Observed Phase Velocity Research Articles

Life and death of the Bose polaron

Spectroscopic and interferometric measurements complement each other in extracting the fundamental properties of quantum many-body systems. While spectroscopy provides precise measurements of equilibrated energies, interferometry can elucidate the dynamical evolution of the system. For an impurity immersed in a bosonic medium, both are equally important for understanding the quasiparticle physics of the Bose polaron. Here, we compare the interferometric and spectroscopic timescales to the underlying dynamical regimes of the impurity dynamics and the polaron lifetime, highlighting the capability of the interferometric approach to clearly resolve polaron dynamics. In particular, interferometric measurements of the coherence amplitude at strong interactions reveal faster quantum dynamics at large repulsive interaction strengths than at unitarity. These observations are in excellent agreement with a short-time theoretical prediction including both the continuum and the attractive polaron branch. For longer times, qualitative agreement with a many-body theoretical prediction which includes both branches is obtained. Moreover, the polaron energy is extracted from interferometric measurements of the observed phase velocity in agreement with previous spectroscopic results from weak to strong attractive interactions. Finally, the phase evolution allows for the measurement of an energetic equilibration timescale, describing the initial approach of the phase velocity to the polaron energy. Theoretically, this is shown to lie within the regime of universal dynamics revealing a fast initial evolution towards the formation of polarons. Our results give a comprehensive picture of the many-body physics governing the Bose polaron and thus validates the quasiparticle framework for further studies.

Estimation of the Sedimentary Basin Structure of the Southeastern Part of the Ishikari Lowland, Hokkaido, Inferred from Receiver Function

There is a very thick (>10 km) sedimentary basin in the Ishikari lowland, the central Hokkaido. This thick sedimentary basin affects strong ground motions observed from earthquakes, e.g., the 2018 Hokkaido Eastern Iburi Earthquake occurred in the southeastern part of the Ishikari lowland. Such a thick sedimentary basin structure is difficult to explore by using microtremor array or seismic reflection. This study investigated the sedimentary basin structures at 16 strong motion stations in the southeastern part of the Ishikari lowland by using the borehole-to-surface spectral ratio and the receiver function method based on the observed strong-motion records. First, at the KiK-net stations, the surface-to-borehole spectral ratio was obtained from observed strong-motion records. The S-wave velocity structure models between the surface and the borehole seismograph were estimated by fitting theoretical spectral ratios to the observed one. Next, the velocity structure models between the surface and the seismic bedrock were derived from the receiver function calculated from the observed records. We used the downhill simplex method to obtain the velocity structure model from the receiver function. The inverse analysis with three different initial models resulted in variations in the estimated velocity structure models. Synthetic velocity waveforms of the aftershock of the 2018 Eastern Iburi earthquake were calculated based on the estimated velocity structure models. The synthetic velocities (0.1-1 Hz) agree with the S waves of the observed records. The synthetic waveforms obtained from three models at each site were very similar. We compare the resulted velocity models with the microtremor array surveys carried out by previous studies. The fundamental mode phase velocities calculated from the velocity structure models obtained from the receiver functions correspond to the observed phase velocity from the microtremor array survey in higher (>0.4-0.8 Hz) frequencies. However, in lower frequencies (<0.4 Hz), the observed phase velocities at HKD184, IKRH03, and IBUH03 agree with the higher-mode phase velocities than the fundamental ones. This agreement suggests that the higher-mode Rayleigh waves dominate the observed phase velocities in the lower frequencies. The velocity structure models estimated from the receiver function analysis indicated that the low Vs (<1.1 km/s) layer deposited thickly in the western part of the Ishikari lowland and the middle Vs (1.1<Vs<2.7 km/s) layers were thick in the eastern part of the lowland. The depth of the estimated seismic bedrock was 12 km at HDKH04, located in the southeastern part of the lowland, which was the deepest between the study sites.

