Plasma Wave System Research Articles

A comparison between the Earth's and Mars' bow shocks detected by the Phobos Plasma-Wave System

Among the scientific instruments on the Phobos spacecraft, one was expected to study the plasma and wave phenomena in the Martian environment: the Plasma-Wave System, PWS. Here we report on the PWS plasma and electric-field spectrum measurements during two Martian bow shock crossings by Phobos 2: one is located near the subsolar point and the other near the dusk terminator. A comparison is also made with the Earth's shock crossed by Phobos 1. As at Earth, three main regions were identified: the upstream region, the shock transition region and the downstream region. A shock foot boundary is often observed in front of the bow shock. This foot is known to be associated with gyrating ions reflected from the shock. Electric-field spectra are presented and tentatively interpreted. The dynamic spectrograms shown in this report differ from the ones published previously, for the fact that the filter channels are not sampled at the same time has been taken into account.

The HARP plasma experiment on-board the Phobos 2 spacecraft: Preliminary results

The HARP differential electrostatic analyzer measured thermal and supra thermal electron and ion fluxes and distributions in the Mars environment. High bit-rate data obtained around the pericenter passage on elliptical orbits early in February 1989 will be highlighted, and correlation with results of other experiments will be discussed. A similarity between the HARP total count rate variations and the spacecraft potential measurements of the Plasma-Wave System experiment is established. The variation of electron energy spectra is discussed in various regions of the solar wind interaction with Mars. Tentative results on electron and ion anisotropy are presented, making use of regular variations of the ratio of count rates of two sensor heads looking in perpendicular directions. Magnetic field data of the MAGMA magnetometer are used for comparison.

The nonlinear description of a plasma maser

A plasma–microwave system was proposed by one of the authors for the production of microwave radiation. The device exploits the electromagnetic instabilities of nonequilibrium plasmas. During single-mode operation of the plasma maser, the resonant particles become trapped and cease to radiate before a substantial fraction of the energy available could be extracted from the plasma in the form of radiation. (This is not true for multimode operation. During multimode operation, the total energy available is distributed over a large number of resonant lines and the amplitudes of the various frequency components of the waves are not at all large enough to cause particle trapping and subsequently saturation. Thus, the linear theory remains valid until the anisotropy disappears due to radiation.) One should, therefore, consider the interaction of the bulk plasma with the nonresonant monochromatic electromagnetic wave in order to estimate the total energy transferred from the plasma to the wave. In the framework of the linear treatment, due to the resonant nature of the instability considered in previous work by one of the authors, the transfer of energy from the nonresonant plasma to the wave is zero. Consequently, the efficiency of the device during single-mode operation can only be evaluated after considering the nonlinear evolution of the plasma–microwave system. In this paper we describe in some details the nonlinear evolution of the plasma–wave system using a single particle analysis. The current density J is calculated using the second moment of the particle distribution function (evaluated using Liouville’s theorem of conservation of phase space density along particle orbits), and the energy balance equation is solved numerically.

Jovian plasma sheet density profile determined by low frequency radio and plasma waves

By using PRA (Planetary Radio Astronomy), PWS (Plasma Wave System) and MAG (Magnetometer) data from Voyagers 1 and 2, essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is estimated. The PRA experiment gives information about the plasma wave polarization. The emission was found to be left hand polarized when the spacecraft was in the northern magnetic tail lobe, and right hand in the southern lobe. The PWS experiment enables the determination of a density profile of the Jovian plasma sheet from the knowledge of the low frequency cutoffs of nonthermal continuum radiation observed at 3 frequencies (f=562 Hz, f=1.0 kHz, f=1.78 kHz). The results are discussed and compared with those obtained from other investigations.

The Plasma Wave System on Phobos

The Phobos-2 spacecraft has been inserted in a Mars orbit to rendezvous with one of the two Martian satellites, Phobos, in early 1989. The Plasma Wave System (PWS) is part of the scientific payload; this instrument will record, for the first time in a Martian environment, the spectra of natural waves with an electric dipole in the frequency range 0-150 kHz and measure the ionospheric plasma density distribution with a Langmuir probe. In-flight data show that the system performs well and that PWS should meet its scientific goals.

First measurements of plasma waves near Mars

HERE we report preliminary results from electic field measurements in the environment of Mars using the plasma-wave system on board the Soviet spacecraft Phobos 2. It also includes a Lang-muir probe which measured plasma densities. Electron-plasma oscillations observed upstream of the bow shock correspond to a solar-wind density of 2 cm-3. The shock-foot boundary was crossed up to three times on each orbit. The shock ramp was detected at altitudes between 0.45 and 0.75 Mars radii (RM) above the planetary surface. The density increased by about a factor of two at the ramp. The shock position, although variable, seemed to be consistent with previous measurements. The downstream mag-netosheath contained broadband electric-field noise below the plasma frequency. The boundary of the obstacle, or planetopause, was crossed at altitudes of the order of 0.28 RM; the cold plasma density was highly variable within the planetopause and reached the unexpected value of 700 cm-3 on the third orbit, at 0.25 Rm altitude. Bursts of waves with frequencies below the electron cyclotron frequency, possibly in the whistler mode, occur within the planetopause.

