Ionospheric and magnetospheric disturbances caused by impacts of small comets and asteriods

Disturbances in the Earths’s ionosphere and magnetosphere caused by impacts of small comets and asteroids (with diameters from 50–60 m to 1–2 km) are analyzed. Two-dimensional hydrodynamical computations of the passage of a cosmic body through the atmosphere with allowance for deceleration and destruction due to aerodynamic loading and formation of the wake behind the body are performed. The tenuous wake facilitates an upward ejection of the plume (heated air and ablation products of the cosmic body). Numerical simulations of the motion of the plume and of its interaction with the geomagnetic field are performed. It is shown that part of the plume moves at higher than escape velocity. The rising plume operates as an MHD generator. Field-aligned currents heat the ionosphere and change its conductivity. The estimated magnetic variations are on the order of those of typical magnetic storms (for bodies with sizes comparable to the Tunguska meteorite) and are even higher for cosmic bodies with diameters of 200–400 m. Excitation of MHD waves is demonstrated. These disturbances are capable of triggering precipitation of particles from radiation belts and exciting intense electromagnetic noise. Strong oscillations of conducting ionospheric layers propagate radially from the place of impact of the low-velocity part of the plume to large distances from the impact point. For a 1-km body the energy of the high-velocity plume is comparable to that of the Earths’s magnetic field. This causes extremely intense magnetospheric disturbances. However, even 200-to 400-m bodies whose high-velocity part of the plume has energies exceeding 0.4–3 Mt TNT—i.e., much lower than the initial kinetic energy of the intruding body—produce global ionospheric and magnetospheric disturbances.     

Intercomparison among plasma wake models for plasmaspheric and ionospheric conditions

Intercomparisons are made among the angular distributions of ions in the wake of a body moving through a space plasma as computed from three different expressions (models). Both subsonic and supersonic relative flows are considered in order to examine the wake current depletion ratios under conditions realistic for the topside ionosphere and plasmasphere. Results of these comparisons demonstrate the importance of including the thermal flux at low Mach numbers and of taking into account the angular acceptance of ion detectors in making theory-experiment comparisons. Gradients in the angular variations of the fluxes are found to be steeper near the wake-ambient interface than closer to the maximum rarefaction region of all models, although quantitatively there is considerable variation among the models. From examining the variations of the wake depletion ratio parametrically with Mach number and normalized potential over ranges characteristic of the plasmasphere and topside ionosphere, we find considerable variation with both parameters, with sensitivity to normalized potential increasing dramatically with Mach number. Overall, however, the Mach number variation appears to be the more significant over this range of parameters.     

Interactions between the orbiting space shuttle and the ionosphere

This paper presents an analysis of interactions between the space shuttle Orbiter and the ionosphere based on thermal plasma data obtained from a Spherical Retarding Potential Analyzer (SRPA) and a Langmuir Probe (LP) flown on the third Space Shuttle flight (STS-3) in March 1982. While previous work on spacecraft-plasma interactions has dealt with wake effects, the present work deals with effects that are seen in ram conditions that have not been previously discussed. One observation is a higher degree of plasma turbulence than has been reported from unmanned spacecraft measurements that manifests itself as a frequency component at 2.2 kHz in the SRPA signal. We also see unusually high number densities of what appear to be ions with a mass of at least 30 or 32 amu and a temperature in the range 2000–3000 K. Coincident with the enhanced molecular ion species we see the temperature of the thermal electrons elevated to above 5000 K. It is hypothesized that these measurements are evidence for a plasma instability resulting from the motion of the outgassing Orbiter through the ionosphere.     

Electron depletion in the wake of ionospheric spacecraft—A comparison between results from Langmuir probes and antennas

In situ observations of the electron depletion in the wakes of satellites and rockets in the ionosphere using Langmuir type probes and antennas are analyzed and compared. The quantitative degree of agreement between the results is demonstrated and discussed. One consequence is an improved interpretation of results previously presented for the OGO II Satellite wake.     

Are 'Bondi–Hoyle wakes' detectable in clusters of galaxies?

In clusters of galaxies, the reaction of the intracluster medium (ICM) to the motion of the co-existing galaxies in the cluster triggers the formation of unique features, which trace their position and motion. Galactic wakes, for example, are an apparent result of the ICM/galaxy interactions, and they constitute an important tool for deciphering the motion of the cluster galaxies.
In this paper we investigate whether Bondi–Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provides us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies.
We find that significant Bondi–Hoyle wakes can only be formed in low-temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scalelength of these elongated structures is not very large: in the most favourable conditions a Bondi–Hoyle wake in a cluster at the redshift of z =0.05 is 12 arcsec long. However, the X-ray emission of the wake is noticeably strong: the X-ray flux can reach ∼30 times the flux of the surrounding medium. Such features will be easily detectable in the X-ray images of nearby, relatively poor clusters of galaxies by the Chandra and XMM-Newton satellites.     

