In the last decade, large electrostatic potentials of the order of tens of kV have been measured on spacecraft in the Earth's magnetosphere. Observations in space have led to the inference of large potentials on natural objects in the solar system. The result for spacecraft can be material damage and operational interference caused by electrostatic discharges. Natural objects such as dust grains can be disrupted, and their motion influenced by electromagnetic forces. The potential of a body in space is determined by a balance between various charging currents. The most important are transfer of charge from plasma particles, photoemission, and secondary electron emission, with other charging mechanisms sometimes contributing. The currents are affected by the body's charge and motion and by local magnetic and electric fields. Dielectric surfaces may have surface potential gradients which can affect the current balance through the creation of potential barriers. These processes are evaluated for bodies in the solar system and in interstellar space. Expected equilibrium potentials range from a few tenths of a volt negative in the ionosphere to a few volts positive in the quiet magnetosphere and in interplanetary space. However, large negative potentials can occur in hot plasmas such as in the disturbed magnetosphere, especially on shaded surfaces. Potentials in interstellar space can be positive or negative, depending on the properties of the local radiation field and plasma. In regions where there have been measurements of spacecraft potentials the results generally agree with these expectations. Deviations can be attributed to the effects of biased or dielectric surfaces or to the magnetic induction effect in large structures such as antennae. An intensive research effort has been initiated to measure material properties, to study charging and discharge processes, to model the current balance to realistic spacecraft configurations, and to obtain additional data in space. Spacecraft potential control experiments have been carried out using passive methods, such as careful surface material selection, and active methods, such as emission of charged-particle beams. The review closes with a survey of possible astrophysical applications where charging effects may be important.

Goddard Space F l i g h t C e n t e r . W e have d e r i v e d t h e p o t e n t i a l d i s t r i b u t i o n i n a plasma c o n t a i n i n g d u s t g r a i n s where t h e Debyel e n g t h can be l a r g e r o r s m a l l e r than t h e a v e r a g e i n t e r g r a i n s p a c i n g . W e t r e a t t h r e e models f o r t h e grain-plasma system, w i t h the assumption t h a t t h e system of d u s r and plasma i s c h a r g e -n e u t r a l : a permeable g r a i t l model, an impermeable g r a i n model, and a c a p a c i t o r model t h a t does n o t r e q u i r e t h e n e a r e s t n e i g h b o r approximation of t h e o t h e r two models. W e u s e a g a u g e -i n v a r i a n t form of P o i s s o n ' s e q u a t i o n which i s l i n e a r i z e d about t h e a v e r a g e p o t e n t i a l i n t h e system. The c h a r g i n g c u r r e n t s t o a g r a i n a r e f u n c t i o n s of t h e d i f f e r e n c e be,tween the g r a i n p o t e n t i a l and t h i s a v e r a g e p o t e n t i a l . W e o b t a i n e x p r e s s i o n s f o r t h e equilj.brium p o t e n t i a l of t h e g r a i n and f o r t h e gaugei n v a r i a n t c a p a c i t a n c e between t h e g r a i n and t h e plasma.The charge on a g r a i n i s d e t e r m i n e d by t h e p r o d u c t of t h i s c a p a c i t a n c e and t h e grain-plasma p o t e n t i a l d i f f e r e n c e . The t h r e e models g i v e s i m i l a r b u t n o t i d e n t i c a l r e s u l t s .> 2 The r e s u l t s depend p r i m a r i l y on t h e parameter Z = 4711 NC, where X i s t h e Debye l e n g t h , N i s t h e g r a i n c o n c e n t r a t i o n , and C i s t h e g r a i n t o plasma c a p a c i t a n c e . When 2 >> 1, t h e number of c h a r g e s on a g r a i n t h a t i s o n l y charged by plasma c u r r e n t s i s g i v e n b y (*/el) /N] where p i s t h e s q u a r e -r o o t o f i e -t h e ion t a e l e c t r o n mass r a t i o , and n i and n a r e t h e a v e r a g e i o n e a n d e l e c t r o n d e n s i t i e s . The charge on a g r a i n i n s u c h r , e g i o n s i s s e v e r e l y d e c r e a s e d from i t s f r e e space v a l u e . The c h a r g e r e d u c t i o n o c c u r s because t h e plasma e l e c t r o n s a r e d e p l e t e d s o t h a t t h e g r a i n does n o t need t o be a s n e g a t i v e l y charged t o e q u a l i z e t h e ion and e l e c t r o n f l u x e s t o i t s s u r f a c e , d e s p i t e t h e i n c r e a s e d g r a i n t o plasma c a p a c i t a n c e .

[1] Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This

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This paper describes a method for obtaining the abundance and altitude of stratospheric NO• by using ground-based spectroscopy at twilight. The method is then used to study the behavior of NO• over Colorado at 40øN; a companion paper discussed observations elsewhere. Large changes in both the abundance of NO• and in its diurnal variation are commonly seen at mid-latitude and appear to reflect the role of quasi-horizontal transport in the stratosphere on a global scale. stratosphere and its center of mass in altitude. Since the density of NO2 changes during the twilight period, we discuss how this affects the interpretation; the abundance is greater at night than in daytime, since sunlight dissociates NO2 into a mixture of NO and NO2. We then discuss the limits and errors associated with this method for determining the abundance and vertical distribution of NO2 in the stratosphere.As in the earlier work [Noxon, 1975], we employed a scanning spectrometer operated at a resolution of 7 A (not the 5 .• incorrectly stated there); instrumental details are described in the earlier paper. Even a simple spectrometer is adequate for such work owing to the relatively strong intensity of the twilight sky and the lack of any requirement for high spectral resolution. Figure I shows at the top two spectra of the zenith Paper number 9C0466. 5047 5048 NOXON ET AL.: STRATOSPHERIC NO•.