The plasma wave system on Phobos

The plasma wave system on Phobos

Jovian plasma sheet density profile from low‐frequency radio waves

By using planetary radio astronomy (PRA), plasma wave system (PWS), and Magnetometer (MAG) data from Voyager 1 and 2 (V1 and V2), essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is calculated. The PRA experiment gives information about the plasma wave polarization. To determine the density profile of the plasma sheet, we have derived the hinge point position of the plasma disc from MAG data and used the low‐frequency cutoffs observed at three frequencies (562 Hz, 1 kHz, and 1.78 kHz) from the PWS experiment. We show that the hinge point position varies with the solar wind ram pressure, and the plasma disc thickness decreases with distance up to about 60 RJ. The average thickness for an isodensity contour corresponding to 1 kHz is 3.29 RJ for V1 and 3.16 RJ for V2.

An Upper Bound to the Lightning Flash Rate in Jupiter's Atmosphere

Lewis (1980) examined Voyager optical measurements and low-frequency radio-wave observations related to lightning discharges in Jupiter's atmosphere using whistler measurements from the plasma-wave system and a specific set of assumptions. Estimates of the average planetary lightning stroke rate were found to be from 0.04 to 0.0001 flashes per square kilometer per year. In the present paper, the planetary lightning rate is demonstrated to be as high as several tens of flashes per square kilometer per year. The same whistler data are used; however, different physical assumptions about the source area, including whistler paths and whistler amplitude distributions are incorporated.

Pseudo-classical transport I: The particle and energy flux

The transport of particles and energy that accompanies the trapping of electrons by a finite amplitude drift wave is calculated. Starting from the drift kinetic equation, it is shown that, in the limit of small collision frequency, the electron entropy source is stationary with respect to variations in the electron distribution function. This variational principal is employed, together with the full Fokker–Planck collision operators, to evaluate the electron transport coefficients and, hence, the flux of particles and energy across the magnetic field. Explicit expressions for the particle and energy flux are obtained in terms of the parameters of the plasma-wave system. These expressions should be used in place of the usual quasi-linear expressions for the particle and energy flux when the autocorrelation time of the wave spectrum is sufficient to permit the trapping of electrons by individual waves. These pseudo-classical transport rates are found to be smaller than the quasi-linear expressions that they replace.

A plasma wave investigation for the Voyager Mission

The Voyager Plasma Wave System (PWS) will provide the first direct information on wave-particle interactions and their effects at the outer planets. The data will give answers to fundamental questions on the dynamics of the Jupiter and Saturn magnetospheres and the properties of the distant interplanetary medium. Basic planetary dynamical processes are known to be associated with wave-particle interactions (for instance, solar wind particle heating at the bow shock, diffusion effects that allow magnetosheath plasma to populate the magnetospheres, various energization phenomena that convert thermal plasma of solar wind origin into trapped radiation, and precipitation mechanisms that limit the trapped particle populations). At Jupiter, plasma wave measurements will also lead to understanding of the key processes known to be involved in the decameter bursts such as the cooperative mechanisms that yield the intense radiation, the observed millisecond fine-structure, and the Io modulation effect. Similar phenomena should be associated with other planetary satellites or with Saturn's rings. Local diagnostic information (such as plasma densities) will be obtained from wave observations, and the PWS may detect lightning whistler evidence of atmospheric electrical discharges. The Voyager Plasma Wave System shares the 10-meter PRA antenna elements, and the signals are processed with a 16-channel spectrum analyzer, covering the range 10 Hz to 56 kHz. At selected times during the planetary encounters, the PWS broadband channel will operate with the Voyager video telemetry link to give complete electric field waveforms over the frequency range 50 Hz to 10 kHz.

Obliquely propagating unstable whistler waves: a computer simulation

Obliquely propagating unstable electron cyclotron electromagnetic (whistler) waves have been studied with the aid of an especially designed computer simulation experiment. The plasma–wave system considered is homogeneous and infinite, and the plasma is taken to be a mixture of warm (bi-maxwellian) and cold populations. With no cold plasma, the rates of growth of the low k modes maximize for the case of parallel propagation (¸ = 0°;) and decrease as ¸ increases; for higher k modes, the opposite occurs. Changing from k space to ω space the above results indicate that off-angle propagation tends to stabilize the low-frequency whistlers but to destabilize the higher-frequency ones. These results are consistent with linear predictions. In all cases, however, the total electromagnetic energy generated in the system decreases with increasing θ. When cold plasma is added to the system, the total generated electromagnetic energy can maximize for a non-zero propagation angle.