ROMAP: ROSETTA MAGNETOMETER AND PLASMA MONITOR

H.U. AUSTER1,∗, I. APATHY2, G. BERGHOFER3, A. REMIZOV4,7, R. ROLL4,K.H. FORNACON1, K.H. GLASSMEIER1, G. HAERENDEL6, I. HEJJA2, E. KUHRT9,

W. MAGNES3, D. MOEHLMANN9, U. MOTSCHMANN5, I. RICHTER1,H. ROSENBAUER4, C.T. RUSSELL8, J. RUSTENBACH4,6,

K. SAUER4, K. SCHWINGENSCHUH3, I. SZEMEREY4 and R. WAESCH9

1Institut fur Geophysik und Extraterrestrische Physik der Technischen Universitat Braunschweig,Mendelssohnstrasse 3, D-38106 Braunschweig, Germany

2KFKI Budapest, AEKI, P.O. Box 49, H-1525 Budapest, Hungary3Space Research Institute Graz, Schmiedlstrasse 6, A-8042 Graz, Austria

4MPS Lindau, P.O. Box 20, D-37189 Katlenburg-Lindau, Germany5Institut fur TheoretischePhysik der Technischen Universitat Braunschweig

6MPE Garching Giessenbachstrasse, Postfach 1603, D-85740 Garching, Germany7IKI Moscow, Profsoyuznaja Street 84/32, 117810 Moscow, Russia

8IGPP at UCLA Los Angeles, California 90095-1567, USA9DLR, Institut fur Planetenforschung, Rutherfordstrasse 2, D-12489 Berlin, Germany

(∗Author for correspondence: E-mail: uli.auster@tu-bs.de)

(Received 8 February 2006; Accepted in final form 1 September 2006)

Abstract. The scientific objectives, design and capabilities of the Rosetta Lander’s ROMAP instru-ment are presented. ROMAP’s main scientific goals are longterm magnetic field and plasma measure-ments of the surface of Comet 67P/Churyumov-Gerasimenko in order to study cometary activity asa function of heliocentric distance, and measurements during the Lander’s descent to investigate thestructure of the comet’s remanent magnetisation. The ROMAP fluxgate magnetometer, electrostaticanalyser and Faraday cup measure the magnetic field from 0 to 32 Hz, ions of up to 8000 keV andelectrons of up to 4200 keV. Additional two types of pressure sensors – Penning and Minipirani –cover a pressure range from 10−8 to 101 mbar. ROMAP’s sensors and electronics are highly inte-grated, as required by a combined field/plasma instrument with less than 1 W power consumption and1 kg mass.

Keywords: plasma physics, comet, fluxgate magnetometer, electrostatic analyser

1. Scientific Objectives

The primary science objective of the plasma packages onboard the Rosetta mis-sion is to investigate a comet’s interaction with the solar wind. The characteristicstructures of the cometary plasma environment will be studied including the plasmaboundaries, plasma waves, the existence of a magnetic cavity and the evolution ofthese features. The nature of the interaction is considered to depend on the solar-wind activity as well as the location in the solar system because the outgassing ratefrom a comet increases as approaching to the perihelion. The influence of a possibleintrinsic magnetic field of the comet nucleus on the comet-solar wind interactionwill also be studied.

Space Science Reviews (2007) 128: 221–240DOI: 10.1007/s11214-006-9033-x C© Springer 2007

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1.1. SOLAR WIND INTERACTION AS A FUNCTION OF HELIOCENTRIC DISTANCE

Images from the flybys of the Vega and Giotto spacecraft at Comet 1P/Halley in1986 showed that only a small part of the nucleus was active. As a consequence,new models of the internal structure of the nucleus were developed, in whichthe thermal conductivity of the surface material is a key parameter. From suchmodels, the gas production rate F of Rosetta’s former target comet 46P/Wirtanenwas calculated as function of its distance to the Sun. Whereas the predictions ofthe models are almost identical for perihelion, they differ significantly at greaterdistances. At R = 3.5 AU, for example, there is a discrepancy of about four ordersof magnitude (F = 1023 s−1 to F = 1027 s−1). This huge discrepancy demonstratesthe large uncertainties about the internal structure of the nucleus.

