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© Copyright QinetiQ limited 2006
Data and Models for Internal Charging Analysis
Alex Hands
University of Surrey, UK
5th September 2017
SEESAW Conference
Boulder CO
© Copyright QinetiQ limited 2006 25th September 2017, SEESAW Conference, NOAA, Boulder CO
Outline
• Background
• Internal Charging
• Data
• Focus on SURF instrument
• Environment Models
• Focus on FLUMIC and MOBE-DIC
• Charging Models
• Focus on DICTAT
• Experimental Approach
• Future Work
© Copyright QinetiQ limited 2006 35th September 2017, SEESAW Conference, NOAA, Boulder CO
Internal Charging
• Energetic trapped electrons in Van Allen belts pose a threat to satellites through internal charging of
dielectric materials:
• The outer electron belt is extremely dynamic - large changes in flux occur over short timescales, driven
by coronal holes and coronal mass ejections (CMEs)
Radiation belt pumping
e.g. April 2010:
>2 MeV flux at GEO
increases by ~4 orders of
magnitude in a few hours!
Electrostatic charging of spacecraft materials
Electrostatic dischargeEnergy coupling into circuit
Satellite anomaly / outage /failure
© Copyright QinetiQ limited 2006 45th September 2017, SEESAW Conference, NOAA, Boulder CO
Satellite Anomalies
• Example of correlation with GOES >2 MeV electron flux:
Anik E1 & E2,
Intelsat K disabled
DRA-Δ anomalies
Note log scale –
outer belt is highly
dynamic due to
buffeting by the
solar wind
© Copyright QinetiQ limited 2006 55th September 2017, SEESAW Conference, NOAA, Boulder CO
In-situ measurements of electron current
SURF Instrument (carried on STRV1d GTO mission)
~MeV electrons in inner belt
(in 2000 but no longer?)
Inner & outer
belts clearly
visible
© Copyright QinetiQ limited 2006 65th September 2017, SEESAW Conference, NOAA, Boulder CO
Giove Spacecraft
• Technology Demonstrator Satellites for Galileo Constellation
• Each Carries Space Environment Monitor(s)
• Medium Earth Orbit (~23,500 km, 56°)
Giove-B:
• Launched Apr 2008
• One instrument:
• SREM
Giove-A:
• Launched Dec 2005
• Two instruments:
• Merlin
• Cedex
© Copyright QinetiQ limited 2006 75th September 2017, SEESAW Conference, NOAA, Boulder CO
Merlin radiation monitor
• Suite of detectors
• Launched in 2005 on
Giove-A
• Still operating successfully
“The Merlin Space Weather Monitor and its Planned Flight on the Galileo System Testbed Satellite (GSTB-V2/A),” K. A. Ryden, C. S. Dyer, P. A. Morris, R. A. Haine and S. Jason IAC paper IAC-04-IAA.4.9.3/U.6.04, Published in Proceedings of IEC Congress Vancouver, Canada, 2004.
© Copyright QinetiQ limited 2006 85th September 2017, SEESAW Conference, NOAA, Boulder CO
Merlin-SURF
Successor SURF detector now has three charging plates
• Top Plate: 0.5mm thick, 0.5mm Al shielding above
• Middle Plate: 0.5mm thick, 1.0mm Al shielding above
• Bottom Plate: 1.0mm thick, 1.5mm Al shielding above
0.0E+00
5.0E-16
1.0E-15
1.5E-15
2.0E-15
2.5E-15
3.0E-15
0 2 4 6 8 10
E (MeV)
Pla
te c
urr
en
t (A
/m2)
Top plate
Middle plate
Bottom plate
Response
functions
Free from proton contamination
(v. small opposite polarity
currents during SEPE)
Peak response ~0.5 – 4 MeV
© Copyright QinetiQ limited 2006 95th September 2017, SEESAW Conference, NOAA, Boulder CO
SURF Data
• Giove-A / Galileo orbit is in the heart of the outer belt
• Perfect location for internal charging currents GALILEO altitudeGALILEO altitude
MERLIN-GIOVE A: CHARGING CURRENTS DUE TO TRAPPED ELECTRONS
0.00
0.05
0.10
0.15
0.20
29/12/2005 15:36:00 29/12/2005 20:24:00 30/12/2005 01:12:00 30/12/2005 06:00:00 30/12/2005 10:48:00 30/12/2005 15:36:00
Date
Cu
rren
t (p
A/c
m2)
0
5
10
15
20
25
30
L (
Re)
:
B (
kG
au
ss)
Shield 0.5mm, collector 0.5mm
Shield 1.0mm, collector 0.5mm
Shield 1.5mm, collector 1.0mm
B (model)
L (model)
First Day: First 6 months:
