Mode Coupling Process Research Articles

Investigation of Coronal Mass Ejections. I. Loop-type with Arcade Flare between the Fixed Legs, and Bubble-type Due to Flare Blast Waves

AbstractIn this paper, we give arguments that there are two types of coronal mass ejection (CME).The first type of CME discussed here is the ‘loop-type’, whose occurrence is related to an arcade flare somewhere between the footpoints. It was found that there were pre-event magnetic connections between the flare location and the locations of the footpoints of a CME of this type, and that these connections disappeared after the event. This suggests that the footpoints of loop-type CMEs are special prescribed points, and this was verified by the observation that the footpoints do not move in this type of CME.The other type of CME is the ‘bubble-type’, which is associated with the flare blast from explosive flares. We confirmed the association of this type of CME with the so-called EIT (Extreme Ultra-violet Imaging Telescope) waves, but the velocity of expansion of the bubble is twice or more greater than that of the EIT waves depending on events. Although EIT waves were widely considered to be Moreton waves viewed by SoHO/EIT in the solar activity minimum period, recent simultaneous observations of both have revealed that the EIT wave is something different from the Moreton wave, and propagates separately with a velocity less than half that of a Moreton wave.We therefore propose a new overall picture: the bubble-type CMEs are the flare-produced MHD blast waves themselves, whose skirt is identified as a Moreton wave. EIT waves may be interpreted as follows: the slow-mode gas motions from the source cause secondary longwavelength fast-mode waves which are trapped in the “waveguide” in the low corona. The secondary long-wavelength wave in the fast-mode, which is trapped in the low corona, has a slower propagation velocity due to the nature of the waves trapped in a “waveguide”. This trapped wave induces slow-mode motions of the gas through a mode-coupling process in the high chromosphere, where the propagation velocities of the fastand slow-mode waves match.Three-dimensional MHD simulations for these two types of CME are in progress, and are previewed in this paper.

Experimental determination of attractor dimension of E×B turbulence

The time series analysis method for chaotic attractor dimension by Grassberger and Procaccia [P. Grassberger and I. Procaccia, Physica D 9, 189 (1983)] was applied to fluctuations due to an E×B rotational flute mode in the Columbia Linear Machine (CLM) [G. A. Navratil, J. Slough, and A. K. Sen, Plasma Phys. 24, 184 (1982)]. After frequency domain filtering, the results show that the fluctuations carry a dimension close to three. A dimension of three supports a possible three-wave mode coupling process revealed in bi-coherence studies. The Grassberger Procaccia method was also performed on cases where partial feedback suppression was applied to the instability. It is shown that feedback does not alter the attractor dimension of the instability, supporting previous findings that the linear feedback only affects the linear drive of the instability, and not its non-linear saturation mechanism.

Grazing-incidence-based hollow taper for infrared laser-to-fiber coupling

Based on grazing incidence, a simple uncoated hollow glass taper as an alternative technique for efficient infrared laser-to-fiber coupling is presented. Because of the mutual action of the direct parallel laser excitation, the mode-coupling process, and mode filtering, the hollow taper serves as a mode converter that transforms the highly multimode intensity distribution of the input laser emission into a high-quality smooth profile at the taper output. When an uncoated Pyrex-glass hollow taper is used for direct launching, without any intermediate focusing elements, of a powerful infrared Er:YAG laser emission (λ=2.94 μm), we obtain high laser-to-taper (81%), taper-to-fiber (96%), and total laser-to-fiber (78%) coupling efficiencies.

Plasma maser effect in magnetosphere plasma associated with MHD turbulence

The interaction of an ion acoustic wave with a kinetic Alfven wave is considered through the plasma maser effect. The plasma maser effect as an energy upconversion mode coupling process, is found to be effective in drawing energy from low frequency MHD turbulence in the resonant mode to the non-resonant ion acoustic wave turbulence in the magnetosphere plasma near the Earth's foreshock region. The calculated growth rate for the ion acoustic wave shows that one of the dominant turbulences in the region of the Earth's foreshock is excited as it draws energy from MHD turbulence through the plasma maser effect.