The new target of the Rosetta mission is comet 67P/Churyumov-Gerasimenko(CG). So far its activity was measured reliably only close to perihelion (e.g. A’Hearnet al., 1995; Crovisier, 2002; Feldman et al., 2004; Makinen, 2004). Estimations ofthe production rate up to 3.5 AU require the use of thermal models for the cometarynucleus. Based on the thermal model by Kuhrt and Keller (1984) Motschmann andKuhrt (2006) developed a production model where the best fit to the observationaldata around perihelion is obtained by assuming an active area of 1.6 km2. A lowthermal conductivity of 0.001 W/Km has been applied. This value is consistent withresults form Hale Bopp observations (Kuhrt, 2002) and deep Impact measurements(A’Hearn et al., 2005). Figure 1 shows that the activity develops from about F ∼

Figure 1. Water production rate F of comet CG in dependency on the heliocentric distance rh . Thenegative values correspond to pre-perihelion distances. The solid curve represents results of a thermalmodel (Kuhrt and Keller, 1994). A thermal conductivity at the surface of the nucleus of 0.001 W/Kmand an active area of 1.6 km2 are assumed. Data points are from A’Hearn et al. (1995) (plus signs),Crovisier et al. (2002) (diamond), Feldman et al. (2004) (triangle) and Makinen (2004) (squares)(Motschmann and Kuhrt, 2006).

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Figure 2. Churyumov-Gerasimenko at 3.5 AU. (a) Heavy ( =cometary) ion number density. (b) Solarwind ( = proton) number density. (c) Magnetic field strength. (d) Cut through the nucleus along thez-axis at x = 0. The interplanetary field points in +Y direction (into the plane). The Alfven Machnumber is 10. No intrinsic cometary field is assumed. The main features are the initial part of thecycloidal tail of cometary ions, a weak Mach cone in the solar wind density, the magnetic pile-up,and the sharp increase of the cometary density at z ∼ 0.25 c/ωp = 62.5 km. This heavy ion densityjump indicates a two-ion plasma boundary where the cometary ions are accelerated up to a criticalpoint, with only weak re-action to the solar wind. At a later stage of high cometary activity this jumpevolves into the cometopause, when it has the property of an ion composition boundary (Motschmannand Kuhrt, 2006).

1026 s−1 at 3.5 AU to about F ∼ 3×1027 s−1 near 1AU. The corresponding evolutionof the plasma features is shown in Figures 2 and 3. Figure 2 shows a very faintcoma with disturbances of the magnetic field in the same order as the interplanetarybackground field at 3.5 AU. At 1.75 AU an extended coma with bow shock, ioncomposition boundary and the onset of the formation of a diamagnetic cavity canbe found (Figure 3). The magnetic field piles up at least to 5 times the interplanetarybackground field.

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Figure 3. Churyumov-Gerasimenko at 1.75 AU. (a)–(d) See caption to Figure 2. The solar wind showsa significant decrease inside the heavy ion density jump which is now located at about z ∼ 2 c/ωp =500 km from the subsolar point and z ∼ 4 c/ωp = 1000 km above the nucleus. The solar wind protonsand the cometary ions are well separated, justifying reference to this heavy ion density boundary as thecometopause, which now has the signature of an ion composition boundary. The Mach cone extendssymmetrically around the whole interaction region, forming the bow shock. The pile-up region is nowpositioned between the shock and the ion composition boundary. The magnetic field penetrates theion composition boundary by diffusion only and no longer by advection. Consequently, the magneticfield decreases beyond the ion composition boundary, indicating the onset of a diamagnetic cavity(Motschmann and Kuhrt, 2006).

1.2. MAGNETIC PROPERTIES OF THE COMET

Whether small bodies like comets or asteroids posses a magnetic field depends ontheir composition, origin and thermal evolution. Previous asteroid missions providenonuniform results. Galileo flybys of asteroids Gaspra and Ida revealed that at leastGaspra is magnetic (Kivelson et al., 1993; Baumgartel et al., 1994; Kivelson et al.,1995; Blanco-Cano et al., 2003; Simon et al., 2006). A magnetization of asteroid

