A brief synopsis of the natural environments that play a role in spacecraft charging is presented. Environments that cause both surface and internal charging are discussed along with the mechanisms involved. The geosynchronous and low altitude regions of the Earth's magnetosphere/ionosphere are considered and simple descriptions of each environment presented. As material properties are critical to the charging process, definition of material properties important to charging, which can be affected by the environment, are also described. Several space experiments are proposed that would help fill the gaps in the knowledge of the performance of materials in a charging environment

Gabriel, S. B.

Garrett, H. B.

English

NASA Technical Reports Server

N89- 23554
AN OVERVIEW OF CHARGING ENVIRONMENTS
S. B. Gabriel and H. B. Garrett
Jet Propulsion Laboratory
California Institute Of TechnoloEy
Pasadena, California
495
https://ntrs.nasa.gov/search.jsp?R=19890014183 2020-03-20T03:13:54+00:00Z
NATURAL ENVIRONMENTS THAT CONTRIBUTE TO CHARGING
This paper presents a brief synopsis of the natural environments that
play a role in spacecraft chargin E. Environments that cause both surface
and internal chargin E are discussed alon E with the mechanisms involved. The
Eeosynchronous and low altitude (< 1000 km) reEions of the Earth's
maEnetosphere/ionosphere are considered and simple descriptions of each
environment presented. As material properties are critical to the charEin E
process, definition of material properties important to charEinE, which can
be affected by the environment, will also be described. Finally, several
space experiments are proposed that would help fill the Eaps in our
knowledEe of the performance of materials in a charEin E environment.
FiEure 1 lists the m_m__or natural environments that contribute to
charEin E. This list is not comprehensive and has been selected on the basis
of those environments that interact directly with surfaces (surface
charEinE) or throuEh surfaces (internal charEinE) to cause the production of
high electric fields. Because of this rather restricted definition,
environments such as contaminant molecules, x-rays, electron ion beams, and
cosmic rays have not been included.
Surface:
• Thermal Plasma
• High Energy Electrons (1-100 keV)
• UV/EUV Radiation
• Magnetic Field
• Neutral Particles
• Internal"
• High Energy Electrons (>/100 keY)
FIGURE 1
496
ROLE OF ENVIRONMENTS IN CHARGING
Figures 2 a) and b) show schematically the interactions between the
environments and the surface that cause surface and internal charEinE,
respectively. In surface charEinE, the process is Eoverned by current
balance (I) (i.e. in the steady-state, the net sum of all the currents to
the surface must be zero and the equilibrium potential will satisfy this
condition). To first order these currents comprise the currents from the
thermal or low enerEy plasma (including any ram ions), the hiEh enerEy
electrons and ions, the secondary electrons emitted by impactin E hiEh energy
electrons and ions and photo-electrons released due to the incident UV/EUV
radiation. The presence of a maEnetic field and space charge can affect the
escape of low enerEy secondary photoelectrons.
For internal charEinE, the primary process is the build up of neEative
charge (electrons) in or on isolated surfaces inside the spacecraft body
caused by penetration of the external surfaces by hiEh energy (> I00 keV)
electrons (2). As shown in the fiEure, charEe can accumulate on in
unErounded conductors or insulators and cables as well. This charEe can
produce high electric fields and induce breakdown.
uv,,euv P_otons
r_t_ ;auseEnllss,on_..Ph toelectron
_,clh
/ _ _ _ nermal Ptas_]a
5.ha,, _L, / !
' E ecl'ons
Red L_,e
... ,C:i{ ./
C a "rl, eh, _ , ,el , Cu,,ent aa, lance Equal,on
Escape ot I = o
eq 5econda_,, EmlSS,O_
Coe t _ _t
FIGURE 2a
M ,,,rT,,_, _,_,L_, i, r,_rr _ '
Trapped Cha,ge
FIGURE 2b
497
THERMAL PLASMA
The presence of a high density, low energy plasma can significantly
affect surface charging. Qualitatively, this can be understood by thinking
of this plasma as a good conductor which "drains off" any charge
accumulation thus preventing high surface potentials. Figures 3a) and b)
(3) are presented to illustrate that the thermal plasma electron density
varies by many orders of magnitude in the magnetosphere ionosphere regions.
At one extreme, in the geosynchronous region, densities are as low as 1 cm -3
while at 300 km in the F region they can be as high as 106cm -3. These large