Recurrence of coronal holeSamples peak of
belt every ~7 hours
© Copyright QinetiQ limited 2006 105th September 2017, SEESAW Conference, NOAA, Boulder CO
SURF Data
• 2005 – 2014:
© Copyright QinetiQ limited 2006 115th September 2017, SEESAW Conference, NOAA, Boulder CO
Most recent SURF data
Jan – Aug 2017
Strong signal in recent weeks
Aug 29th Large Coronal Hole
© Copyright QinetiQ limited 2006 125th September 2017, SEESAW Conference, NOAA, Boulder CO
Existing Environment Models
• Several models describe the Van Allen belts:
• AE8:
• Industry standard for decades
• Static model – no flux variability
• Inadequate for internal charging
• AE9:
• Multiple data sources
• Comprehensive statistics
• Complex (many input parameters & run options)
• FLUMIC:
• Worst-case model for internal charging
• Based primarily on GEO data (not near peak)
• User-friendly but not up-to-date
• MOBE-DIC:
• Based on MEO data
• Extrapolated to other parts of outer belt
• Various others targeted at specific environments/orbits
Vette (1991)
Johnston et al. (2014)
This Talk
© Copyright QinetiQ limited 2006 135th September 2017, SEESAW Conference, NOAA, Boulder CO
FLUMIC
• Empirical model developed specifically for
internal charging (2000)
• Based mainly on data from STRV and
GOES in 1980s and 1990s
• Give ‘worst-case’ 1-day flux envelope as
function of:
− B
− L
− fraction of solar cycle
− fraction of year (seasonal)
• Latest version 3.0 (available on SPENVIS)
• ALE (Anomalously Large Event) version
for ‘worst case’
13
Flux envelope varies
with apparent solar
cycle modulation in
GOES data
© Copyright QinetiQ limited 2006 145th September 2017, SEESAW Conference, NOAA, Boulder CO
FLUMIC – key features
1. Covers inner and outer electron belts2. Flux output depends on date
Seasonal
modulation
Solar cycle
modulation
3. Simple exponential spectrum
4. It’s easy to use!
© Copyright QinetiQ limited 2006 155th September 2017, SEESAW Conference, NOAA, Boulder CO
Model of Outer Belt Electrons for Dielectric Internal Charging (MOBE-DIC)
Instrument response functions used
to derive flux based on assumed
exponential spectrum
0.0E+00
5.0E-16
1.0E-15
1.5E-15
2.0E-15
2.5E-15
3.0E-15
0 2 4 6 8 10
E (MeV)
Pla
te c
urr
en
t (A
/m2)
Top plate
Middle plate
Bottom plate
• New model based on MEO Electron fluxes extracted from SURF data
Create reduced series of average
flux at equatorial peaks (L≈4.7)
𝒇 𝑬 = 𝑨 × 𝒆−
𝑬𝑬𝟎
(Note: harder
spectra in more
extreme events)
Fit baseline spectra at three exceedance
probabilities - 90%, 99% & 100%
These three spectra form the
basis of the MOBE-DIC model
© Copyright QinetiQ limited 2006 165th September 2017, SEESAW Conference, NOAA, Boulder CO
Extrapolating to other L-Shells
• Inclination of Giove-A orbit means higher L shells only encountered at higher latitudes
• Need to renormalise non-equatorial fluxes:
L≈4.7 at equator
Assume Vette function (like AE8 and FLUMIC)
[Scaling is (slightly) L-dependent but not energy-dependent]
Equatorial Flux1. All Flux
2. Fit ‘envelope’ to renormalised data (at each energy)
→ Energy-dependent L-Shell profile
© Copyright QinetiQ limited 2006 175th September 2017, SEESAW Conference, NOAA, Boulder CO
Extrapolating to other L-Shells
• Final (energy-dependent) L-shell profile (3 < L < 8)
FLUMIC function used below
L=4.5 (no Giove data)
Normalised to L=4.7: Normalised to L=6.6:
(NB slightly modified version used for integral flux)
© Copyright QinetiQ limited 2006 185th September 2017, SEESAW Conference, NOAA, Boulder CO
Comparison to GOES data
• Compare model to cumulative probability density functions from data:
>2 MeV flux adjusted to
L=6.6 and for dead-time
effects
(Meredith et al., 2015)
MOBE-DIC prediction for ‘100%’
(worst case) at GEO for >2 MeV
flux is:
2.34 x 105 e/cm2/s/sr
Theoretical upper limit
(Koons et al. 2001)…
2.34 x 105 e/cm2/s/sr !!
