Planetary Plasma Research Articles

On the dayside region between the shocked solar wind and the ionosphere of Mars

The objective of this paper is to explore the structure and properties of the region between the shocked solar wind and the planetary plasma on the dayside of Mars. This study is based on the observations obtained by the plasma instruments carried on board the Phobos 2 spacecraft. These instruments have independently identified several plasma boundaries in the dayside magnetosphere of Mars, the boundaries are located close to each other but exhibit different plasma features. This investigation leads us to conclude that a dayside region exists on Mars with the following characteristics: (1) the bulk of the shocked solar wind protons are deflected, (2) the total magnetic field increases, (3) a plasma depletion region is formed, (4) both shocked solar wind and planetary plasma are present, (5) accelerated electrons and heavy ions are also present, (6) intensive wave activity is seen in the 5 to 150 Hz frequency interval, and (7) a current layer and associated magnetic shears can be identified. We name this region, located between the ionosphere and the shocked solar wind, the dayside boundary layer or dayside mantle. Its key features are similar to those of the Venus mantle; the variety of names given to the Martian plasma boundaries merely reflects its complicated structure. We conclude that the two boundary layers at Venus and Mars are very similar and that the physical processes (i.e., the interaction of the shocked solar wind with the planetary plasma) at work within these boundary layers are probably similar.

Multiple shocks near Mars

Mars presents us with an example of a magnetosheath in which the interaction of the solar wind with planetary plasma of heavy ions results in a generation of strong nonlinear bi-ion MHD waves. These waves provide oscillations in the momentum exchange between protons and heavy ions and may give rise to multiple shock-like structures. One-dimensional hybrid simulations of plasma flow interaction with a “heavy ion obstacle” were performed to study the generation of shock waves in bi-ion plasma. A differential motion of protons and heavy ions leads to bunching of heavy ion flow picked up by solar wind. Plasma bunching arises because of a resonant interaction between magnetosound waves excited by beam and “slow” waves of a spatial charge in the heavy ion flow. Both kinds of waves grow, steepen and gradually evolve to shocks.

The Martian Magnetosphere - A Laboratory For Bi-Ion Plasma Investigations

The Martian space gives us an example of the direct interaction of the solar wind dominated by protons and the planetary plasma of heavy ions. Due to the lack of the intrinsic magnetic field the solar wind exposes the extended exosphere/atmosphere of Mars far from the bow shock. Photoionization, electron impact and charge exchange lead to the creation of newly born ions which may affect the incoming solar wind. At large distances the most abundant neutral components are molecular and atomic hydrogen, which could reach very high altitudes as the consequence of a weak gravitational attraction of Mars. It was found that pickup protons modify significantly the structure of the ion foreshock and, probably, the bow shock itself due to their effective reflection at the bow shock (Dubinin et al., 1994; Dubinin et al., 1995). Fast oxygen atoms originated in the process of the dissociative recombination of molecular oxygen, which dominates at the ionosphere heights additionally contribute to the neutral exosphere. Closer to the planet, the shocked solar wind interacts with a dense oxygen atmosphere, which produces an effective obstacle to the solar wind. In addition to the features related with the extended atmosphere, the importance of the ion gyroradius effects is also unique at Mars. The pick-up gyroradius of O + ions exceeds the characteristic size of the system and the classical MHD description is violated. In such Large Larmour Radius (LLR) configurations a relative streaming of different ion species is admissible and leads to a strong coupling between the solar wind protons and heavy planetary ions. In this paper we focus on effects of heavy ion population on dynamics of the solar wind. In section 2, a description of the MHD approach for two ion species is given. In section 3, we discuss mechanism of coupling between both species moving with different speeds. In section 4, the problem of critical points and shocks in bi-ion flows is considered. In section 5, we briefly discuss phenomenon of the’ obstacle boundary’ near Mars. In section 6 we derive the dispersion relations for low-frequency waves in bi-ion plasma composed of the solar wind as a core plasma and a minority of unmagnetized heavy ions which are in a relative streaming. In section 7, dispersion analysis of whistler waves is made. Application of these results to the Martian space is given. Observations and results of modeling are presented to illustrate our findings.