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Braille was detected at Deep Space 1 encounter (Richter et al., 2001) by a signaturein the magnetic field measurement which can be explained by an asteroid dipolefield. On the other hand the first measurement on a surface of an asteroid, namelyon 433 Eros (Acuna et al., 2002) shows the absence of a global scale magnetizationat. No measurements exist to characterise the magnetic properties of comets. Theclosest approach of former missions was too far away to investigate a magneti-zation directly. The cometary plasma environment was clearly dominated by theinteraction of the solar wind with the cometary atmosphere/ionosphere because allformer flybys occurred at close perihelion distances. For the first time, the Rosettamission will provide magnetic field measurements close to a nucleus when thecometary activity is not fully developed and on the cometary surface. If the nucleusis not protected by an atmosphere produced by outgassing, the solar wind inter-acts directly with a possible intrinsic field. An origin of remanent magnetisation ofcomets could be magnetic minerals such as iron-nickel, magnetite and pyrrhotite,which were magnetised by relatively strong magnetic fields in the early solar nebula(e.g. Sugiura and Strangway, 1988). If there is a generic relationship between therefractory components of asteroids and comets, the magnetism of cometary nucleicould be caused by material exhibiting a natural remanent magnetisation (NRM)in much the same way. Whether or not such material is present in cometary nucleiis still under debate.

Experimental results (mass spectrometry of particles escaping from 1 P/Halley)and theoretical models both highlight primary and possibly secondary magneticminerals. One should expect the more pristine bright cometary regions to becharacterised by rather primary magnetic material, whereas the dark fraction-ated regions should be enriched by secondary magnetic material. More specif-ically, the following magnetic minerals/materials are being considered: Fe3O4

(magnetite), Fe Ni (metal) and (Fe, Ni)0.9S (pyrrhotite) as major carriers inthe bright regions, and magnetite and a Fe S Ni Si O-rich phase in the darkregions. This is probably the main carrier of the NRM in bulk samples ofC1-chondrites.

The growth of fractal aggregates from collisions between small dust grainsis generally accepted to be the first step in the formation of planetesimals andcometesimals in the early Solar System. Until now, the grain-grain interactionsconsidered within this scenario were only mechanical and electrostatic in nature.If magnetised material were present at this stage, as is suggested by meteoriticand asteroidal evidence, magnetic interactions between dust particles should alsobe taken into account. It has been shown experimentally (Nuth et al., 1994) andnumerically (Nubold and Glassmeier, 1999, 2000) that magnetised grains tend tobuild elongated structures of low fractal dimension and non-vanishing magneticmoment. Figure 4 summarises the results from numerical work on the build-upof aggregate magnetisation (μcluster) from individual dust grains with a magneticmoment μparticle. If enough magnetic material is available, this process may lead tocm- or even m-sized magnetic structures, which ROMAP might be able to detect.

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Figure 4. Accretional remanence (μ = μcluster/μparticle) of growing magnetic dust aggregates vs.number of particles (N) in a numerical simulation (Nubold and Glassmeier, 2000).

Remanent magnetisation of primitive objects such as comets could thus be called‘accretional remanence’.

1.3. REQUIREMENTS FOR INSTRUMENTATION

To achieve the scientific objectives, magnetic field as well as particles in a largeenergy range should be measured to characterise the plasma properties. Thereforeboth Orbiter and Lander of the Rosetta mission are equipped with plasma packages.While ROMAP measures on the surface of the comet nucleus, the plasma packageonboard the Orbiter (Glassmeier et al., this issue) provides data from various regionsof the plasma environment of the comet. Due to power and weight limitations themeasurement onboard the Lander is limited to magnetic field (MAG) and low energyparticles (Simple Plasma Monitor – SPM) which measure solar wind protons andelectrons.

The magnetic field of solar wind and cometary environment has to be measuredfrom DC to seconds. Furthermore the coordinated observations of IMF disturbancesby the lander and orbiter magnetometer in the frequency range up to 30 Hz can beused to determine the global electrical conductivity of the cometary nucleus (Dyal,1973). Therefore a fluxgate magnetometer with a maximum sampling rate of 64 Hzhas been proposed. The required accuracy can be derived by our simulations. Theblue lines in the left bottom panels in Figures 2 and 3 indicate that an accuracy inthe order of the solar wind magnitude is necessary to investigate the evolution ofplasma environment. For 4 AU this corresponds to an accuracy requirement of 1nT.Resolution, stability and frequency range of a fluxgate magnetometer are no prin-cipal limitation. To keep this accuracy under the extreme measurement condition

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onboard a cometary Lander–namely limited boom length and wide temperaturerange – are the particular design goal of this experiment.