Good agreement between MOBE-
DIC and GOES at 99% and 100%
(slightly worse at 90% due to
conservative L-shell envelope)
Unsurprisingly, excellent
consistency at L=4.7 (MEO)
© Copyright QinetiQ limited 2006 195th September 2017, SEESAW Conference, NOAA, Boulder CO
Comparison to FLUMIC
Differential Spectra Integral Spectra
MOBE-DIC gives harder
spectrum at peak of MEO
(close to 100% level at 1 MeV)
Good agreement at GEO
(FLUMIC in between 99% &
100% MOBE-DIC level)
© Copyright QinetiQ limited 2006 205th September 2017, SEESAW Conference, NOAA, Boulder CO
MOBE-DIC: Implementation
• MOBE-DIC model is defined by a set of parameters and simple equations
• At present simple spreadsheet implementation:
• Public version available on request ([email protected])
• To be made available via Spenvis…
Easy to use!
© Copyright QinetiQ limited 2006 215th September 2017, SEESAW Conference, NOAA, Boulder CO
Internal Charging Models
• Various models exist for calculating internal charging (1-D and 3-D)
• For example:
• DICTAT
• NUMIT (1-D & 3-D)
• MCICT
• SPIS-IC
• Based on same basic electrostatic equations:
𝐸 =𝐽𝑑𝜎
. 1 − 𝑒− 𝑡 𝜏𝜏 =
𝜀0𝜀𝑟𝜎
Exponential build-up of
electric field (or surface
voltage), eventually
reaching equilibrium
Ele
ctr
ic F
ield
Time
© Copyright QinetiQ limited 2006 225th September 2017, SEESAW Conference, NOAA, Boulder CO
DICTAT
• 1-D structure
• Electron transport – Weber & Sorensen formulae
• Temperature effects
• Radiation induced and field enhanced
conductivity
• Cable and Flat geometries
• Various grounding arrangements
• Electric field calculated in ten layers
22
Electron
environment
ShieldingMaterials
properties
Radiation
induced
conductivity
Field
enhanced
conductivity
Thermal
effects on
conductivity
Permittivity
Density
Electron
depositionDielectric
geometry
Electric
field
calculation
ESD event ?
Charging
current
Orbit propagator
FLUMIC
Optional
© Copyright QinetiQ limited 2006 235th September 2017, SEESAW Conference, NOAA, Boulder CO
DICTAT
• Structure on SPENVIS:
Spectrum
Material
parameters
E-field as a function of time
(e.g. for varying shielding thickness)
© Copyright QinetiQ limited 2006 245th September 2017, SEESAW Conference, NOAA, Boulder CO
Sensitivity Analysis
Material properties have dominant impact on charging behaviour:
Key conductivity equations:
𝜎𝑅𝐼𝐶 = 𝑘𝑝 . 𝐷 ∆
All very important factors even without environment
variation
(RIC = radiation-induced conductivity)
© Copyright QinetiQ limited 2006 255th September 2017, SEESAW Conference, NOAA, Boulder CO
Real Environment Variability
• Ryden et al. (following similar work by Bodeau) used SURF currents to analyse charging response to
real environment under different conductivity assumptions:
Relaxation time constant:
2 days
Peak E-field = April 2010
Relaxation time constant: 20
days
Peak E-field = December 2006
Timing of peak electric field
varies from material to material !
© Copyright QinetiQ limited 2006 265th September 2017, SEESAW Conference, NOAA, Boulder CO
Experimental Approach
• Internal charging behaviour can be recreated in the lab – e.g. using electron accelerator or, alternatively,
radioactive beta source
• At Surrey University we use strontium-90 in Realistic Electron Environment Facility (REEF):
Vacuum
Chamber
housing ~3
GBq source
0.01
0.1
1
10
0.1000 1.0000 10.0000Energy (MeV)
Cu
rre
nt (p
A/c
m2)
REEF (30mm) REEF (187.5mm)
Average GEO (AE-8min) NASA worst case GEO
Worst case GEO (FLUMIC)
Dynamic range
encompasses
worst case
Sr-90 spectrum
extends up to ~2.2
MeV electrons
© Copyright QinetiQ limited 2006 275th September 2017, SEESAW Conference, NOAA, Boulder CO
REEF Setup
• Intensity varied by raising/lowering source housing:
(Surface potential measured with
non-contact Trek probe)
Long-term measurements of charging response:
1200 hours ≈ 2 months
continuous exposure
© Copyright QinetiQ limited 2006 285th September 2017, SEESAW Conference, NOAA, Boulder CO
Future Plans: EMU & CREDANCE
• Merlin-SURF instrument has been operating successfully for >11.5 years (and still going!)
