K. J. Harker

Radiation from long pulse train electron beams in space plasmas

A previous study of electromagnetic radiation from a finite train of electron pulses is extended to an infinite train of such pulses. The electrons are assumed to follow an idealized helical path through a space plasma in such a manner as to retain their respective position within the beam. This leads to radiation by coherent spontaneous emission. The waves of interest in this region are the whistler slow (compressional) and fast (torsional) Alfven waves. Although a general theory is developed, analysis is then restricted to two approximations, the short and long electron beam. Formulas for the radiation per unit solid angle from the short beam are presented as a function of both propagation and ray angles, electron beam pulse width and separation and beam current, voltage, and pitch angle. Similar formulas for the total power radiated from the long beam are derived as a function of frequency, propagation angle, and ray angle. Predictions of the power radiated are presented for representative examples as determined by the long beam theory.

Magnetic fields in the vicinity of pulsed electron beams in space

Abstract Most measurements of the radio frequency emissions from artificial electron beams in space have been made in close proximity to the beam. This paper presents theory and results for the near-field radiation associated with the operation of a pulsed electron beam, taking into account the effects of the ionospheric plasma. The beam is modeled as an infinite train of square-wave pulses and the radiation is obtained by adding coherently the radiation from each individual electron in the idealized helical trajectory assumed by the beam as it traverses the magnetized plasma. The mathematical solution is obtained by taking the Fourier transform in space and time of Maxwells equations and the driving modulated beam current, and then taking the inverse transform of the resulting electric field.Typical electric field strengths are presented for a range of modulation frequencies extending from below the ion cyclotron to the electron cyclotron frequency.

Waves generated by pulsed electron beams

Abstract During the Spacelab-2 flight of July, 1985, electron beams (1 keV, 100 mA) square-wave modulated at ELF and VLF were emitted from the space shuttle. The wave fields generated by the beam were monitored by a free-flying sub-satellite at distances up to 300 m perpendicular to the beam. The amplitude of the magnetic and electric fields were modulated by the spin of the satellite. This modulation allows the study of the wave polarization. Results for a 7 min duration beam sequence in which the beam was pulsed at 1.22 kHz are presented and compared with recent predictions for wave-fields stimulated by ideal helical electron beams propagating in a magnetized plasma. It is found that the predicted magnetic field amplitude of the first harmonic of the beam pulsing frequency (below the lower hybrid frequency) is in agreement with observations, however, the predicted and the observed polarizations are entirely different. For the higher harmonic components (above the lower hybrid frequency), theory predicts order of magnitude larger magnetic field amplitudes than observed. It is concluded that the theory does not adequately model the full distribution of the radiating current.

Ground level signal strength of electromagnetic waves generated by pulsed electron beams in space

Abstract A theoretical study has been made of the signal strengths at ground level of waves generated by pulsed electron beams in space. Such beams might be generated by pulsed electron guns aboard either a satellite or a rocket. The radiated energy is first calculated by an improved version of a theory based on coherent spontaneous emission. This theory evaluates the electric and magnetic field strengths and power fluxes in the far field by applying asymptotic expansion techniques. With this information at hand, the power flowing out within a cone whose apex is located at the gun position is calculated. Finally, the intersection of the rays in this cone with the Earths surface is determined by using Snells law considerations. Ground signal levels are calculated for typical ionospheric conditions as a function of pulsing frequency for fixed beam voltage and for voltage adjusted for resonance between the waves and the particles. For short beams, the ground level signal strengths are relatively insensitive to the wave particle resonance condition, but for longer beams the associated peaking of the signal level begins to be observed. Finally, these results are compared against ambient noise levels to determine under which circumstances these ground signals can be detected.