The subject of surface potentials on natural and artificial bodies in space is reviewed with particular emphasis on recent developments. Following a brief survey of work done up to the early 1970s on the charging of astrophysical objects in interstellar and interplanetary space and spacecraft charging, the various charging mechanisms in space are examined, including the collection of plasma particles, photoemission, and secondary electron emission by electron impact and ion impact, and the effects on charging of nonisotropic plasmas, wakes, and environmental magnetic and electric fields are considered. The concept of equilibrium potential is discussed, along with differential charging, potential barriers and discharge processes. The measurement of spacecraft potentials is then considered, and recent work on spacecraft potential modification and control by active and passive means is presented. Finally, astrophysical applications where charging effects may be important are discussed, and areas for further work are indicated.

http://iopscience.iop.org/article/10.1088/0034-4885/44/11/002/pdf

Potentials of surfaces in space

The buildup of static charge on satellite surfaces is an important issue in the utilization of satellite systems. The analysis of this phenomenon has required important advances in basic charging theory and the development of complex codes to evaluate the plasma sheaths that surround satellites. The results of these theories and calculations have wide application in space physics in the design of systems and in the interpretation of low-energy plasma measurements. In this review, those aspects of charge buildup on satellite surfaces relevant to the space physics community are summarized. The types of charging processes, models of charge buildup, satellite sheath theories, and charging observations are described with emphasis on basic concepts.

The charging of spacecraft surfaces

The use of emissive Langmuir probes in unmagnetized and weakly magnetized multidipole plasmas is investigated. It is shown that plasma potential, plasma electron temperature, and probe temperature can be determined from one probe characteristic curve. Data indicate that the inflection point of the current‐voltage curve gives the plasma potential to an accuracy the order of the probe temperature Tw/e for weak probe emission. Effects of space‐charge limiting and contamination of the probe are presented.

Inflection-point method of interpreting emissive probe characteristics.

This paper presents a review of the distribution, nature and properties of dust particles in space, their charging due to plasma and other currents and the effects of their presence on plasma behaviour.

http://iopscience.iop.org/article/10.1088/0031-8949/45/5/010/pdf

The physics of dusty plasmas

Effects of space-charge limitation have been considered for an analytical estimation of plasma potential with use of emissive probes, taking account of a correct expression of emission current under space-charge limited condition. The analytical results are obtained for the floating potential method of emissive probe and for the differential emissive probe method, both without and with an oscillating ac potential. In the case of the floating potential method, the operating condition is discussed for different plasma parameters at several probe temperatures. These results indicate that the floating potential of the emissive probe gives us the plasma potential to an accuracy of the order of the plasma electron temperature Te/e for a strong probe emission. In the case of the differential emissive probe method, the relations among space-charge effects, the ratio of thermoelectron emission current to plasma electron collection current, the plasma electron temperature, the probe temperature, and the amplitude o...

Effect of space-charge limited emission on measurements of plasma potential using emissive probes

A systematic investigation of the effects on probe response caused by electrons emitted with a half‐Maxwellian distribution is performed. In the absence of collisions a theory for spherical and cylindrical probes in a plasma of infinite extent is formulated, based on orbit calculations. A simple method is developed which greatly facilitates the solution of the space‐charge‐limited emission problem. The structure of the resulting double sheath is analyzed in detail. Effects of trapped particles are found to be unimportant under practical situations. Solutions of current‐voltage characteristics and space‐charge‐limited current are presented for the case of thermionic emission. Theories in the transitional and continuum regimes for a spherical probe in a weakly ionized gas are formulated by the moment method. For a dense plasma with a free fall sheath the emission is usually both space charge and diffusion limited.

Effects of Electron Emission on Electrostatic Probes at Arbitrary Pressures

The electrical effects of a charged sphere immersed in weakly ionized gas of infinite extent are analyzed on the basis of kinetic theory. Approximate solutions are obtained by the application of a moment method in which velocity space is split on the basis of exact orbit calculation, self‐consistent with the field. The theory is shown to reproduce both the collisionless and the continuum results in the appropriate limits. Detailed analysis is performed asymptotically for small Debye length but arbitrary mean free path. For quantitative calculations a completely absorbing probe surface at a highly negative potential is chosen. Criteria for the collisionless or collision‐dominated sheath are obtained explicitly. The voltage‐current characteristics are presented in a form useful for direct prediction of electron number density in terms of measured voltage and current, provided the mean free path is known. If the mean free path is only estimated, the results can be used directly to check self‐consistency of that estimate.

Asymptotic Theory of a Spherical Electrostatic Probe in a Stationary Weakly Ionized Plasma

The collisionless expansions of spherical and cylindrical gas clouds into an ambient gas are investigated. General expressions for the higher moments are derived and evaluated. The solutions exhibit effects not apparent in previous work on collisionless expansions bounded by vacuum. Mass flow, temperature and velocity exhibit propagating maxima which decay in amplitude and diffuse with time. Time constants for the duration of the expansions are obtained as a rough criterion for possibility of shock formation. Comparisons are made with the Euler solution and a short time collisional solution obtained by numerical integration of the Boltzmann equation with a model collision term.

Collisionless Expansion of Gas Clouds in the Presence of an Ambient Gas

A theoretical analysis of steady, normal shock structure in binary gas mixtures of inert, monatomic gases is given. The analysis is based on the moment method of kinetic theory. The governing equations of this two‐temperature‐two‐velocity model are in no way restricted by initial concentration of the species, mass ratio of the species, or Mach number. The system of equations which describes the structure includes the conservation of mass of each species, the conservation of momentum and energy of the mixture, the transfer of momentum and peculiar energy equations, the evolution of the stress moment of each species, and the heat flux moment of each species. The nondimensional parameters which characterize the phenomenon are identified as the number density ratio in the free stream η = (nj/ni)−∞, the mass ratio θ = mi/mj, and the mixture Mach number in the free stream M−∞. By choosing various combinations of these parameters, five different types of structures are proposed. By further restricting the light ...

Structure of Normal Shock Waves in Gas Mixtures

An asymptotic theory is formulated to describe the effects of surface electron emission on the behavior of a stationary, collision‐dominated, weakly ionized gas in the vicinity of a spherical probe. Limits where an electron energy balance yields either constant or variable electron temperatures are examined. When the microscopic emission current is of the order of the product of the plasma random current and the small normalized sheath thickness associated with no emission and isothermal electrons, the theory, under certain restrictions, follows the corresponding no‐emission formalism; the results, however, are different due to the altered surface boundary conditions. In particular, moderate emission can suppress the saturation behavior usually exhibited by the current‐voltage characteristics at highly negative probe potentials. When the microscopic emission current is of the order of the plasma random current itself, even the overall structure of the flow field may exhibit significant variations. In the cold ion limit, at a given level of massive emission, the nonisothermal theory predicts a saturation of the electron current at highly negative probe potentials due to the divergence of the quasi‐neutral potential drop, whereas the isothermal theory does not. When the ion and electron temperatures are of the same order of magnitude, the potential drop across the sheath becomes comparable to that across the quasi‐neutral region.

George K. Bienkowski

Papers

Effects of Electron Emission on Electrostatic Probes at Arbitrary Pressures

Asymptotic Theory of a Spherical Electrostatic Probe in a Stationary Weakly Ionized Plasma

Collisionless Expansion of Gas Clouds in the Presence of an Ambient Gas

Structure of Normal Shock Waves in Gas Mixtures

Continuum theory for the interaction of a weakly ionized gas with an emitting surface