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Planetary and Space Science 58 (2010) 287–299

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The fluxgate magnetometer of the BepiColombo MercuryPlanetary Orbiter

K.-H. Glassmeiera,�, H.-U. Austera, D. Heynera, K. Okrafkaa, C. Carrb, G. Berghoferc,B.J. Andersone, A. Baloghb, W. Baumjohannc, P. Cargillb, U. Christensenf, M. Delvac,

M. Doughertyb, K.-H. Fornac-ona, T.S. Horburyb, E.A. Lucekb, W. Magnesc, M. Mandead,A. Matsuokah, M. Matsushimaj, U. Motschmanni, R. Nakamurac, Y. Naritaa, H. O’Brienb,

I. Richtera, K. Schwingenschuhc, H. Shibuyak, J.A. Slaving, C. Sotinl, B. Stolla,H. Tsunakawaj, S. Vennerstromm, J. Vogtn, T. Zhangc

aInstitut fur Geophysik und extraterrestrische Physik, Technische Universitat Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, GermanybImperial College, London, UK

cSpace Research Institute, Austrian Academy of Sciences, Graz, AustriadGeoForschungsZentrum, Potsdam, Germany

eApplied Physics Laboratory, The Johns Hopkins University, MD, USAfMax Planck Institute for Solar System Research, Germany

gNASA Goddard Space Flight Center, MD, USAhISAS/JAXA, Japan

iInstitut fur Theoretische Physik, Technische Universitat Braunschweig, GermanyjTokyo Institute of Technology, Japan

kKumamoto University, JapanlUniversity of Nantes, France

mDanish Meteorological Institute, DenmarknJacobs University Bremen, Germany

Received 14 March 2008; received in revised form 13 June 2008; accepted 30 June 2008

Available online 25 July 2008

Abstract

The magnetometer (MAG) on the Mercury Planetary Orbiter (MPO) of the joint European–Japanese BepiColombo mission to planet

Mercury is a low-noise, tri-axial, dual-sensor, digital fluxgate instrument with its sensors mounted on a 2.8-m-long boom. The primary

MPO/MAG science objectives are to determine the spatial and temporal structure of the magnetic field in the Hermean system, in

particular the structure and origin of the intrinsic magnetic field of Mercury. MPO/MAG has a dynamic measurement range of �2000nT

with a resolution of 2 pT during operation along the near-polar orbit of the MPO spacecraft around Mercury. MPO/MAG is designed to

provide measurements with rates between 0.5 and 128 vectors/s. In cooperation with its sister magnetometer instrument, MMO/MGF on

board the BepiColombo Mercury Magnetospheric Orbiter (MMO), MPO/MAG will be able to distinguish between temporal and spatial

magnetic field variations in the magnetically closely coupled Hermean system.

r 2008 Elsevier Ltd. All rights reserved.

PACS: 96.30.Dz; 96.12.Hg; 07.55.Ge

Keywords: Mercury; Planetary magnetic field; Magnetosphere; Fluxgate magnetometer

e front matter r 2008 Elsevier Ltd. All rights reserved.

s.2008.06.018

ing author. Tel.: +49531 391 5215.

ess: kh.glassmeier@tu-bs.de (K.-H. Glassmeier).

1. Science objectives

A prominent feature of planet Mercury is its globalmagnetic field. First detected during the Mariner 10 flybys

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in 1974/1975 (Ness et al., 1974) its existence was recentlyconfirmed by the first flyby of the MESSENGER space-craft (e.g. Solomon et al., 2006) at Mercury on 14 January2008. It was found that the intrinsic magnetic field of theinnermost planet in the solar system is almost identical towhat it was 34 years ago (Anderson et al., 2008). Aftercorrecting for the contribution from the solar windinteraction, the mean dipole has the same intensity towithin a few percent, generating an equatorial surface fieldof about 230 nT.

Magnetic fields measured in the proximity of a planethave four major contributions: an external magnetic fieldgenerated by the interaction of the solar wind with theplanet, an induced magnetic field due to temporal changesof the external field in the electrically conducting planetaryinterior, remanent magnetization of material in theplanetary crust, and, as a major contribution, a globalmagnetic field generated by a dynamo process within theplanet. Measuring the magnetic field in Mercury’s environ-ment, separating, and identifying the various contributionsis the major science objective of the Mercury PlanetaryOrbiter (MPO)/magnetometer (MAG) instrument.

1.1. External magnetic fields

Though the strength of the Hermean magnetic field issmall, a well developed magnetosphere has been observed byMariner 10 (Ness et al., 1974). The mean stand-off distanceof its magnetopause is 1.7 Hermean radii ðRMÞ. This impliesthat the Chapman–Ferraro or magnetopause currentsproduce a significant magnetic field in the magnetosphereof about 60nT at the planetary surface (e.g Glassmeier et al.,2007b). Compared to the field of internal origin, about230 nT, this is a contribution which cannot be neglected.At Earth the magnetopause currents contribute an about20nT field in a 31000 nT background field.

Not only the magnetopause currents will contribute tothe total field but other currents are expected to flow, too.Slavin et al. (1997) report about the existence of field-aligned currents in Mercury’s magnetosphere. Thesecurrents need to be closed within the magnetosphericplasma, in the electrically conducting crust or, for example,in a photo-ionization layer close to the surface (e.g.Glassmeier, 1997; Janhunen and Kallio, 2004; Ip andKopp, 2004). Other currents of importance will be themagnetotail current or partial ring currents. In any case,they will have an important impact on any magnetic fieldmeasurement. Furthermore, the magnetospheric system isreacting strongly to the ever changing solar wind withstrong field variations expected within the magnetospherewith the magnetopause from time to time even reaching theplanetary surface (e.g. Siscoe and Christopher, 1975).