The energy ranges of particle sensors have been specified considering the tech-nical and mass restriction. Nevertheless electrons and ions shall be measured from afew tens of eV up to a few keV to investigate solar wind and cometary fluxes. Sincethe comet and consequently the orientation of the instrumentation of the Lander isrotating, the pointing of the ion spectrometers deviates from the solar wind maindirection. For this reason the field of view (FOV) of the sensors especially for ionshas been selected rather wide.

To meet this requirement one electron and two ion spectrometers have been in-stalled on the Lander. Energy ranges from 40 eV to 8 keV for ions and from 0.3 eVto 4.2 keV for electrons can be measured with one three-layer hemispherical ana-lyzer for all three spectrometers. These ranges cover electron velocities from about300 to 40000 km/s, proton velocities from about 100 to 1200 km/s and watergroupvelocities from about 20 to 300 km/s. These are just the velocity ranges which areto be expected by hybrid code simulations of Bagdonat et al. (2004). The two ionspectrometers are equipped with deflection plates and accommodated perpendicu-lar to each other. This allows the determination of flux direction according to theinstrument. A Faraday cup to measure ion flux at extremely high values completesthe sensor arrangement.

Important for the scientific output is the coordinated operation of both plasmapackages which has to be tuned to the mission phases. During cruise both magne-tometers can be used to analyse spacecraft disturbances. After Lander release, es-pecially during the descent to comet Churyumov-Gerasimenko the Rosetta Landercan measure the magnetic field dependent on distance to the surface and thus revealpossible remanent magnetisation and temporal variations can be separated by theparallel measurement onboard the Orbiter. After landing the coordinated opera-tion of plasma packages onboard Orbiter and Lander provide the opportunity toinvestigate the evolution of the plasma environment at two points simultaneously.

2. Instrument Description

The combined magnetic field and plasma sensor is mounted on a small boom, twopressure sensors are accommodated on the Lander balcony. The sensor electronics,a small Digital Processing Unit as well as a high-voltage generator for the plasmasensor and an independent one for the and pressure sensors are located inside thewarm Lander compartment. Sensor parameters are given in Table I.

2.1. ROMAP SENSORS

The magnetic field is measured by a vector compensated ringcore fluxgate mag-netometer designed by TU-Braunschweig. The sensor consists of two entwinedringcores plus pick-up coils and Helmholtz coils for each sensor axis. Avoiding

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TABLE I

ROMAP instrument parameters.

Type of sensor Parameter Value

Fluxgate Magnetometer Dynamic range ±2000 nT

Resolution 10 pT

Sensor noise <5 pT/√

Hz at 1 Hz

Frequency range 0. . .32 Hz

Offset drift <0. 1nT/◦C

Electrostatic, Channels Ions 2 CEM

Hemispherical

Analyzer

Electrons 1 CEM

Energy range Ions 40 . . . 8000 eV

Electrons 0.35 . . . 4200 eV

Energy resolution Ions 7%

Electrons 15%

Field of view Ions 100 ◦ × 100 ◦

Electrons 15 ◦ × 60 ◦

Entrance area Ions 0.08 and 0.10 cm2

Electrons 0.17 cm2

Energy steps 32 or 64, log. scaled

Max. count rate 106 counts/s

Exposition time 40. . .1000 ms

Faraday cup Ion integral energy distribution up to 2000 eV

Resolution (current mode) ±4.10−14–±5.10−10 A

Field of view 140 ◦ × 140 ◦

Energy steps 16 steps

Entrance area 6 cm2

Penning Sensor Range 10−8–10−3 mbar

Electric field 106 V/m

Magnetic field 700 Gauss

Pirani Sensor Range 10−3–10 mbar

Bridge resistors 1 k Ohm

separate mechanical support allows the external field on the ringcore positionto be compensated for with high homogeneity and low mass (the overall sen-sor weight is 40 g). Dynamic feedback fields as well as offset fields of up to2000 nT can be generated in order to compensate for Lander and/or Orbiter DCstray fields. The determination of Lander and Orbiter offsets could be done dur-ing the cruise phase using non-compressible waves in the solar wind (Hedgecock,1975). Parallel measurements by the Lander and Orbiter magnetometers during the

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Figure 5. Combination of electrostatic analyser, magnetometer and Faraday cup sensors.

