Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM H. NILSSON1,∗ , R. LUNDIN1 , K. LUNDIN1 , S. BARABASH1 , H. BORG1 , O. NORBERG1,2 , A. FEDOROV3 , J.-A. SAUVAUD3 , H. KOSKINEN4 , E. KALLIO4 , P. RIIHELÄ4 and J. L. BURCH5 1 Swedish 2 Present Institute of Space Physics, P.O. Box 812, SE-981 28 Kiruna, Sweden address: Swedish Space Corporation, Esrange Division, P.O. Box 802, SE-981 28 Kiruna, Sweden 3 Centre d’Etudes Spatiale des Rayonnements, Toulouse, France 4 Finnish Meteorological Institute, Helsinki, Finland 5 Southwest Research Institute, San Antonio, Texas, USA (∗ Author for correspondence: E-mail: [email protected]) (Received: 29 March 2006; Accepted in final form: 18 August 2006) Abstract. The Ion Composition Analyzer (ICA) is part of the Rosetta Plasma Consortium (RPC). ICA is designed to measure the three-dimensional distribution function of positive ions in order to study the interaction between the solar wind and cometary particles. The instrument has a mass resolution high enough to resolve the major species such as protons, helium, oxygen, molecular ions, and heavy ions characteristic of dusty plasma regions. ICA consists of an electrostatic acceptance angle filter, an electrostatic energy filter, and a magnetic momentum filter. Particles are detected using large diameter (100 mm) microchannel plates and a two-dimensional anode system. ICA has its own processor for data reduction/compression and formatting. The energy range of the instrument is from 25 eV to 40 keV and an angular field-of-view of 360◦ × 90◦ is achieved through electrostatic deflection of incoming particles. Keywords: plasma, solar wind, ions 1. Introduction The plasma tail is the visible consequence of plasma effects on a comet. The plasma tail consists of an energized and outflowing ionized cometary matter. Plasma energization and outflow results from the exchange of energy and momentum between the solar wind plasma and the partially ionized coma that develops and expands as the comet approaches the Sun. The coma is ionized by the solar EUV/UV radiation but also by impinging solar wind electrons (impact ionization). Ionized cometary atoms/molecules are subsequently energized by non-thermal escape processes, processes related with the transfer of solar wind energy and momentum. Ion drag is yet another process that results in the accompanying acceleration of neutral gas from the cometary coma. The first spacecraft to encounter a comet, the comet Halley, confirmed the abovementioned relationship between the solar wind and a comet (e.g. Mukai et al., Space Science Reviews (2007) 128: 671–695 DOI: 10.1007/s11214-006-9031-z C Springer 2006 672 H. NILSSON ET AL. 1986; Balsiger et al., 1986; Korth et al., 1986; Johnstone et al., 1986). The solar wind interaction with a comet is quite similar to the solar wind interaction with unmagnetized planets such as Mars and Venus, albeit in a much weaker gravitation environment. Using this analogy Lundin and Nilsson (2004) presented a model of the solar wind energy and momentum exchange with comet 67/P Churyomov– Gerasimenko based on the knowledge gained from missions to Mars. References to the latter are e.g. Luhmann and Schwingenschuh (1990) (single particle pickup) or Pérez-de Tejada (1987), Lundin and Dubinin (1992), Lundin et al. (1991) (massloaded energy and momentum transfer). ESA’s Rosetta mission to the comet 67P/Churyumov-Gerasimenko will be the first spacecraft to orbit a comet, performing in-situ measurements, remote sensing, and providing measurements on the comet surface by a lander. Rosetta was launched on 2 March 2004 and after more than ten years in space will rendezvous with comet 67P/Churyumov-Gerasimenko. Rosetta carries a suite of plasma instruments, the Rosetta Plasma Consortium (RPC), which are responsible for in-situ investigations of the cometary plasma environment, from the time Rosetta approaches the frozen nucleus until perihelion more than one year later. The Ion Composition Analyzer (ICA) is one of the five RPC instruments that will fulfill the RPC scientific objectives. For a description of the RPC instrument suite, see Trotignon et al. (1999), Carr (2006). ICA is a development of earlier versions of ion mass spectrographs for space plasma physics missions, but it has been specifically tailored for use on the Rosetta spacecraft, taking into account that Rosetta is a 3-axis stabilized platform. The Rosetta mission also requires very low instrument mass and power consumption, together with a more than one year mission lifetime a decade after launch. Plasma instruments such as ICA are essential for studies of the structuring of matter in a cometary environment. As already mentioned, the neutral gas