1.2. Induced magnetic fields

Electromagnetic induction within Mercury’s interior hasbeen discussed by e.g. Suess and Goldstein (1979), Hood

and Schubert (1979), Grosser et al. (2004) or Glassmeieret al. (2007b). The solar wind driven variability of theHermean magnetospheric current system causes significantinduced magnetic fields of the order of 20 nT at the surface.These fields are induced within the planet and support theinternal field origin in inhibiting the solar wind impinge-ment on the surface. Care is required in separating theinduced-field contribution from the dynamo generatedfield, since improper separation implies that the lattercannot be accurately determined.

1.3. Crustal remanent magnetization

The magnetic fields of the Moon and planet Mars aredue to a remanent magnetization of crustal materials ofthese bodies. Also Earth has a significant crustal magne-tization. Not much is known yet about the crustal field ofMercury. Aharonson et al. (2004) discuss the possibilitythat a strong non-uniform magnetization might even beable to explain the measured Hermean magnetic field.However, any future investigation on the importance ofsuch a field requires more information about rock proper-ties in the crust, its thickness, and the thermal evolution ofthe planet (e.g. Breuer et al., 2007).

1.4. Dynamo generated fields

One of the key equations of magnetohydrodynamicdynamo theory is the so-called induction equation (e.g.Backus et al., 1996; Stevenson, 2003):

q~Bqt¼ r � ~U � ~Bþ ZD~B. (1)

Here ~B and ~U denote the magnetic field and the flowfield, respectively; Z ¼ 1=m0s is the magnetic diffusivity ands the electric conductivity of the medium in which thedynamo is operating. The conversion of kinetic intomagnetic energy is the result of the first term on the rightside. However, not every flow field ~U is suitable to supportthe dynamo process. A convenient way to estimate whethera given physical system can operate a dynamo is themagnetic Reynolds number Rm ¼ jr � ~U � ~Bj=jZD~Bj �vL=Z, where v and L denote the characteristic velocityand spatial scale of the system. For a dynamo to work onerequires Rmb1. If this condition is not fulfilled magneticdiffusion very effectively acts against field amplification(e.g. Busse, 2000; Stevenson, 2003).Taking terrestrial values for the electrical conductivity,

s ¼ 5� 105 S=m and the fluid velocity, U ¼ 10�3 m=s,and a Hermean core radius of 1860 km (e.g. Spohn et al.,2001), gives one a magnetic Reynolds number of the orderof 1200, sufficiently large to expect a dynamo processto operate in the interior of Mercury. However, thisreasoning requires that at least part of the Hermean core ismolten, exhibits a large enough electrical conductivity,and is rigorously convecting with sufficiently largefluid speeds. This later condition implies the existence of

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a super-adiabatic temperature gradient within the Hermeancore, supporting the required convection. This shortdiscussion demonstrates that a couple of requirements arenecessary conditions to allow a planet like Mercury tooperate a dynamo process. Thus, studying a planetarymagnetic field provides deep insights into the spatio-temporal structure of the planet.

However, the measured Hermean surface field strength isabout 30 times smaller than expected for a terrestrial-typedynamo (e.g. Christensen, 2006). Stanley et al. (2005)suggested a thin-shell dynamo to explain this weak field.Using numerical experiments they show that a thin-shelldynamo for Mercury’s field is possible with toroidal fieldsbeing more efficiently produced through differential rota-tion than poloidal fields generated via interaction of theconvection flow with the toroidal fields. As the poloidalfields generated are also of smaller scale, they are spatiallysignificantly attenuated and the remaining surface field isweak.

Another solution for the weak Hermean field problemwas offered by Christensen (2006) who argues that theouter regions of Mercury’s liquid core are stably stratified.This implies a deep seated dynamo generating a strongmagnetic field dominated by rapidly fluctuating small-scalecontributions, attenuated by the stably stratified outer coreregions. At the surface only the weaker dipole andquadrupole contributions can be detected by a spacecraft.Further possibilities to explain the weak Hermean field arealso discussed by Heimpel et al. (2005) and Takahashi andMatsushima (2007).

A novel approach to the Hermean dynamo problem isdiscussed by Glassmeier et al. (2007a). Because theHermean magnetosphere is rather small and the strengthsof any magnetospheric currents are comparable to theirterrestrial values (Connerney and Ness, 1988; Slavin et al.,1997; Glassmeier, 2000; Korth et al., 2004) the magneticeffects of these currents close to the planetary surface andwithin the planetary interior are significant and cannot beneglected. In their model of a feedback a–O dynamo,Glassmeier et al. (2007a) demonstrate that the external fieldof magnetospheric origin may have an influence to thedynamo itself and can explain the weak observed surfacefield.

Yet another possibility is discussed by Stevenson (1987)and Giampieri and Balogh (2002) who consider a thermo-electric dynamo to generate Mercury’s magnetic field. Fora more elaborated discussion of the current knowledge ofthe Hermean dynamo problem see Wicht et al. (2007) andreferences therein.

