CHAPTER

DEEP DIELECTRIC CHARGINGAND SPACECRAFT ANOMALIES 16Shu T. Lai*,†, Kerri Cahoy‡, Whitney Lohmeyer‡, Ashley Carlton‡, Raichelle Aniceto‡, Joseph Minow§

MIT Space Propulsion Laboratory, Cambridge, MA, United States* Boston College, Newton, MA, United States†

MIT Space Systems Laboratory, Cambridge, MA, United States‡ NASA Marshall Space Flight Center, Huntsville, AL,

United States§

CHAPTER OUTLINE

1 Introduction ................................................................................................................................. 419

2 What Is Deep Dielectric Charging? ................................................................................................ 420

2.1 The Roles of Ions ........................................................................................................422

3 Space Environments ..................................................................................................................... 423

3.1 Trapped Radiation ......................................................................................................423

3.2 Temporal Variation of the Radiation Belts .....................................................................423

3.3 Relative Role of Electrons and Ions ..............................................................................424

4 Deep Dielectric Charging and Discharging ..................................................................................... 426

5 Dependence from Geomagnetic Indices: Dst and Kp Index ............................................................... 428

6 Delay Time ................................................................................................................................... 428

7 Discharge Event Parameters ..........................................................................................................429

8 Spacecraft Design Guidelines ........................................................................................................429

9 Conclusion ................................................................................................................................... 431

Acknowledgment ................................................................................................................................ 431

References ........................................................................................................................................ 431

1 INTRODUCTIONThe space radiation environment presents hazards to satellites and the services they provide by causing

internal and surface charging, displacement damage, accumulated dose effects, etc., which have led to

many documented satellite anomalies (e.g., Baker, 2000; Fennell et al., 2001; Frederickson, 1996).

This chapter focuses on internal charging, which is one of the main causes of space environment-

related anomalies (Koons et al., 1999). We present the physical mechanisms and environments that

may lead to internal charging, an overview of some notable on-orbit spacecraft anomalies that are at-

tributed to internal charging, and conclude with spacecraft design guidelines that can be used to mit-

igate internal charging threats.

Extreme Events in Geospace. https://doi.org/10.1016/B978-0-12-812700-1.00016-9

# 2018 Elsevier Inc. All rights reserved.419

2 WHAT IS DEEP DIELECTRIC CHARGING?Deep dielectric charging (also called internal, or bulk charging) occurs when high-energy electrons and

ions (MeV and higher) penetrate satellite shielding materials and deposit charge on internal spacecraft

components. The terms “deep dielectric” or “bulk” charging refer to charge densities that accumulate

within insulating (or dielectric) materials when exposed to penetration radiation. The more generic

term “internal charging” includes charge densities that accumulate on the surfaces of conducting

materials within the shielding afforded by the outer structure of a spacecraft. Both processes result

in electric fields within spacecraft structures and materials and represent a threat to arcing which

can damage spacecraft components.

The physics of internal charging for an insulator exposed to radiation environments is described by

a set of two equations (Sessler, 1987; Sessler et al., 2004)

r �E¼�r2Φ¼ ρ

κε0(1)

∂ρ∂t

¼�r � JR + JCð Þ (2)

where the Eq. (1) is Poisson’s equation that relates the electric field, E, and electric potential, Φ, to thecharge density, ρ, within the material. The continuity equation in Eq. (2) relates the charge density

accumulating in the material to the incident radiation (electron) current density, JR, and the conductioncurrent density, JC. The parameters κ and ε0 are the dielectric constant for material exposed to the

charging environment and permittivity of free space, respectively. Eqs. (1), (2) together describe

the magnitude and direction of a time and spatially dependent electric field within a material due to

the accumulation of charge from an external radiation source and loss of charge from the material

through conduction. Typically, only electron currents are considered for internal charging because ions

at similar energies as the electrons don’t penetrate deeply into materials. Ions with sufficient energy to

penetrate deep into the materials typically exhibit very low flux compared to the electrons with the

same penetration depth. Thus, ions are not a significant contributor for internal charging currents in

most internal charging environments. The role of ions will be considered in Section 2.1. Electron cur-

rents are given by

JC ¼ σE¼ σdark + kpdγ

dt

� �αh iE (3)

where the bulk conductivity of the insulator σ is divided into the σdark conductivity (in the absence of

exposure to photons or charged particles) and a radiation induced conductivity that depends on the dose

rate (dγ/dt) generated by interaction of the radiation field with the dielectric material. The parameter kpis the coefficient of radiation-induced conductivity and the exponent 0.5<α<1 depends on the energy

distribution of electron trapping states in the insulating material. Evaluation of internal charging re-

quires solving both a radiation transport problem to compute the charge deposition and radiation dose

rates within the material and the electrostatic problem to obtain the electrostatic fields and potentials

due to the accumulating charge density.