outflow from a comet near perihelion will very quickly become controlled by electromagnetic forces when the solar UV- and solar wind impact ionization of the neutral gas becomes sufficiently high. Non-thermal heating/acceleration processes will be responsible for the high velocity outflow within the structured plasma tail. It will be the objective of ICA to study these outflow processes, to determine the magnitude of the outflow and the physics behind it. The Rosetta mission will have unique science capabilities, one being to study the evolutionary stages of atmospheric outflow process from approximately 3 AU to its perihelion. However, the most important science will probably be related to the unexpected new discoveries that most certainly will be made during more than a year in orbit in the nebulous, dusty and unpredictable comet environment, an environment that bears many signs of the early solar system. To study the dusty plasma in this environment is one important aspect, to study the various plasma regimes of the cometary magnetosphere (Bowshock, cometopause, mass-loading boundary, chemical boundary, inner coma) is another. For pick-up processes the newly formed comet ions are picked up by the solar wind and acquire the same bulk speed as the solar wind flow. In such a case the mass of the cometary ions can be inferred from the energy of the particles as RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 673 was done for example by Mukai et al. (1986), Johnstone et al. (1986). However, as demonstrated from observations by Balsiger et al. (1986), Korth et al. (1986), establishing the origin of, and the physics related with the solar wind – comet interaction, a mass resolving ion spectrometer is required. This is what ICA provides for the Rosetta mission. 2. Scientific Objectives The basic objective of the ICA instrument is in-situ measurements of the interaction between the solar wind and cometary particles. The mass per charge and energy per charge, as well as the travel direction of individual positive ions, are measured. ICA will contribute to many of the common scientific objectives of RPC (Carr, 2006, this issue). In order to meet the scientific objectives the ICA instrument is designed to meet the following requirements: 1. ICA will determine ion distribution functions for the major solar wind and cometary ion species. 2. ICA has a mass-resolution sufficient to enable the distinction between major ion constituents (e.g. H+ , He++ , He+ , O++ , O+ , CO+ 2 ) within an energy range from about 25 eV/e – 40 keV/e, though not all species can be resolved at all energies. 3. ICA will measure “dusty” plasma components in the sense that rather large mass per charge can be measured in some measurement modes. 4. ICA will distinguish relatively heavier ionized atom and molecular species from + + the cometary ionosphere (e.g. O+ , N+ , H2 O+ , N+ 2 , O2 , CO2 ), picked-up by the solar wind flow or streaming out by mass-loaded acceleration processes, from the ions of the solar wind. ICA will not be able to distinguish between ions with small difference in ion mass such as O+ , N+ and H2 O+ , but the general group of ions will be identified. For the energies corresponding to pick-up ions the emphasis of the instrument is on plasma dynamics and the role of the heavier ions in this respect. 3. Instrument Description The ICA instrument (Figure 1) is based on the design of three earlier versions of this type of instrument. Those are the TICS instrument flown on the SwedishGerman research satellite Freja which was operated between 1992–1996 (Eliasson et al., 1994), the IMIS instrument which was part of the ASPERA-C experiment on the ill-fated Mars-96 mission, and the IMI instrument on the Japanese Nozomi mission to Mars (Norberg et al., 1998). The instrument also has heritage from the ASPERA experiment flown on the Soviet spacecraft Phobos-2 to Mars (Lundin 674 H. NILSSON ET AL. Figure 1. Photograph of the ICA Flight Model (FM). et al., 1989). Furthermore an almost identical mass resolving ion spectrometer has been flown on the Mars Express mission, the IMA sensor of the ASPERA-3 instrument (Lundin et al., 2004; Lundin and Barabash, 2004). IMA has performed well so far during the Mars Express mission. One more copy, only slightly modified, is part of the ASPERA-4 instrument (Barabash and The ASPERA-4 Team, 2006) on Venus Express. ICA consists of two parts. The cylindrical sensor unit contains an electrostatic entrance angle selection filter, an electrostatic filter for energy-per-charge analysis, a mass spectrograph, the detector assembly, and high voltage supplies. The data processing unit contains the interface between the sensor and the Plasma Interface Unit (PIU). The PIU is the common telemetry, command, and power interface between all RPC instruments and the spacecraft, Carr (2006), this issue. 