1.5. Determination of Gauss coefficients

The field generated by either of the above-mentioneddynamo processes is generally of global scale, that isa spherical harmonic expansion of the field at the planetarysurface or in low-altitude orbit would be dominatedby low degree Gauss coefficients. Dipole, quadrupole,

and octupole contributions determine the field’s topology.This is an important means to discriminate betweenthe various field sources mentioned. Any remanentmagnetization of the Hermean crust would most likelycontribute to the higher degree Gauss coefficients. At Earththe crustal field contribution is larger than the dynamo onefor degrees above 13, while at Mars and the Moon thespectrum is dominated by higher degree contributions ofcrustal origin. Low-degree contributions, indicative fordynamo origin, are almost negligible at Mars and Moon(e.g. Purucker, 2008).The importance of proper determination of the Gauss

coefficients is also demonstrated by using the magnetic orMauersberger spectrum Rn to estimate the depth of thedynamo or source region. The Mauersberger spectrum isdefined as

Rnðr; rdynÞ ¼ ðnþ 1Þ=ðrdyn=rÞ2nþ4X1m¼0

fðgmn Þ

2þ ðhm

n Þ2g, (2)

where r and rdyn denote the radius of the reference sphereand the outer radius of the dynamo or source region,respectively; g and h are the Gauss coefficients and n and m

denote degree and order of the spherical harmonicsconsidered. The spectrum is a measure of the mean squaredmagnetic field contributed by the multipoles of degree n inthe spherical harmonic expansion.Postulating the magnetic spectrum at the dynamo depth

enables one to determine the source depth from a measuredmagnetic spectrum at the reference sphere, for example theplanetary surface. With the hypotheses of narrow-scaleflow and a dynamically weak magnetic field near the top ofthe planetary dynamo region Voorhies et al. (2002) andVoorhies (2004) have been able to estimate the depths ofthe terrestrial and Martian core with considerable accuracyusing the following magnetic spectrum:

Rnðrp; rdynÞ /nþ 1=2

nðnþ 1Þ� ðrdyn=rpÞ

2nþ4X1m¼0

fðgmn Þ

2þ ðhm

n Þ2g,

(3)

where rp is the planetary radius. With rp ¼ 2439 km, theHermean planetary radius, and two different values forthe radius of the dynamo region, rdyn ¼ 1869 and 1135 km,the resulting magnetic spectrum is displayed in Fig. 1. Thelarger value corresponds to the Hermean core radiussuggested by Spohn et al. (2001), the smaller one isconsistent with the radius of the unstable region of the corein the dynamo model discussed by Christensen (2006). Thespectrum is rather sensitive to a variation of the sourcedepth, that is the radius of the dynamo region.For comparison the surface Mauersberger spectrum of a

uniform Gauss coefficient distribution at the core radius isalso shown in Fig. 1. It is apparent how important a properdetermination of the magnetic spectrum, especially itsdipole, quadrupole, and octupole coefficients is for ageophysical interpretation of planetary magnetic fieldmeasurements. A detailed determination of the Gauss

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coefficients of the Hermean magnetic field is thus the primeobjective of MPO/MAG.

2. Instrument design

To determine Mercury’s magnetic field to the targetedaccuracy of harmonic coefficients up to order of at least 4,and to separate the external and internal sources of themagnetic field, the magnetic field vector along the MPOorbit has to be measured with an accuracy of 0.1 nT atvariable rates up to 10 vectors/s. Full orbital coverage is

0 5 10 15 20Degree n

100

101

102

103

104

105

106

Rn

(nT2 )

Hermean Magnetic Spectrum

Model Source Depth__________1860 km

-----------------1135 km

Uniform Distribution-..x..-..x..-..x..-

Fig. 1. Modelled magnetic spectra for Mercury illustrating the depen-

dence of the spectrum from the source depth and the Gauss coefficient

distribution at the dynamo region surface.

Nadir pointing direction

Radiator pointi

Fig. 2. Schematic view of the MPO spacecraft with its deployed m

required, including operation in eclipse. The final perfor-mance of the investigation also depends on the achievedsystem-level requirement on magnetic cleanliness at thelocation of the MPO/MAG sensors and the attitudedetermination of the spacecraft with respect to the planet.To fulfill these requirements a robust and space-provenfluxgate magnetometer with two boom-mounted sensors(see Fig. 2), a robust programmable digital fluxgateelectronics, a powerful processing unit, and an experimentinternal power supply unit are selected.This hardware of the instrument is provided as a

collaborative effort between the Principle Investigatorteam at the Institut fur Geophysik und extraterrestrischePhysik of the Technische Universitat Braunschweig(IGEP), the Space Research Institute of the AustrianAcademy of Sciences in Graz (IWF), the Space andAtmospheric Physics Group of the Imperial College inLondon (SPAT), and the Institute of Space and Astro-nautical Sciences of JAXA (ISAS).Two identical magnetometers are used in order to

determine the influence of the spacecraft magnetic fieldon the observations. Due to different distances of thesensors to the spacecraft body, spacecraft fields can beseparated from external fields. The so-called ‘dual-magnet-ometer’ technique (e.g. Neubauer and Schatten, 1974)relaxes the magnetic cleanliness requirements on thespacecraft to a level which is one order of magnitudeabove the required accuracy of the magnetic fieldmeasurement.The fluxgate magnetometer principle has emerged as the

optimum compromise method for magnetic field measure-ment in space, since such instruments are rugged, low-in

ng direction

Radiator panel

MPO/MAG

sensor

agnetometer boom. The spacecraft is always nadir pointing.