The propagation of the high-energy particles is governed by the Bethe-Bloch equation (Garrett and

Whittlesey, 2012; Ziegler, 1999):

420 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

S Eð Þ¼�dE

dx¼ 4πZ2ne

mv2α2 log

2mv2

1�β2

� ��β2� logI

� �(4)

where S(E) is the stopping power, which is the energy loss per unit distance of particle penetration intomatter.

S Eð Þ¼�dE

dx(5)

The range R, which means the depth, of penetration by a charged particle of initial energyEp is given by

R¼Z Ep

0

1

S Eð ÞdE (6)

Using Eqs.(4)–(6), one can plot the range, or penetration depth, for a given projectile and a given

material.

Fig. 1 shows that the penetration depth of electrons is about two orders of magnitude as ions of about

the same energy. For a typical spacecraft wall with a thickness of 100 mils (2.54 mm) of aluminum,

electrons need energies in the range 0.5–5 MeV to penetrate, and protons need energies of

10–100 MeV. For a more in depth discussion, see (Ziegler, 1999).

One of the key characteristics of deep dielectric charging is the time it takes to build up electric

fields within an insulating material or on a conductor compared to the time it takes for charge to leak

100,000

mil

Al(g/cm2)10,000

1000

100

10

1

0.1

0.010.01 0.1 1

Dep

th in

alu

min

ium

,(e

lect

rons

and

pro

tons

)(m

il)

10

Energy (MeV)

100 1000

<~10 kÅ

<~10 um

<~10e-milp-mil

<~1

<~0.1

<~0.1 mm

<~1 mm

<~10 mm

<~0.1 m

<~1 m

metric

FIG. 1

Electron (solid) and proton (dashed) penetration depth in aluminum (in mils on the left y-axis and in meters on the

right y-axis) for a range of energies (0.01–1000 MeV). For 100 mils (2.54 mm) of aluminum, protons need to have

energies above about 20 MeV and electrons need to have energies above 1 MeV to pass through.Reprinted with

permission from Garrett, H., Whittlesey, A., 2012. Guide tomitigating spacecraft charging effects. In: JPL Space Science and Technology

Series, Inc. John Wiley & Sons, Pasadena, CA.

4212 WHAT IS DEEP DIELECTRIC CHARGING?

off the material. An estimate of this time constant can be obtained from a simple one dimensional,

planar model for internal charging of an insulator where the electric field E at a depth X in a dielectric

is given by Garrett and Whittlesey (2000).

ε dE=dtð Þ + σE¼ JR (7)

The parameters E, JR, and ε, and σ are the electric field, radiation current density, absolute permittivity

of the material, and σ the volume conductivity of the insulating material described in Eq. (3). A solution

to this equation assuming that JR and σ are independent of time is

E¼E0 exp �σt=εð Þ+ JR=σð Þ 1� exp �σt=εð Þð Þ (8)

where E0 is the electric field at t¼0. The absolute permittivity of the material is related to the permit-

tivity in a vacuum ε0 by the dielectric constant κ¼ε/ε0. Using this definition in Eq. (8), the time con-

stant for charging of the material is estimated by τ¼ε/σ or τ¼κε0/σ. The charges deposited inside the

dielectrics can stay inside for hours, days, or weeks (Rodgers and Sorenson, 2012; Bodeau, 2010). If the

dielectric’s resistivity is high, the rate of charge build up can overcome the leakage rate of the material

and large charge densities can accumulate within a material. In addition to the properties of the dielec-

tric, the time it takes for internal charging to occur depends on the duration of the elevated high-energy

flux environment and the material thickness. Internal electric fields, material temperature, ionization

properties, and history of discharges are also important factors (Lai, 2012).