3.1. PRINCIPLES OF M EASUREMENTS A cross-section of the ICA instrument is shown in Figure 2. Particles enter the instrument through a 360◦ aperture covered with a grid. Behind the grid is an electrostatic acceptance angle filter, a deflection system the purpose of which is to deflect particles coming from elevation angles between 45◦ and 135◦ with respect to the vertical symmetry axis. Particles passing the acceptance angle filter continue into the electrostatic analyzer (ESA). Ions within a swept energy pass band will RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 675 Figure 2. Cross-section of the ICA sensor showing the main components of the instrument. pass the ESA. The ions are then deflected in a cylindrical magnetic field set up by permanent magnets; the field deflects lighter ions more than heavy ions away from the centre of the analyzer. The ions finally hit a microchannel plate (MCP) and are detected by an anode system. The direction and mass per charge of ions are measured simultaneously. The magnet assembly can be biased with respect to the ESA to post-accelerate ions. This post-acceleration enables a selection of both mass range and mass resolution. ICA is mounted such that the field-of-view is centered on a plane that contains the Sun for nominal pointing direction, and also the comet when in the vicinity of it (see Section 5.1 for further discussion on the field-of-view of the instrument). The instrument characteristics are given in Table I. 3.2. ICA S ENSOR The ICA sensor measures the energy, mass, and arrival angle of positive ions. The sensor covers the energy range 25 eV to 40 keV in stepped energy sweeps; each sweep can contain up to 96 exponentially spaced steps. Particles enter the sensor through a grid that can be biased negatively to 12 V relative to the spacecraft, to repel thermal electrons and accelerate thermal ions into the instrument. This grid can also be grounded to the spacecraft potential. Behind the grid is an electrostatic acceptance angle filter, a deflection system whose purpose is to deflect particles coming from elevation angles between 45◦ and 135◦ with respect to the vertical symmetry axis. Particles passing the acceptance angle filter continue into the energy filter, a spherical “top-hat” electrostatic analyzer (ESA). The dimensions of the ESA is 45.0 mm (center radius) and 2.2 mm (distance between the two plates). The ESA allows the passage of ions with energies within a prescribed passband. 676 H. NILSSON ET AL. TABLE I Summary of expected ICA performance The small deviations encountered in reality are discussed in Section 5.2. Quantity Energy Range Resolution Scan Angle Range (FOV) Resolution Temporal resolution 2D distribution 3D distribution Geometric factor Per 22.5◦ sector Per 360◦ sector Range 25 eV to 40 keV E/E = 0.07 32 (solar wind), 96 (otherwise) 90◦ × 360◦ (2.8 π sr) 5.0◦ × 22.5◦ (16 elevation steps × 16 sectors) 4 s (12 s full energy range) 64 s (192 s full energy range) 6 × 10−4 cm2 sr 1 × 10−2 cm2 sr The outer hemisphere if the analyzer is kept at a fixed low voltage (from 0 to −11.25 V), while the inner hemisphere is stepped through the high energy part of the voltage sweep, from 4 kV down to less than 100 V. During the lower part of the energy sweep the situation is reversed, the inner hemisphere is kept at a fixed potential and the potential on the outer hemisphere is stepped down towards ground potential. The reason for this is to allow for highest possible definition of the low energy steps. Ions transmitted through the ESA enter the mass analyzer through a 7 mm wide slit. The slit somewhat reduces the possible paths particles can take through the mass analyzer and thus increases the mass resolution, but at the expense of some reduction in particle count rates. The mass analyzer uses a cylindrical magnetic field which deflects the ions outward, away from the central axis of the analyzer system. The magnetic system consists of 16 radially oriented permanently magnetized Neodymium-Iron-Boron magnets. To keep the stray magnetic fields low, the 16 magnets are matched to have the same magnetic field strength to within ±1% accuracy. The magnetic field strength is approximately 0.1 T. The dimensions of the magnet assembly and the field strength have been optimized so that all ions with energies greater than about 2.7 keV will pass through the analyzer and produce a mass image on the MCP. The location of mass spot boundaries (at the Full Width Half Maximum) on the anode rings is described by M0 = −6.019 + 2.523 · G − 0.031 · G 2 − R (1) M1 = −2.902 + 2.780 · G − 0.043 · G 2 − R (2) RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 677 Figure 3. Empirical correction to the radius where particles of a given energy per charge will hit the detector surface for different ion masses and post-acceleration as indicated in the plot. G=√ 103 (E/Q[V ] + PAC [V ]) · M/Q (3) where R is described by Figure 3, E is the energy, Q the charge and M the mass of the measured particle and PAC the used post-acceleration voltage. See Section 5.2 for examples of results from the calibration. The whole magnet assembly can be biased with a post-acceleration voltage between 0 and 4 kV. The postacceleration is used to vary the mass resolution of the instrument. The use of a magnetic analyzer and an adjustable post-acceleration makes the instrument simple and dependable, a very important aspect for a mission like Rosetta. Particles striking the front of the 100 mm diameter MCP (two stacked plates biased at 2.8 kV at the front side) produce an electron shower at the back side of the MCP. The electron shower is detected by an imaging anode system. A system of 32 concentrical anode rings behind the MCP measures the radial impact position (representing ion mass), whereas 16 sector anodes measure the azimuthal impact position (representing ion entrance angle). The 32 ring anodes and the 16 sector anodes are each connected to a charge-sensitive preamplifier of the MOCAD type. When the preamplifier output of both a ring and a sector is high, coincidence circuitry implemented in a Field Programmable Gate Array (FPGA) resolves the impact coordinates of each detected signal. The coordinates are used by another FPGA to update a 32 × 16 memory position with one every time a particle is detected in a given ring-sector position. Two memories are used to allow for double buffering of data. While data is being 678 H. NILSSON ET AL. sampled into one memory, the other is read by the instrument’s data processing unit. 3.3. ICA E LECTRONICS AND DATA P ROCESSING UNIT The ICA electronics can be divided into two main groups, high voltage power supplies for biasing of the various electrostatic filters and the MCP assembly, and digital electronics to handle the instrument operations. Figure 4 shows a block diagram of the ICA instrument. The high voltage supplies are laid out on two round boards in the cylindrical sensor part of the instrument. The high voltages are achieved by transforming an input voltage of 25 V to approximately 500 V, and subsequent chains of diode-capacitor voltage multipliers. ICA has two separate high voltage power supplies. One supply is dedicated to keeping the front side of the MCP biased to approximately 3 kV, while the other one supplies all other high voltages as fixed raw voltages. These raw voltages are then regulated by the use of high voltage opto-couplers. The electrostatic entrance filter can be stepped between ±2.5 kV to an accuracy of 1.2 V using a 12-bit D/A-converter and the electrostatic energy filter is stepped between 0 and 4.5 kV also using a 12-bit D/A-converter and a high voltage opto-coupler. As described above, high accuracy at low voltage settings is achieved by fixing the 4.5 kV supply at a voltage of about 100 V, and using a 11.25 V supply with a 12-bit D/A-converter connected to the outer ESA hemisphere to step the lower energy range. The post-acceleration voltage is also taken from the raw 4.5 kV voltage and set to the desired voltage using an opto-coupler. The digital electronics performs the following main functions: (a) Reading data from the double-buffered sensor memory to the CPU and processing the data. (b) Feeding the IEEE 1355 serial interface to the PIU with processed and formatted data. (c) Receiving commands on the serial interface from the PIU. (d) Controlling the high voltage power supply settings and monitoring voltages and temperatures. The digital electronics are built around an MA37150 processor. The on-board software runs from a 1 Mbit large bit-error corrected RAM. Flight software is stored both in 256 kbit PROM and in 4 Mbit EEPROM to enable patching of the software. An 8 Mbit memory is used both as working area for the CPU, and as a buffer for formatted data to be sent to the PIU. Data is transmitted to and from the PIU via an IEEE 1355 serial interface. ION LV Grid Voltage Control Grid +12V -12V GND HV Elevation. Analyser HV Elevation Analyser Main HV Supply +/- 1.5 kV -3 kV -4 kV +28V Main B Grid HV Control HV Monitoring Temp sens Post. acc HV Control +12V -12V GND A N O D E Mass Sector 32 pcs +5V -5V Preamps +5V -5V 16 pcs Serial I/F test connector ROM Signal Conditioner +5V PIU I/F DATA CACHE +5V -5V Sensor Controller CPU RAM Figure 4. Block diagram of the Ion Composition Analyzer (ICA). LV Energy Separation E HV Energy Separation -3 kV MCP Bias +28V Bias DPU unit Temp Sensor TxD RxD +28V +12V -12V +5V -5V GND ICA-PIU I/F connector HV-inhibit connector RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 679 680 H. NILSSON ET AL. 4. On-Board and Ground Data Handling Every sample period (approx. 