From the Akebono wave measurements, we have shown that Z-mode electromagnetic waves are excited in the equa- torial plasmasphere during geomagnetically disturbed pe- riods. Z-mode waves tend to be more intense than UHR waves, indicating that these Z-mode waves are excited di- rectly through a wave-particle interaction rather than the mode conversion from electrostatic waves. A possible can- didate of the source population is energetic electrons in- jected into this region by the intense large-scale electric ﬁeld during geomagnetic storms. From the trajectory trac- ing, electrons in the premidnight tail region are accelerated perpendicular to the ambient magnetic ﬁeld and conﬁned around the geomagnetic equator because of the conserva- tion of the ﬁrst and second adiabatic invariants. Since the in- tensity of Z-mode and UHR waves is associated with the de- velopment and decay of the ring current, ring current elec- trons are a probable candidate for the free energy source of these waves after injected into the equatorial plasmasphere. In order to investigate whether equatorially mirroring electrons excite Z-mode plasma waves directly, linear growth rates of Z-mode waves have been calculated nu- merically in the high-density backgrounds of the plasma- sphere. Z-mode waves interact with tens of keV electrons with large pitch angles through the higher-order cyclotron resonance. From the calculations of spatial growth rates, Z-mode waves are ampliﬁed 33.8 dB. This magnitude is
The steepening of whistler-mode waves has been recently studied for waves observed at the frequency of approximately 0.2 electron cyclotron frequency in the inner magnetosphere (Yoon et al. 2014), but it has been rarely studied in the past. The harmonic waves observed near the Moon can provide important clues for under- standing the nonlinearity of whistler-mode waves in a plasma medium. Moreover, the lunar atmosphere and dust environment are investigated in the recent LADEE mission. We expect that the present study of the harmonic waves will expand our understanding of the intrinsic plasma behaviors around the Moon to the solar wind. The possibilities of the harmonics should be verified by comprehensive observations of multi-spacecraft in the future work. The quantitative effects on the waves and the steepening rate are left to be examined.
Inspired from above literature, whistler mode waves have been studied theoretically in this paper. As Pandey et al.  studied the effect of cold plasma injection on whistler mode instability triggered by perpendicular AC electric field at Uranus, we will investigate whistler mode waves by injection of hot electron beam in the magnetosphere of Uranus. In this paper, the beam particles considered hot due to thermal anisotropy and relativistic effect. Also the relativistic effect of beam, affecting the velocity of beam particles, will also be studied while considering the presence of perpendicular AC electric field to magnetic field and by using the method of characteristic solutions and kinetic approach, the detailed derivation and calculations has been done for dispersion relation and growth rate for magnetosphere of Uranus. Parametric analysis has been done by changing various plasma parameters: temperature anisotropy, AC frequency, ratio of n c /n w and relativistic factor to study their effects in the
10 Read more
The theory of magnetohydrodynamic (MHD) waves in structured plasma was de- veloped in the 1970s and 1980s (e.g. Zajtsev & Stepanov 1975; Roberts 1981b,a; Edwin & Roberts 1982, 1983b). During the past decades, various MHD wave modes were conﬁdently detected with modern instruments, and were exploited for seismo- logical applications (see reviews by, e.g., Nakariakov et al. 2005; De Moortel & Nakariakov 2012). Standing fast kink waves were detected in the closed coronal loops (e.g. Nakariakov et al. 1999; Aschwanden et al. 1999; White et al. 2012) and in post-ﬂare arcades (Verwichte et al. 2005). The global (fundamental) kink mode was implemented to estimate the magnetic ﬁeld strength (so called MHD seismology, e.g. Nakariakov & Ofman 2001). Propagating fast kink waves were found in the coronal loops (Williams et al. 2002; Tomczyk et al. 2007; Van Doorsselaere et al. 2008). Trapped fast sausage modes, due to the existence of the cut-oﬀ wavenumber (Nakariakov et al. 2003), are only supported by suﬃciently thick and dense loops. Some spatially-resolved radio imaging observations were consistent with the features of fast sausage waves (e.g. Asai et al. 2001; Melnikov et al. 2005). Standing slow mode waves were detected as intensity and Doppler-velocity oscillations in the post- ﬂare loops using spectrometric data (Wang et al. 2003). Propagating slow mode waves were conﬁdently detected near the footpoints of active region loops (e.g. De Moortel et al. 2000; Marsh et al. 2009; Wang et al. 2009a; Verwichte et al. 2010; Yuan & Nakariakov 2012, and Chapter 4).