The induced electric field may exceed the breakdown threshold for the material, causing electro-

static discharge (ESD) in the insulating material (Baker, 2000; Fennell et al., 2001; Frederickson, 1996;

Garrett and Whittlesey, 2012). It is generally accepted that an internal electric field E* of 106–108 V/min dielectric materials may pose a risk for internal discharge. The resulting discharge is potentially haz-

ardous directly to the material, or indirectly to other spacecraft components, causing spacecraft com-

ponent anomalies.

2.1 THE ROLES OF IONSThe role of electrons penetrated deep inside dielectrics has been known for years. The electrons pen-

etrated inside provide high internal electric fields which may be responsible for spacecraft anomalies in

orbits exposed to MeV electron fluxes in the outer radiation belts. The high internal electric fields are

necessary for deep dielectric charging. The roles of ions have not received much attention. We discuss

two roles for ions in the internal charging process as follows.

The flux of electrons is about two orders of magnitude higher than that of ions because of the mass

difference. During exposure to a high-energy electron environment, the electrons deposited deeply in-

side a material build up high electric fields. Days after the exposure, low energy ions are attracted to the

surface of the dielectric by the electric fields generated by the internal charge density and deposit

charge inside the material on the surface or at relatively shallower depths within the material. The

double layer gradually formed inside the dielectric material enhances the internal electric fields.

Another role of high-energy ions is enhancing the probability of discharges (Fig. 2). It is generally

believed that a high-energy heavy particle penetrating inside may cause a single event upset (SEU).

A SEU may cause displacement damage or a sudden increase in ionization density in a local region,

or track, enhance the conductivity there. In addition, if a high-electric field is already present in that

422 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

region, the increase of ionization density together with the high-electric fields, may enhance the

mobility of the electrons there. As a result, a discharge may occur.

3 SPACE ENVIRONMENTSIn this section, we examine environments that may lead to deep charging.

3.1 TRAPPED RADIATIONHigh-energy particles can be found in the trapped energetic particle environment in Earth’s magnetic

field, a region known as the Van Allen radiation belts. The belts differ in distribution and energy for

protons and electrons. They are generally located at about 1 RE–4 RE with energies of about

0.1–400 MeV and at about 2 RE–8 RE with energies of about 0.4–4.5 MeV for protons and electrons,

respectively (Hastings and Garrett, 1996). The AE8 and AP8 models (and now AE9/AP9 models) are

typically used to predict the variations in the belts (Sawyer and Vette, 1976; Vette, 1991; SPENVIS, n.d.).

3.2 TEMPORAL VARIATION OF THE RADIATION BELTSFig. 3 shows the modeled radiation belt fluxes during solar maximum. A similar figure can be obtained

for solar minimum. The AE8 and AP8 models (now the updated AE9 and AP9 models) allow us to

MeVelectrons

MeVions E

(A) (B)

(C) (D)

+

+ + + + + + + + + + + + + + + + + + + + +

+ ++

+ + +

+ + +

E E

Slow ions

Anodized discharge

FIG. 2

(A) Penetration of high-energy electrons and ions into dielectrics. (B) Electric field build-up inside the dielectric

material. (C) Ions coming in at shallower depths enhancing the internal electric field. (D) A spontaneous discharge

may or may not occur if the electric field reaches a critical value. An external factor, such as a very high-energy

particle, or a meteor, impact, may induce a discharge (Lai, 2012).

4233 SPACE ENVIRONMENTS

estimate radiation belt fluxes at solar maximum and solar minimum, and hence the variation between

these two cases. It should be noted that solar maximum and solar minimum do not necessarily reflect

the solar cycle variation of outer belt electron fluxes. For example, we often see the highest fluxes in the

declining phase of the cycle when high-speed solar wind streams are more stable and long-lived

(Wrenn et al., 2002).

It is important to also note that the temporal variation of the electron fluxes in the belts often ex-

hibits many periods of rapid intensification (minutes to hours) followed by slow decay (weeks)—as

shown in Fig. 4. The temporal behavior is a significant issue in assessing deep dielectric charging

threats to spacecraft in MEO and GEO orbits inside the radiation belts.

As shown in Fig. 4, the high-energy (2–6 MeV) electron fluxes in the radiation belts (especially the

outer belt) do not vary smoothly with the solar cycle. The fluxes vary according to solar events, such as

the arrival of a CME. In the solar cycle declining phase, very energetic particles from cosmic rays can

sometimes come in. As will be discussed later, spacecraft anomalies related to deep dielectric charging

and discharging occur more often during the declining phase of the solar cycle. Further details of ra-

diation belt variations including solar cycle dependence and role of wave-particle interactions can be

found in the Chapters 13 and 14 of this volume.