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power and mass, and offer high precision (e.g. Zanettiet al., 1994; Acuna, 2002; Balogh et al., 2001; Carr et al.,2005; Anderson et al., 2007; Auster et al., 2007, 2008;Glassmeier et al., 2007c). The basic operating principle ofthe fluxgate magnetometer is well known and documented(Acuna, 2002; Auster, 2008). A soft-magnetic core, usuallytoroidal in shape, is wound with a coil and driven intosaturation with an excitation AC. The external magneticfield (which is to be measured) distorts the symmetry of themagnetic flux in the core, which generates a signal at evenharmonics of the excitation frequency, the amplitude ofwhich is proportional to the external magnetic field. Thissignal is picked-up by a sense coil wound around the core.By feeding back a current into the sense coil proportionalto the measured signal, the ambient field is backed-off andthe sensor operates in null-mode, thereby improvinglinearity.

The sensor design, using two ring cores for threemeasurement axes, is shown in Fig 3. Typically, the secondharmonic of the drive frequency is notch-filtered, amplified,synchronously detected, integrated, and used to drive thefeedback current. It is this current which is proportional tothe ambient magnetic field. The output of a traditionalanalog magnetometer is a voltage which may be digitizedby an analog-to-digital (AD) converter. In such a design,the signal processing to extract the second-harmonic signalis performed by an analog circuit. This is an efficient andwell-developed method, however, the filtering and syn-chronous detection stages all show temperature dependen-cies. By replacing the analog processing with a digitalsystem, this signal processing is performed as a softwarefunction in the digital domain. As well as eliminating someof the temperature effects, this provides an inherentflexibility in the design, since the processing parameterscan be modified by software.

The basic design of the digital technique magnetometeris presented in Fig. 4, which also shows a comparisonwith the traditional analog design. The fluxgate sensoroutput signal is digitized four times per excitation period

Fig. 3. The MPO/MA

(about 10 kHz) by a 14 bit AD converter. Subsequently, thefiltering, synchronous detection, and integration areperformed by software, resulting in a calculated value forthe feedback current which is used to null the ambient field.The feedback current is controlled by a 16 bit digital-to-analog (DA) converter. The field proportional value(21 bit, resolution 2 pT), which is transmitted to theinstrument controller, consist of the sum of the weightedAD- and DA-converter values.Digital fluxgate magnetometers as described are success-

fully operated on the Rosetta Lander (Auster et al., 2007),the Venus Express spacecraft (Zhang et al., 2006), and thefive Themis probes (Auster et al., 2008). These develop-ments have validated the basic principles of the technique,and demonstrated the enhanced temperature stability.In addition to the sensors front end electronics, the

instrument includes an Instrument Controller Unit (ICU),and a power supply for provision of secondary voltages tothe sensor electronics, the ICU and sensor heaters.Functionally, the MPO/MAG instrument is largely auton-omous in operation, requiring a minimum of commandingonly for selection from a set of science operations modesand corresponding telemetry bit-rates.The instrument controller will be based on the ESA-

provided Remote Terminal Controller (RTC) Application-Specific Integrated Circuit (ASIC). The software is writtenin C language. A boot-up routine will execute out of aProgrammable Read Only Memory (PROM). Once theinstrument health has been checked out the code image willbe loaded into the Random Access Memory (RAM),typically on telecommand, and the instrument will re-bootout of the RAM. The PROM can subsequently be disabledto save power. As an option, the RAM code can be loadedeither from the Electrically Erasable Programmable Read-Only Memory (EEPROM) in the instrument or from theHighly Integrated Control and Data System (HICDS)software. Again, this is selectable by telecommand. Onceoperating, the instrument software executes a control loopwhich continuously polls the various data sources and data

G sensor design.

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Sensor Amplifier

F f/2

Feedback

Instrument Controller

Analogue to Digital Converter

Analogue FluxgateMagnetometer Electronics

Sensor Amplifier

Feedback

Instrument Controller

Analogue to Digital

Converter

Digital Fluxgate Magnetometer Electronics

Digital to Analogue Converter

Digital Controller (FPGA)

Filter IntegratorSynchronous

Detector

Clock f÷2

Drive

Drive

Fig. 4. The MPO/MAG block diagram.

Table 1

MAG/MPO resource requirements

Mass 2530 g total

Sensor (incl. thermal hardware) 2� 270 g

Harness 720 g

Electronics 1270 g

Dimensions

Sensor 82:4� 82:4� 122:7mm

E-box 162� 169� 96:6mm

Power consumption 4.6W

Data interface to spacecraft Spacewire

Sample rate 0.5–64 vectors/s

(128 vectors/s in burst mode)

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299292

sinks and implements all necessary control, monitoring,and data processing functions in a time-multiplexedmanner. There is no need for any real-time operatingsystem within the ICU, since the repetitive and simplenature of the tasks allows for the maximum load to becalculated and hence the processor performance to bespecified.

Clearly, there is a strong dependence between hardwarebuffering of data, data production rates and processorperformance. A trade-off study has been performed todetermine the optimum design. The principal operatingtasks of the software are summarized below:

Data rate to HICDS Max: 12 700bit/s

Min: 20 bit/s

Typical: 110 bit/s

Data volume/orbit 3Mbit

� Sensor electronics communication: Data reception,sensor electronics setup, sensor temperature sampling. � Inflight calibration: Six calibration modes for instru-

ment checkout and calibration purposes are implemen-ted.

� Spacecraft communication via Spacewire interface:

Telecommand reception, decoding and acknowledging,science and housekeeping data transmission, timesynchronization, exception handling, software updateand memory management, Spacewire interface servi-cing.

� Instrument internal tasks: Data processing and filtering,

data compression, data packet generation, packet timestamping, packet buffering, event generation.