The cool wake around 4C 34.16 as seen by XMM–Newton

We present XMM–Newton observations of the wake–radio galaxy system 4C 34.16, which shows a cool and dense wake trailing behind the host galaxy of 4C 34.16. A comparison with numerical simulations is enlightening, as they demonstrate that the wake is produced mainly by ram pressure stripping during the galactic motion through the surrounding cluster. The mass of the wake is a substantial fraction of the mass of the X-ray halo of an elliptical galaxy. This observational fact supports a wake formation scenario similar to that recently demonstrated numerically by Acreman et al.: the host galaxy of 4C 34.16 has fallen into its cluster, and is currently crossing its central regions. A substantial fraction of its X-ray halo has been stripped by ram pressure, and remains behind to form the galaxy wake.     

Jupiter's ionosphere and magnetosphere

The distribution of ionization and the temperature variation in the Jovian ionosphere is determined by simultaneously solving the momentum and chemical equations for electrons, ions and neutrals together with their respective heat transport equation. The boundary conditions at the bottom of the ionosphere are chosen in accordance with recent infra-red and occultation measurements. The ionosphere is hotter than previously thought. The electron temperature may be as high as 1500 K. A considerable flux of particles can escape from the ionosphere. These particles are trapped in the Jovian magnetosphere by a two-stream instability. A Gledhill disk will form. The variation of plasma density along Io's orbit is calculated.     

Theoretical density distribution of plasma streaming around a cylinder

The distribution of plasma density around a metallic cylinder in a collisionfree plasma is determined by the system of Vlassov equations and the Poisson equation. The plasma may have a streaming velocity perpendicular to the cylinder axis producing a wake behind the cylinder. In the region outside a thin double layer at the cylinder surface the problem allows some simplification. Then numerical solutions become possible-even if the streaming velocity is low. Some numerical solutions are presented. A magnetic field produces an asymmetric shape of the wake, if the gyration radius of the ion becomes comparable to the cylinder radius.     

The Schumann resonance: A tool for exploring the atmospheric environment and the subsurface of the planets and their satellites

The propagation of extremely low frequency (ELF) electromagnetic waves and resonance phenomena in the Earth atmosphere has been extensively studied, in relation with ionospheric dynamics, and thunderstorm and lightning activities. A similar investigation can be performed for any other planet and satellite environment, provided this body is wrapped into an ionosphere. There are, however, important differences between Earth and other bodies, regarding the surface conductivity, the atmospheric electron density, the ionospheric cavity geometry, and the sources of electromagnetic energy. In a first approximation, the size of the cavity defines the range of the resonance frequency; the electron density profile, up to the upper atmospheric boundary, controls the wave attenuation; the nature of the electromagnetic sources influences the field distribution in the cavity; and the body surface conductivity, which gives the reflection and transmission coefficients, indicates to what extent the subsurface can be explored. The knowledge of the frequencies and attenuation rates of the principal eigenmodes provides unique information about the electric properties of the cavity. Instruments capable of monitoring the electromagnetic environment in the ELF range are, therefore, valuable payload elements on balloons, descent probes and landers. We develop models for selected inner planets, gaseous giants and their satellites, and review the propagation process of ELF electromagnetic waves in their atmospheric cavities, with a particular emphasis on the application of the Schumann resonance observation to subsurface studies. The instrumentation suitable for monitoring the electromagnetic environment in geophysical cavities is briefly addressed.     

Simulation of the solar wind interaction with non-magnetic celestial bodies

Laboratory experiments are described to simulate the solar wind flowing around nonmagnetic planets for three cases: non-conducting and ideally conducting planets, and a planet with a gaseous shell. A glass sphere was used as a model of a non-conducting planet (the Moon). Spatial distributions of plasma density and magnetic field strength that have been obtained agree with the data from measurements in space. However, the magnetic field does not increase before the rarefaction wave in the model experiment. A field increase was observed only for a conducting lunella: this argues in favour of the existence of a high conduction region on the Moon. A wax ball was used to model phenomena on the day-side of non-magnetic celestial bodies with a gaseoue shell (Venus, comets). Its surface easily evaporates in the plasma flow, and ionized evaporation products form an artificial ionosphere. The magnetic field frozen in the plasma flow is shown to be a determinative factor in the formation of a sharp ionospheric boundary. The supersonic plasma flow that interacts with the ionosphere gives rise to a stationary shock wave.     

Disappearing ionospheres on the nightside of Venus

Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. We have correlated data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. We suggest mechanisms which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.     