Lander’s ejection, descent and surface operations will provide additional input forinflight calibration. The main part of the SPM sensor is a hemispherical electrostaticanalyser with two channeltrons (channel electron multipliers, CEMs) for ion mea-surement and one for electron measurement. Deflection plates at the ion channelentrance provide the angular resolution. Despite the SPM’s small size, it has highsensitivity and resolution and wide field of view. Using CEMs in counting mode,the electrostatic analyser measures electron and ion distribution in a wide energyrange (E/q = 0–8 kV). Hemispherical deflection plates analyse the energy in 32or 64 steps. All major plasma parameters such as electron, proton and proton bulkvelocity, density and isotropic temperature can be derived. A retarding grid Faradaycup sensor measures currents owing to fluxes of low energy charged particles ona collector plate. It measures the ‘reduced’ velocity distribution of the plasma be-cause of its inherent integration over velocities contained in a plane of differentialthickness perpendicular to the axis of the sensor. As the sensor is not differentialin angle, it requires relatively low data rates. For a given orientation, it providesdifferential information in velocity space only along a direction perpendicular tothe modulator grid (Lazarus et al., 1993). All of the sensors are integrated withinone spherical sensor head. Figure 5 shows the sensor compartment. The head ismounted on a 60 cm-long boom that hinges on the upper edge of the Lander struc-ture and is launch-locked on the Lander balcony. After opening the launch lock,the boom will be deployed by two springs inside the hinge.

Additional two pressure sensors are included in the ROMAP sensor arrange-ment. The sensors are moved from Lander experiment COSAC to ROMAP tooptimise long term operation of pressure sensors. Two pressure sensors are selected

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Figure 6. Pressure measurement by Penning and Pirani sensors.

to cover the whole pressure range from 10−8 to 101 mbar. For the range from 10−8

to 10−3 mbar an ionising system (Penning) is deployed while for the range from10−3 to 101 mbar a heat conduction sensor (Minipirani) is available. Both sensors(see Figure 6) are located on the balcony. SPM and Pressure sensors were designedby MPS Lindau.

The Penning System for the low pressure range is realised by measurement ofthe current of ions and electrons to the cathode and anode. The ions and electronsare generated by impact of electrons with atoms and molecules in an area with highmagnetic field strength between cathode and anode. The number of ions generatedis proportional to the pressure. The initial generation of first electrons after switchon of the sensor is performed by a type of field emission or just by chance. It cantake some minutes for the generation of sufficient electrons at low pressure. Thevoltage between anode and cathode is around 3 kV. The ion current is measureddirectly by registration of the power consumption of the high voltage converterwhich supplies the Penning sensor.

The Minipirani is just a resistor, heated up to 120 ◦C. The heat power is nec-essary to compensate heat loss due to cooling by surrounding gas. Other effectslike radiation loss, heat conduction of the connection wires and limited gas heatconduction at higher pressure define the range for high resolution. The electronicis very simple. The heated resistor is part of a full bridge, balanced by an operationamplifier.

2.2. ROMAP ELECTRONICS

The ROMAP electronics consist of two boards inside the common electronics box.The central part of the sensor electronics on the first board is a field programmablegate array (FPGA) that controls analogue-to-digital (ADC) and digital-to-analogue(DAC) converters.

The 16-bit analogue-to-digital converters digitise science and housekeeping datafrom all four sensors. In Figure 7, this data flow is represented by dotted lines. The

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Figure 7. ROMAP block diagram.

analogue elements of traditional fluxgate magnetometers – such as filters and phase-sensitive integrators – are replaced by fast digitisation of the sensor AC-signal andthe subsequent data processing in FPGAs (Auster et al., 1995). In this way, massis saved without loss of accuracy and the resolution of the ADC is increased by thesquare route of the ratio between sampling frequency (double excitation) and thedata rate. Therefore the resolution is restricted only by the sensor noise (<5 pT/

√Hz

at 1 Hz) (Fornacon et al., 1999) and not by the electronics. Compensation fields forthe magnetometer and high-voltage steps for the electrostatic analyser and Faradaycup are controlled via digital-to-analogue converters (dashed lines in Figure 3).The sensor electronics were developed by Magson GmbH (Berlin, D). The high-voltage generator (developed by KFKI, Budapest, H) is in a separate shielded boxon the front panel of the common electronics box. The controller is located on thesecond ROMAP board. It controls MAG and SPM, stores their data output andimplements the interface to the Lander Command and Data Management System(CDMS). It triggers the measurement cycle of the magnetometer, implements thedigital magnetometer algorithm, controls the magnetometer feedback and generatesdata frames. For the SPM sensors, the controller implements the counting logicfor electrons and ions, samples Faraday cup data, generates SPM data frames,controls the high-voltage parameters (energy, elevation), controls the channeltronhigh-voltage supply and computes the plasma parameters. In the parameter mode,only the sums of the rows and columns of the sampled ion and ion-current arrays aretransmitted. The controller is based on an RTX2010. Address decoder, reset logic,