differences in density have 2 major implications:
1) At low altitudes (< 1000 km), even in the presence of high energy
electron population, (- i0 keV) such as the precipitating
electrons in the high latitude auroral oval regions, high
spacecraft potentials are unlikely to occur.
2) Charging calculations are quite different in the two regions. At
geosynchronous altitudes the ratio of characteristic dimension of
the spacecraft to the Debye length (R) is << 1 whereas at 300 km
the reverse is true (i.e. R _1), leading to the so-called thick
and thin sheath approximations. In the former, space-charge
effects can be neglected (ie. the charge density in the sheath
region can be set to O) while the latter necessitates the
inclusion of space-charge effects.
I ooo
500
F2
2' 103 _04 135 106 TO7
Eleclru_1 C_HCerlhd_r:c, rl _e, _ ,r',s ccrr13_
FIGURE 3a FIGURE 3b
498
HIGH ENERGY ELECTRONS
Figures 4a) and b) show representative spectra for the high energy
eleccron population at geosynchronous (4) orbit and for a discrete auroral
arc (5), respectively. With respect to surface charging, the most notable
feature is the presence in both cases of electrons with energies of 10 key
and greater; typically one can describe these populations in terms of a
Maxwellian distribution function with a characterstic temperature, T e.
Simple charging analysis demonstrates that, in the absence of significant
thermal plasma and a photoelectron current, the spacecraft potential is
directly proportional to the mean temperature of the electrons. As electron
temperatures can vary from 1-20 keY, potentials of 1-20 kV are possible and
indeed, have been observed.
103 i
-- Dlslr_buhon gunct,on
_,_ .... Ma_we_han F,I
2 Maxwelhan Fit
c 1° _
1 Electzons
_, I ,;raG+ _.
,o ++! '_t
101 102 103 104 105 106
ENERGY (eV)
FIGURE 4a
'°+I \
10 - 27 Electrons
'_ 11 t26183
"_ 49843 UT
10- 28
__ 10 - 29
_ _o- 30
g
c
a
10 - 32
10 - 33
10 100 1000 10000
Energy in eV
J
100000
FIGURE 4b
499
UV/EUVRADIATION
The photoelectron current, emitted from the surface by the impact of
the incident high energy photons, can make an important contribution to the
overall current balance. This is particularly true at higher altitudes,
where, for example, at Eeosynchronous altitude in the absence of a
significant thermal plasma, the photoelectron current plays a dominant role.
The photoelectron current is a function of satellite material, solar flux,
solar incidence angle and satellite potential (6) In figure 5, adapted from
reference 7, is a composite plot of: W(E), the electron yield per photon;
S(E), the solar flux; and their product, H(E), the total photoelectron
yield, as a function of energy, E, for aluminum oxide.
1022 -
._. 1020
I
>
(D
'W'--
; 1018
O9
I
E
o 1016
c-
I.U
1014 -
1012 -
10 2
c
O
o 100
c.
O..
09
£
lO-2
c
m_'10-4
10-6
8
0
5 10
.......... S
--H
_ W
...... % ........... 0°. ....... . ...........
tI
15
E (eV)
20 25
FIGURE 5
500
MAGNETIC FIELD
The Earth's magnetic field can be approximated by a magnetic dipole
located near the centre of the Earth. The dipole moment is M = 0.312 G R E 3
where R E is the radius of the Earth, and the dipole is directed so that the
magnetic south pole on the Earth's surface is located in northern Greenland
(geographic coordinates: 78.5 ° N, 291 ° E). The spatial distribution of the
dipolar magnetic field strength beyond the surface of the Earth is given by
(8) :
B - B E JR1 "3 [ 4-3 cos2h 11/2
cosbh
where R is the radial distance measured from the center of the Earth, B E =
0.312G is the equatorial field at g _ RE , and h is the magnetic latitude.
Figure 6 shows a schematic of the Earth's magneto ionosphere indicating
the various domains. The solid lines represent the magnetic field lines
which can be seen to be distorted from a purely dipolar pattern due to the
interaction with the solar wind. In terms of charging environments, the
magnetic field can be viewed as playing essentially three roles: 1) it is a
major factor in determining the shape and location of the charging regions
(i.e. the domains shown in figure 6); 2) it can affect the escape of
photoelectrons or secondary electrons emitted from the surface (9) 3) it can
introduce anisotropy in particle fluxes.
m4_.
SOLAR
WIND
D D
PLASMA- PLASMA- POLAR VAN ALLEN
SPHERE SHEET BELTS
FIGURE 6
501
NEUTRAL ATMOSPHERE
Figure 7 shows the neutral atmosphere environment up to an altitude of i000
km. The important feature of this environment is that the neutral density is
approximately two to three orders of magnitude higher than the electron ion