162 Read more
ducts. At altitudes below the turning point of an O‐ mode pump wave, near the upper hybrid resonance, upper hybrid and lower hybrid turbulence can lead to the formation of field‐aligned density striations (FAS). Once FAS are formed, O‐mode waves are mode‐ converted to upper hybrid waves, enhancing the turbu lence, leading to anomalous absorption of the pump wave and efficient bulk heating of the electrons. Measurements from the EISCAT heater in Tromsø by Rietveld et al.  directly verify the increase in elec tron temperature by 3000 K whereas the ion tempera ture was enhanced only by 100 K. Later experiments by Blagoveshchenskaya et al.  show electron tempera tures reaching close to 6000 K when the HF pump approaches magnetic zenith. Although the experimental facilities at HAARP cannot directly measure electron temperature, they have been able to detect descending artificial ionized layers (DAIL) [Pedersen et al., 2009], the generation and descent of which were modeled extensively by Eliasson et al. , indicating that the DAILs required bulk heating of the electrons to ~4000 K to be subsequently accelerated by Langmuir turbu lence to suprathermal speed and ionize the neutrals.
19 Read more
gate and drain waveforms have a common parity. The red and blue waveforms represent the waveforms monitored on the gate and drain lines, respectively. Because of the parasitic resistances of L Lg and L Ld , the pulses are simply attenuated in Fig. 5(a). As suggested in Eq. (12), the FET attenuates the pulses even more in Fig. 5(b). Similar waveforms for the π-mode inputs are shown in Fig. 6. For the pinch-off case, the amplitudes of the pulses decrease in Fig. 6(a) similar to that for the c-mode. Moreover, the amplitude gain by the FET contribution is established in Fig. 6(b). It is thus concluded that the π-mode gains the amplitude of the supporting waves, while the c-mode waves are attenuated. Fig. 7 shows four subsequent spatial waveforms for the π-
11 Read more
p r o p a g a tio n , and t h e r e f o r e h e a tin g , a re imposed by th e e x is te n c e o f c u t o f f s a t 0)^ = (jO^ f o r th e 0 mode, end pe pe = a)(o) ± e ) f o r th e X mode, The X mode has a c o ld plasma resonance a t th e upper h y b r id fre q u e n c y ^UH ~ ( w f g + ^ ^ ) ^ , whereas th e 0 mode, b e in g ind e p e n d e n t o f Bp, does n o t possess such a re sona nce . However, b o th waves a re found t o have h o t plasma resonances a t O) = n ^ ^ , (n = 1 , 2 , . . . ) . As u s u a l, these resonances have a mode c o n v e rs io n i n t e r p r e t a t i o n ( C a ir n s and Lashmore- D a v ie s , 1982, 1 983). In th e v i c i n i t y o f th e upper h y b r id o r second harm on ic resonance th e X mode i s c o n v e rte d t o a B e r n s te in wave w hich w i l l be c y c l o t r o n damped f o r n onzero k „ , w h ile th e fu ndam en tal
159 Read more
The upper layer circulation resembles a mode-1 depres- sion wave for the concave case and a mode-1 elevation wave for the convex case. This result means that the surface signatures in synthetic aperture radar (SAR) imagery should resemble these waves, but would likely be weaker and more intermittent. The divergence ahead of the convex wave and the convergence behind it should produce a dark/light pattern (smooth/rough) in SAR imagery; the opposite should be true for the concave wave (Alpers, 1985; Liu et al., 1998; Chang et al., 2008). While mode-2 waves have been observed in moderate resolution imaging spectroradiometer (MODIS) imagery (Yang et al., 2009), they have not yet been observed in SAR imagery.