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TABLE II

ROMAP resources requirements

Resources Experiment part Requirements∑

Mass MAG sensor 40 g

SPM sensor 120 g

Pressure sensor 110 g

Boom + hinge + cable 80 g

Launch lock 40 g

Pressure harness 50 g

Electronics in CEB 360 g

(interface, analogue, controller,

HV-box, connectors, frontplate)

Pressure E-Box 130 g 930 g

Power Ssensor electronics 350. . .550 mW

Controller 180 mW

Penning electronics 100 mW

Pirani electronics 50 mW

HV-part 200 mW <900 mW

Telemetry rate surface mode

MAG 70 bits/s

SPM 30 bits/s 80 bits/s

Slow mode

MAG 70 bits/s 68 bits/s

Fast mode

MAG 4400 bits/s 4369 bits/s

clock generators, control signal generator, watchdog logic and CDMS interface areintegrated within an FPGA. Controller hard- and software were developed by theSpace Research Institute Graz.

Detailed numbers for the required resources are listed in Table II.

3. Commissioning Results

3.1. FUNCTIONALITY

Magnetometer, electrostatic analyser, Faraday cup and Pirani sensor are workingperfectly. All housekeeping values are nominal. Commanding, onboard data pro-cessing and telemetry interface are free of errors. The SPM high voltage works onlyat the three lower levels. At the upper two HV levels the instrument was resetted

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by sparks probably due to outgassing inside the warm compartment where the HVelectronics is located. For the same reason the high voltage for the penning sensorcould not be switched on.

During commissioning all magnetometer function are successfully tested. Dueto the sensor position between Lander and Orbiter the absolute measurement of thesolar wind field is affected by the spacecraft bias field. Nevertheless field varia-tion in a frequency range between 0.001 and 32 Hz can be measured. Figure 8shows solar wind fluctuations dominated by Alfven waves since |B| is nearlyconstant during the period shown in this figure. After commissioning the mag-netometer was switched on at all Lander functional tests as well as during Earthflyby and Draconide encounter. Further operation are planed for Mars and Asteroidflybys.

Our recent test phases indicate that the instrument is working as planned. How-ever, the SPM sensor is mounted on the Lander in such a position that all sensorsare pointing towards the orbiter wall during cruise phase. This implies that we arenot able to get any real scientific data before Lander release. It is only the electronanalyser which gave us some data which can be interpreted as photoelectrons andpartly as solar wind electrons.

Figure 8. 2 h of solar wind magnetic field data measured by ROMAP fluxgate sensor during com-missioning. Data are in spacecraft coordinate system and offset corrected by Hedgecock method.

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Figure 9 shows the summary plot of electron spectra for different cycles (2 to 86).Blue, green, and black lines indicate modelled results using a Maxwellian spectrumfor the electron fluxes. Probably, the instrument detects three types of electronfluxes: photoelectrons from some part of the spacecraft (blue), which is below apotential of 9 V relative to the spectrometer, photoelectrons from the entrance of thespectrometer (green), and a very small contribution from the solar wind electrons(black). In each case n, T and Us (potential of electron flux source according toelectron spectrometer) are calculated approximating the electron distribution as aMaxwellian. All spectra were measured at minimum HV applied on CEMs (level= 1 of 5). This implies that at least the electron channeltron is as sensitive as it wasduring the last ground calibration. Also, we can say that the CEMs survived formore than two year being in open air before launch.

The two ion spectrometers Ion1 and Ion2 show no response from the ion detectorsbecause their entrances are almost closed. Only a few noise signals (1 bit out of16) are detected, which indicates that the charge amplifiers are alive. For the samereason there is no response from the Faraday Cups.

Figure 9. Distribution function of electrons measured by the electrostatic analyser.

