C. S. Wu

Bio: C. S. Wu is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Electron & Electromagnetic radiation. The author has an hindex of 20, co-authored 64 publications receiving 2363 citations. Previous affiliations of C. S. Wu include University of Science and Technology of China & National Central University.

A theory of the terrestrial kilometric radiation

Abstract: During magnetospheric substorms, electrons with energies of about 1 keV are injected from the plasma-sheet region into the auroral region. A fraction of these energetic electrons can precipitate into the upper atmosphere, and the rest are reflected because of the mirror effect of the convergent geomagnetic field. It is found that these reflected electrons can result in the amplification of electromagnetic waves via a relativistic normal cyclotron resonance. This process may explain the recently discovered terrestrial kilometric radiation.

Effects of Finite Plasma Beta on the Lower-Hybrid-Drift Instability,

Abstract: The local dispersion relation for the lower-hybrid-drift (LHD) instability is derived and analyzed, taking into account the finite-beta effects associated with transverse electromagnetic perturbations as well as with resonant and nonresonant electron-orbit modifications due to magnetic-field gradients The influence of finite-beta effects on the LHD instability is calculated in a fully self-consistent manner for arbitrary values of electron-ion temperature ratio, local beta, cross-field ExB velocity/ion thermal speed ratio, and other plasma parameters Stability properties are investigated analytically for the case of cold electrons, and the local dispersion relation is solved numerically in the parameter regime of most interest for high-density plasma pinches The results show that for all parameter regimes studied, the net effect of finite plasma beta is to reduce the maximum growth rate of the LHD instability, although the details can vary, depending on the plasma parameters Except in the limit where the electron/ion temperature ratio tends to zero, it is found that there is a critical value of plasma beta above which the LHD instability is completely stabilized

Effect of finite ion gyroradius on the fire‐hose instability in a high beta plasma

Abstract: In this paper, a generalized kinetic dispersion equation that supports various hydromagnetic waves and instabilities is derived. The general dispersion equation is derived under the usual assumption of hydromagnetic perturbations [i.e., ‖ω‖2≪Ωi2, and (kzνA/Ωi)2≪β∥i, where Ωi and νA are the ion gyrofrequency and Alfven speed, respectively, and β∥i is the parallel ion beta], but for arbitrary values of the quantity λi=(k⊥ρ⊥i)2/2=(k⊥νA/Ωi)2 β⊥i/2 that appears in the dielectric tensor. Here, ρ⊥i refers to the mean ion gyroradius, and β⊥i is the perpendicular ion beta. Otherwise, the dispersion equation is fairly general with no additional approximation, such as ignoring certain off‐diagonal dielectric tensor elements (which is usually done in the literature) employed. In the subsequent numerical analysis, special attention is paid to the fire‐hose instability in a high beta plasma. The numerical results reveal that the conventional treatment of the fire‐hose instability (i.e., taking zero ion gyroradius limit...

Time scales for formation and spreading of velocity shells of pickup ions in the solar wind

Abstract: This paper discusses the process of assimilation (pickup) by the solar wind of newly ionized atoms and molecules. Generally, the pickup process is considered to evolve in three stages: (1) the initial interaction of newly created ions with the interplanetary magnetic field to form the ring-beam distribution; (2) pitch angle scattering of the ring beam to form a hollow shell; and (3) slower velocity diffusion to form a partially filled-in shell distribution. Using numerical simulations of turbulence such as would occur naturally in the solar wind and such as would be encountered near cometary bow shocks, the processes of shell formation and evolution are studied, and the results are used to estimate the time scales for shell formation and diffusion in several situations of recent observational interest, the interstellar He data obtained by AMPTE and cometary ion pickup distributions obtained by various spacecraft at comets Giacobini-Zinner and Halley.

Generation of type iii solar radio bursts in the low corona by direct amplification

Abstract: An alternative scenario to the plasma-emission model is proposed for coronal type III solar radio bursts. According to this model, the radio bursts are produced inside a magnetic flux tube with density depletion by a direct amplification of electromagnetic waves with frequencies near the electron gyrofrequency and its harmonics. The amplification mechanism is the cyclotron-maser instability driven by a beam of flare-generated streaming electrons. In the present discussion, a depletion factor of approximately 102 near the chromosphere is assumed. The essential point is that in order to produce the electromagnetic waves near the fundamental electron gyrofrequency, the present model requires 0.1 ≤ fp/fg ≤ 0.4 (where fp and fg denote the plasma frequency and gyrofrequency, respectively) in the source region. The propagation of an amplified wave is initially confined within the magnetic flux tube until the wave arrives at a point where the local exterior cutoff frequency is equal to the exiting wave frequency. The proposed model is spurred by the consideration that above an active region where the emission is presumed to originate, the ambient magnetic field is strong enough that, in contrast to conventional theories, it cannot be ignored. Preliminary analysis leads to some encouraging results, on the basis of which we may resolve a number of long-standing issues raised by observations. The proposed scenario also implies a fundamentally different interpretation of the observed frequency drift in the dynamic spectrum.

