Shu T. Lai

Aspects of Spacecraft Charging in Sunlight

This paper is an overview of spacecraft charging in sunlight. The daylight photoelectron flux emitted from spacecraft surfaces normally exceeds the ambient electron flux. As a result, charging of spacecraft surfaces to positive voltage is expected to occur in sunlight. Indeed, spacecraft are often observed to charge to low positive voltages in sunlight. However, spacecraft can charge to high-level (kiloelectronvolts) negative voltages in sunlight. Why do spacecraft charge negatively in sunlight? One chief reason concerns differential charging between the sunlit and dark sides. For a satellite with dielectric surfaces, an electric field builds up on the shaded surfaces and then wraps around to the sunlit side to form a potential barrier that suppresses the photoemission. A monopole-dipole (for zero spin) or monopole-quadrupole model (for fast spin) describes the differential charging potential distribution due to blocked photoelectrons. It is shown that these cases are similar to a more general multipole potential field in that the surface node potentials satisfy an approximate linear relation. These cases are all driven by the shade side charging so that the onset for charging is approximately the same in sunlight or eclipse if conduction currents through the spacecraft can be neglected. If conduction currents are important, potential barriers can develop on the dark side, leading to suppression of the secondary emission currents and modification of charging onset. The results were briefly compared with observations. Another important reason for negative charging concerns reflectance. Highly reflective mirrors generate substantially reduced photoemission so that current balance can be achieved without barrier formation. The onset for charging in this case depends strongly on the reflectivity. The critical temperature for charging of surface materials under space substorm conditions with different ratios of photoemission current to electron ambient current, corresponding to varying satellite surface reflectivity values, was calculated. Numerical results, which show that with substantially reduced photoemission, highly reflective surfaces charge in sunlight with the critical temperature for onset decreasing with increasing reflectivity, are presented

A critical overview on spacecraft charging mitigation methods

Interactions between hazardous space plasmas and spacecraft surfaces often result in spacecraft charging. Spacecraft charging may disturb the scientific measurements onboard, affect communications, control, and operations of spacecraft, and may be harmful to the health of the electronics on the spacecraft. Several mitigation methods have been proposed or tested in recent years. This paper presents a critical overview on all of the mitigation methods known to date: 1) passive methods using sharp spikes and high secondary emission coefficient surface materials and 2) active methods using controlled emissions of electrons, ions, plasmas, neutral gas, and polar molecules. Paradoxically, emission of low-energy positive ions from a highly negatively charged spacecraft can reduce the charging level, because the ions tend to return and may generate secondary electrons which then escape. We discuss the advantages and disadvantages of each of the methods and illustrate the ideas by means of examples of results obtained on SCATHA and DSCS satellites. Finally, mitigation of deep dielectric charging is briefly discussed.

Dependence of electron flux on electron temperature in spacecraft charging

Two important observations when the onset of spacecraftsurface charging occurs are (1) the electron flux measured in the high-energy (above several keV) channels onboard rises and (2) the ambient electron temperature rises above a critical value. We show by means of an analytical model that the two behaviors are consistent with each other.

Charging of fast spinning spheroidal satellites in sunlight

We present models for a fast spinning, dielectric coated, spheroidal (prolate or oblate) spacecraft charging in sunlight. This work is a generalization of previous treatments of sunlight charging of spherical satellites. The main difference is that the spacecraft geometry can be characterized by a shape parameter that sets the aspect ratio of the spheroids. The models are based on an expansion of the Laplacian potentials external to the spacecraft surface in terms of products of standard Legendre polynomials, describing the polar angle dependence, and modified Legendre functions of the second kind, representing the “radial” behavior. The potential distributions are discussed relative to the corresponding monopole-dipole and monopole-quadrupole configurations in spherical geometry. A Taylor expansion is developed for the potentials when the shape parameter is large (the spherical limit) and expressions are also given in the opposite limit, when the shape parameter goes to zero. As in the spherical case, th...

Effects of Solar UV on Spacecraft Charging in Sunlight

Abstract : Spacecraft surface charging is determined by the balance of currents. Photoelectron currents from spacecraft surfaces greatly exceed the ambient electron or ion currents and therefore are often of prime importance for charging in sunlight. The authors present a brief overview of several aspects of spacecraft charging in sunlight. For a conducting spacecraft at geosynchronous altitudes, charging in sunlight is usually up to a few positive volts only. If the spacecraft is in areas where the solar ultraviolet radiation is strong and the ambient electron density is low, the spacecraft can charge to a few tens of positive volts. For a non-conducting spacecraft at geosynchronous altitudes, the dark side can charge to hundreds or thousands of negative volts as a result of the collection of ambient electrons. There exists a critical electron temperature governing the onset of negative voltage charging. The sunlit side initially tends to charge to low positive volts. The high negative voltage of the dark side may wrap around the sunlit side forming a potential barrier blocking the photoelectrons emitted from the sunlit surfaces. As a result, the sunlit side may also charge to negative voltages. The critical temperature for this differential charging to occur is approximately the same as for eclipse charging. Depending on the spin axis with respect to the sun direction, monopole-dipole or monopole-quadrupole potential distributions may occur. For spacecraft with high surface reflectance, the photons do not deposit enough energy to generate photoelectrons. As a result, the surface can charge to high negative voltages in sunlight without invoking differential charging. In this case, the critical temperature is changed, depending on the reflectance and the photo-emissivity of the surface.

The Mott transition as a cause of anomalies on spacecraft

In the Mott transition, an insulator with a critically high donor density undergoes a sudden transition to become a conductor. On the other hand, it is known that sufficiently high electric fields applied to dielectrics at ordinary charge densities can cause breakdown. The author conjectures that the critical donor density can be lowered by applying high electric fields to the insulators. Using a simple model encompassing the concepts of Debye, Poole, and Frenkel, the author has obtained a continuous curve connecting the two critical points, viz., critical density and critical field. The motivation of this work is spacecraft anomalies. They briefly review the circumstances of the most notable spacecraft failures in recent years. Although the true causes of the failures are probably system design specific and will probably never be known, it is likely that the failures were due to breakdowns or discharges in semiconductors or dielectrics. Irradiated dielectrics and semiconductors have defects and dangling bonds playing the role of donors. High electric fields are built by electrons deposited inside the dielectrics. With the use of the curve obtained, it is concluded that the insulator-conductor transition can occur at lower donor densities and lower electric fields than previously thought.