The SO2 atmosphere and ionosphere of Io: Ion chemistry,atmospheric escape,and models corresponding to the Pioneer 10 radio occultation measurements

Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO₂Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO₂ atmosphere with a surface density of 14 × 10¹⁰ cm^?3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO₂⁺, the major ion produced, is converted rapidly to SO⁺ and in turn to S⁺ by reactions with the dissociation products of SO₂. Ion chemistry leads to the formation of S⁺ as the dominant ion at and above the ionospheric peak. Na⁺ would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S⁺ ions would escape from Io in the wake region at a rate of up to 10²⁶ sec^?1.     

Some aspects of electric field mapping in the auroral ionosphere

The mapping of the spectra of electrostatic field below 300 km altitude is theoretically calculated for a horizontally stratified auroral ionosphere. Perpendicular electric fields of large scale size are the same for different altitudes of the ionosphere. However, electric fields of small scale size vary with altitude and decrease drastically when the scale size is smaller than a certain value which depends on altitude. These results are similar to those observed by satellites above 300 km altitude. In the case of a homogeneous anisotropic ionosphere, analytical results are obtained for the penetration of electric field into the ionosphere as a function of wavenumber. The “smoothing” of the electric field when penetrating a horizontally stratified ionosphere is demonstrated. The smallest possible scale of parallel electric field structure within the ionosphere is found. Also presented is a method of finding the smallest horizontal length with which the electric field can penetrate the ionosphere with little distortion. For an average conductivity model, this length is found to be about 1 km. Finally, the mapping of packets of electric field to the ground is constructed.     

Current Sheath Studies in a Small Plasma Focus Operating in Hydrogen–argon Mixtures

We present preliminary time and space resolved studies of current sheath formation in Plasma Focus discharges, using a novel array of non-invasive magnetic field probes. The experiments are performed in a Mather type plasma focus, operating at 2 kV. The discharge is formed between a hollowed anode and six symmetrically arranged cathode rods. The array of small magnetic probes is located along the cathode rods. The probes are of millimeter size. They are shielded behind the rods, as to minimize capacitive coupling to the anode electrode, and allow non-perturbing measurements to be made. A simple analytical model of current sheath evolution is used to analyze the probe signals. The experiments have been performed in pure Hydrogen and Hydrogen with Argon mixture, at pressures from below 0.2 Torr upwards. The effect of the Argon mixture on the current sheath structure is investigated with the probe array. It is found that at constant mass density operation, the increase in the percentage of Argon results in a thinner sheath, with steeper current profile.     

The solar wind interaction with Venus through the eyes of the Pioneer Venus Orbiter

Venus has no internal magnetic dynamo and thus its ionosphere and hot oxygen exosphere dominate the interaction with the solar wind. The solar wind at 0.72 AU has a dynamic pressure that ranges from 4.5 nPa (at solar max) to 6.6 nPa (at solar min), and its flow past the planet produces a shock of typical magnetosonic Mach number 5 at the subsolar point. At solar maximum the pressure in the ionospheric plasma is sufficient to hold off the solar wind at an altitude of 400 km above the surface at the subsolar point, and 1000 km above the terminators. The deflection of the solar wind occurs through the formation of a magnetic barrier on the inner edge of the magnetosheath, or shocked solar wind. Under typical solar wind conditions the time scale for diffusion of the magnetic field into the ionosphere is so long that the ionosphere remains field free and the barrier deflects almost all the incoming solar wind. Any neutral atoms of the hot oxygen exosphere that reach the altitude of the magnetosheath are accelerated by the electric field of the flowing magnetized plasma and swept along cycloidal paths in the antisolar direction. This pickup process, while important for the loss of the Venus atmosphere, plays a minor role in the deceleration and deflection of the solar wind. Like at magnetized planets, the Venus shock and magnetosheath generate hot electrons and ions that flow back along magnetic field lines into the solar wind to form a foreshock. A magnetic tail is created by the magnetic flux that is slowed in the interaction and becomes mass-loaded with thermal ions.The structure of the ionosphere is very much dependent on solar activity and the dynamic pressure of the solar wind. At solar maximum under typical solar wind conditions, the ionosphere is unmagnetized except for the presence of thin magnetic flux ropes. The ionospheric plasma flows freely to the nightside forming a well-developed night ionosphere. When the solar wind pressure dominates over the ionospheric pressure the ionosphere becomes completely magnetized, the flow to the nightside diminishes, and the night ionosphere weakens. Even at solar maximum the night ionosphere has a very irregular density structure. The electromagnetic environment of Venus has not been well surveyed. At ELF and VLF frequencies there is noise generated in the foreshock and shock. At low altitude in the night ionosphere noise, presumably generated by lightning, can be detected. This paper reviews the plasma environment at Venus and the physics of the solar wind interaction on the threshold of a new series of Venus exploration missions.     