3.3 RELATIVE ROLE OF ELECTRONS AND IONSBecause of the mass difference between ions and electrons, the electron flux exceeds the ion flux in

space by typically two orders of magnitude. When a severe geospace storm hits a spacecraft, the

AP8In geomagnetic coordinates

0° longitude

8 7 6 5 4 3 2 1 0 1

RE

2 3 4 5 6 7 8

106

104

102

100

107

105

103

101

10 MeV integral flux(cm2-s)−1

1 MeV integral flux(cm2-s)−1

AE8

FIG. 3

Integral flux (cm2-s)�1 of protons (>10 MeV) and electrons (>1 MeV) as a function of altitude (where 1

RE¼6371 km) in Earth orbit at 0° longitude, using the AP8/AE8 models at solar maximum. Generated by using

the AE8/AP8 models in SPENVIS (Sawyer and Vette, 1976; Vette, 1991; SPENVIS, n. d.). The mapping is done in

magnetic coordinates given here in earth radii. There are up-to-date versions: AE9 and AP9. For illustration

purpose, it is sufficient to use AE8/AP8.

424 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

electrons and ions penetrate into the dielectric materials. For electrons and ions in the energy range of

1 MeV to several tens of MeV, the electrons penetrate deeper than the ions, as shown in Fig. 1. As a

result, the electric fields building up inside depend very much on the electrons accumulating inside.

The accumulation depends at the same time on the leakage rate, which is slow for dielectric materials.

In other words, the electron accumulation plays the role of building up the internal electric fields. The

accumulation takes time.

Fluence F is the integration of flux J over a period of time T

F Tð Þ¼Z T

0

J tð Þ dt (9)

Relating to the information in Fig. 2, let us take the radiation-belt region in red, where the flux Jis 107 electrons cm2s�1. Such a flux J contributes a fluence F¼1011 electrons cm2 in 104 s or

about 3 h. The actual accumulation depends also on the charge leakage, which is slow for dielectrics

materials. Therefore, leakage slows the build-up of the internal electric field and lengthens the time

needed for reaching the critical electric field E*.If the electric field built up inside reaches a critical value E*, typically 106�108 V/m depending on

the material, then a spontaneous discharge may occur. If a spontaneous discharge does not occur, the

electric fields may continue to build up to higher values, forming an extremely hazardous situation. It

takes time to build up the situation, which is unstable. Any disturbance applied to the situation may

trigger a discharge. Because of the time needed for accumulation and for a probability of disturbance,

such a discharge occurs with a delay.

When the peak of the severe geospace storm has passed, lower energy ions, being attracted by the

electric fields, may come in and deposit at shallow depths. As a result, the internal electric fields are

enhanced to even higher values. The probability of a discharge becomes higher than at the peak of the

severe geospace storm. In other words, the protons play the role of enhancing the internal electric fields

and potentially triggering the deep dielectric discharging. Again, the discharge occurs with a delay.

Delay is a characteristic of deep dielectric charging and discharging.

A typical example of such a delay is hours, days, or weeks. Such a delay time is not a physical

constant. It depends on the rate and duration of the incident energetic electron flux which varies in

SAMPEX: E=2–6 MeV electron fluxes7

4

L va

lue

1

1993 1999 2005

Innerbelt

Outerbelt

Year

FIG. 4

Long-term (12-year) plot from SAMPEX showing temporal variation of the two radiation belts.

4253 SPACE ENVIRONMENTS

time at various locations in geospace, as well as the electrical properties of the material which control

the rate at which charge may be lost, including electrical resistivity, temperature, and aging of the

materials.

Extremely high energy protons of energies reaching many tens of MeV may penetrate well

inside a material or spacecraft structure (Fig. 1). Suppose a beam of high-energy protons or ex-

tremely energetic cosmic ray ions (100 MeV, more or less) hit the dielectrics, which happen to be

deeply charged a priori; the ions may penetrate deeply into the dielectrics generating a region, or

trail, of ionization. Such a scenario may trigger a discharge. In other words, high-energy proton

beams or cosmic-ray ions may play the role of triggering deep dielectric discharging. Although

such a scenario requires a delay for charging accumulation followed by a rare probability of

high-energy ion penetration and deposition. In this way, the ions play the roles of (1) enhancing

the internal electric fields because of the differential ranges between electrons and ions and trig-

gering is instantaneous, and (2) increasing the probability of triggering a discharge. Therefore, the

delay time for an ion storm, or ion beam, to generate a discharge is comparably short, perhaps a

day or a few hours. Again, it is not a physical constant but depends on the material properties, rate

of depositions, etc.