� Sensor heating control. � System control: Clock control for sensor electronics and

instrument controller, power management (powerdown, power save modes), functional monitoring,system initialization.

The instrument power supply unit is a non-redundant unitwhich is located on a single electronics card at the bottomof the box. It provides the six secondary voltages requiredby the MAG instrument.MPO/MAG resource requirements as well as its main

instrument parameters are given in Tables 1 and 2,respectively.

3. Test and calibration

Testing and calibration of the MPO/MAG instrumentwill be done by procedure developed for missions likeROSETTA, Double Star, VenusExpress, or THEMIS(Glassmeier et al., 2007c; Auster et al., 2007, 2008; Carret al., 2005; Zhang et al., 2006). A variety of test facilities

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Table 2

MAG/MPO instrument specification

Type of sensor Vector compensated fluxgate ring core

sensor with digital signal processing

Heritage Rosetta, Venus Express, Themis

Range �2000nT dynamic

�5000nT compensation

Resolution 2 pT

Noise 10 pT=ffiffiffiffiffiffiffiHzp

at 1Hz

Temperature range/calibrated

Sensor �100 to 200 �C

Electronics �55 to 80 �C

Offset stability

vs. time o0:5 nT=100hvs. sensor temperature o50 pT=�Cvs. electronics temperature o50 pT=�C

Axis alignment

Mechanical tolerance o1�

Knowledge of axes direction o0:1�

Stability of axes direction o0:1�

0.02.04.06.08.0

10.012.014.016.018.0

pT/S

qrt (

Hz)

XYZ

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

°C

nT

XYZ

-60 -10 -40 -90 -140 -190

-60 -10 -40 -90 -140 -190

Fig. 5. Noise performance of the MPO/MAG prototype sensor in the

frequency range 1–2Hz (top) and sensor offset (bottom) as a function of

temperature. X , Y , and Z denote the three components in a sensor related

coordinated system.

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299 293

are available in Germany and Austria developed for theseformer missions. The facilities in MAGNETSRODE, closeto Braunschweig and the facility in Graz are equipped withchambers in which the sensor temperature can be activelycontrolled. For purposes of the MPO/MAG these facilitiesare reworked to increase its temperature range up to200 1C. In MAGNETSRODE (Glassmeier et al., 2007c)scale values and axis alignment can be calibrated, while inthe Graz facilities offset stability and noise can be checked.Other facilities in Berlin, Braunschweig and Jeserigerhuttenare used to check the dependency of instrument parameteron the electronics temperature, to determine the magneticaxis absolutely versus a mechanical reference system, andto perform measurements of the Earth magnetic fieldvariations in parallel to a standard instrument (see Austeret al., 2007 for further details). First tests have been donewith an MPO/MAG sensor prototype in the Graz facilities.Noise performance and offsets were measured (Fig. 5). Theoffsets changes are less than �1 nT over the full tempera-ture range tested (�60� ! þ200 �C). The noise level,measured between 1 and 2Hz decreases from 4210 pT=ffiffiffiffiffiffiffiHzp

at room temperatures to 328 pT=ffiffiffiffiffiffiffiHzp

at theexpected maximum working temperature of þ180 �C beingin orbit around Mercury.

4. Thermal concept

Prime technical drivers for the BepiColombo MPO/MAG instrument are extreme and fast changing environ-mental conditions, especially the large solar irradiation.This irradiation depends on Mercury’s distance to the Sun(0.3AU at perihelion, 0.47AU at aphelion), the position ofthe MPO spacecraft on its elliptical near-polar orbitaround Mercury (inclination: 90� � 2�, pericenter height400� 20 km, apocenter height 1508� 20 km), the orienta-tion of the nadir pointing spacecraft, and the orientation ofthe boom-mounted sensors with respect to the Sun. To

limit the thermal loads the BepiColombo mission isdesigned to have its apocenter at the sub solar pointduring Mercury’s perihelion and its pericenter at thesubsolar point during aphelion.At perihelion MPO/MAG sensors are exposed to solar

irradiation of about 14 500W=m2 and infrared irradiationfrom the hot surface of Mercury of about 4600W=m2 (Fig.6). In contrast, during eclipse there is only a total infraredirradiation from Mercury of 6W=m2. Eclipse duration is42min maximum at aphelion, the orbital period being2.3 h. These conditions represent a major thermal stress tothe magnetometer sensor. The worst hot case expected isfor orbital phases of 45� and 315�, where irradiation fromthe Sun with 13 100W=m2 is still strong and the irradiatedarea of the sensor maximizes.This thermal situation imposes two main constraints to

the MPO/MAG sensors. First, the total temperature at thesensors should not exceed 200 �C because all materials andmanufacturing processes are qualified for this uppertemperature; second, a temperature variability of less than0:5 �C=min is required in order to guarantee the requestedaccuracy.To meet these constraints the thermal protection concept

consists of a thermal shield to protect the sensor fromsolar irradiation and an optimally thermally isolatedsensor inside the shield (see Fig. 7). Because there aretwo sources of irradiation, the sensors are mounted on theboom in such a way that the base plates are mounted on

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225° 270°

315°

90°

45°

180°

0.4667 AU6280 W/m2

0.3237 AU13100 W/m2

0.3075 AU14489 W/m2

Fig. 6. Schematic view unto the ecliptic plane with the orbit of Mercury around the Sun as well as the BepiColombo Mercury Planetary Orbiter around

Mercury. BepiColombo’s polar orbit is indicated by the black line connecting the blue circles symbolizing the spacecraft in its periherm and apoherm

positions. Distance to the Sun and the expected total solar irradiation are given.