A case study of a thermally ducted undular mesospheric bore accompanied by ripples over the western Himalayan region

An undular mesospheric bore event has been recorded over the western Himalayan region in O(1S) 557.7 nm airglow images on a clear and moonless night of 02 October 2018 using a multi-wavelength all-sky imager at Hanle, Leh Ladakh, India (32.77°N, 78.97°E). The bore has a prominent leading dark front followed by trailing waves and it propagates with a mean observed phase velocity of ~31 ± 5 m/s. It also shows a small-scale undulation and clockwise rotation in its phase front. In order to understand the evolution of the bore, vertical temperature profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard TIMED satellite and HWM14 wind maps are used. SABER temperature shows a mesospheric inversion layer prior to the occurrence of the bore event which acted as a thermal duct layer to guide the propagation of the bore. The analyses also suggest that inhomogeneity in the duct depth at different parts of the bore’s horizontal extension could lead the small-scale undulations in the bore phase fronts. The same can also be responsible for the rotation of the bore. Additionally, the peak separation of the bore’s trailing waves suggests that large-scale gravity wave interaction with pre-existing thermal duct could be the potential source for the generation of the undular bore at mesospheric height. Furthermore, the present results indicate that neutral instabilities and weakening duct layer in the path of the bore propagation might have accelerated the faster dissipation of the bore’s energy and consequently suppress its long-distance horizontal propagation.

Variability of deconvolved bedrock motion of the 2011 off Pacific Coast of Tohoku Earthquake around the K-NET Tsukidate station considering uncertainty in shallow S-wave velocity model from inversion of Rayleigh wave phase velocity

A large acceleration with a peak amplitude of 2700 cm/s2 was recorded at the K-NET Tsukidate station (MYG004) during the 2011 off Pacific Coast of Tohoku Earthquake. Nonetheless, there was little structural damage around the station. In this study, we estimate the engineering bedrock’s motion variability due to the main shock, deconvolved from the observed surface motions, considering uncertainties in shallow S-wave velocity (Vs) profiles. The Vs profile uncertainty at the strong motion station is estimated with observed Rayleigh wave phase velocities, from microtremor explorations, using the Markov-chain Monte Carlo inversion. We obtain upper and lower possible values of the estimated bedrock spectral accelerations and the ground motion proxies at MYG004 using the uncertainty of the Vs profile. The estimated bedrock peak ground acceleration ranges are 610–930 cm/s2 and 2000–3100 cm/s2 for the east–west (EW) and north–south (NS) directions, respectively. Meanwhile, the ranges of the peak ground velocities are about 35–40 cm/s and 90–120 cm/s for the EW and NS components, respectively. The results indicate that the minimum and maximum possible values of the deconvolved spectral acceleration and the ground motion proxies are not only affected by Vs profile uncertainty but also by surface ground motion characteristics. Moreover, the spectral velocities at a station in the main part of Tsukidate Town are also estimated with the observed phase velocity. They have predominant peaks at periods shorter than 0.5 s, suggesting only minor structural damage in the city center.

Microtremor exploration for shallow S-wave velocity profiles at stations in local strong motion network in Bursa, Yalova, and Kocaeli in north-western Turkey

We conducted microtremor array surveys for shallow S-wave velocity profiles at 20 sites in Bursa, Yalova and Kocaeli provinces in the north-western part of Turkey to provide fundamental data to assess the seismic hazard in the area. All of the measurement sites were positioned very close to strong motion stations belonging to the Disaster and Emergency Management Presidency of Turkey (AFAD) in order to further understand site amplification factors in strong motion records. Of the 20 study sites, two were located in Yalova, four in Bursa and 14 in Kocaeli.We temporarily installed two small arrays to obtain simultaneous records of vertical microtremors. Then, the spatial autocorrelation method was applied to retrieve Rayleigh wave phase velocity curves in a frequency range from 1 to 30 Hz from the array records. The phase velocities in the western part of the Kocaeli area are low across a wide frequency range, while relatively high phase velocities are found in the eastern part of the Kocaeli province. The phase velocities in the Yalova and Bursa provinces are widely distributed suggesting large variations in soil conditions. The observed phase velocity curve at each site was inverted to a one-dimensional (1D) S-wave velocity profile to a depth of 100 m, using a hybrid heuristic inversion method. All the S-wave velocity profiles in the eastern Kocaeli area are similar; however, the sites in the western Kocaeli and Yalova-Bursa areas have profiles with different features from the others.Finally, we discuss amplification factors for S-waves using the inverted profiles. The dominant fundamental periods of the amplification factors were distributed in a frequency range from 0.7 to 5 Hz. The profiles obtained are also used to map average S-wave velocities in the study area, with an addition of existing data at strong motion stations of the AFAD.

