Sally K. Ride

On the interaction between the shocked solar wind and the planetary ions on the dayside of Venus

Around Venus the planetary ionosphere is directly exposed to the shocked solar wind. The interaction takes place in a broad region surrounding the dayside ionosphere, called the mantle, where the shocked solar wind plasma and the plasma of planetary origin have equally important roles. In this paper both the experimentally determined characteristics and the microphysics of the mantle are discussed in detail. It is shown that as a result of the interaction between the two plasma populations, a modified two-stream instability develops, and waves are excited with a frequency of a few times the lower hybrid frequency. The polarization of the waves is almost perpendicular to the magnetic field. The stabilization of the higher-frequency part of the wave spectrum is the result of transverse wave convection in the particular sheet-like geometry of the mantle. The interaction of these waves with planetary ions and electrons is described within the framework of a nonlinear model in which the saturation of the modified two-stream instability is due to induced scattering of the waves on cold planetary ions. The effective collision frequency between the shocked solar wind protons and planetary ions is also calculated; it is shown how this leads to ion pick up and heating. Other macroscopically observable effects of these processes are electron acceleration along the magnetic field and ionospheric heating. The experimental data collected in the dayside mantle of Venus by the instruments carried onboard the Pioneer Venus Orbiter are compared to our model. It is believed that the observations support the scenario presented.

Reducing slip in a far‐infrared free‐electron laser using a parallel‐plane waveguide

There are several interesting applications for a picosecond, far‐infrared (100–1000 μ) free‐electron laser. One obstacle to its development is the slip that occurs between the electron beam and the radiation. This can be reduced by operating the laser in a parallel‐plane waveguide, and choosing the laser parameters and transverse guide dimension such that the group velocity of the wave nearly matches the axial velocity of the electrons. The laser wavelength depends on both the electron energy and the waveguide dimension, and the laser can be tuned by varying either. Both the tuning characteristics and the slip as a function of wavelength are different from those of a conventional free‐electron laser.

Laser synchrotron radiation as a compact source of tunable, short pulse hard X-rays

Abstract A compact laser synchrotron source (LSS) is proposed as a means of generating tunable, narrow bandwidth, ultrashort pulses of hard X-rays. The LSS is based on the Thomson backscattering of intense laser radiation from a counterstreaming electron beam. Advances in both compact ultraintense solid-state lasers and high brightness rf linac beams make the LSS an attractive compact source of high brightness X-rays, particularly at photon energies beyond ∼ 30 keV. The spectral flux, brightness, bandwidth and pulse structure are analyzed. In the absence of filtering the spectral bandwidth in the LSS is typically ⪅ 1% and is limited by electron beam emittance and energy spread. Two configurations of the LSS are discussed, one providing high peak power and the other high average power.

Excitation of VLF waves by an electron beam injected into the ionosphere

We propose a scenario for radiation of electromagnetic whistlers as a result of the injections of a modulated electron beam into the ionosphere. We show that for plasma parameters typical of the ionosphere and electron beam parameters typical of electron guns carried into space aboard rockets or spacecraft, the interaction can result in the instability of electrostatic waves driven by Cherenkov resonance between the waves and the beam. A linear stability analysis was carried out for two limiting cases: a wide and a narrow electron beam; the wave excitation is shown to develop at typical distances of the order of 1 km. Nonlinear evolution of the instability was investigated by a combination of analytical and numerical methods. Electron beam energy is efficiently transformed into the electrostatic mode. The mechanism of electromagnetic mode generation is then dipole radiation from the electron beam, which is strongly bunched as a result of the electrostatic instability. The efficiency of radiation is a strong function of the ratio of the electron beam velocity to electron Alfven velocity (v{sub ae} = cw{sub ce}/w{sub pe}); the possibility of increasing the efficiency of radiation is discussed. 34 refs., 9 figs.

A laser accelerator

We show that a laser can efficiently accelerate charged particles if a magnetic field is introduced to improve the coupling between the particle and the wave. Solving the relativistic equations of motion for an electron in a uniform magnetic field and superposed, circularly polarized electromagnetic wave, we find that in energy-position phase space an electron traces out a curtate cycloid: it alternately gains and loses energy. If, however, the parameters are chosen so that the electrons oscillations in the two fields are resonant, it will continually accelerate or decelerate depending on its initial position within a wavelength of light.A laboratory accelerator operating under these resonant conditions appears attractive: in a magnetic field of 105 Gauss, and the fields of a 5×1012 W, 10 μm wavelength laser, an optimally positioned electron would accelerate to 700 MeV in only 10m.