Base plate

Stand-off

OSR

Sensorhousing Connectors

CFRPstructure

Fig. 7. MPO/MAG sensor with thermal shield.

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299294

the anti-nadir side of the boom (Fig. 2). This mountingminimizes the infrared irradiation from the planet. Such amounting allows to optimize the thermal shield for themajor remaining radiation source, solar irradiation. Thedesign of the thermal shield requires a low absorptance(ratio of absorbed radiation to the impinging irradiation)in the solar spectrum, that is ao0:3, and a high thermalemittance (ratio of emissive power to the power of a blackbody at the same temperature) in the infrared, that is�40:8. The shield is realized by mounting six optical solarreflectors (OSR) on each side of the sensor (Fig. 7).The end-of-lifetime (EOL) values of the solar absorptanceand thermal emittance of the OSRs used are a ¼ 0:2and � ¼ 0:85, respectively. The OSRs are glued withELASTOSILs S692 on the support structure madeof high temperature carbon fiber reinforced plastic(HT-CFRP). Between the OSRs and the CFRP structurean aluminium foil is placed to cover the gaps between themirrors and to support the heat transfer between theilluminated and the non-illuminated sides.

Neglecting any conductive transfer to the interface,thermal modelling shows that this design keeps thetemperature low enough. Assuming a steady state behaviorat the extreme positions the sensor is still able to survive.At perihelion we expect a temperature of 192 1C, andat aphelion of 121 1C. Passing through orbital phases of45� and 315�, the positions of maximum irradiation (seeFig. 6), the temperature is expected to increase up to202 1C.

To meet the second constraint, minimized heat exchangebetween sensor and environment, three parameters need tobe minimized: heat radiation from the sensor vs. the shield,heat conduction via the stand-off to the base plate, andheat conduction through the electrical harness. To reducethe radiative heat exchange to the shield the surface of the

sensor is furnished with a low-� material. In addition, ahigh temperature multilayer insulation material, a 15-layerHT-MLI, is placed between the inner side of the CFRPstructure and the sensor. The conductive heat flow to thebase plate is minimized by a 50mm polyetheretherketon(PEEK) stand-off (Fig. 7). Finally, the sensor harness(AWG32) is routed through the stand-off and ends atconnector pins at an outer ring. Here the sensor can be

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Modelled Sensor Temperature 10

20

30

40

50

pera

ture

[°C

]

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299 295

connected to the boom harness. Through this design wehave a total harness length of about 150mm betweensensor and boom interface.

To check the applicability of the outlined thermalharness design extensive thermal modelling is done. Weuse the thermal analysis software ThermXL of the AlstomPower Technology Center for steady state and transientanalysis. This software package uses a lumped parametermodel containing thermal inertia, inner power dissipation,and the distribution through radiative and conductive linksbetween the defined nodes. The heat loss from the sensor tothe thermal shield and the base plate has been calculatedfor two different temperatures differences (Table 3). Inboth cases the common temperature of base plate andthermal shield is set to 0 1C; the two sensor temperaturesassumed are 10 and 150 1C. Two temperature differencesare discussed here because losses due to thermal conduc-tion are proportional to the temperature differenceT sensor � T shield, where T sensor and T shield are the sensorand shield temperatures, respectively, while losses due toradiation proportional to T4

sensor � T4shield due to the

Stefan–Boltzmann law. Compared to the heat losses givenin Table 3 the heat production by the sensor itself is of theorder 30mW, leading to a temperature increase of 10220�

when switching-on the instrument.For both temperature differences the conductive heat

loss is larger than the radiated one. The sensor harness isstill the main path for heat transfer. This has designconsequences because we aim at decoupling the sensortemperature from the boom temperature and like to haveas stable as possible thermal conditions for the sensors.Heat exchange via radiation is easily reduced by the use oflow �-materials, especially MLI. It is the conductive heatloss dominating the thermal behavior. It cannot be avoidedfor mechanical and electrical reasons. It just can beminimized, using highly insulating materials for themechanical accommodation of the sensor, and long andthin cables for the electrical connections, causing large heatabsorption and little heat conduction, respectively.

Above considerations show that the base plate tempera-ture mainly defines the equilibrium temperature of thesensor. Using the thermal modelling software and assum-ing a constant base plate temperature of 150 1C alongMercury’s orbit, we calculate a sensor temperature of164 1C at perihelion, 125 1C at aphelion, and at thepositions of maximum irradiation 172 1C. For terrestrialconditions and a base plate temperature of 0 1C the sensoris expected to cool down to about �40 �C.

Table 3

Heat loss for two different temperature differences

0� ! 10� 0� ! 150�

Conduction to interface

PEEK stand-off 0.005W 0.075W

Sensor harness 0.015W 0.225W

Radiation to thermal shield 0.003W 0.096W

During an orbital period of 2.3 h the MPO/MAG sensorwill be exposed to fast changes in its irradiation environ-ment. To estimate the thermal behavior of the sensorwithin this changing irradiation environment, a transientanalysis was performed. This is in particular of importancefor the eclipse phase. For the model studies a sensortemperature of 164 1C (the maximum temperature expectedduring perihelion passage) and a base plate (or boom)temperature of 150 1C were assumed for the pre-eclipsephase. During eclipse solar irradiation is switch-off andonly infrared radiation from the planet is taken intoaccount. We furthermore assume a base plate temperatureof 0 1C during eclipse as a worst case scenario. Theresulting drop of the sensor temperature was modelled at0.42 1C/min, which is below the design goal of 0.5 1C/min,indicating the suitability of the selected design.Our thermal modelling results, however, depend criti-