Experimental observation of electron-acoustic wave propagation in laboratory plasma

In the field of fundamental plasma waves, the direct observation of electron-acoustic wave (EAW) propagation in laboratory plasmas remains a challenging problem, mainly because of heavy damping. In the Magnetized Plasma Linear Experimental device, the wave is observed and seen to propagate with the phase velocity ∼1.8 times the electron thermal velocity. A small amount of cold, drifting electrons, with the moderate bulk to cold temperature ratio (≈2−3), is present in the device. It plays a crucial role in reducing the damping. Our calculation reveals that the drift relaxes the stringent condition on the temperature ratio for wave destabilization. Growth rate becomes positive above a certain drift velocity even if the temperature ratio is moderate. The observed phase velocity agrees well with the theoretical estimate. Experimental realization of the mode may open up a new avenue in the EAW research.

Crustal S-wave structure around the Lake Van region (eastern Turkey) from interstation Rayleigh wave phase velocity analyses

This study focuses on the average crustal and the upper mantle structure throughout the Lake Van region of eastern Turkey. The study aimed to investigate the structure with the fundamental mode interstation Rayleigh wave phase velocities from the local and the regional earthquakes recorded by Kandilli Observatory and Earthquake Research Institute stations. Considering back azimuth differences of each source and station path, six different broadband station pairs and 27 earthquakes were selected to determine the 1-D shear-wave velocity structures throughout the region by using an interstation method (slant stacking technique). The linearized least squares algorithm was used to obtain the 1-D shear-velocity model that best fit the observed phase velocity dispersion curve. The normalized statistical resolution matrix was calculated to measure the reliability of the solution. Inversion results revealed that the solution quality of the upper crust is weak due to the high resolution lengths. The average shear-wave velocities in the lower crust scale down to approximately 3.5 km/s. It was inferred that this low-velocity zone shown in the lower crust may be associated with widespread volcanism. Final 2-D S-wave velocity models obtained from the inversion revealed that the crust-mantle boundary is ~42 km, and shear velocities vary from 3.6 to 4.2 km/s. Furthermore, the upper mantle (~45-70 km) velocities are slower than globally suggested models (e.g., IASP91), and this is possibly related to shallow hot asthenospheric material.

Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

AbstractSeismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low‐velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat flow data and mantle‐xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one‐dimensional shear‐velocity profiles that are consistent with phase‐velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1‐D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity‐depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low‐velocity zone in the upper mantle and that temperature structures that are consistent with constraints from mantle xenoliths yield phase‐velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km.

Estimation of S-Wave Velocity Profiles at Lima City, Peru Using Microtremor Arrays

Microtremor exploration was performed around seismic recording stations at five sites in Lima city, Peru in order to know the site amplification at these sites. The Spatial Autocorrelation (SPAC) method was applied to determine the observed phase velocity dispersion curve, which was subsequently inverted in order to estimate the 1-D S-wave velocity structure. From these results, the theoretical amplification factor was calculated to evaluate the site effect at each site. S-wave velocity profiles at alluvial gravel sites have S-wave velocities ranging from ∼500 to ∼1500 m/s which gradually increase with depth, while Vs profiles at sites located on fine alluvial material such as sand and silt have Swave velocities that vary between ∼200 and ∼500 m/s. The site responses of all Vs profiles show relatively high amplification levels at frequencies larger than 3 Hz. The average transfer function was calculated to make a comparison with values within the existing amplification map of Lima city. These calculations agreed with the proposed site amplification ranges.