Energetic electrons in the dayside mantle of Venus

The purpose of the current study is to review the available and relevant experimental data in order to examine whether there is any evidence of accelerated electrons above the dayside ionosphere of Venus, where the shocked solar wind encounters the planetary plasma, that is in the dayside mantle. Such energetic electrons were seen around Mars in the corresponding region, and theoretical studies also predict their existence at Venus. Direct measurements, however, are not available, because the retarding potential analyzer (ORPA) onboard the Pioneer Venus Orbiter had an upper energy limit of 60 eV. Therefore we were forced to look for indirect evidences. We have reexamined the ORPA data and the magnetic field measurements in the dayside mantle. We have concluded, using these indirect indicators, that this energetic electron population is very likely to be present in the dayside mantle of Venus.

The interaction of the shocked solar wind and the planetary ions at Mars

In this study we investigate that region of the dayside magnetosphere of Mars where we may expect that the shocked solar wind comes into direct contact with the planetary ions, and where energy and momentum transfer from the solar wind to planetary plasma may take place. It is our intention to analyse whether the dominant microphysical processes are similar to those observed in the equivalent regions of the magnetosphere of Venus, that is in the dayside mantle of Venus. In this study we present results obtained along the first elliptic orbit of the Phobos 2 spacecraft. Our first conclusion is that around Mars it is the magnetic barrier region that corresponds to the dayside mantle of Venus. In this region at Mars wave excitations are observed, dominantly in the 5 to 50 Hz region; accelerated heavy ions and electrons were also detected. The major conclusion of this study is that despite several observed differences in the way that Venus and Mars interact with the solar wind, the dominant plasma features in the magnetic barrier region of Mars and the dayside mantle of Venus are very similar, indicating that probably the underlying physics is also similar.

Behavior of heavy ions in a collisionless parallel shock generated by the solar wind and planetary plasma interactions

The role of heavy ions in the collisionless parallel shock of Venus and Mars is investigated where the solar wind interacts directly with the planetary ionosphere. We consider O+ ions in the planetary plasma and He++ ions (α particles) in the solar wind as the heavy‐ion species. By solving a linear dispersion relation of the ion cyclotron beam instability, which provides a dissipation in the parallel shock, we find that the dominant frequency and wavenumber are not much modified when the heavy ions are included in a realistic amount. Particle simulations are performed in order to examine the effects and dynamical behavior of the heavy ions in the parallel shock. The re‐formation of the parallel shock is observed, which confirms that the electromagnetic waves that make up the parallel shock are not affected by the heavy ions. Notable pickup of O+ ions by electromagnetic waves is not detected. Instead, strong heating of the solar wind He++ ions is observed in the parallel shock region.

Macroparticle simulation of collisionless parallel shocks generated by solar wind and planetary plasma interactions

An implicit‐particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one‐dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right‐hand circular polarization that propagate upstream (R− waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re‐forms. The right‐hand circularly polarized electromagnetic waves that propagate downstream (R+ waves) are excited at the shock ramp. Nonlinear wave‐particle interaction between the solar wind and the R+ waves causes wave condensation and density modulation. These R+ waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat‐topped velocity distribution along the magnetic field owing to the ion acoustic‐like electrostatic waves.

Planetary plasma envelope rotation and the nature of atmospheric electricity

In this paper new results on the nature of atmospheric electricity are given in the framework of an exact model solution. These results are connected with the non-rigid rotation of the planet-plasmasphere system. For such a system, the magnetized rotating planet and the surrounding plasma envelope form an electric generator with an output voltage across the atmospheric layer. The output voltage is shown to be approximately equal to the unipolar voltage. The values of the atmospheric electric field and current obtained are close to the values obtained experimentally.

Structuring of the transition region (plasma mantle) of the Martian magnetosphere

Measurements made by the ASPERA plasma instrument onboard the Phobos‐2 spacecraft have shown that the transition from the Martian magnetosheath occupied mainly by the shocked solar wind to the boundary layer dominated by planetary plasma occurs by a sequence of step‐like drops and jumps of the proton bulk velocity and the magnetic field value. The plasma is stratified on alternating layers occupied either by protons or by heavy ions. The resemblance with shock‐like structures is emphasized by the fact of proton temperature enhancements which replicate drops of the bulk velocity. The observed striations are considered as peculiar discontinuities typical for multi‐ion plasma, or flute‐like modes which may be excited at the boundary of the Martian magnetosphere.