cally on the thermal parameters of the materials used. Tocheck the validity of these parameters the thermal designwas experimentally checked at the 10 solar unit vacuumchamber of the Institute of Space and AstronauticalScience (ISAS) of the Japanese Space Agency JAXA. Thesensor and its thermal shield were prepared with 9 PT1000thermal probes for exact measurement of thermal behaviorof components and parts.As an example Fig. 8 displays results for the following

test setup: the base plate is strongly connected to thethermally control test chamber interface, whose tempera-ture is rapidly dropping from 85 to �100 �C during the testwithin 30min, solar irradiation is switched-off, standardcopper wiring is used, and the sensor temperature ismeasured.For comparison, this experimental setup was also

modelled with both, copper and brass wiring (an attractivealternative to reduce the conductive heat flux) assumed.Evidently our experimental results agree well with themodelled results. The measured temperature drop rate of0.59 1C/min stays close to the design goal of 0.5 1C/min.

brass wire copper wire

Measured Sensor Temperature copper wire

Time [h]

-20

-10

0Tem

0 0.2 0.4 0.8 1 1.2 1.4 1.6 1.8 20.6

Fig. 8. Measured and modelled sensor temperatures for the thermal

design used for the MPO/MAG experiment.

ARTICLE IN PRESSK.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299296

5. Comments on magnetic field analysis

Proper calculation of spherical harmonic expansioncoefficients requires the separation of the actually mea-sured field into its external and internal contributions(Giampieri and Balogh, 2001; Korth et al., 2004; Scuffhamet al., 2006). The external contribution is of particularimportance in the Hermean system as discussed above. Theclassical procedure to analyze a magnetic field of planetaryorigin involves the determination of its multipole spectrumin terms of a spherical harmonic expansion of a scalarmagnetic potential C. This potential is related to themagnetic flux density ~B via

~B ¼ �rC. (4)

The scalar magnetic potential has contributions fromexternal and internal sources with respect to a referencesurface, usually assumed to be the planetary surface:

C ¼ Cint þCext. (5)

The spherical harmonic expansion then reads

Cint ¼ RM

Xnmax

n¼1

RM

r

� �nþ1Xn

m¼0

½gmn;i cosðmjÞ þ hm

n;i sinðmjÞPmn ,

(6)

Cext ¼ RM

Xnmax

n¼1

r

RM

� �nXn

m¼0

½gmn;e cosðmjÞ þ hm

n;e sinðmjÞPmn .

(7)

Here r; y and j denote the radial distance from the centerof the planet, polar distance, and azimuth; Pm

n denotes asemi-normalized (Schmidt) associated Legendre polyno-mial of the argument cosðyÞ, and subscripts i and e denoteGauss coefficients of the internal and external contribu-tions. Any measurement of the magnetic vector field ~B on areference sphere allows a separation into external andinternal contributions using a proper combinationof horizontal and vertical field components (e.g. Backuset al., 1996).

However, using a scalar magnetic potential to describethe magnetic field implies that the measurements are takenin a current-free environment as can be seen from thefollowing short discussion. The Helmholtz theorem statesthat any sufficiently smooth, rapidly decaying vectorfield can be decomposed into irrotational (curl-free)and solenoidal (divergence-free) component vector fields(e.g. Duschek and Hochrainer, 1961). In general themagnetic induction should be decomposed into

~B ¼ �rCþ r � ~A, (8)

where ~A is the magnetic vector potential. Taking the curl ofEq. (8) and using Ampere’s law gives one

r � ~B ¼ r � r � ~A ¼ m0~j, (9)

where the displacement current is disregarded; ~j and m0denote the local electric current density and permeability of

free space, respectively. Only if~j 0 the vector potential ~Amay be assumed to vanish and therewith ~B remainscompletely defined by its scalar potential.From single-point measurements in space it is impossible

to calculate the local vorticity of the magnetic induction inorder to determine the local current density. Withoutknowledge of local currents it is impossible to distinguishplanetary from magnetospheric sources. At the Earthsurface, for example, one can neglect any electric currentsin the atmosphere. But measurements in space need toconsider magnetic field contributions due to local electriccurrents. In the terrestrial case extensive modelling ofionospheric and magnetospheric current systems is done toremove corresponding field contributions from measuredfields (e.g. Lesur et al., 2008).At Mercury not much is known about these external

current systems. First attempts to model them aredescribed by Connerney and Ness (1988), Korth et al.(2004), or Scuffham and Balogh (2006). Conditions atMercury are more difficult than in the terrestrial case.First, solar wind driven variations are severe and cause arapidly varying magnetospheric environment probably notaccessible to any modelling effort. Second, the size of themagnetosphere is comparable to the thickness of theChapman–Ferraro current layer in the magnetopause.Studies of the terrestrial magnetopause indicate a bound-ary thickness of a few hundred kilometers (e.g. Phan andPaschmann, 1996; Paschmann et al., 2005), that is thecurrent layer can be considered as a thin-current sheet. TheHermean magnetopause can be assumed to be of similarwidth as demonstrated by Russell and Walker (1985). Thisimplies that the current carrying boundary layer fills aconsiderable volume of the small Hermean magnetosphericcavity. This, in turn, means that most of the planned MPO/MAG observations are done in a non-current-free envir-onment, which puts into question usage of the classicalmagnetic field description by a scalar magnetic potential.the r � r � ~A contribution in Eq. (9) needs to be takeninto account.To provide an estimate of the errors introduced by