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3.2. SPACECRAFT DC DISTURBANCES ON MAGNETIC FIELD MEASUREMENT

The magnetic properties of the Lander are measured in the frame of environmentaltest at IABG Ottobrunn, Germany. A Lander generated bias field of 1000 nT instowed and of less than 100 nT in deployed sensor position was predicted. Afterthese measurements the range of the ROMAP magnetometer was fixed to ±2000 nT.Unfortunately a part of the APX experiment containing a permanent magnet waslate exchanged. Magnetic tests at ESTEC before shipment showed an additionalbias field at the stowed sensor position of about 5000 nT. To avoid the saturation ofthe magnetometer during cruise a compensation magnet was installed on the bottomside of the base plate during the launch campaign in Kourou (accommodation seeFigure 10). Thus the remaining bias field [1400 nT, 800 nT, −1000 nT] could bepressed below 2000 nT.

Nevertheless the magnetic field will be influenced during cruise by these magnetsbecause the magnetic moments of the magnets depends on temperature (typically0.1%/ ◦C). Temperature dependencies of up to 5 nT/ ◦C have to be taken into accountas long the boom is stowed. After boom deployment these dependency is neglectable(<50 pT/ ◦C) because the distance of the sensor to the magnets increases by a factorof 5.

3.3. SPACECRAFT AC DISTURBANCES ON MAGNETIC FIELD MEASUREMENT

Two types of AC disturbances could be detected during Commissioning, the influ-ence of the reaction wheels and the influence of the Lander supply current.

Figure 10. Accommodation of ROMAP sensor and main DC disturbance sources (distances in mwith respect to spacecraft reference point).

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The magnetic influence of the reaction wheels (frequency of about 10–30 Hz)could be measured by both Orbiter and Lander magnetometers in fast mode di-rectly. Amplitudes are 30 nT (peak-peak) at ROMAP and 0.8 nT at the Orbitermagnetometer (RPC-MAG) sensor position. This influence can be separated eas-ily because the distinct frequency corresponds with the rotation periods of thewheels. In slow mode (1 Hz averages) aliasing effects up to 1 nT are still visibledue to the limited damping factor (−30 dB) of the simple boxcar filter. It has tobe taken into account during cruise. After Lander separation this type of AC errordisappears.

The Lander supply current generates a magnetic field of 1.7 nT/mA at ROMAPstowed sensor position [−0.9 m, 0.3 m, 1.2 m] given in s/c coordinates. The identicalsignature with a sensitivity of about 8 pT/mA could be observed at RPC-MAGsensor position [−2.3 m, 0.7 m, −0.4 m].

This type of disturbance could be investigated during a period in which theLander heater system was switched each 30 s (see Figure 11). The reason for thesupply current dependency is a ground loop caused by a non well electrical isolatedconnection of Lander structure and Orbiter structure. Using a dual magnetometerapproach the disturbances can be separated and removed during cruise.

In Figure 11 the cleaning procedure is illustrated for the X-component for a onehour interval. The large regular disturbance is generated by switching of variousheater (each 65 mA) of the Lander Thermal Control Unit. The upper panels showthe disturbance measured by ROMAP and RPC, the lower panel the cleaned data.After Lander separation also this type of disturbance disappears.

So both detected AC disturbances are orbiter generated and not relevant for thefinal measurement sequences.

4. Measurement Scenario

4.1. INSTRUMENT MODES

The instrument will make its measurements in three different modes. In the slowmagnetometer mode, the magnetic field vector will be transmitted with 1 Hz res-olution. In the fast magnetometer mode, the sampling rate is fixed at 64 Hz. Byvarying of calibration factors the measurement range can be extended by a factor oftwo. In both magnetometer modes, the high-voltage for electrostatic analyser andFaraday cup is switched off.

The SPM sensors are active in the surface mode only. In this mode, magnetometer(50 ms) and SPM (200 ms) work sequentially. Resolution settings, exposition time,the ratio between SPM raw and parameter data and high voltage level can be definedby commands. An automatic calibration sequence is implemented which increasesthe high voltage stepwise to check the sensitivity of the channeltrons which mightbe influenced by aging.

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Figure 11. Removal of heater current generated magnetic field disturbances by dual magnetometermethod. Upper panel: Magnetic field measured by ROMAP (inboard); Middle panel: Magnetic fieldmeasured by RPC-MAG (outboard); Lower panel: Magnetic field after application of dual magne-tometer technique.

The high voltage for the penning sensor can be activated independently fromthe selected mode. Penning as well as Pirani values are transmitted continuouslyas housekeeping information.