The Planetary Plasma Interactions Node of the Planetary Data System

Five years ago NASA selected scientists at UCLA to form the Planetary Plasma Interactions (PPI) Node to help the scientific community locate, access and preserve particles and field data from planetary missions. Since planetary plasma data are varied and require expertise in many areas the PPI Node is distributed with an Outer Planets Subnode at the University of Iowa, an Inner Planets Subnode at UCLA and a Radio Astronomy Subnode at GSFC. The PPI Node has tried to serve the science community by providing them with high quality data products. It has worked with missions and individual scientists to secure the highest quality data possible and to thoroughly document it. The PPI Node has validated the data, placed it on long lasting media and made sure it was properly archived for use. So far it has prepared and archived over 10 11 bytes of data and has produced 171 CD-ROMs with peer reviewed data. In so doing an efficient system has been developed to prepare and archive the data and thereby enable to steadily increase the rate at which the data are archived. Although the PPI Node produced a substantial archive during the initial five years, it has an even larger amount of work in progress. This includes preparing CD-ROM data sets with all of the Voyager, Pioneer and Ulysses data at Jupiter and Saturn. It is also completing the Pioneer Venus data restoration. The Galileo Venus archive and radio science data from Magellan will be prepared early in 1995. It is assisting the Small Bodies Node of PDS in the preparation of comet data and with the preparation of data from the comet Shoemaker-Levy 9 collision with Jupiter. Asteroid data from Gelileo will also be archived. In addition to providing the data, users have been provided with software tools to manage and read the data which are computer, operating system and data format independent. Scalable systems have been developed so that the same software used to manage and access the data for the entire PPI Node can be used by individual investigators to manage the data on a single CD-ROM thereby greatly reducing the software development effort for both the PPI Node and users. This software is delivered with the disks. The PPI Node data holdings are available over the Internet. They can be accessed through the World Wide Web (WWW) at Universal Resource Locator (URL) http://www.igpp.ucla.edu/ssc/pdsppi/Welcome.html. Users without a WWW browser can use the WWW interface by signing into the host “pdsppi.igpp.ucla. edu” as “pdsuser”.

The sheath/ionosphere boundary layer at Venus

At Venus the interaction of the shocked solar wind and cold planetary ions takes place in the dayside mantle. The shocked solar wind is a warm, drifting Maxwellian plasma whereas the planetary plasma is cold; the plasma in the mantle is strongly magnetized. The coexistence of these two populations is unstable, and it leads to wave excitations that organize the energy and momentum exchange between the shocked solar wind and the plasma of planetary origin. The source of the free energy is the solar wind. The intensive wave activity seen in the 100 Hz channel of the wave instrument onboard the Pioneer-Venus Orbiter in the dayside mantle region of Venus can be identified as almost electrostatic VLF waves excited by the kinetic branch of the modified two-stream lower hybrid instability. The waves interact with the particles, and the planetary plasma is heated and accelerated outside the ionosphere, close to its upper boundary. This way solar wind scavenges the ionosphere, and planetary ions leave the planetary magnetosphere. A portion of the wave energy is capable of penetrating directly into the ionosphere and heating it.

The solar wind interaction with Mars: A review of results from previous soviet missions to Mars

The three first orbiters of Mars: Mars-2 and Mars-3 in 1971–1972 and Mars-5 in 1974 provided the first simultaneous measurements of plasma and magnetic field in Martian environment. Their orbits were not very favorable for investigation of solar wind-Mars interaction, at best only marginally penetrating the obstacle to the shocked solar wind flow. Yet in spite of limited capabilities of then developed instruments, sampling rate and amount of data collected, results of the data analysis quite convinsingly showed existence of a detached bow shock, very thick tail with the downstream flow of planetary plasma, high altitude of dayside obstacle to the solar wind flow with underlying “cushion” of planetary plasma imbedded in moderately strong magnetic field, and boundary layer with traces of pick-up ions in the ionosheath. These results strongly suggested the importance of the interaction with the Martian atmosphere and allowed a first estimate of atmospheric losses due to the interaction with the solar wind. Arguments for the existence of a weak intrinsic magnetic field were presented, not very convincing ones however. Previously reported results are strongly supported by recent and much more informative Phobos-2 measurements.