• Continuous direct measurement of MEO charging currents
• Successor instrument – Environmental Monitoring Unit (EMU) launched on one of FOC
Galileo GNSS satellites in November 2016
• Eight charge-collecting plates
• Wider energy response: 0.1 to >10 MeV
• Data will be analysed as part of ESA GALEM project
• Upgrade to MOBE-DIC planned
• Cosmic Radiation Environment Dosimetry and Charging Experiment (CREDANCE) – to be
launched in 2018 on SpaceX Falcon Heavy Rocket as part of Space Environment Testbed
(SET-1) payload on AFRL DSX Spacecraft
• Merlin-type instrument
• Eccentric orbit covering slot region: 6000 x 12000 km
• SPHERE monitor under development (Surrey Proton, Heavy Ion & Electron Radiation
Monitor) – collaboration between SSC and SSTL
EMU
CREDANCE
© Copyright QinetiQ limited 2006 295th September 2017, SEESAW Conference, NOAA, Boulder CO
Summary
• Internal Charging remains a significant threat to spacecraft operating in the trapped radiation belts
• SURF detector continues to provide direct and uncontaminated measure of in situ charging currents
• FLUMIC and MOBE-DIC models aim to provide user-friendly guide to worst case environment for
charging effects
• DICTAT provides simple 1-D analysis of charging behaviour based on user-supplied material properties
• Lack of accurate knowledge of material parameters is a key uncertainty in modelling internal charging
behaviour & risk assessment of satellite vulnerability
• Laboratory testing can help both by discovery of material properties and realistic measure of charging
behaviour in low intensity environments
• Future instruments will help reduce environment uncertainty
© Copyright QinetiQ limited 2006 305th September 2017, SEESAW Conference, NOAA, Boulder CO
Thank You !
© Copyright QinetiQ limited 2006 31
Spare Slides
31
© Copyright QinetiQ limited 2006 325th September 2017, SEESAW Conference, NOAA, Boulder CO
Electron environment specification
Interested in ‘worst case’ electron
environments (>10 hours)
Models based on measurement over
long periods
• Orbit specific e.g. NASA HDBK spec,
MEO model
• Extrapolated to all regions e.g. FLUMIC,
AE9 (new!)
Note: AE8 and other average models
are not applicable
32
NASA recommended ‘worst case’
geostationary electron flux
spectrum (circles) alongside the
anomalously large event (ALE)
spectrum defined in the FLUMIC3
model (squares).
‘NASA’
FLUMIC
© Copyright QinetiQ limited 2006 335th September 2017, SEESAW Conference, NOAA, Boulder CO
Worst Case Statistics
• Use derived flux time series to create cumulative distribution functions (CDFs) at discrete
energies in the range 0.5 – 3 MeV (peak of instrument response)
𝒇 𝑬 = 𝑨 × 𝒆−
𝑬𝑬𝟎
(Note: harder
spectra in more
extreme events)
Fit baseline spectra at three confidence
levels - 90%, 99% & 100%:Create reduced series of average
flux at equatorial peaks (L≈4.7)CDFs based on equatorial flux:
These three spectra form the
basis of the MOBE-DIC model
© Copyright QinetiQ limited 2006 345th September 2017, SEESAW Conference, NOAA, Boulder CO
Extrapolating to other L-Shells
• Equatorial spectra at L≈4.7 form the basis of the model
• Need to derive profile of L-shell to extrapolate, however…
• L-Shell profile is not stable, e.g.:
SURF on STRV1d (Ryden et al. 2001)
Van Allen Probes, REPT
(Baker et al. 2014)
AE8AE9
FLUMIC
Van Allen Probes, ERM (Maurer et
al. 2013)
© Copyright QinetiQ limited 2006 355th September 2017, SEESAW Conference, NOAA, Boulder CO
Comparison to existing models
• AE9:
• NASA Handbook 4002A