Dynamics of dust in a plasma sheath and injection of dust into the plasma sheath above Moon and asteroidal surfaces

We have examined single dust particle dynamics in a plasma sheath near the surface of solid bodies in space, considering conditions which resemble those of planetary system bodies, when photoelectric effect can be neglected. The forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. As the dust particles will charge negatively in the sheath, these forces will act in opposite directions and may balance.The charge delay of a moving dust particle is responsible for many of the interesting dynamical properties, and we show that for a stationary plasma, dust motion is unstable to about one Debye length out from the surface of the solid body. This part of the sheath will therefore be devoid of dust particles as they will either fall down, escape completely from the solid body or collect and make damped oscillations at stable positions in the outer part of the sheath. With increasing plasma bulk speed towards the surface, the inner unstable part of the sheath will decrease in thickness.The sources for the dust in the sheath are assumed to be mainly ejecta from meteorites and micrometeorites, but may also, for the smallest solid bodies, be from electrostatic levitation of very small dust particles. We have for different sizes of solid bodies calculated the sizes of ejecta that can be floated in the sheath. For the solar wind plasma, the suspended dust particles range from less than 1 m for the Moon to about 80 m for an asteroid with radius 1 km. These particles create a dust atmosphere.The results in this paper hold when the dust particle density is so low that the charges on the dust particles do not contribute significantly to the total space charge; a higher density will lead to a modification of the sheath.Our calculations show that ejecta below a certain size will be accelerated in the sheath and totally escape from the body even if they have near zero initial vertical velocity, while ejecta above this size will need a much larger velocity to escape. This is especially significant for the small solid bodies (radius of order km and less) which will therefore act as important sources of micronsized dust. This could be of significance for the dust production and the size distribution of dust in planetary ring systems.     

Laboratory observations of electron temperature in the wake of a sphere in a streaming plasma

A parametric study was performed of electron temperature variation in the wake of a conducting sphere in a streaming plasma. The flow conditions were varied as follows: the ambient electron temperatures in the range 850–2450 K; the ambient electron densities in the range 5 × 10⁴?7 × 10⁵/cm³; and body potentials relative to plasma potential in the range of + 1.7 to ?2.8 V for an ion beam energy of ～4 eV. Electron temperature enhancements were observed which ranged up to 200 per cent above ambient in the nearest proximity of the body surface. The magnitude of the enhancement depends upon the ambient density, temperature and body potential.     

The stability of inner cometo-sheath with large Larmor radius effects

An investigation of the linear stability of the cometary inner sheath, the boundary layer surrounding the ionopause which separates the outflowing unmagnetized plasma from an inflowing magnetized plasma, has been carried out by taking into account the large Larmor radius effects. The structure of the boundary layer is determined by the balance between an outward ion-neutral collisional drag force and an inward magnetic stress. The eigenvalues and the eigenfunctions are obtained numerically by treating the cometary ionosphere as a layer of finite thickness, bounded by the contact surface, i.e., the diamagnetic cavity boundary. Certain limiting cases of the wave equations are also discussed. In general, the cometary ionosphere is structurally linearly unstable and the effects of recombination, photoionization, plasma pressure, though stabilizing are unable to quench the instability completely. The large Larmor radius also has a destabilizing effect on the system. The instability of the cometosheath is further proved by the _c/_i assuming a value greater than 30 that is sufficient for the convection of perturbations down into the cavity surface and this is in agreement with the observations of ripples in the ionopause.     

Symmetric theory of probe-plasma interactions

The theory of interactions between a probe and the surrounding plasma at rest is developed in a spherically and in a cylindrically symmetric model (probe theory). The theory is based on the Vlasov-Poisson system; a general numerical program was developed to solve this system by means of an iterative procedure. Various ambient plasma and charged particle emission properties are described by the complete set of boundary conditions for the distribution functions in the phase space. By use of this numerical method, potential and space charge density in the whole surroundings of the probe as well as the current densities of all plasma constituents are calculated self-consistently.Furthermore, the regions of the phase space with particle trajectories of the same kind can be approximated depending on the plasma properties. Then, the current densities can be estimated analytically. This approach to the problem yields self-consistent approximations and is the only stringent derivation of the thick sheath and of the thin sheath approximation of the classical Langmuir theory. These approximations are generalized with respect to the charged particle emission from the surface.The symmetric probe theory is applied to the following problems of spacecraft environment and spacecraft charging: (i) a spacecraft in the ionosphere with very negative surface potential, (ii) a spacecraft in the solar wind with strong photoelectron emission, and (iii) a spacecraft in the transition region of comet Halley with very strong secondary plasma emission.