Solar wind-driven plasma flows in the Venus and Mars ionospheres

A review of the experimental evidence of planeta y plasma flows in the Venus and Mars ionospheres is presented. It is argued that similar processes are responsible for the plasma flows observed in both planets and that they are related to the transfer of momentum of the shocked solar wind to the planetary ionospheres. Pickup ions born in the solar wind and scattered into the latter region through momentum scattering interactions with local turbulent wave fields are assumed to be the carriers of that momentum. Calculations of a balance condition between the height-integrated momentum flux of the planetary plasma flow and the height-integrated deficiency of momentum flux exhibited by the shocked solar wind that streams around both planets lead to flow speeds in good agreement with measurements.

Probability phenomena due to separatrix crossing

The effect of a small or slow perturbation on a Hamiltonian system with one degree of freedom is considered. It is assumed that the phase portrait ("phase plane") of the unperturbed system is divided by separatrices into several regions and that under the action of the perturbations phase points can cross these separatrices. The probabilistic phenomena are described that arise due to these separatrix crossings, including the scattering of trajectories, random jumps in the values of adiabatic invariants, and adiabatic chaos. These phenomena occur both in idealized problems in classical mechanics and in real physical systems in planetary science and plasma physics contexts.

Stationary Drift-Rossby Vortices in Shear Flows

Stationary vortex solutions of the Charney-Hasegawa-Mima equation describing planetary Rossby waves and plasma drift waves are analyzed for background flow with velocity shear. The solutions are obtained by Larichev-Reznik's method that divides the entire plane into two regions by a circle on which the internal and external solutions of linear equations are connected. The boundary conditions on the circle are satisfied correctly by adding a vorticity-free flow to the external background shear flow. Explicit flow patterns, dipolar, tripolar and quadrupolar, are given for several kinds of the background shear flows; the monopolar vortex like the Jovian Red Spot could not be obtained by this method. Another method is given, to derive a nonlinear equation for the stream function when the equilibrium density profile and shear flow in a plasma are prescribed.

Source location of the narrowbanded radio bursts at Uranus: Evidence of a cusp source

While Voyager 2 was inbound to Uranus, radio bursts of narrow bandwidth (< 5 kHz) were detected between 17–116 kHz by both the Planetary Radio Astronomy (PRA) and Plasma Wave (PWS) experiments. These R‐X mode bursts, designated n‐bursts, were of short duration (about 250 msec), tended to occur when the north magnetic pole tipped toward the spacecraft, and increased in occurrence with increasing solar wind density. In this report, we present an explicit determination of the burst source location based upon fitting the region of detection at high and low frequencies to field‐aligned, symmetric cones. The region of good fits was located between the north magnetic pole and the rotational pole, corresponding approximately to the northern polar cusp. Based upon the emission power, it is suspected that at certain times large amounts of auroral input power may originate in this cusp.

Remote sensing of planetary plasma by means of natural radio emissions

Voyagers 1 and 2 discovered radio emissions at Jupiter, Saturn and Uranus which were similar to those already studied in the Earth's magnetosphere. Terrestrial myriametric radiation, TMR, emanates from electrostatic upper hybrid, ESUH, waves, the conversion being believed to occur as described by the linear mode conversion window, LMCW, theory. A technique, based on the theory, has been devised which allows TMR, observed by satellites, to be used to remote-sense the source regions so as to obtain the location and plasma density of the source plasma. The technique can also be applied to study the sources of the analogous radiations observed at the other planets: Jovian kilometric radiation and narrowband electromagnetic emissions at Saturn and Uranus. The results of using this technique with the four planetary radiations are presented. They provide further evidence for the validity of the LMCW theory and for the importance of plasma density gradients in the conversion process.

Emissions of plasma waves from VLF to LF ranges in the magnetic polar regions - new evidences obtained from the data of the Ohzora (EXOS-C) satellite.

