Phase Velocity Of Electromagnetic Wave Research Articles

The influence of the ionized medium on synchrotron emission spectra in the solar corona

Suppression of synchrotron emission at low frequencies owing to the influence of the ionized coronal plasma is proposed to account for the observed low frequency cutoff in type IV radio bursts Since in a plasma the index of refraction is frequency dependent and less than unity, the phase velocity of electromagnetic waves is greater than c at low frequencies with the result that synchrotron emission is suppressed. A calculation is made of the synchrotron emission resulting from high-energy electrons radiating in the coronal plasma, and these spectra exhibit sharp low-frequency cutoffs similar to those observed in type IV meter and decimeter radio bursts.

Superphase velocity effects caused by a single particle

The effects that take place when a particle traverses a medium of refractive index n(ω) with a velocity v, are discussed. The particle can be an electron or a dipole and its velocity v is referred to the phase velocity of electromagnetic waves in the medium c/ n(ω) as being sub- or superphase velocity. All equations are specific cases of the equations of conservation of momentum and energy of both the photon and particle. The emission of photons without incident photons is shown to yield the Čerenkov and normal and anomalous Doppler effects. Scattering by moving electrons and dipoles is treated at some length, including the normal and inverse Compton effects, ionospheric scattering, anomalous absorption and anomalous anti-Stokes scattering.

The Doppler Effects of the Drifting Plasma in Semiconductors

The Doppler effects of injected carriers of semiconductor are detected in microwave fields and applied to the measurement of the drift velocity of semiconductor plasma. The beat frequency (Δf) between incident and Doppler shifted microwave signals is theoretically shown to be given by Δf=(2vd/cp)f0, where vd and cp are the drift velocity of injected carrier and the phase velocity of electromagnetic wave of frequency f0 in a wave guide, respectively. This relation is verified experimentally by the observation of the Doppler signals in 35 G.c/s as well as by the measurement of the drift time between two tapered wave guides of 10 G.c/s. The drift velocity of the injected carriers can be measured over a wide range of electric field strength. In lower electric field strengths the drift velocity is equal to the ambipolar drift velocity, but in high electric fields it deviates from the ambipolar velocity and finally saturates to a constant value.

Instabilities in a Plasma-Beam System Immersed in a Magnetic Field

The interaction of an electron or an ion beam moving with velocity $\ensuremath{\beta}c$ through a stationary plasma in the presence of a static magnetic field is investigated theoretically under the assumption that the plasma is cold, the intensity of the beam is very small, and that the beam, as well as the waves resulting from the interaction, are aligned along the direction of the magnetic field. It is well known that an electron beam moving with velocity exceeding the phase velocity of electromagnetic waves in a stationary plasma is capable of exciting such waves and these waves (circularly polarized) rotate in the same direction and at the same angular frequency as the gyroelectrons of the beam. It is shown in this investigation that there is an apparent reversal in the direction of rotation of these gyroelectrons as seen by the stationary observer. Consequently, the excited wave has a circular motion in the same direction as perturbed stationary ions. The frequency $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\omega}}$ of the excited wave satisfies the inequality $|\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\omega}}|l{\ensuremath{\Omega}}_{i}$, where ${\ensuremath{\Omega}}_{i}$ is the gyrofrequency of perturbed stationary ions. Similarly, a wave excited by an ion beam has a circular motion in the same direction as perturbed stationary electrons. The frequency ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\omega}}}^{i}$ of the latter wave satisfies the inequality $|{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\omega}}}^{i}|l{\ensuremath{\Omega}}_{e}$, where ${\ensuremath{\Omega}}_{e}$ is the gyrofrequency of perturbed stationary electrons. It is shown that an electron (ion) beam moving in the direction of the magnetic field can excite only a wave having negative (positive) helicity. The reverse situation occurs for a beam moving against the magnetic field. General relationships are formulated and illustrated graphically for determining the frequency and growth rate of waves which can be excited by an electron or an ion beam in any magnetized cold plasma. A particular case is illustrated in which an incident ion beam interacts with a relative dense plasma such as occurring in thermonuclear discharges, ionosphere, and interstellar space. It is shown that in such cases there is an excitation of hydromagnetic waves for a wide range of velocities of the ion beam.

Precision Determination of the Velocity of Electromagnetic Waves

THE free-space phase velocity of electromagnetic waves in vacuo has been measured by means of a millimetre-wave interferometer operating at a frequency of 72 Gc./s., corresponding to a wave-length of about 4 mm. The result is: c0 = 299 792.50 ± 0.10 km./s.

Phase-Velocity of Electro-Magnetic Waves along the Ground

IN a recent publication, Colwell and his co-workers have recorded the results of experiments to determine the group velocity of electro-magnetic waves along the ground1. These experiments indicated that the speed with which a short pulse of radiation passed along the ground between two points situated some kilometres apart was considerably less than the velocity of light in free space. Their measurements were made on frequencies in the neighbourhood of 2,000 kilocycles per second.