Analysis of effects of active sources on observed phase velocity based on the thin layer method

In the wave field induced by active sources, the observed phase velocity of surface waves is influenced by both mode incompatibility (i.e. non-planar spread of surface waves is idealized as plane waves) and body waves. Effects of sources are usually investigated based on numerical simulations and physical models. Several methods have been proposed to mitigate the effects. In application, however, these methods may also have difficulties since the energy of the body waves depends on soil stratification and parameters. There are multiple modes of surface waves in layered media, among which the higher modes dominate the wave field for soils with the irregular shear velocity profiles . Considering the mode incompatibility and the higher modes, we derive analytical expressions for the effective phase velocity of the surface waves based on the thin layer stiffness method, and investigate the effects of the body waves on the observed phase velocity through the phase analysis of the vibrations of both the surface waves and the body waves. The results indicate that the effective phase velocity of the surface waves in layered media varies with the frequency and the spread distance, and is underestimated compared to that of the plane surface waves in the spread range less than about one wavelength. The oscillations that appeared in the observed phase velocity are due to the involvement of the body waves. The mode incompatibility can be ignored in the range beyond one wavelength, while the influence range of the body waves is far beyond one wavelength. The body waves have a significant influence on the observed phase velocity of the surface waves in soils with a soft layer trapped between the first and the second layers because of strong reflections. ► The effective phase velocity varies with the frequency and the spread distance. ► The effective phase velocity is underestimated within one wavelength. ► The mode incompatibility can be ignored in the range beyond one wavelength. ► The body waves induce the oscillations in the observed phase velocity. ► The influence range of the body waves is far beyond one wavelength.

Repeatability of multimode Rayleigh‐wave dispersion studies

Rayleigh‐wave dispersion is used to study the near‐surface elastic properties of a thick, lacustrine clay to approximately 10 m depth. Ten repeated sets of Rayleigh dispersion curves were obtained through late spring to early fall. A variety of methodologies were used to extract the dispersion curves, but a modified frequency–ray parameter (f − p) method most successfully yields dispersion curves for the first three Rayleigh modes. The Rayleigh‐wave velocities varied from 100 to ∼350 m/s with frequency over the band from 75 to 10 Hz. Over this band, these velocities did not measurably vary during the study period. The observed phase velocity curves were inverted with P‐wave and density values obtained from shallow coring to obtain the shear‐wave velocity structure at the site down to &gt; 14 m. This case study highlights the robust, repeatable, nature of surface wave dispersion methods when care is taken in the acquisition of field data.

An observation of a fast external atmospheric acoustic‐gravity wave

In November 1999 a new near‐IR airglow imaging system was deployed at the Starfire Optical Range outside of Albuquerque, New Mexico. This system allowed wide angle images of the airglow to be collected, with high signal to noise, every 3 seconds with a one second integration time. At approximately 1000 UT on November 17, 1999, a fast wavelike disturbance was seen propagating through the OH Meinel airglow layer. This wave had an observed period of ≈215 seconds, an observed phase velocity of ≈160 m/s and a horizontal wavelength of ≈35 km. This phase velocity is among the fastest yet reported using an imager viewing the OH Meinel bands, while the wave period is among the shortest. Simultaneous Na lidar wind and temperature data from 80 to nearly 110 km altitude allow the intrinsic properties of the wave to be calculated. The Einaudi and Hines [1970] WKB approximation for the acoustic‐gravity wave dispersion relation was used to calculate the wave's intrinsic properties. Using this approach indicates that the observed disturbance was an external acoustic wave in the 90 to 107 km altitude region and an external gravity wave at other altitudes between 80 and 90 km. Using model atmospheric data for altitudes below and above this altitude regime indicates that the wave is essentially external everywhere except perhaps in narrow regions around 80 and 105–110 km. This is confirmed using a more exact full‐wave model analysis. The observations and model results suggest that this wave was not generated in the troposphere and propagated up to the mesosphere, but rather near 100 km altitude where it was possibly generated by a Leonids meteor.

