, the reflection and energization of protons at these shock waves is not 'instantaneous', neither is it a one-step process: it is performed through repeated crossings of the shock front.

Sarris, E. T.↗

NASA NTRS1974

A comparison between theory and experiments on non-linear three-wave interactions in plasmas

Quantitative comparisons between theory and experiments on non-linear three-wave interactions in a nearly homogeneous plasma are presented in this paper. First, the theoretical spatial behaviour of signal and idler waves excited by a large amplitude pump wave is predicted, with damping of all waves included. These results are then specialized to allow detailed quantitative comparisons to be made with available experimental data on parametrically excited ion-acoustic waves. Where good agreement is obtained, the comparison indicates further measurements required to verify more completely the basic theory of three-wave interactions in plasmas. Where wide disagreement is obtained, the comparison indicates possible directions in which the theory might be improved.

Peng, Y.-K. M.↗

NASA NTRS1975

Flux limit on cosmic ray acceleration by strong spherical pulsar waves

The study investigates the acceleration of cosmic rays by superrelativistic plasma waves. The pulsar wave field imposed upon the particles is that of a magnetic dipole wave including azimuthal dependence and near field terms. The refractive index of the plasma is assumed to be constant everywhere. Two distinct zones appear: in the near zone spherical effects dominate, particles may be decelerated; in the far zone the results for the motion differ noticeably from the vacuum case. From a WBK-like treatment for a spherical S wave, it is concluded that for a wave solution to be valid even near the light cylinder, the wave can carry a sufficiently small particle flux.

Asseo, E.↗

NASA NTRS1976

Jovian electron bursts - Correlation with the interplanetary field direction and hydromagnetic waves

The bursts of relativistic electrons detected on Pioneer 10 upstream from Jupiter and within 400 Jovian radii of the planet have been found to be correlated with the interplanetary magnetic field. In three examples, electrons with energies between 3 and 6 MeV escaping from Jupiter's magnetosphere were observed only when the interplanetary magnetic field was along the Jupiter-spacecraft line. Large-amplitude interplanetary waves with characteristic periods of 10 min were found to be well correlated with intervals during which the field was along the Jupiter-spacecraft line. Abrupt changes in the field away from the preferred direction caused equally abrupt terminations of the waves with an accompanying reduction in the electron flux. These results are consistent with propagation of the electrons from Jupiter to Pioneer along the magnetic field lines. Hydromagnetic wave generation by Jovian charged particles, presumably the relativistic electrons themselves, as they travel upstream, appears to be an attractive explanation for the origin of the waves. At the observed frequency, hydromagnetic waves are Doppler-shifted to the gyrofrequency of the relativistic electrons. A plasma instability that appears capable of explaining the observations is a cyclotron overstability that occurs when the velocity of runaway electrons exceeds the velocity of hydromagnetic waves.

Smith, E. J.↗

NASA NTRS1976

A comparative study of three techniques for using the spectral matrix in wave analysis

Three techniques - power spectral analysis, coherency analysis, and wave analysis - for studying ULF waves in the magnetosphere are described. These techniques, while mathematically dissimilar, are all based on the assumption that, at any single frequency, a single plane wave, propagating along the normal to the plane, is present. Wave polarization parameters provided by these techniques are important in the development of models of wave generation. A comparative study of the three techniques was carried out using simulated data involving known wave and noise properties and real ULF event data from ATS 1 and 6.

Arthur, C. W.↗

NASA NTRS1976

Waves in active and passive periodic structures - A review

The theory and recent applications of waves in periodic structures are reviewed. Both the Floquet and coupled waves approach are analyzed in some detail. The theoretical part of the paper includes wave propagation in unbounded and bounded active or passive periodic media, wave scattering from periodic boundaries, source radiation (dipole, Cerenkov, transition, and Smith-Purcell) in periodic media, and pulse transmission through a periodic slab. The applications part covers the recent development in a variety of fields: distributed feedback oscillators, filters, mode converters, couplers, second-harmonic generators, deflectors, modulators, and transducers in the fields of integrated optics and integrated surface acoustics. Work on insect compound eyes, mechanical structures, ocean waves, pulse compressions, temperature waves, and cholesteric liquid crystals, and particles interaction with crystals is briefly reviewed, especially in the case of zeolite crystals and superlattices. Recent advances in fabrication techniques for very fine gratings are also covered.

Elachi, C.↗

NASA NTRS1977

Plasma wave electric fields in the solar wind - Initial results from Helios 1

Plasma-wave measurements by Helios 1 show that the electric-field intensities in the solar wind are usually very low, much lower than those for comparable measurements near earth, where particles moving upstream from the bow shock often cause large disturbances in the solar wind. The most commonly occurring plasma wave detected by Helios is a sporadic emission at frequencies from about 1 to 10 kHz, between the electron and ion plasma frequencies. These waves are thought to be ion acoustic waves Doppler-shifted upward in frequency from below the ion plasma frequency. The maximum electric-field intensity of these waves is a few hundred microvolts per meter. At higher frequencies, from about 20 to 100 kHz, electron plasma oscillations are detected at frequencies near the local electron plasma frequency. These electron plasma oscillations are more intense, with field strengths sometimes as large as a few millivolts per meter, but occur very infrequently. Both the ion acoustic waves and the electron plasma oscillations show a tendency to occur at higher frequencies closer to the sun but no pronounced variation in intensity with radial distance from the sun. In four cases, electron plasma oscillations have been found in association with type III radio bursts.