violating the current-free constraint, we use a simplemodel of the magnetosphere and the Chapman–Ferrarocurrent system, displayed in Fig. 9. The magnetosphereis approximated as a sphere. The Chapman–Ferrarocurrents are circular sheet currents on this sphere(Fig. 9a). In order to create a current filled space thecurrent density is radially extended with exponentiallydecreasing intensity towards the planet (Fig. 9c); variationwith polar distance is sinusoidal (Fig. 9b). These currentsgenerate a magnetic field which can be calculatedanalytically using a decomposition into poloidal andtoroidal field modes (e.g. Backus et al., 1996; Engels andOlsen, 1998). The internal magnetic field is assumed to bethat of an axisymmetric dipole with g01 � �230 nT. Usingthe condition of a closed magnetosphere, that is assumingthat the normal magnetic field component at the magne-topause vanishes, parameters of the Chapman–Ferraro

ARTICLE IN PRESS

=

Fig. 9. Sketch of a simple model magnetosphere. r is the radial distance from the center of the planet, RM is one planetary radius and RMP is the

magnetopause distance. For further explanation see the text.

Fig. 10. Comparison of the observed dipole field coefficient (g01observed) and

the true internal (g01true) moment determined along circular orbits with

varying orbital radii.

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299 297

current system are determined. The magnetopause islocated at 1:7RM, with a thickness of 500 km assumed,which is also the spatial decay length of the currentdistribution towards the planet; the exponential decay scaletowards the magnetosheath is set to 50 km. The maximumcurrent density is jf;max ¼ 2:8� 10�7 A=m2.

In Fig. 10 we compare the results of the sphericalharmonic analysis of modelled magnetic field data taken onpolar circular orbits around Mercury with the radii varyingfrom the planetary surface up to the magnetopausedistance of 1:7RM. The resulting dipole coefficient g0

1

strongly depends on the radius. For measurements takenclose to the planet the coefficient is very close to that onetheoretically expected, that is the field is dominated by theinternal source. However, for orbits closer to the magne-topause the field is dominated by magnetopause currents.These currents generate a magnetic field which strengthensand weakens the primary field within and outside themagnetospheric cavity, respectively. In the terrestrial casethe magnetopause currents are assumed to represent asheet current system. Only when crossing the magneto-pause the magnetic field suddenly exhibits a jump from itsmagnetospheric to its near-zero value on the solar windside. At Mercury we expect this transit to be smoother withthe internal field being more and more compensated whenapproaching the magnetopause. Thus, the dipole momentderived from our simple model is approaching zero andproper determination of g0

1 fails.As a sample case we will briefly present this effect on the

data analysis of magnetic field measurements taken along asingle elliptical and polar MPO orbit (Fig. 11). The trueinternal dipole coefficient used is g0

1 ¼ �230 nT. Thetheoretically expected field change clearly deviates fromthe modelled one the further the spacecraft approaches themagnetopause. Determination of g0

1 using data from allorbital positions gives an internal coefficient ofg01 ¼ �199:6 nT. This deviation clearly shows the shielding

effect of the radially distributed magnetopause currentsand the violation of the current-free condition.We conclude that local currents along the spacecraft

orbit have a significant influence on the magnetic fieldrepresented in terms of spherical harmonics. This requires acareful removal of any external field contribution. Severalsteps can be taken to achieve this goal. Data taken overmany orbits and covering almost the same longitude can bestacked and averaged to remove temporal variations of theexternal contributions. Further information on thesetemporal variations and the actual state of the Hermeanmagnetosphere will be provided by the magnetometer

ARTICLE IN PRESS

Fig. 11. Comparison of magnetic field changes along an elliptical polar orbit in the simple model Hermean magnetosphere.

K.-H. Glassmeier et al. / Planetary and Space Science 58 (2010) 287–299298

experiment on the BepiColombo Mercury MagnetosphericOrbiter, MMO/MGF (Baumjohann et al., 2008).Both, MPO/MAG and MMO/MGF will provide updatesof Mercury’s magnetospheric magnetic field models (e.g.Korth et al., 2004; Scuffham and Balogh, 2006) andimprove the reduction task. Finally, since the classicalspherical harmonic analysis is susceptible to local currentsmore robust analysis methods need to be investigatedtaking into account the r � r � ~A term. Work of Backuset al. (1996) applying a Mie decomposition to vectormagnetic field data and Mayer and Maier (2006) using avector wavelet approach are promising in this field ofstudy.

6. Summary and conclusions

Magnetic field measurements at Mercury will providesignificant insight into the structure and dynamics of thisplanet. The fluxgate magnetometer system onboard theBepiColombo Mercury Planetary Orbiter is designed toprovide the necessary high-precision measurements.As compared to other planetary missions a major designdriver for MPO/MAG are the harsh environmentalconditions close to the Sun. The thermal concept outlinedwill allow to meet the scientific requirements. Analysis andinterpretation of the measurements requires additionalefforts to separate the internal field from its externalcontributions in the highly variable Hermean magneto-spheric system. Extensive modelling of the magnetosphere,development of new analysis tools as well as intensivecooperation with MPO/MAGs sister experiment, theMMO/MGF fluxgate instrument onboard the BepiColom-bo Mercury Magnetospheric Orbiter (Baumjohann et al.,2008), will provide the means to achieve the scientific goalof the experiment.

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