A continuous-wave Raman silicon laser

Abstract: Achieving optical gain and/or lasing in silicon has been one of the most challenging goals in silicon-based photonics because bulk silicon is an indirect bandgap semiconductor and therefore has a very low light emission efficiency. Recently, stimulated Raman scattering has been used to demonstrate light amplification and lasing in silicon. However, because of the nonlinear optical loss associated with two-photon absorption (TPA)-induced free carrier absorption (FCA), until now lasing has been limited to pulsed operation. Here we demonstrate a continuous-wave silicon Raman laser. Specifically, we show that TPA-induced FCA in silicon can be significantly reduced by introducing a reverse-biased p-i-n diode embedded in a silicon waveguide. The laser cavity is formed by coating the facets of the silicon waveguide with multilayer dielectric films. We have demonstrated stable single mode laser output with side-mode suppression of over 55 dB and linewidth of less than 80 MHz. The lasing threshold depends on the p-i-n reverse bias voltage and the laser wavelength can be tuned by adjusting the wavelength of the pump laser. The demonstration of a continuous-wave silicon laser represents a significant milestone for silicon-based optoelectronic devices.

Relativistic theory of wave‐particle resonant diffusion with application to electron acceleration in the magnetosphere

Abstract: Resonant diffusion curves for electron cyclotron resonance with field-aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single-ion plasma, and a numerical scheme is developed for the more realistic case of a multi-ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth's magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler (R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere.

Current disruption in the Earth's magnetosphere: Observations and models

Abstract: Observations and models of current disruption in the Earth's magnetosphere are briefly reviewed. At the approach of current disruption onset, the cross-tail current sheet shows a rapid growth in the current density, a large upsurge in the duskward ion bulk speed to nearly the ion thermal speed, an increase in the plasma pressure and its isotropy, a rise in the plasma beta, and a decrease in the current sheet thickness to a length scale comparable to the thermal ion gyroradius. During current disruption, there are (1) large changes in the local magnetic and electric fields, (2) significant magnetic and electric fluctuations over a broad frequency range, (3) magnetic field-aligned counterstreaming electron beams, (4) ion energization perpendicular to the magnetic field, and (5) reduction in the cross-tail current by an amount similar to that built up during the growth phase. Observations further indicate that regions of local reversal of the north-south magnetic field component are not necessarily sites of intense particle energization. Remote sensing of disruption activities shows that at least some current disruptions are not caused by a disturbance propagating earthward from the tail beyond 10 RE downstream. The timescale involved is comparable to or shorter than the ion gyroperiod. Current disruption thus has spatial and temporal scales outside the MHD regime. Several models for current disruption are briefly discussed. Two roles are considered for the cross-field current instability proposed for current disruption. It can provide anomalous resistivity for magnetic reconnection as advocated by the traditional viewpoint or act singly to instigate global changes of the magnetosphere during the initial substorrn expansion phase. The latter role is elaborated by showing that the instability may modify significantly the local current density and any such process will alter the force equilibrium in the current sheet and give rise to an efficient plasma and energy transport on a global scale. Furthermore, such a process can generate field-aligned current with intensity comparable to those associated with an auroral breakup arc at substorrn expansion onset. This scenario leads to a new emphasis that in addition to magnetic reconnection, rapid conversion of magnetic energy into particle energy in magnetotail systems may take place without a magnetic X line or separatrix playing the key role in energy conversion.

Potential waves for relativistic electron scattering and stochastic acceleration during magnetic storms

Abstract: The possibility of electron stochastic energization to relativisitic energies (≥ 1 MeV) via resonant wave-particle interactions during a magnetic storm is explored. The minimum electron energy Emin for cyclotron resonant interaction with various electromagnetic waves is calculated for conditions representative of storm-times. Since Emin > 1 MeV for resonance with L-mode ion cyclotron waves, intense stormtime EMIC waves could contribute to relativistic electron loss, but not acceleration. Inside the plasmapause whistler mode waves, and highly oblique magnetosonic waves near the lower hybrid frequency, can resonate with electrons over the important energy range from ∼ 100 keV to ∼ 1 MeV. In low density regions outside the plasmapause, the whistler, RX, LO and Z modes can resonate with electrons over a similar energy range. These waves have the potential to contribute to the stochastic acceleration of electrons up to relativistic energies during magnetic storms.

Alfven Waves in the Solar Corona

Abstract: Alfven waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Sun's corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfven waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfven waves may carry sufficient energy.