The data of natural plasma waves obtained from the initial three month observations of the planetary plasma sounder (PPS) system are investigated, focusing on VLF to LF range phenomena in the polar ionosphere. It is disclosed that the dynamic spectrum of the auroral VLF hiss phenomena consists of the main portion and wings. The main portion whose lowest frequency is less than 1kHz is associated with the leaked AKR phenomena, suggesting that the main portion is generated at the acceleration region of the auroral particles. The wings that have a cut-off frequency at several kHz, higher than the main portion, are formed as the result of the cut-off at the lower hybrid resonance frequency in side area of the source region. These features suggest that the auroral VLF hiss emissions are generated through the Landau type interactions with the auroral particles in the large wave number domain of the whistler mode which is of the nature of a electrostatic plasma wave. The detailed features of the LF hiss emissions are derived for the first time from the data with high time resolution in time and frequency. The auroral LF hiss emissions in a frequency range from 100kHz to 450kHz have weak and relatively steady dynamic spectra showing a clear contrast with the leaked AKR phenomena which shows the very intense and highly variable feature. It is found that the generation of the LF hiss is sometimes taking place at the level of 500km altitude. The upper hybrid mode waves are also generated in the polar ionosphere by the electron beams streaming down from the acceleration region, and the waves are observed in the form of locally trapped Z-mode waves that are converted while propagation through the inhomogenous media.

Plasma conditions in the polar ionosphere observed in SPW experiments of PPS system on board the Ohzora (EXOS-C) satellite.

The data of the stimulated plasma wave experiments (SPW) obtained from the planetary plasma sounder (PPS) aboard the Ohzora (EXOS-C) satellite have been analyzed for the initial three months. From the ionograms of SPW experiments 1) vertical profiles of the topside ionosphere, 2) irregularity of the plasma distribution and 3) identification of the modes of the naturally emitted plasma waves have been obtained. The studies on these subjects lead to the following results.(i) In the polar region ionosphere, the auroral particle precipitations generate the intense plasma waves even in the satellite level.(ii) Associated with the generations of the intense plasma waves caused by the particle precipitation, there are irregularities of the plasma distribution in the topside ionosphere which become the origin of the scattering effects on the transmitted RF pulses.(iii) Enhancement of the electron density takes place caused by the enhanced precipitation of the auroral particles in the auroral region. At the same time remarkable heat input which is indicated by increasing of the electron temperature is also confirmed.(iv) Near the auroral zone in the lower latitude side than the auroral regions, there is also the region where the heat input is taking place in milder manner than the case of the auroral regions.(v) Large energy input into the auroral zone triggers a relatively large scale disturbance propagating in the polar cap ionosphere in the form of the traveling ionospheric disturbance (TID).

Leaked AKR and terrestrial hectometric radiations discovered by the plasma wave and plantetary plasma sounder experiments on board the Ohzora (EXOS-C) satellite - instrumentation and observation results of plasma wave phenomena.

Planetary plasma sounder (PPS) experiments system is installed in the Ohzora (EXOS-C) satellite which was launched on February 14, 1984, in an orbit with initial apogee and perigee of 865km and initial perigee of 354km, respectively, and initial inclination of 74.6°. Utilizing orthogonal 40m (tip to tip) antennas and swept frequency receivers in the range from 100kHz to 16MHz and using a swept frequency VLF receiver in the range from 500Hz to 20kHz, PPS is operated for two main purposes. One operation is for the reception of natural plasma waves, both in the high frequency and the VLF ranges, and the other operation is for active experiments using the high power (300W maximum) transmitter of the RF pulse where, in the normal mode, the swept frequency signal from 100kHz to 16MHz is modulated with a pulse width of 244μsec. During the first three months after the deployment of the 40m antennas which was made on May 16, 1984, data from more than 500 orbits have been obtained. From these data the following phenomena have been discovered:1) Leaked component of auroral kilometric radiation, 2) Terrestrial hectometric radiation from the topside region of the ionosphere.In the frequency range from 100kHz to 450kHz, there exist emissions separated from LF Hiss; the cause of the emissions is attributed to leakage of the auroral kilometric radiation. The leaked components are believed to be produced by the conversion of Z-mode waves to whistler mode waves. With cooperative operation of the stimulated plasma wave experiments (SPW), it is shown that an enhancement of terrestrial hectometric radiation is related to irregularities of plasma density. This result also suggests that the conversion of Z-mode waves to L-O mode waves is taking place near the source regions.