10 Read more
Damping of ship whipping vibrations following a slam due to wave impact is traditionally assumed to be primarily of material or struc- tural origin. However, several mechanisms of energy dissipation to the surrounding water exist, including gravity and acoustic waves. Nei- ther transports much energy for the lowest frequency modes, in which the acoustic wavelength may be an order or magnitude greater than the ship length whereas the gravity wavelength is at least an order of magnitude shorter than the ship beam. However, the acoustic damp- ing ratio increases as the fourth power of frequency, becoming signif- icant for higher frequency modes. This paper investigates at what
12 Read more
Abstract. We present high-resolution, three-dimensional simulations of rotation-modified mode-2 internal solitary waves at various rotation rates and Schmidt numbers. Ro- tation is seen to change the internal solitary-like waves ob- served in the absence of rotation into a leading Kelvin wave followed by Poincaré waves. Mass and energy is found to be advected towards the right-most side wall (for a Northern Hemisphere rotation), leading to increased amplitude of the leading Kelvin wave and the formation of Kelvin–Helmholtz (K–H) instabilities on the upper and lower edges of the de- formed pycnocline. These fundamentally three-dimensional instabilities are localized within a region near the side wall and intensify in vigour with increasing rotation rate. Sec- ondary Kelvin waves form further behind the wave from ei- ther resonance with radiating Poincaré waves or the remnants of the K–H instability. The first of these mechanisms is in accord with published work on mode-1 Kelvin waves; the second is, to the best of our knowledge, novel to the present study. Both types of secondary Kelvin waves form on the same side of the channel as the leading Kelvin wave. Com- parisons of equivalent cases with different Schmidt numbers indicate that while adopting a numerically advantageous low Schmidt number results in the correct general characteristics of the Kelvin waves, excessive diffusion of the pycnocline and various density features precludes accurate representa- tion of both the trailing Poincaré wave field and the intensity and duration of the Kelvin–Helmholtz instabilities.
15 Read more
In Chapter 6 Charpit’s method was used to solve the WKB equations to find the behaviour of the fast and slow waves along a characteristic curve. This resulted in two sets of ordinary differential equations which were solved numerically using a fourth-order Runge-Kutta scheme. This allowed the path of incoming wave to be found for various starting points along the x-axis. In line with the numerical simulations these showed that the fast wave slows as it approaches the magnetic null point, causing the edges of the wave to refract around the null point (in agreement with McLaughlin and Hood (2004)). Conversely the slow wave was shown to stretch out along the magnetic field lines as it approaches the null point, meaning that only a small section of the wave actually enters the mode-conversion region. The solutions found using Charpit’s method were also used to predict the position of the incoming fast wave (as done in McLaughlin and Hood (2006)) and slow wave, and to predict the position of the transmitted wave from the mode-conversion region. These are in excellent agreement with the numerical simulations. It was also shown that when taking a cut along x = 0 these predictions are identical to those found using the characteristic wave speeds. In Chapters 3 and 4 the one-dimensional model with a uniform, background magnetic field was used as a first step in building up to examine the two-dimensional coronal null point problem. In reality, the mode- conversion region in this type of set-up is unlikely to lie in the corona where the plasma β is typically very low ( O 10 −4 ) and is more likely to lie in the chromosphere or strong field regions in the photosphere. In
193 Read more
In Sect. 5, we considered the case of kink mode dispersion in which the common assumption of the long wavelength limit no longer applies. The eﬀect of the long wavelength approximation breaking down could be seen, and yet overall the behaviour was still quite well described by the analytical profiles. Considering various methods of using the profiles for seismological inver- sions in Sect. 4, we demonstrated simple yet robust methods for obtaining accurate (error 10%) estimates of some of the loop parameters. Indeed, even the less appropriate methods produced seismological estimates with errors less than a factor of 2. In this regard, the use of damped kink oscillations is a promising seismological tool, subject of course to su ﬃ ciently good data. This data would ideally resemble the numerical simulations pre- sented here, i.e. accurate measurements of many wavelengths. However, we have shown that even just a few wavelengths can be suﬃcient for accurate inversions if they have suﬃciently low noise.