density (i.e. the atmosphere is weakly ionized). At higher altitudes where
the mean free path for collisions between neutrals and electrons is very
large, collisionless probe theory can be applied to surface charging
calculations, while at lower altitudes, collisional theory is probably
needed and ionization may be of importance. Little attention has been paid
to charging calculations at altitudes between 100-200 km where the thermal
plasma density is low but where auroral precipitations of high energy
electrons are still found. This may because it has been assumed that the
simultaneous increase in the local thermal plasma density caused by the
precipitating auroral electrons is of sufficient magnitude to prevent
significant charging. In addition, at these altitudes, satellites cannot
orbit for long periods due to aerodynamic drag. However, with the advent of
the proposed tethered satellite systems (e.g. TSS2) which will be able to
trail a downward deployed platform to altitudes of about 100 km, this
problem warrants more detailed study.
E
2_
XD
<
1000
900 t
8OO
I\\\\ !700
L " 1600 _. _ ,
\ \'
500 - "\ "
400- \'_"|
300 -
200 -
100 -
0
-- O1
02
--- N2
..... Ar
.... H1
........... He
-- -- Total
1 I I k
102 104 106 108 1010 1012 1014 1016 1018 1020
Number Density cm-3
FIGURE 7
5O2
INTERNAl CHARGING
HIGH ENERGY TRAPPED ELECTRONS
FiEure 8 (10) shows empirical radiation belt electron fluxes at L =
1.4. The important feature to note here, as far as internal charEin E is
concerned, is the presence of siEnificant fluxes at enerEies between 0.i and
1.0 MeV. These enerEetic electrons can penetrate liEhtly shielded parts of
the spacecraft and accumulate on unErounded cables, conductors or
insulators, located inside the spacecraft. If the resultant electric field
rises to a hiEh enough value, breakdown can occur. The ranEes of 0.i and
1.0 MeV electrons in aluminum are about 3 and 70 mils, respectively.
106 .... I ' ....... I ' ' '
>
¢,
,.:_ 105
(,,9
04
E
C
"6 1o4
X
IJ_
103
i5
"r"
0
_ 102
W
101
T r
,_,OGO 5 Days 130-190 1968
• OV1-13 Days 139-198 1968
:, OSO 4 Days 79-86 1968
OGO 3 Days 250-300 1966
• OV3-3 Days 250-280 1966
OGO 1 -_ Day 300 1964
-- Electron Models
' t ' '1 i I I i I I I I[
0.06 0.1 0.2 0.4 0.6 1.0
AE-5 1967
E (MeV)
FIGURE 8
5O3
TIME VARIATIONS OF
GEOPHYSICAL PHENOMENA
Nearly all of the charging environments described previously show
dynamic variations both temporally and spatially. Consequently, in many
cases, modellin E of an environment has to be done statistically with averaEe
and worst case environments be_n E defined. Further, the environments are
not independent but form a coupled and hiEhly-complex interactinE system.
Figure 9 shows order of magnitude time variations for some of the phenomena
associated with charEin E environments.
Geophysical
Event
1. Solar Cycle
2. Geomagnetic Storms
3. Substorms
4. Magnetic Pulsations
5. Plasma Boundary Crossings
6. Plasma Waves
Time
Period
11 Years
1-10 Days
1-3 Hours
Sec-Min
Sec-Min
mS- #S
FIGURE 9
504
POTENTIAL IN SITU MATERIAL
CHARGING EXPERIMENTS
For many of the adverse interactions between the space environments and
spacecraft materials described here, the critical parameters required to
accurately model the physical response are currently lackin E or poorly
known. In particular, for spacecraft charEinB, 6 key parameters are
required as a function of material and, as some of the interactions result
in lon E term variations in properties, time. These are
I)
2)
3)
5)
6)
Bulk and surface electrical properties
Secondary emission coefficients for electrons and ions
Backscatter properties for electrons
Photoemission properties
Sputterin E characteristics
Arc breakdown properties
To date, these properties are known only through Eround tests or, in situ,
by indirect methods--primarily through variations in parameters to fit
charEin E observations. Given the hypothesized variations over time of these
parameters due to radiation damaEe , contamination, etc., in situ
measurements are vital to a proper understandin E of how materials behave
over the lon E term in a charEin E environment.
505
POTENTIAL IN SITU MATERIAL
CHARGING EXPERIMENTS
Three experiments are presented in Figure 10 that would make possible
measurements of the in situ material properties relevant to spacecraft
charEin E. First, for surface charEin E it is necessary to study the in situ
secondary, backscatter, and photoemission properties. AlthouEh Eround
experiments are useful for this purpose, it is not likely that the in situ