125 ms), the ICA sensor produces a mass-anglecount matrix of the size 32 × 16 × 16 bits, for a total of 8192 bits per sample or 64 kbits per second. To reduce this large amount of data to the bit rates allocated to ICA (5, 100 and 1000 bits/s, see Section 4.2), the on-board CPU performs both data reduction and data compression. The basic measurement cycle consists of performing 16 energy sweeps at different settings of the acceptance angle filter, in order to step through the full field of view of the instrument. The energy sweep contains either 32 or 96 steps. A full measurement cycle is completed in 192 s. 4.1. ICA ON-BOARD DATA REDUCTION The data is first reduced from a 16-bit representation to 8-bits using a logarithmic compression. Directions shadowed by the spacecraft are masked out to avoid sending invalid data. Options are available to mask out certain mass-channels which show large noise levels as well as certain acceptance angles which cannot be reached for certain energy levels due to finite resolution of the digital-to-analog converters controlling the energy-acceptance filter, see Section 5.2. Depending on the selected mode, the data is then reduced by different methods. Basically, the following two methods are used: (a) The 32 mass channels are grouped together based on calibration data, and the sum of the grouped mass channels (representing a certain ion such as H+ or a certain ion mass range) is transmitted instead of individual mass channels. This will be referred to as mass-lookup tables and is used when the number of transmitted mass channels is less than 8. For 8 and 16 mass channels a simple integration of neighboring mass channels is used. (b) Summation of angular bins, both the 16 stepped elevation angle and the 16 sector directions (see Section 5.1 for further discussion of the field-of-view). The reduced data is finally compressed with a loss-less compression algorithm based on the Rice method (Rice et al., 1993). This compression reduces the data with approximately a factor of 5. Due to the data compression, the output data packets have variable length, and are buffered until enough compressed and time-stamped packets are available to fill a fixed length packet transmitted to the PIU. The buffer is also used to allow for higher than average production of data for some time. If data is coming in to the buffer faster than it is being removed by the TM stream an overflow will occur and data will be lost. The instrument can automatically adjust the reduction so that it fits the current efficiency of the compression. As an RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 681 TABLE II Minimum modes suitable for a TM rate of 5 bit/s. Indicated masses are the number of mass ranges transmitted by using lookup-tables except for the Mexm mode in which case all 32 physical mass channels are transmitted. Azimuth and elevation angles are reduced by integration from an original value of 16. Integration is never performed for energy, but the Mspo mode uses a subset of 32 energies. Mode Index Masses Azimuth angles Energies Elevation angles Max sets Idle Mspo Msis Mexm 0 2 4 5 2 6 32 1 1 1 32 96 96 1 1 1 15 5 5 adjustable high and low “watermark” in the buffer is exceeded, the data reduction is changed accordingly. 4.2. ICA O PERATIONAL M ODES ICA can produce data at three different data rates, 5 (minimum mode), 100 (normal mode), and 1000 (burst mode) bits/s. These three telemetry modes can be used together with a number of operational modes. The energy scan can be stepped either through 96 steps to cover the full energy range, or used in a reduced 32 step mode. The 32 step mode can be used to increase temporal resolution while studying ions with limited energy distributions, such as the solar wind. The post-acceleration voltage can be switched between high and low voltage settings, to measure ions with both low and high mass per charge at the best possible mass resolution. In addition to these operational modes, different data reduction schemes can be applied to reduce the sensor data to fit the three allocated bit rates. Since ICA uses data compression, the size of the compressed data will vary depending on the achieved compression. The CPU can be allowed to autonomously change the data reduction scheme in case the output data rate from the instrument differs too much from the desired