4.2. OPERATION DURING FLYBYS; DESCENT AND ON COMETARY SURFACE

During Rosetta’s cruise phase, ROMAP will measure in the slow magnetometermode. Measurement sequences during cruise will be used for sensor inflight cal-ibration. Both magnetometers aboard the Orbiter and Lander have to measure inparallel. Particularly during periods when Lander and Spacecraft are active (activeand passive checkouts), magnetometer data from ROMAP helps to distinguishbetween external fields and spacecraft disturbances. First science data could be ex-pected during Earth, Mars and asteroid flybys. During the descent phase, ROMAP’smagnetometer measures the magnetic ‘fall off’ profile, which provides reliable in-formation about the comet’s internal magnetic structure, if any.

SPM will start its measurements some minutes after landing. After a short ini-tial measurement interval of 20 min, measurement cycles lasting typically 4 h are

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expected for the long-term mission on the surface. The fast magnetometer modewill be used only for measurements in parallel with the Permitivity Probe (E-field)of the Lander’s SESAME instrument.

4.3. DATA PROCESSING

Data processing and archiving is done by the Rosetta Lander Science Operationsand Navigation Center (SONC) located at CNES in Toulouse. The SONC data pro-cessing system takes as input raw telemetry data (packets) and raw attitude and orbitfiles from the DDS (data distribution system) located at ESOC. The SPM raw dataare decommutated into ions energy and angle distributions (currents and counts),Faraday cup current and electron energy distribution (counts). The magnetic fieldvectors in instrument frame are extracted, time stamped in UTC, converted tophysical units (nT) and stored in the SONC database. These data are furthermorecalibrated (offset, sensitivity and alignment calculation) and rotated into differentcoordinate systems (EMEJ2000, ECLIPJ2000, SM, GSM, GSE, MSO). The cal-ibration can be performed with different sets of calibration data (alignment andsensitivity matrices and offset vectors) that are stored in the SONC data base. Thisallows the use of different calibrations for different time intervals. The calibrateddata are not stored at SONC but produced on the flight from the raw data and aselected calibration set. However calibrated data produced by SONC are archivedat Planetary Science Archive (ESAC) in PDS format.

5. Summary

Magnetometer, plasma monitor, Pirani sensor and all electronic parts controllingthe instrument are working perfectly. Only the high voltage of penning sensor couldnot be activated up to now. During cruise the accuracy of the magnetic field mea-surement is limited by spacecraft/Lander disturbances. During cruise both magne-tometers, onboard Lander and Orbiter are used to investigate the spacecraft field. Ifthe ROMAP boom will be deployed shortly after separation of the Lander, space-craft AC-fields disappear and the bias field on the deployed sensor position canbe determined by comparison with the magnetic field measurement onboard thenearby Orbiter. The expected bias field after deployment will be less than 100nT and its temperature dependency about <0.1 nT/ ◦C. During descent no otherLander activities are performed, so that the new determined offsets should bevalid until landing. So the local field on the landing side can be determined. Ifthe magnetometer discover an intrinsic magnetic field of Comet 67P/Churyumov-Gerasimenko, it would be the first detection of magnetism at a comet and wouldyield important insights about it composition and evolution. After landing the mag-netometer works as Variometer. Therefore bias field changes due to various Lander

ROMAP: ROSETTA MAGNETOMETER AND PLASMA MONITOR 239

operation between ROMAP measurement sequences don’t affect the scientificobjectives.

The measurement of the plasma monitor is still limited by the sensor accom-modation during cruise. Before separation the Orbiter is in the field of view of thesensor. Only photoelectrons and partly solar wind electrons can be measured. Butalso with shielded entrance the pristine functionality of ion channels and Faradaycup has been proved. Noise and sensitivity are still as good as it was at groundcalibration.

Acknowledgements

The authors wish to thank Olaf Hillenmaier, Ronald Kroth, Rainer Enge and IrmgardJerney for providing the electronics, Bernd Chares, Steffen Ebert, Manfred Erdmannand Ulrike Ragnit for designing, manufacturing and integration of the instrumentmechanics, Christiane Stuntebeck, Joachim Block and Rainer Schutze for the biasfield compensation and Joelle Durand and Daniel Popescu for data processing. Thework at the Technical University of Braunschweig is financially supported by theDeutsches Zentrum fur Luft- und Raumfahrt.

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