Demonstration experiment of a laser synchrotron source for tunable, monochromatic X-rays at 500 eV

A demonstration experiment of the laser synchrotron source (LSS) is being planned and constructed to generate short-pulsed, tunable X-rays in the range of ∼500 eV by Thomson scattering of laser photons from a relativistic electron beam. Laser photons of λ = 1.06 μm are Thomson backscattered by a 4.5 MeV electron beam from an S-band RF electron gun. The laser photons are derived from a 15 J, 3 ns Nd:glass laser. The RF electron gun is being constructed for initial operation using a thermionic cathode. It will be upgraded with a photocathode to produce high quality electron beams with high current and low emittance. The X-ray pulse structure consists of ∼10 ps long micropulses within a laser pulse width dependent macropulse. The estimated X-ray photon flux is ∼1018 photons/s, and the number of photons per macropulse is ∼108.

Thomson Backscattered X-Rays from an Intense Laser Beam

Abstract We have formulated and obtained analytical expressions for Thomson backscattered X-ray radiation for an electron beam incident on a linearly polarized electromagnetic undulator at a small angle. The analytical expressions are valid for fundamental and harmonics with arbitrarily large laser intensities. The intensity distribution pattern is evaluated numerically.

Upstream wave activity at comet P/Grigg-Skjellerup

Comet P/Grigg-Skjellerup is a comet with a moderate gas production rate. Quasi-linear theory, which has successfully described waves upstream of the bow shock of comet P/Halley (a comet with a much higher gas production rate), is not appropriate in this case. In this paper, we develop the nonlinear theory of laminar MHD wave activity upstream of the bow shock of P/Grigg-Skjellerup using a combination of analytical and numerical methods. The nonlinear saturation mechanism is the trapping of cometary ions, resulting in their bounce oscillations over pitch angle. For the specific case of the Giotto encounter with P/Grigg-Skjellerup, during which the solar wind propagation was almost transverse to the magnetic field, the excitation of left-hand circularly polarized waves propagating toward the comet is initially dominant. Later on, as a result of pitch angle scattering of resonant particles, we find that waves propagating toward the Sun are also excited. The numerical solutions show that under the action of waves propagating in both directions the cometary ions acquire a spread in energy. The results of this theoretical analysis are compared with measurements made during the Giotto encounter with P/Grigg-Skjellerup and are found to be in good qualitative agreement.

Excitation of nonlinear Alfvén waves by an ion beam in a plasma: 1. Right‐hand polarized waves

A self-consistent theory of MHD wave excitation in the solar wind plasma by a cold ion beam moving along the ambient magnetic field is developed. Using analytical and numerical technics, we investigate the linear stage of the exponential growth of wave amplitudes, their saturation due to trapping or pitch angle diffusion of resonant ions and the steepening of the wave profile due to plasma nonlinearity. The derivative nonlinear Schrodinger equation was used to describe the nonlinear input of plasma particles, and a hybrid method of incomplete numerical simulation was used to investigate strongly nonlinear motion of resonant particles. We have studied the temporal behavior of wave energy for waves propagating in both positive and negative directions in relation to the magnetic field, directions and we have found that in cases under consideration the steepening profile developed only for right-hand polarized waves. The physical mechanism for wave steepening is higher harmonic generation due to plasma nonlinearity.

Wave activity near Pluto

In this paper, we analyze wave activity near Pluto. The situation is somewhat analogous to that near comets: molecules (in this case methane) escape from the atmosphere; some are phono-ionized, and a comet-like mass-loading of the solar wind plasma takes place. But the wave activity due to planetary ion interaction with the solar wind is quite different from that near comets. In the weak interplanetary magnetic field near Pluto, there is no MHD wave excitation; instead, we show that ion acoustic waves are excited. We construct a nonlinear theory of ion-acoustic wave excitation. These waves are excited due to a dissipative instability. The frequency, characteristic growth rate, and spatial scale of the instability are calculated. The main physical mechanism for wave amplitude saturation is the smearing of the velocity distribution function of the beam ions. We use a quasilinear approach to determine the characteristic scale for the electric wave amplitude.