4 DEEP DIELECTRIC CHARGING AND DISCHARGINGDeep dielectric charging is not directly related to spacecraft anomalies. In a hazardous environment,

deep dielectric charging occurs and builds up the internal electric fields. A spontaneous discharge may,

or may not, occur even when the electric field reaches sufficiently high values. A triggering impetus

may bring forth the occurrence of a discharge. If very high energy cosmic ray particles, including pro-

tons or heavier elements, impacting on the dielectric impregnated with deep charging may trigger a

discharge. A meteor impact may also trigger such a discharge.

In the laboratory, it has been repeatedly demonstrated with ease that a piece of plexiglass irra-

diated by MeV electrons for a long time may, or may not, undergo a spontaneous discharge

(Fig. 5). This situation is a metastable equilibrium, which is unstable. Likewise, a supercharged di-

electric material may, or may not, readily undergo spontaneous discharge. However, touching the

deeply charged plexiglass with a grounded wire or impacting it with a hammer can readily induce

a discharge (Fig. 5).

The solar cycle is an important factor in determining the likelihood of spacecraft anomalies related

to deep dielectric charging. It has been found (Wrenn et al., 2002; Lohmeyer and Cahoy, 2013) that

occurrence of these anomalies is higher during the declining/minimum phase of a solar cycle (Fig. 6).

Two factors could contribute to the result. First, as noted above, outer radiation belts tend to intensify

during the declining/solar minimum phase. It could explain the build-up of fluences required to pro-

duce a discharge. Second, during the declining phase of a solar cycle, the high-energy cosmic ray par-

ticles can penetrate into the Earth’s geospace more readily (Mewaldt, 2013; Bazilevskaya et al., 2014).

Therefore, there is higher probability of high-energy cosmic particles on the spacecraft surfaces during

this period. The cosmic particles may initiate discharges in deeply charged dielectrics, but this factor is

a hypothesis which needs further examination.

426 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

FIG. 5

Triggering a discharge in a precharged plexiglass.Reprinted with permission fromBert Hickman, http://capturedlightning.com.

0 0

20

40

60

80

100

120

1996 1998 2000 2002 2004

Year

Sun

spot

num

ber

Num

ber

of A

nom

alie

s

2006 2008 2010 2012

1

2

3

4

5

Sm

oothedsunspot num

ber

Satellite fleet ASatellite fleet B

FIG. 6

Yearly total of SSPA anomalies. The sunspot curve is smoothed. Most of the anomalies occurred during the low

period of the solar cycle (Lohmeyer and Cahoy, 2013).

4274 DEEP DIELECTRIC CHARGING AND DISCHARGING

5 DEPENDENCE FROM GEOMAGNETIC INDICES: DST AND KP INDEXThe disturbance storm time (Dst) is an index of magnetic activity giving information about the strength

of the ring current and other magnetospheric currents and is derived from a network of near-equatorial

geomagnetic observatories. During quiet time (Dst) typically takes on values that fluctuate around

0 nT. Negative Dst indicates a geomagnetic storm (e.g., Dst of �500 nT or less is considered an

extreme storm). The Kp index is the averaged value of measurements by globally distributed sensors

on the ground, which is used to characterize the magnitude of geomagnetic storms. The Kp index

ranges from 0 (quiet) to 9 (extreme geomagnetic activity). It does not indicate the local flux rates

at the location of a spacecraft. Unlike surface charging, discharges due to deep dielectric charging

do not necessarily occur quickly in response to the storm level. It may take time to build up the internal

electric field. Dst and Kp indices, and also Ae index, are therefore not usually considered when looking

at discharge risks.

6 DELAY TIMEIt has been observed that spacecraft anomalies often occur after, but not during, the peak energetic

electron flux. With charge storage and decay time in dielectrics, the charge buildup peaks with a delay

time. As a case in point, the GOES-5 satellite (Baker et al., 1987) experienced high fluence of high-

energy electrons for days before an anomaly occurred (Fig. 7A) and both Equator-S and Galaxy 4 fail-

ures behaved similarly (Fig. 7B).