Coupling between High-Frequency Plasma Waves in Laser-Plasma Interactions

Experimental evidence for the coupling between two electron plasma waves having nearly the same frequency but greatly differing in wave number is presented using time and wave-number resolved spectra of Thomson scattered light from the plasma. The qualitative features of the measured {ital w}({ital t},{ital k}) spectra are predicted by a Lagrangian fluid description and reproduced in particle simulations. These show that the daughter waves generated in this mode coupling process take the energy preferentially from the large {ital k} wave without significantly affecting the small {ital k} plasma wave.

On the conservation of the nonresonant quanta in plasma–maser interaction

This paper presents an investigation on the conservation of the nonresonant quanta in a new mode-coupling process, namely, the plasma-maser interaction. Considering interaction of ordinary mode electromagnetic wave with Langmuir turbulence in a magnetized plasma, it is shown that the anisotropy induced by the external magnetic field is primarily responsible for the nonconservation of the number of quanta in an open system. In contrast, the number of quanta of the high frequency wave is conserved in a closed system.

Plasma emission by a nonlinear beam instability

A new theory for electromagnetic radiation emission with wave frequency at the plasma frequency and/or its harmonic (known as the plasma emission) is presented. According to the new theory, the radiation emission takes place as a result of combined effects of the excitation of electrostatic waves by a nonlinear beam instability, and the concurrent conversion into electromagnetic waves by a nonlinear mode coupling process. The underlying physical mechanism for the nonlinear beam instability is an interaction of energetic electron beam with enhanced ion-acoustic or Langmuir turbulence. The turbulence, which is treated as intrinsic in the present analysis, also gives rise to the nonlinear mode coupling process between the electrostatic and electromagnetic modes. That is, the two processes (nonlinear instability and mode coupling) occur concomitantly. An important aspect of the present theory is that the effective growth rate associated with the radiation is comparable to the usual beam–plasma (or bump-in-tail) instability growth rate, which makes the radiation emission process very efficient, a feature very different from any standard theory.

Mode coupling effects on alpha-particle-driven long wavelength Alfvén wave instability

It is demonstrated both analytically and numerically that mode couplings play an important role in the nonlinear evolution of alpha-particle-driven long wavelength Alfvén wave instabilities. The mode coupling process is characterized by a beat between two linearly unstable Alfvén waves having opposite frequencies, which generates a linearly stable, static (zero frequency) mode. The backreaction of the static mode tends to stabilize the Alfvén instabilities by eliminating the phase shift between the alpha pressure and the Alfvén fluctuations.

Mode-coupling processes in polymethyl methacrylate-core optical fibers

Aperture, attenuation, and far-field radiation diagrams have been studied on short lengths of commercial polymethyl methacrylate- (PMMA-) core optical fibers. The spectral attenuation of the PMMA constituting the core has been carefully measured and compared with the attenuation that may be calculated from optical properties of PMMA. Accurate extrinsic attenuation spectra have been obtained. Moreover, the dependence of the far-field radiation diagram on the fiber length and on the launching incidence of a laser beam has been studied on this side of the mode-equilibrium length. The analysis of these diagrams, first performed with the Gloge mode-coupling model, has been improved with the hypothesis that the mode-coupling processes are a result of light diffraction by structural anomalies in the core. The average size and form of these structural anomalies has been evaluated. They may be longitudinal microcracks of the PMMA coming from stress relaxation, which occurs during the fiber-drawing process.

Random Multimode Optical Media: I. Mode Coupling Process in Slab Waveguide with Stochastic Wall Perturbations

Abstract The scope of the paper is to present in explicit form the mode coupling coefficients in slab waveguide, in the case when both boundaries of guiding core or gradient refractive index distribution are perturbed by random deviations from an ideal model. The central idea is to present the relationships between the processes, depicting the waveguide deformations and the estimators of the mode coupling processes. The correlation functions and the spectral density functions of both groups of processes are linearly interdependent; we examine the complex coefficients of these relations. In particular, the TE modes are analysed in detail in step-index slab. For the case of parabolic profile (n(x) = A(1 − Bx2), within the core) the method which derives the statistic integral form of coupling coefficients is presented (also for guided TE modes). All modes are assumed to be monochromatic, i.e. time-dependence is exp (−iωt).