value. In the tables below, these automatic changes correspond to changing the index number within the chosen mode (table) for Normal mode (NRM, Table III), burst high angular resolution mode (HAR, Table IV) and burst mass matrix mode (EXM, Table V). For these modes the time for one full data block is 192 s. (sweep through 16 elevation angles × 96 energy channels × 125 ms dwell time) and no changes to the instrument settings are ever done in the middle of a data format. For the minimum formats shown in Table II a different approach to data reduction is used. The time of a minimum data format is 16 minutes and the number of reduced spectra which can be fit into the telemetry stream is transmitted, all others are dropped. The maximum number of sets possible is shown in the seventh column of Table II. 682 H. NILSSON ET AL. TABLE III Normal modes suitable for a TM rate of 100 bit/s. Indicated masses are the number of mass ranges transmitted by using lookup-tables. Azimuth and elevation angles are reduced by integration from an original value of 16. Integration is never performed for energy. Mode Index Masses Azimuth angles Energies Elevation angles NRM-0 NRM-1 NRM-2 NRM-3 NRM-4 NRM-5 NRM-6 NRM-7 8 9 10 11 12 13 14 15 6 6 6 6 6 6 3 3 16 16 16 16 8 4 4 4 96 96 96 96 96 96 96 96 16 8 4 2 2 2 2 1 TABLE IV Burst modes suitable for a TM rate of 1000 bit/s with emphasis on angular resolution. Indicated masses are the number of mass ranges transmitted by using lookup-tables. Azimuth and elevation angles are reduced by integration from an original value of 16. Integration is never performed for energy. The sector angular resolution is preserved as far as possible at the expense of elevation angle and mass resolution. Mode Index Masses Azimuth angles Energies Elevation angles HAR-0 HAR-1 HAR-2 HAR-3 HAR-4 HAR-5 HAR-6 HAR-7 16 17 18 19 20 21 22 23 16 16 16 8 4 2 2 2 16 16 16 16 16 16 8 8 96 96 96 96 96 96 96 96 16 8 4 4 4 4 4 4 The mass lookup table of ICA sorts ion mass ranges according to Table VI. 4.3. ICA ELECTRICAL G ROUND SUPPORT EQUIPMENT The ICA Electrical Ground Support Equipment (EGSE) is a PC running the Linux operating system and power supplies to deliver regulated voltages to the instrument. The EGSE interfaces to ICA through an electronics card simulating the Plasma Interface Unit (PIU). A hardware driver implemented as a Linux kernel module takes care of the communication between the PIU simulator and the PC via a serial port. Data is then made available to display software from a ring buffer via read-calls RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 683 TABLE V Burst modes suitable for a TM rate of 1000 bit/s, with emphasis on mass resolution. Azimuth and elevation angles are reduced by integration from an original value of 16. Integration is never performed for energy and mass in this mode. Mode Index Masses Azimuth angles Energies Elevation angles EXM-0 EXM-1 EXM-2 EXM-3 EXM-4 EXM-5 EXM-6 EXM-7 24 25 26 27 28 29 30 31 32 32 32 32 32 32 32 32 16 16 16 16 8 4 2 2 96 96 96 96 96 96 96 96 16 8 4 2 2 2 2 1 TABLE VI ICA mass lookup table, used when fewer than 8 mass channels can be transmitted in the TM stream. Lookup number 0 1 2 3 4 5 Ion M/Q H+ 1 >O+ 32 O+ 16 He+ 4 He2+ 2 O2+ 8 to a device (/dev/tm). Most display software run under the X Windows System. A special X-event routine is available as a means of communicating the availability of new data packets to the display software. This enables the X display software to be idle when no new data is available instead of continuously polling the device. Various software modules have been developed to graphically display instrument housekeeping and science data, as well as providing for sending commands (writecalls to /dev/tm) through an easy-to-use graphical user interface. 5. Data from ICA In order to interpret the data from the ICA ion spectrometer the user must know the above information about the data formats and data reduction modes. Furthermore the field-of-view in spacecraft coordinates must be known (Section 5.1) as well as the instrument response (geometric factor as well as angular acceptance filter performance, Section 5.2). 684 H. NILSSON ET AL. Figure 5. Definition of the sector numbering and elevation angle numbering of the ICA instrument. The instrument coordinates are also shown. 