102

104

21 April 1 May 10 May

1998

20 May

105

106

107

108

Ele

ctro

ns/c

m2 -

day-

sr

109

1010

1011

101

Ele

ctro

n (c

ount

s/s)

100

10–1

Jul

(A) (B)13 14 15 16 17 18 19 20 21

1984

22 23 24 25 26

~Background levels

5–7.5 MeV

3–5 MeV

GOES–5failure

S/C 1982-019 (~70° W)

GEOSTATIONARY ORBIT

GOES Electrons E > 2 MeV

Long-duration electron event(Key to deep-dielectric charging)

Equator-Sfailure

Galaxy 4failure

FIG. 7

(A) High-energy electron counts during the period July 13–26, 1984. The satellite GOES-5 failed after, but not

during, the peak flux high energy electron flux (Baker et al., 1987).(B) Galaxy-4 and Equator-5 spacecraft failure

events showing delays of failures occurring after the peak of the electron fluence (Baker et al., 1998).

428 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

7 DISCHARGE EVENT PARAMETERSIt is difficult to define an extreme deep dielectric charging event. One can define a space environment

in which deep dielectric charging is likely to occur. One can also define a catastrophic discharge event

as the result of deep dielectric charging.

When dielectric materials accumulate significant charge, they can become a hazard. However, there

may be no impacts to the material or component until a discharge occurs. Discharge occurrences de-

pend on several parameters:

1. The penetration depth of a charged particle from the environment depends on the energy of theincoming particle and the material properties. For 1 mm of aluminum, a proton must have

approximately 10 MeV, and an electron must have 0.6 MeV.

2. The particle energy distribution. For incoming electrons and ions of approximately the same energy,

the deposited charges form a double layer inside because of the difference in penetration depths of

electrons and ions. The double layer not only enhances the internal electric field but also cancels the

net electric field just outside the dielectricmaterial. In this situation, any surfacemeasurementwould

indicate a low electric field but the internal field may be building up to a high value.

3. The fluence—the integration of the incoming flux over a period of time—is also a key parameter

(Han et al., 2005). The balance between the incoming and leakage flux results in a delay time τ forthe internal field to build up to a critical value E*. The delay may be hours, days, or longer (Ziegler,

1999). For example, (Lohmeyer and Cahoy, 2013; Lohmeyer et al., 2015) found that anomalies in

the dielectric of high-power amplifiers were statistically more likely 14–21 days following high-

energy electron flux events. A discharge may not occur during a high flux period, but may instead

occur during a following lower flux period, due to the build-up of charge over time.

4. One major parameter is the critical electric field for dielectric breakdown, E*¼106 to 108 V/m. It is

possible, but unlikely, to have an extreme electric field E≫E*, inside the dielectric without anydischarge. Such a situation is anunstable equilibrium. It requires another parameter to causea discharge.

5. Another parameter is the dielectric material age. The E* value decreases as the dielectric material

ages. Small cracks may develop in the material, and multiple small local discharges may also lower

E* as tiny discharge paths may remain.

6. The leakage flux of the dielectric material is a critical parameter. Because the flux at high energies

(0.6 MeV or higher) is lower than at lower energies, it can take time to build up high electric field

inside. The time development of the charge build up is governed by the competition between the

incoming flux and the leakage flux. It depends on the conductivity of the material and may change

in space gradually as a result of material temperature, space radiation, charge density deposition,

and other factors (Hanna et al., 2013).

7. Any disturbance, such as the impact of a high-energy proton, or heavy ion, penetration, a meteorite

passing through, or a small discharge leading to avalanche ionization, may cause a significant

discharge (Lai, 2012).