13 Read more
heating could potentially be a more eﬃcient mechanism (Najmi et al., 2016). Stochastic heating of charged particles can occur transverse to a magnetic ﬁeld in the presence of large-amplitude electric ﬁeld gradients (Balikhin et al., 1993; McChesney et al., 1987; Stasiewicz et al., 2000) exceeding a threshold amplitude above which the individual particle orbits become unstable and diverge with time. A one-dimensional (1-D) Vlasov simulation study (Najmi et al., 2016) has demonstrated the viability of such a heat- ing scheme at the UH layer, whereby electrons are eﬃciently energized via stochastic heating by short-wavelength, large-amplitude EB waves gener- ated through the parametric decay of trapped UH waves in a magnetic ﬁeld-aligned density striation (FAS). Submeter, supersmall FAS are believed to be produced by electrostatic processes (Gurevich & Zybin, 2006; Milikh et al., 2008; Najmi et al., 2014) within a few seconds of the O-mode pump wave being initiated and later evolve to be a few meters across but tens of kilometers in length along the magnetic ﬁeld lines (Kelley et al., 1995; Franz et al., 1999).
13 Read more
are given in Fig. 12. In the forward-propagating long waves, the kinetic energy was all in the mode-1 form because of its propagation in front of the ISWs, indicating the genera- tion of the forward-propagating long waves corresponds to a cascading process from higher to lower modes. Figure 11 shows that the energy was mainly mode-1 in the oscillat- ing tail and the amplitude-modulated wave packet, but weak mode-2 signals were also present. The presence of mode-2 energy for the oscillating tail is reasonable because they have shorter wavelengths and slower phase speeds than the mode- 2 ISW and propagate following the mode-2 ISW (Akylas and Grimshaw, 1992; Vlasenko et al., 2010). The modal struc- tures of forward-propagating long waves for different times show its mode-1 nature was stable during the evolution of mode-2 ISWs in the background shear current (Fig. 12a and b). In the rear of the mode-2 ISW, the modal structures of trailing waves transformed slightly with time (Fig. 12c–g), and the wave-induced currents are more and more concen- trated around the pycnocline when the amplitude-modulated wave packet propagates far away from the mode-2 ISW. 4.3 Energy loss of mode-2 ISWs
15 Read more
Abstract. – A dynamical eﬀect of coherent backscattering is predicted theoretically and supported bycomputer simulations: The distribution of single-mode delaytimes of waves reﬂected bya disordered waveguide depends on whether the incident and detected modes are the same or not. The change amounts to a rescaling of the distribution bya factor close to √ 2. This eﬀect appears onlyif the length of the waveguide exceeds the localization length; there is no eﬀect of coherent backscattering on the delaytimes in the diﬀusive regime.