3D Site Effects: A Thorough Analysis of a High-Quality Dataset

This article is a contribution toward filling the large gap that currently exists between theoretical and observational studies of site effects. We present a detailed analysis of 10 events recorded by a dense temporary seismograph network that operated in the small (roughly 1.3 by 0.4 km) alluvial valley of Parkway, New Zealand. We computed f - k spectra for the three recorded components of those 10 events as a function of frequency and time window along the seismograms. Our results show that a major part of the energy in the records propagates as surface waves. The small size of the basin explains the observation that small phase velocity waves appear shortly after direct P - and S -wave arrivals. Both Love and Rayleigh waves are present. It was difficult to isolate individual wavetrains, but directions of propagation are dominated by southerly directions, especially at lower frequencies and earlier times along the seismograms. The inversion of observed phase velocity dispersion is in very good agreement with available information on the subsoil structure for this valley. A comparison with a different interpretation of one event from our dataset shows clearly how misleading it can be to analyze partial observations. The large variability of ground motion in the time domain contrasts with the very stable results obtained for this dataset in the frequency domain and indicates that we still have a long way to go before we are able to incorporate locally generated surface waves into the frame of modern building codes.

地表摩擦が重要なケルビン波のCISK

As a step toward an understanding of the mechanism of super cloud cluster (SCC), the effect of surface friction on the instability of Kelvin wave is discussed, together with the condition of the vertical profile of parameterized convective heating. In this paper (Part I), linear theory is developed with an equatorial beta-plane model in which the amount of convective heating is assumed to be proportional to the vertical velocity at the top of the boundary layer. Results from the eigenvalue problem suggest that the effect of surface friction is, at least under such parameterization, indispensable for essential understanding of SCC; this study is along the same line as Wang (1988) with respect to the importance of surface friction. Issue is focused on three unstable modes. One is a stationary mode excited with much heating at lower levels, and identified with conditional instability of the first kind (CIFK). The other two are Kelvin modes that contrast with each other; one is destabilized by enough heating at upper levels, even without surface friction (referred to as NFK: Non-Frictional Kelvin mode), and the other is destabilized by moderate heating at lower levels under the control of surface friction (FK: Frictional Kelvin mode). NFK has phase velocities of more than 10 ms -1 , and its growth rates are on the order of 10 -3 s -1 at small wavelengths. On the contrary, FK is characterized by smaller phase velocities of less than 10 ms -1 , and by only slight dependence of its growth rates on wavelength. In addition, the convergence in the boundary layer exhibits a phase shift slightly eastward relative to the convergence aloft; this is the key feature that induces effective feedback between convection and the wave. In view of the large phase velocity and the small preferred scale, NFK may be interpreted as a Kelvin mode of the type that has been treated in many previous studies on SCC. Difficulties in explaining the observed phase velocity and preferred scale of SCC are inherent in NFK. By contrast, FK can give a realistic phase velocity and preferred scale, and clear up major difficulties in the previous studies. Inclusion of surface friction is expected not only to serve as the missing link of the past linear theories of SCC, but also as a basis of future non-linear experiments with explicit moisture processes.

MHD mode structure and propagation in the ASDEX tokamak

The Mirnov oscillations observed in the ASDEX tokamak are analysed with regard to the generally accepted interpretation scheme according to which (1) Mirnov oscillations are caused by currents flowing parallel to the magnetic field on rational magnetic surfaces and (2) the field perturbation is frozen within the plasma. If the second statement holds, the frequency is obtained from the profiles of the electron density, the ion temperature and, if applicable, the toroidal or poloidal rotation velocity. It is shown that there are modes which are consistent with the above interpretation. On the other hand, mode coupling is observed. Mode coupling is also invoked to reconcile the experimental findings with the predictions of a theory based on statement (1); according to this theory, there is a poloidal variation of both the phase velocity and the amplitude of the Mirnov oscillations. While the observed phase velocity fits well into this picture, the poloidal variation of the amplitude cannot be ascribed to only one mode in the majority of cases. In addition, the possibility of coherent MHD activity due to currents in the scrape-off layer is discussed.