8 SPACECRAFT DESIGN GUIDELINESTwo sets of design guidelines for electrostatic discharge (ESD) hazards have been established. The

first, and most commonly cited, is captured in NASA-HDBK-4002A (2011), which specifies a 10-h

safe fluence level of �2�1010 electrons/cm2 (Violet and Frederickson, 1993). The safe-fluence level

4298 SPACECRAFT DESIGN GUIDELINES

is derived from the Combined Release and Radiation Effects Satellite (CRRES), which was launched

into an elliptical orbit in 1990 to study fields, plasmas, and energetic particles through the Van Allen

radiation belts. CRRES was equipped with an Internal Discharge Monitor (IDM), which recorded deep

dielectric charging events (Fig. 8) (Frederickson, 1996; Hanna et al., 2013). Using the CRRES data,

(Frederickson et al., 1992) suggested a threshold for deep dielectric discharge risk at 2�1010elec-

trons/cm2 for 10 h because CRRES experienced no ESD events when the accumulated fluence insidethe IDM over a 10-h orbit was less than 2�1010 electrons/cm2 (Frederickson et al., 1992).

Wrenn and Smith (1996) define two thresholds for space hazards. Threshold I states that significant

probability of hazard exists when >2 MeV daily electron fluence outside the spacecraft exceeds

3.8�109 electrons/cm2. Threshold II states that an extremely significant probability of hazard exists

when >2 MeV daily electron fluence exceeds 3.8�1010 electrons/cm2 (Wrenn and Smith, 1996).

These thresholds are based upon empirical correlations of anomalies with the external electron flux

data. However, keep in mind that the >2 MeV electron flux outside the spacecraft is only a proxy

for the actual electron fluence reaching the source of ESD inside the spacecraft. The range of a

2 MeV electron is about 170 mils (4.3 mm) aluminum (Fig. 1). If the shielding between the ESD source

and external environment is less than that, then lower energy electrons can penetrate the shielding and

contribute to the charging of the ESD source. There are far more electrons at energies lower than

2 MeV, so the >2 MeV external flux level may understate the total flux of electrons reaching the

ESD source if the shielding is substantially lower than 170 mils aluminum.

Recent findings by Bodeau (2010) and Lohmeyer et al. (2015) discuss statistical correlations be-

tween high-energy electron fluence over periods of 1–3 weeks prior to anomalies (Bodeau, 2010).

Lohmeyer et al. (2015) confirm that a 26-component set of solid state power amplifier anomalies

on eight geostationary communications satellites cluster at a higher than expected rate when

>2 MeV electron fluence for 14 and 21 days prior to the anomalies was high compared to all

>2 MeV electron fluence measurements for similar durations in the 1996–2012 interval. A Monte

Carlo analysis conducted on daily >2 MeV electron fluence measurements show a higher rate of oc-

currence of anomalies after high fluence than expected by chance.

Orbits 6 - 1066

Orbital electron fluence, per sq cm

2 × 1010 1 × 1011 5 × 1011

100

Pul

ses

per

orbi

t

0

10

FIG. 8

Discharge pulses from orbits 6 to 1066 on the CRRES satellite as a function of the orbital electron fluence in

electrons per cm2.Reprinted with permission from Frederickson, A.R., Holeman E.G., Mullen, E.G., 1992. Characteristics of

spontaneous electrical discharging of various insulators in space radiations. IEEE Trans. Nucl. Sci. 39(6), 1773–1782.

430 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES

9 CONCLUSIONHigh-energy electrons and ions that have penetrated dielectric materials may remain inside for a long

time because of low conductivity of the material. Electrons of 0.6 MeV can penetrate into 1 mm of

aluminum, for example. The buildup of charges inside may be slow because of the low flux of

high-energy electrons and ions. When high fluence of high-energy electrons arrives, they may not im-

mediately generate a spacecraft anomaly but will continue to accumulate charges inside. When the

charge accumulated reaches a certain level, the probability of a discharge becomes high. For typical

dielectric materials, the critical electric field E* for dielectric breakdown is 106–108 V/m. From the

CRRES experience, the critical fluence of electrons of>2 MeV is 2�1010 electrons/cm2 for 10 h.

From experience of spacecraft failures, the delay time for spacecraft failure to occur after exposure

to the peak of the high-energy electron flux is about one to several days.

These critical numbers are not physical constants. They depend on the satellite material properties

such as conductivity, radiation induced conductivity, geometry, aging of material, history of partial

discharges, etc. The dependency is also on the space environment properties such as the electron en-

ergy, the locality of the satellite, the duration of high-energy electron event, the duration of a satellite in

a certain location, the solar activity, the phase of the solar cycle, etc.

ACKNOWLEDGMENTWe thank the reviewers for helpful comments.

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432 CHAPTER 16 DEEP DIELECTRIC CHARGING AND SPACECRAFT ANOMALIES