ISSN:0048-6604    

DOI:10.1029/RS019i002p00453

年代:1984

数据来源: WILEY

摘要:

A theoretical study has been made of the electromagnetic radiation arising from pulsed electron beams. The study assumes an electron beam which has a well‐organized spatial structure determined by a fixed trajectory in a magnetic field and on/off pulsing governed by the electron source. From this model the electromagnetic radiation is determined by adding coherently the radiation from each individual electron in the helical stream. The radiation per unit frequency interval is determined, as well as the radiation per unit solid angle, as a function of both propagation and ray angles, electron beam pulse width and separation, total number of pulses, and beam current. As expected for a coherent process, it is found that the radiated power varies at the square of the beam current. The relatively high efficiency of the beam in producing electromagnetic radiation is illustrated by consideration, among others, of a 1‐keV, 100‐mA beam used in recent experiments on the space shuttle. For these parameters the total radiated power per steradian is calculated at selected angles to be greater than 1% of the total beam power carried as electron kinetic energy. These results provide a useful theoretical basis for planning future electron beam experiments in space pl

ISSN:0048-6604    

DOI:10.1029/RS019i002p00454

年代:1984

数据来源: WILEY

摘要:

The first active beam‐plasma experiments utilizing the space shuttle were carried out in March 1982 as part of the NASA Office of Space Science mission on the third space shuttle flight. A fast pulse electron generator emitted a 1‐keV, 100‐mA electron beam in either a continuous (dc) mode or an on/off modulated mode in the ELF to HF frequency range. Stimulated electrostatic and electromagnetic waves and associated plasma effects were measured with the plasma diagnostics package as it was maneuvered through and near the beam by the remote manipulator system. For the dc beam the wave spectrum was electrostatic, was peaked in the 300‐ to 500‐Hz range with a spectral density of 4 × 10−3V2m−2Hz−1, and was unpolarized. Above the nominal lower hybrid resonance frequency the intensity decreased but was polarized. Strong emissions near the electron gyrofrequency and the plasma frequency were occasionally detected. The background spectrum was similar to the dc case for VLF and ELF modulations, but the emissions at the modulation frequency were more electromagnetic and more intense with field strengths

ISSN:0048-6604    

DOI:10.1029/RS019i002p00471

年代:1984

数据来源: WILEY

摘要:

An electron beam emitted from the Office of Space Sciences 1 pallet on STS 3 was pulsed with specially designed very low frequency (VLF) formats in an attempt to generate whistler mode waves. Modulated operations of the beam emitted by a fast pulse electron generator (FPEG) were initiated during times of magnetic conjunctions between STS 3 and the high‐altitude DE 1 satellite equipped with broadband VLF receivers. Coordinated FPEG/VLF modulation and DE 1 wideband data acquisition were achieved in 12 different cases. No evidence of any waves generated by FPEG were detected on the DE 1 analog wideband data. However, it is shown that in all of the cases, either the STS 3 attitude was such that the emitted electrons struck the main body of the vehicle, or it was not possible for whistler mode waves to propagate from the STS 3 location up to the vicinity of the DE 1 satellit

ISSN:0048-6604    

DOI:10.1029/RS019i002p00487

年代:1984

数据来源: WILEY

摘要:

Computer simulations are performed to investigate beam‐plasma physics in the Space Experiment With Particle Accelerators, which is planned for active and interactive experiments in the earth's upper atmosphere and magnetosphere using a high‐power electron gun on board the space shuttle. Using a two‐dimensional electromagnetic particle simulation code (EM2), we study the divergence and propagation of the electron beam through the ambient magnetoactive plasma as well as the plasma wave excitation by the electron beam. First, we apply the EM2 code to a one‐dimensional model where thexaxis is taken perpendicular to the static magnetic field. We find a strong excitation of a slow extraordinary wave around the electron beam and a propagation of a pulselike ordinary wave packet excited as an impulse response to the onset of the beam injection. Second, we apply the EM2 code to a two‐dimensional model where thex‐yplane is taken transverse to the static magnetic field. The detailed behavior of the beam as well as the background cold plasma is studied. The electron beam is locally injected in thex‐yplane with a drift velocityVz. The electron beam initially expands in thex‐yplane owing to an electrostatic field produced by the excess charge of the beam electrons, and it subsequently oscillates with the frequency of the slow extraordinary wave which is excited around the beam. Uniformity in thez; direction is assumed, producing a self‐consistent two‐dimensional simulation as a preliminary to a later three‐dimensional study where this con