Gurnett, D. A.↗

NASA NTRS1977

Observations of ion cyclotron waves within the plasmasphere by Hawkeye 1

A survey of the plasma wave data from the Hawkeye 1 spacecraft has been performed in search of ion cyclotron waves associated with the scattering and loss of ring current ions within and near the plasmapause. During an 18-month period, encompassing about 270 orbits, a total of five events have been found with clearly detectable electric and magnetic fields at frequencies below the proton gyrofrequency. Comparisons of the electric and magnetic field amplitudes for these events provide strong evidence that these waves are ion cyclotron waves. All five events occurred during recovery phases of magnetic storms inside or very close to the plasmapause boundary. The results of this survey confirm and are consistent with the earlier identification of ion cyclotron waves by the Explorer 45 satellites. The Hawkeye 1 observations show that ion cyclotron waves of substantial amplitude occur at magnetic latitudes well away (about 28 deg) from the magnetic equator.

Kintner, P. M.↗

NASA NTRS1977

Application of the wavenumber jump condition to the normal and oblique interaction of a plane acoustic wave and a plane shock

The kinematics of normal and oblique interactions between a plane acoustic wave and a plane shock wave are investigated separately using an approach whereby the shock is considered as a sharp discontinuity surface separating two half-spaces, so that the dispersion relation on either side of the shock and the wavenumber jump condition across a discontinuity surface completely specify the kinematics of the problem in the whole space independently of the acoustic-field dynamics. The normal interaction is analyzed for a stationary shock, and the spectral change of the incident wave is investigated. The normal interaction is then examined for the case of a shock wave traveling into an ambient region where an acoustic disturbance is propagating in the opposite direction. Detailed attention is given to the consequences of the existence of a critical shock speed above which the frequency of the transmitted wave becomes negative. Finally, the oblique interaction with a fixed shock is considered, and the existence and nature of the transmitted wave is investigated, particularly as a function of the angle of incidence.

Kleinstein, G. G.↗

NASA NTRS1977

Coherent wave induced particle precipitation into the upper atmosphere

A description is presented of the results of a computer simulation involving a study of the particle precipitation induced by coherent VLF waves in the magnetosphere. The results of a computation for a 10 picoweber/sq m wave amplitude are shown in a graph. The precipitated flux for three different energies is given in a table. For 1.5 keV the energy deposition rate is about 0.8 erg/sq cm-sec, almost as intense as a moderate aurora. It is concluded that significant energy is deposited by a wave of 10 picoweber/sq m intensity. Such a wave amplitude is representative of highly coherent VLF wave types that are found in the magnetosphere. On the basis of the considered results it appears that controlled VLF wave injection in the magnetosphere could be an important and useful tool to study the coupling processes between the atmosphere and magnetosphere.

Inan, U. S.↗

NASA NTRS1977

Stimulated plasma waves in the ionosphere

The reported discussion is concerned with longitudinal waves associated with electron motions. These waves are easily stimulated in the ionosphere by rocket- and satellite-borne RF sounders. Most of the observations of stimulated plasma waves in the ionosphere are based on ionograms obtained from the sounders carried on board five satellites, including Explorer 20, Alouette 1 and 2, and ISIS 1 and 2. The majority of the observations can be explained by considering the propagation of the sounder-stimulated plasma waves. Attention is given to aspects of plasma wave dispersion, linear phenomena, plasma wave instabilities and nonlinear phenomena, unexplained phenomena, diagnostic applications, geophysical and astrophysical applications, and a number of experiments planned for the future.

Benson, R. F.↗

NASA NTRS1978

An instability of finite amplitude circularly polarized Alfven waves

A demonstration is presented that a finite-amplitude circularly-polarized Alfven wave is generally unstable in a MHD fluid. The wave decays by a four-wave coupling process in which the daughter waves are forward propagating random density and magnetic fluctuations and a backward-propagating magnetic wave. For parameters typical of the solar corona and the solar wind (thermal to magnetic energy density ratios between 0.1 and 1, and values between 0.1 and 0.9 for the ratio of magnetic energy density of the initial Alfven wave to that of the background magnetic field), large decay rates are found.

Goldstein, M. L.↗

NASA NTRS1978

Transmission of Alfven waves through the earth's bow shock - Theory and observation

From both theoretical and experimental bases, the transmission of Alfven waves through the bow shock is investigated. The theory of Alfven wave transmission through fast MHD shocks is extended to all cases of incident wave vectors. Particular consideration is given to Alfven waves propagating parallel to the ambient magnetic field with field perturbations polarized in the plane formed by the ambient magnetic field and the shock normal. An analysis is also made of magnetic field and plasma data from Explorer-35 in the vicinity of the bow shock. It is suggested that hydromagnetic waves are present in all of the 14 shock crossings studied, and that in upstream regions of at least 6 crossings, predominantly Alfvenic fluctuations exist. Average amplitudes of these fluctuations are measured on either side of the shock and the enhancement is measured by comparing their levels. Theoretical and experimental findings are compared and the apparent discrepancy in amplification factors may be explained by the strong damping of any transmitted magnetoacoustic modes downstream with relatively little damping of any transmitted Alfven waves.

Hassam, A. B.↗

NASA NTRS1978

Observation of gravity waves during the extreme tornado outbreak of 3 April 1974

A continuous wave-spectrum high-frequency radiowave Doppler sounder array was used to observe upper-atmospheric disturbances during an extreme tornado outbreak. The observations indicated that gravity waves with two harmonic wave periods were detected at the F-region ionospheric height. Using a group ray path computational technique, the observed gravity waves were traced in order to locate potential sources. The signals were apparently excited 1-3 hours before tornado touchdown. Reverse ray tracing indicated that the wave source was located at the aurora zone with a Kp index of 6 at the time of wave excitation. The summation of the 24-hour Kp index for the day was 36. The results agree with existing theories (Testud, 1970; Titheridge, 1971; Kato, 1976) for the excitation of large-scale traveling ionospheric disturbances associated with geomagnetic activity in the aurora zone.

Hung, R. J.↗