Compatibility of AC and DC
magnetic field measurements
on Solar Orbiter and on Solar
Probe Plus: lessons learned
T. Dudok de Wit1), G. Jannet1), S. Harris2), U. Auster3), S. Bale2),
F. Berthet1), P. Brown4), J. Connerney5), P. Fergeau1), K. Goetz6),
S. Harvey2), T. Horbury4), V. Krasnoselskikh1), R. Kroth7),
H. Lewtas4), P. Martin1), R. McDowall5), H. O’Brien4), D. Sheppard5)
1)LPC2E,
Orléans, 2)SSL, Berkeley, 3)TU Braunschweig, 4)Imperial College,
London, 5)NASA/GSFC, 6)Univ. of Minnesota, 7)Magson GmbH, Berlin
1. Why is magnetic compatibility an issue ?
Magnetic sensors (AC and DC) are extremely sensitive to currents circulating in
and around the spacecraft. Because of these EMC/EMI issues, they need to be
accomodated on a boom, several meters away from the spacecraft body.
Both instruments also interfere with each other, but often have to share the same
boom (Solar Orbiter, Solar Probe Plus, JUICE ?, ...) What minimal separation
do we need to meet the science objectives ?
AC magnetometer (search-­‐coil)
~1m
Location of
magnetometers on
Solar Orbiter
DC magnetometer (fluxgate), outboard
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2. Compatibility issues with Solar Orbiter
Solar Orbiter has two types of magnetic sensors
MAG — a pair of DC (fluxgate) sensors from Imperial College, London
SCM — an AC (search-coil) sensor from LPC2E, Orléans.
Both instruments presently are only 1 m apart. Is a longer boom needed ?
What should we do to reduce the noise level ?
To answer these questions we carried out interference tests between the AC
sensors for Solar Orbiter & Solar Probe Plus (from LPC2E) and several DC
sensors for:
•
•
•
Solar Orbiter (from Imperial College)
Solar Probe Plus (from GSFC) not detailed here
Themis & BepiColombo (from TU Braunschweig) not detailed here
3
3. Test procedure
Five major concerns are
Impact of MAG driver (tens of kHz) on SCM
Impact of MAG harness on SCM
Impact of MAG heater (powered at several tens of kHz) on SCM
Impact of SCM ferrite on MAG magnetic budget
Impact of boom drum on SCM (for Solar Orbiter)
Example with MAG sensor
for Solar Orbiter
Tests were carried out in various configurations
variable sensor-to-sensor distance (30 cm to 2 m)
SCM antenna
SCM preamp.
Y
variable sensor orientation
X
variable sensor to harness distance (1 cm to 10 cm)
MAG sensor
4
4. Test facility & setup
All interference tests were carried
out in a magnetically shielded
chamber at the Chambon-la-Forêt
(France) facility.
test setup
Power
Supply
MAG Sensor w/
Integral Heater
+15?
Breakout Connector
GSEOS & IDL
Router
MAG - Harness
Length - TBD
MAG Bd
Laptop
Ethernet
Connect to
Instrument (P8)
MISG
Power (P1)
Ground Plane
Power
Adapter
Wall
Outlet
Signal
Generator
Dual Channel
Preamp
Shielded Twisted Pair
Spectrum
Analyzer
Laptop
SCM – Coax Cable
SCM
Antenna
Test Heater
Battery
Magnetically Shielded Chamber
5
5. Instrument characteristics
AC search-coil magnetometer (SCM)
Triaxial sensor that measures 3 components of the
magnetic field from 10 Hz to 20 kHz, and one
component from 1 kHz to 1 MHz.
Preamplifier and heater are built into the sensor
foot
MAG test model
for Solar Probe
Plus (GSFC)
SCM prototype for
Solar Orbiter (single
antenna shown)
DC fluxgate magnetometer (MAG)
Triaxial instrument that measures 3
components of the magnetic field from DC
to 128 Hz (or more).
Includes driver, preamplifier and heater.
MAG prototype for
Solar Orbiter
(Imperial College)
Analogue (SolarOrbiter, Solar Probe Plus) or
digital (BepiColombo, Themis)
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6. Science requirements
Affordable noise levels are driven by the science requirements.
These are determined from typical solar wind turbulence levels from 1 to
0.28 AU (from Helios, Cluster, Messenger, ...)
3
10
2
10
amplitude [nT / Hz1/2]
at 0.28 AU
Expected level of
solar wind
turbulence at 0.28
AU
Shocks
1
10
0
10
−1
10
−2
Whistlers
10
−3
Type II/III
radio
bursts
10
−4
10
−5
10
−6
10
−2
10
−1
10
0
10
1
10
2
3
4
10
10
10
frequency [Hz]
5
10
6
10
7
10
7
7. Sensor-to-sensor interference
The main sources of interference are the MAG drive frequency
(15.3 kHz for Solar Orbiter) and heater frequency (131.2 kHz)
noise level [nT / Hz1/2]
Drive
fundamental
heater
fundamental
Drive 9th and
th
11th harmonics
Drive 7
harmonics
heater
3rd harmonic
Drive harmonics
Spectral response of
SCM MF sensor with
nearby MAG.
frequency [Hz]
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8. Effect of MAG driver
Test 1 : effect of MAG driver on SCM sensor
(driver frequency: 15.3 kHz)
Amplitude decays approximately as (separation)-3, as expected
Worst orientation is Y, best orientation is Z
For a noise floor from science requirement (3x10-5 nT/Hz1/2 + 20dB margin)
separation must be > 1 m
−1
noise level measured with
MF sensor (10 kHz-1 MHz)
amplitude of peak of driver
nT / Hz1/2
10
x orientation
y orientation
z orientation
noise floor
−2
10
−3
10
−4
10
0
0.5
1
separation [m]
1.5
2
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9. Effect of MAG heater
Test 2 : effect of MAG heater on SCM sensor
(driver frequency: 65 kHz, 100% pulse with modulation)
Amplitude decays approximately as (separation)-3, as expected
Best orientation is X and Y
For a noise floor from science requirement (3x10-5 nT/Hz1/2 + 20dB margin)
separation must be > 0.6 m
−1
noise level measured with
MF sensor (10 kHz-1 MHz)
amplitude of peak of heater
nT / Hz1/2
10
x orientation
y orientation
z orientation
noise floor
−2
10
−3
10
−4
10
0
0.5
1
separation [m]
1.5
2
10
10. Effect of MAG cable
Test 3 : effect of MAG cable proximity on SCM sensor
(main effect comes from heater frequency @ 65 kHz)
Amplitude decays approximately as (cable separation)-3, as expected
Best orientation is X and Y
For a noise floor from science requirement
cable separation must be > 10 cm
−1
drum
SCM
to be avoided !
drum with cable loop
nearby SCM on Solar
Orbiter
amplitude of peak
nT / Hz1/2
noise level measured with
MF sensor (10 kHz-1 MHz)
10
x orientation
z orientation
x (no cable)
z (no cable)
−2
10
−3
10
−4
10
5
10
15
cable separation [cm]
20
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11. Conclusions
Separation between MAG and SCM must be ≥ 1 m in order to meet
science requirements, both on Solar Orbiter and on Solar Probe Plus. ***
This is crucial ! ***
Separation between SCM and harness should be ≥ 15 cm but
effects can be alleviated by proper cabling (twisted cables, careful
grounding)
Same conclusions apply for all MAG models (digital / analogue), with
the lowest interference noise levels for the Themis model and the highest
for the Solar Probe Plus model.
in addition
SCM has no significant impact on MAG for separation > 50 cm.
no measurements are possible without an electromagnetically clean satellite !
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