ISSN:0048-6604    

DOI:10.1029/RS019i002p00496

年代:1984

数据来源: WILEY

摘要:

Wave‐particle interactions produced by a low‐energy electron beam injected into the ionosphere are experimentally examined in a space simulation plasma chamber. The energy range of the beam is from thermal (∼0.1 eV) to 4 eV. An electrostatic VLF wave with a monochromatic frequency well below the LHR frequency is excited. The beam‐excited wave propagates almost perpendicular to the beam flow direction. The phase velocity of the excited wave is much slower than the ion sound speed in the background plasma. A standing wave is observed within a hollow region surrounded by a conic beam in the direction perpendicular to the conic axis, but the wave inside the hollow cone is found to propagate along the conic axis. The frequency changes of the emissions observed in the previous rocket experiment are successfully reproduced when the beam injection angle from the magnetic field is

ISSN:0048-6604    

DOI:10.1029/RS019i002p00503

年代:1984

数据来源: WILEY

摘要:

We study the linear growth and nonlinear saturation of electromagnetic radiation amplified by a hot (5–20 keV) population of electrons possessing a loss cone velocity distribution in the presence of a cold (20–500 eV) electron population. A relativistic electromagnetic simulation code is used to study the emission process. Three cases are presented in detail to illustrate the generation process of auroral kilometric radiation. The first case, which has an electron plasma frequency ωpe= 0.2ωce(electron cyclotron frequency) and possesses a double loss cone distribution, exhibits a strong narrow peak of the fast extraordinary mode (Xmode) radiation just above the X mode cutoff frequency. The second case with ωpe= 0.2ωceand a single loss cone distribution shows a preferred direction of propagation for the amplified radiation. The third case with ωpe= 0.5ωceshows a peak in the ordinary mode (Omode) radiation. In all cases, the radiation saturates by turbulent scattering of resonant particles into the

ISSN:0048-6604    

DOI:10.1029/RS019i002p00509

年代:1984

数据来源: WILEY

摘要:

A kinetic cyclotron maser instability associated with a hollow‐beam distribution function is studied. The instability differs from that discussed for the gyrotron device in two respects: in the present case the momentum dispersion is substantial, and furthermore there exists a low‐energy background plasma. On the basis of physical arguments we can demonstrate that the hollow‐beam distribution is far more unstable than the loss cone distribution which has been extensively investigated in recent years. A criterion for maximum growth rate is established on the basis of consideration of the resonance ellipse. The validity of this criterion is supported by the results of numerical calcul

ISSN:0048-6604    

DOI:10.1029/RS019i002p00519

年代:1984

数据来源: WILEY

摘要:

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all‐sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty‐eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron densityNecontours, from the satellite altitude down to theFregion ionization peak, were obtained along N‐S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all‐sky camera images. Density cavities (regions whereNe<100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral‐arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudin

ISSN:0048-6604    

DOI:10.1029/RS019i002p00527

年代:1984

数据来源: WILEY

摘要:

ISIS 1 topside sounder receiver observations that reveal examples ofOmode auroral kilometric radiation (AKR) are presented. They correspond to locations outside the low‐density source region of intense AKRXmode emission. The propagation modes are identified by comparing the natural radiation wave cutoffs with the local resonant and wave cutoff phenomena stimulated by the sounder transmitter. TheOmode AKR is the dominant emission in these regions of relatively high electron density, but it is considerably weaker than the intenseXmode AKR observed to emanate from low‐density cavities above the auroral regions. In addition to the fundamentalOmode, second and third harmonic bands of radiation have also been detected. These harmonics associated withOmode AKR are less intense than the harmonics associated withXmode AKR. It is difficult to explain the variety of harmonic AKR observations (Xas well asOmode) on the basis of present AKR theor

ISSN:0048-6604    

DOI:10.1029/RS019i002p00543

年代:1984

数据来源: WILEY