surfaces will actually retain their Eround values followin E 1on E term
exposure to space. This experiment (referred to here as the Electrical
Properties DeEradation experiment) would alternately expose samples to the
ambient environment and then to a probe capable of directly measurin E the
secondary emitted electrons and ions--one potential confiEuration would be a
carousel tray for the samples with a commercially available secondary
emission probe. The second experiment would measure natural and, if
necessary, simulated surface arcs with the objective of locatin E them on the
test surface and estimatin E the conducted and radiated emissions (referred
to here as the ElectroStatic Discharge experiment). Such instruments have
been flown in the past on SCATHA but, for various reasons, did not return
sufficient information on the arc discharEes to unambiEuously define their
characteristics or their locations.
• Electrical Properties Degradation Experiment
• Secondary Emission Properties
• Photoemission Rate
• Surface Electrical Properties
• Electrostatic Discharge Experiment
• Arc Properties
• Surface Location
• Surface Conditions at Time of Arc
• Surface Damage
• Internal Discharge Monitor
• Internal Arc Characteristics
• Material Bulk Property Changes
FIGURE i0
5O6
POTENTIAL IN SITU MATERIAL
CHARGING EXPERIMENTS
The third experiment, the Internal Dischrge Monitor, is planned to fly on
the CRRES spacecraft in 1990 in a simplified form to monitor arc discharges
on samples inside a protected tray assembly. As in the case of CRRES, more
advanced forms of this instrument will also need to be flown through the
radiation belts to obtain useful information in a short time period. All 3
instruments would need supporting environmental sensors to monitor the
natural environment--particularly the relevant particle and EUV fluxes.
Finally, while the EPD and ESD instruments might be suitable for low-
altitude, short duration missions (although the longer the exposure, the
better), the IDM will clearly require multi-year missions.
507
CONCLUSIONS
In this paper we have identified and provided brief descriptions of the
major natural environments that contribute to charging. The intent has been
to provide an overall picture of the charging environment but in a very
short space. The result is a necessarily oversimplified and idealized view
of a highly complex interdependent physical system; in essence, only
allusions have been made to some of the realistic features such as temporal
variability and particle flux anisotropies.
The importance of surface properties in determining charging levels
makes it essential to measure these properties in the real environment.
However, to correlate charging levels with the environment and surface
characteristics, simultaneous monitoring of the environment will also be
requlred.
5O8
1)
2)
3)
4)
5)
6)
z)
8)
9)
lO)
REFERENCES
H. B. Garrett, "The Charging of Spacecraft Surfaces", Reviews of
Geophysics and Space Physics, Vol. 19, 4, pp577-616, 1981.
A. R. Frederickson, "Radiation Induced Dielectric Charging", Space
Systems and Their Interactions with Earth's Space Environment, ed.
by H. B. Garrett and C. P. Pike, vol. 71, Progress in Astronautics
and Aeronautics, 1980.
Chiu, Y. J. et al., "An Equilibrium Model of Plasmaspheric Composition
and Density", J. Geophys. Res., vol. 84, 1999, pp 909-916.
Garrett, H. B. and Spitale, G. C., "Magnetospheric Plasma Modelling
(0- 100 keY)", Journal of Spacecraft and Rockets, vol. 22, 3, p231,
1985.
Gussenhoven, M. S. et al., "High-level spacecraft charging in the
low-altitude polar environment", J. Geophys. Res., 90, 11, 009,
1985.
Lucas, A. A., "Fundamental Processes in Particle and Photon
Interactions with Surfaces", Photon and Particle Interactions with
Surfaces in Space, ed. by R. J. L. Grard, D. Reidel, Hingham, MA,
pp3-21, 1993.
Grard, R. J. L., "Properties of the satellite photoelectron sheath
derived from photoemission laboratory measurements", J. Geophys.
Res., 78, pp 2885-2906, 1973.
Spjeldvik, W. N. and Rothwe11, P. L. "The Earth's Radiation Belts",
AFGL-ZR-83-0240, 1983.
Laframboise, J. G., "Calculation of Escape Consents of Electrons
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Field", J. Geophys. Res., 93, A3, pp1933-1943,1988.
Teague, M. J. and Vette, J. I., "The Inner Zone Electron Model AE-5,
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(1972).
509


An overview of charging environments

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