To swim at a steady speed a fish must produce power, primarily to overcome drag. This power is generated by the myotomal muscle on either side of the body. A wave of muscle activation/contraction (detected as an electromyogram, EMG) passes alternately down each side of the body from head to tail. A wave of curvature also travels down the body, as a result of the combined effects of muscle activity, the arrangement and physical properties of the myosepta and skeletal elements, and the interaction between the fish’s body and the reactive forces from the water in which it moves (for a review, see Videler, 1993). The role of the passive myoseptal and skeletal elements is important, but beyond the scope of this review. Because of the interaction between the body and the water, the coupling between muscle activity and body curvature depends upon the characteristic shape of each fish species. The power generated by muscle contraction is converted to thrust, either along the length of the fish, or at the tail, depending upon the swimming mode. The relationship between the waves of muscle activity and body curvature also depends upon the swimming mode adopted by the fish. Muscle fibres lengthen and shorten rhythmically during steady swimming. The timing of muscle activity relative to the phase of the strain cycle determines the force, work and power output of the muscle. Just how body
It is known that surface waves such as Rayleigh waves will interact with a surface defect, and measuring the properties of the transmitted and reflected waves can help to characterise the defect [1-4]. Furthermore, amplitude and frequency enhancements in the near field have been reported previously for a Rayleigh wave incident on a surface defect . This previous work focussed on enhancements arising from defects normal to the sample surface [3,4,6], however, many industrial failures arise from defects that have grown at an angle to the surface. Rolling contact fatigue in rails, for example, commonly propagates at an angle of 25° to the sample surface, and stress corrosion cracking defects can propagate at any given angle to the surface. Both of these defects can cause costly damage to industrial components and can be very challenging to detect. The amplitude and frequency enhancement effects observed in the near field of these defects may be useful to characterise the angle of the defect, and subsequently the defect type, prior to catastrophic damage occurring.
Using the setup explained in Sect. 2, we ran two more simula- tions in which we only changed the width of the boundary shell by using l = 0.75 (wider boundary shell) and l = 0.35 (nar- rower boundary shell) with respect to Table 1. These were anal- ysed in combination with the reference simulation with l = 0.5 described in Sect. 3 to investigate how the mode-coupling and phase-mixing are affected by Alfvén speed gradients. A nar- rower boundary shell leads to a steeper Alfvén speed profile (as all other parameters in the setup have remained unchanged), implying that phase-mixing will become more efficient (i.e. the damping length will be shorter Heyvaerts & Priest (1983)). The mode-coupling process that feeds kinetic and magnetic energy into the shell region also depends on the Alfvén speed profile in the shell region, but now a wider shell (a milder Alfvén speed gradient) leads to more efficient mode-coupling (Pascoe et al. 2010, 2012, 2013). Hence, the deposition of thermal energy is dependent on the combined efficiency of mode-coupling and phase-mixing and how the resistivity effects interact with these mechanisms, so that it is not a priori obvious which configuration will be most efficient. The purpose of this experiment is to assess which effect dominates the dynamics and how this influences the non-ideal effects in the simulation, in particular the heating.
13 Read more
It is very important to identify different guided waves modes for ultrasonic guided waves NDT to diagnose the structure integrity. Traditional guided waves signal proc- essing methods include the wave-form analysis such as wavelet analysis  in the time domain, the two dimen- sional Fourier Transform  in the frequency domain and the short Time Fourier Transform  in the time- frequency domain. Recently, correlation analysis  and Duffing chaotic oscillator  have been applied to iden- tify the small flaw and improved the accuracy of fault detection by guided wave inspection. A Fourier basis provided a poor representation of functions well local- ized in time, and wavelet basis are not well suitable to represent functions whose fourier transforms have a nar- row high frequency support. In both cases, it is difficult to detect and identify the guided wave patterns from their expansion coefficients, because the information is diluted across the whole basis. The wavelet analysis, correlation analysis and duffing chaotic oscillator can be used as different kinds of denoise approaches and they can not identify the different guided wave modes. The Fourier Transform can only get the frequency information. The short Time Fourier Transform can get the guided wave
The derived maps of various parameters of the governing quantities of the underlying model (such as the location of the maxima of the modal function) and the parameters of the weakly non-linear models provide a new insight into qualita- tive features of the propagation and transformations of inter- nal waves of the second mode in the South China Sea. The presented climatologically valid distributions of the phase speed and coefficients at the non-linear terms of Gardner’s equation (1) (or other equations of the family of KdV-type equations) may be used for expressing estimates of various parameters of internal waves of this kind. This includes in- ter alia evaluation of hydrodynamic loads on the seabed (and on offshore engineering structures) created by the propaga- tion of such waves, forecasting of areas and depths strongly affected by the internal wave activity after intense wave gen- eration events, and identification of regions with a very high probability that such waves will break.
16 Read more