Anomalous radiation from Langmuir turbulence

The stability of stationary Langmuir turbulence against a test high-frequency electromagnetic perturbation is studied. The enhanced growth of the electromagnetic radiation through the new mode–mode coupling process is obtained for an open system plasma with a particle source and sink. The growth rate of the electromagnetic mode comes from the direct and polarization mode coupling terms even for an unmagnetized plasma.

The coupling of stimulated Raman and Brillouin scattering in a plasma

The observation of an anti-Stokes satellite in the spectrum of light backscattered from a CO2 laser plasma is reported. Its origin is found to be Thomson scattering of the incident light from a counterpropagating mode-coupled plasma wave. The parent electron and ion waves in the mode-coupling process were driven by stimulated Raman and Brillouin backscattering. The parent and daughter plasma waves were detected by ruby laser Thomson scattering. A computer simulation modeling the experiment shows further cascading of the Stokes backscattered light to lower frequencies, apparently a result of its rescattering from another, higher phase velocity, counterpropagating coupled mode. Comparisons with theoretical predictions are presented.

Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet: 2. Two‐stream waves

Both primary and secondary two‐stream (Farley‐Buneman) waves have been detected by in situ electric field and plasma density probes in the strongly driven daytime equatorial electrojet over Peru. Simultaneous Jicamarca radar observations showed strong vertical and oblique 3‐m type 1 echoes, also indicative of the two‐stream mechanism. The rocket data show the two‐stream region on the topside of the unstable layer to be situated between 103 and 111 km where the electron current was the strongest. This region was characterized by broadband plasma oscillations extending past 1 kHz in the rocket frame. Furthermore, above 106.5 km, where the electron density gradient was stable, a layer of laminarlike, horizontally propagating two‐stream waves was detected. These waves were strongest near 108 km, the altitude where previous measurements have shown the electrojet current over Peru to be strongest. The peak rocket frame frequency of these waves was 25 Hz, and the observed broadband electric field and plasma density amplitudes were 2 mV/m and 1–2% rms, respectively. The electric field amplitudes were likely attenuated by a spatial filtering effect and may have been several times higher. The data suggest that these waves had phase velocities comparable to the electron drift velocity (≃500 m/s) and peak wavelengths (2–3 m) that agree with kinetic theory predictions. Distinct vertically oriented waves which may have been generated by a mode coupling process were also observed by the rocket in this region, in agreement with the simultaneous radar observations. Below the laminar two‐stream layer, in the region of the large‐scale, gradient drift driven horizontal electric fields, secondary two‐stream waves were observed that were driven by localized δE × B plasma drifts. The amplitudes of the horizontally propagating kilometer scale waves were clearly strong enough (10–15 mV/m) to drive vertically oriented secondary two‐stream (type 1) waves, as proposed by Sudan et al. (1973) and observed by the Jicamarca radar between roughly 103 and 106 km. Below 103 km, secondary two‐stream waves could not be generated because ψ (the ratio of the ion and electron collision frequencies to their gyro frequencies) becomes large, despite the fact that the observed large‐scale wave amplitudes remained strong at the lower altitudes. Observations of “flat‐topped” waveforms of the large horizontal electric field structures suggest that secondary two‐stream phase velocities may be saturated because of a limitation of the driving electric fields rather than a process intrinsic to the two‐stream instability itself.

Accurate characterization of the transmittivity of large-diameter multimode optical fibers

We describe an original technique that permits a specially accurate measurement of the variations of the transmittance of optical fibers vs the incidence of launched light. For large-diameter large-aperture fibers, which prefer light power transmission, the transmittance may be modeled using geometric optics. Taking into account the reflection losses at both ends, the core attenuation and a very weak lack of reflectivity at the core-cladding interface, good agreement is obtained between calculated variations and experimental results for the three kinds of fiber tested. The remaining difference has to be attributed to the mode coupling process which may be thus evaluated. The too high attenuation coefficient, which is attributed to the cladding to account for the losses at the core-cladding interface, characterizes in fact a thin perturbed layer where the materials contact. Thus, this method is a good means to evaluate separately the various processes contributing to the attenuation of open beams along the fibers and to determine which process should be worked on to improve the transmittance.