The upper crust low velocity layer; a Rayleigh (Rg) phase velocity study from SE Norway

AbstractA recent surge of interest in short‐period fundamental mode Rayleigh waves (Rg) propagation is motivated by their usefulness for structural mapping and estimation of attenuation (Q) properties of the uppermost crust, their importance in scattering studies and as a nuisance (false alarms) in detection seismology. We have investigated Rg phase velocity dispersion characteristics, using recordings from 16 out of 22 subarrays of the NORSAR array of aperture 100 km. The observed phase velocity in combination with a one‐layer and two‐layers model over halfspace models were inverted using a maximum likelihood scheme. In view of the smoothness and simplicity of the dispersion curves, the number of unknowns were limited to three parameters, namely shear velocities and thickness, using a Poisson's ratio of 0.25 for P‐S velocity coupling and density fixed. In the general array siting area the derived low velocity layer has an average thickness of 1.02 km, and the associated shear velocity is 2.82 km s‐1. The underlying model half space appears to be uniform, and its derived shear velocity is 3.55 km s‐1, which is typical of the upper crust. In contrast to other similar studies, no obvious correlation was found with local geology, which mainly consists of Precambrian crystalline rock sequences. Group velocities were calculated from the structural models and compared to the observational ones. The agreement between calculated and observed group velocities was reasonable.

Interferometric phase velocity measurements in the auroral electrojet

A double-probe electric field detector and two spatially separated fixed-bias Langmuir probes were flown on a Taurus-Tomahawk sounding rocket launched from Poker Flat Research Range in March 1982. Interesting wave data have been obtained from about 10s of the downleg portion of the flight during which the rocket passed through the auroral electrojet. Here the electric field receiver and both density fluctuation ( δn/ n) receivers responded to a broad band of turbulence centered at 105 km altitude and at frequencies generally below 4 kHz. Closer examination of the two ( δn/ n) turbulent waveforms reveals that they are correlated, and from the phase difference between the two signals, the phase velocity of the waves in the rocket reference frame is inferred. The magnitude and direction of the observed phase velocity are consistent either with waves which travel at the ion sound speed ( C s ) or with waves which travel at the electron drift velocity. The observed phase velocity varies by about 50% over a 5 km altitude range—an effect which probably results from shear in the zonal neutral wind, although unfortunately no simultaneous neutral wind measurements exist to confirm this.

Nonlinear Contributions to the Frequency Spectrum of Wind-Generated Water Waves

In the continuous frequency spectrum of wind-generated water waves Fourier components have different origins. At a particular frequency, some will be harmonies resulting from the nonlinear profiles of lower frequency waves, others will be near-free gravity waves. In this paper the relative importance of these two different contributions is studied in the case that the nonlinearities can be treated perturbatively. The calculation starts from a fit to observed spectra: in the sea the JONSWAP spectrum is chosen; in the laboratory a different fit with a sharper fall-off near the spectral peak is taken. The nonlinear corrections are most significant at frequencies larger than twice the peak frequency and increase with increasing frequency. They are determined mainly by the behavior of the spectrum near the peak. The relative importance of the nonlinear contributions increases with decreasing dimensionless fetch. This is in agreement with experimental observations. In the laboratory, with narrower spectra, nearly all of the spectral energy at twice the peak frequency is due to the nonlinear contributions. The observed magnitude agrees reasonably well with our calculated value. In the open-ocean nonlinear corrections are a small fraction of the linear contribution at this frequency. For a nonlinear system the concept of phase velocity loses its meaning in general. Nevertheless, experimentally, nonlinearities will show up as an anomaly in the observed phase velocity. This anomaly is studied. In the laboratory, where the nonlinearities dominate, a large anomaly is expected and this agrees with the observations. In the open sea experimental evidence is conflicting. It is found that several mechanisms tend to suppress the anomaly, so that small deviations from the linear value are obtained.

Two-stream ion-ion instability in an axially bounded ion beam plasma system

When ion beams were injected into axially bounded plasmas, ion waves, whose wavelengths were determined by boundaries, were found to be very distinctly excited. The selection of the excited oscillation frequency was determined by the boundaries such that the spacing of the boundaries was approximately an integer times the wavelength of the ion waves. The observed phase velocity was in fairly good agreement with the theoretical velocity.

Oblique propagation of electron plasma waves in a magnetoplasma

The electron plasma waves which are radiated from a point source and propagate in an oblique direction with respect to an applied static magnetic field are investigated both theoretically and experimentally. The observed phase velocities and wave amplitudes in the oblique directions are explained by considering anisotropic characteristics of a magnetoplasma in which the directions of phase and group velocities are different. The observed phase velocity in the radial direction was found to agree with the theoretical ray velocity.