Rf stabilization of universal drift mode based on plasma-maser theory.

The condition for stabilizing the universal drift mode by superimposing an external high-frequency Trivelpiece-Gould mode is investigated based on a new mode-coupling process (plasma-maser effect). The energy up-conversion from the universal drift mode to the high-frequency rf field stabilizes the low-frequency drift mode. Under suitable conditions, the new method based on plasma-maser interaction is effective at a smaller relative magnitude of the rf field than the one based on a ponderomotive force in a narrow sense.

Plasma-maser effects in plasma astrophysics

The role of a new mode coupling effect (plasma-maser) in space plasma physics is reviewed. The new maser effect, the idea that the resonant electrons with the low-frequency mode can amplify the high-frequency mode, does not require population inversion of electrons. The generation mechanisms of ULF modulated ELF emissions, auroral kilometric radiation, chorus related electrostatic bursts, whistler mode in the solar wind, and type III solar radio bursts are studied based on plasma-maser effect. The forced plasma-maser interaction model reduces to a conservative Lotka-Volterra system. A chaotic behavior of the forced Lotka-Volterra system is obtained. The new mode coupling process has potential importance in attempting to interpret numerous astrophysical radio phenomena.

Non-Ising-Like Effect on the Sound Propagation in a Critical Fluid

Sound attenuation and dispersion near the critical point reveals important aspects of order parameter dynamics hidden in direct observations of order parameter. Until recently it has been believed that the mode· coupling theory of Kawasaki l ) fairly well describes the overall features of sound pro· pagation in single· component classical fluids. However, it has become awared that discrep· ancies between the theory and experimental observations are quite serious. 2 ) Since the original theory is confined to the two heatmode intermediate states, attention has been called to the possible importance of mode coupling processes involving three, four, ... heat·modes in intermediate states in the sound propagation problem. In fact, it is shown ) that taking four heat·mode contribu· tion to sound attenuation and dispersion into account resolves the existing discrepancies. In so doing three heat·mode intermediate states were neglected by simply assuming a built· in symmetry of the Ising model (the lattice·gas model) in which fluctuations in the number density and those in the energy density are statistically independent, whereas this is not so in the real liquid· vapor system. 4 )

Negative Diffusion of Perpendicular Ion Energy in a Magnetized Plasma due to Mode-Coupling Process

It is shown that the mode coupling of drift-wave--type fluctuation via thermal ions in a magnetized plasma can cause a negative diffusion of the ion perpendicular energy across the magnetic field. The resulting diffusion flux is estimated to be of the neoclassical value in the plateau regime.

A complex ray tracing study of ion cyclotron whistlers in the inosphere

The complex ray theory of Budden & Terry (1971) has been used to study ion cyclotron whistlers in the topside ionosphere. A study of mode coupling processes by the use of ray tracing is impossible using standard real ray theory, but they can sometimes be handled by using complex rays and complex space. Losses by coupling have a similar effect to losses by collisional damping, and large coupling losses produce large complex lateral deviation in the ray path, which in turn requires the use of angles of incidence with an appreciable imaginary part to ensure that the ray path ends at a real point. The importance of understanding the branch points of the Booker quartic equation, and the Stokes phenomenon at these points is stressed. Results are presented for calculations in a model ionosphere, which agree quite well with experimental observations.

Theory and numerical simulations on collisionless drift instabilities in three dimensions

The non-linear behaviour of the collisionless drift instabilities and the resultant anomalous plasma diffusion have been studied by means of computer simulations and analytical theory. The simulation model used is a full three-dimensional electrostatic model of cylindrical geometry in an external magnetic field. Full dynamics is employed for ion motion while guiding-centre approximations are used for electron motion, which allows us to use rather realistic plasma parameters in the simulations. The simulation results indicate that a strong turbulence develops through non-linear interaction of the drift instabilities, which results in the formation of convective cells and anomalous particle diffusion. The broad frequency spectrum resembles that observed recently in toroidal confinement devices. Analytical theory is developed, based on the modecoupling process, to explain the generation of convective cells and strong plasma turbulence, along with an estimate of the resultant particle diffusion.