5.1. I NSTRUMENT FIELD- OF -V IEW The workings of the instrument have already been described in Section 3. The sectors and elevation angles defining the different viewing directions resolved by the instrument are referred to using index numbers defined in Figure 5. It is important to note two things: (1) The instrument coordinate system is not the same as the spacecraft coordinate system. Instrument Y is spacecraft −Z and instrument Z is spacecraft Y. The instrument coordinate system is shown here for consistency with the calibration report on ICA by A. Fedorov, CESR, Toulouse. (2) As indicated in Figure 5, particles detected by a certain physical sector of ICA actually come from the opposite direction. This can be understood by following the ion path in the instrument depicted as a blue and a red line in Figure 2. The position of ICA on Rosetta is shown schematically in Figure 6. Sector numbers are now shown for the viewing direction, not the physical location of the sector in the instrument. As can be seen, sectors 10 to 15 and 0 are shadowed by the spacecraft for elevation angle indices from 0 to 7. Solar panels may influence the measurements also for higher elevation indices; this must be checked individually for each event. Figure 6 shows Rosetta in nominal position with the sun in the spacecraft X direction and the comet in the spacecraft Z direction. This is a good viewing situation for ICA. Picked-up cometary ions will likely form shells in phase space, centered on the solar wind bulk drift velocity, so that ions will probably arrive from all different viewing directions for the pick-up case. It is mainly the cold solar wind which can be problematic to measure, but most of the time this can be done by the IES ion spectrometer (Burch et al., 2006). Solar wind flow around the cometary obstacle will likely have an added component in the -Z direction (away from the comet) which facilitates the measurements from the ICA point-of-view. RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 685 Figure 6. Schematic location of ICA on the spacecraft. Also shown are the sectors (10–15 and 0) which for some elevation angles (index 0–7) are shadowed by the spacecraft. Note that sector numbering in the figure corresponds to the viewing direction, not the physical location of the corresponding sector on the detector MCP. 5.2. I NSTRUMENT CALIBRATION The ICA instrument has been calibrated using a vacuum chamber and ion source at the CESR in Toulouse. The characteristics are summarized below and deviations from expected performance commented upon. 5.2.1. Energy Characteristics The instrument was originally designed to be able to measure down to very low energies (1 eV). In order to achieve this, the instrument has two digital-to analog converters for stepping the deflection voltage which regulate the energy selection. One converter controls the outer hemisphere of the electrostatic analyzer and steps in the range 0 V to −11.25 V, whereas the other converter controls the inner hemisphere between 0 and 4000 V. For the inner hemisphere a reference voltage controls the high voltage applied to the sphere through an opto-coupler. For low energies the 686 H. NILSSON ET AL. inner hemisphere is kept constant at −8.7 mV reference value. For energies above 100 eV per charge the situation is the opposite, the outer hemisphere is fixed at −759 mV whereas the inner hemisphere is stepped to cover the full range up to 40 keV. The very lowest energies (a few eV) can be problematic to measure because of spacecraft charging considerations but the outer grid of ICA can be biased which may be used to off-set this effect. In practice the calibration showed that the entrance deflection system (elevation angle filter) limits the lower energies measurable and the onboard energy table uses a lowest energy of 25 eV. The highest desired energy of 40 keV is reached, but not for all elevation angles (see below). The calibration showed that the energy resolution was nominal, i.e. with a E/E of 0.07. 5.2.2. Spatial Characteristics The placement of the ICA instrument and the location of the sectors are shown in Figures 5 and 6. The resolution corresponding to the 16 different sectors of the micro channel plate (MCP) package is termed azimuthal resolution in this document as well as in the ICA archive format. The electrostatic deflection system will bring in particles with some elevation from the plane defined by the MCP into the detector. This angle is termed elevation angle, and its sign and index number is defined in Figure 5. The corresponding angular resolution will be termed the elevation resolution. The azimuth angular resolution is clearly defined by the subdivision of the 360◦ field-of-view in the instrument symmetry plane into 16 sectors (i.e. 22.5◦ resolution), confirmed by the calibration results. The elevation resolution needs to be carefully evaluated using the calibration results. The result is shown in Figure 7. Note that this result was obtained with the ICA cylinder only, not the Digital Processing Unit (DPU). Thus it was not subject to the constraints of the DAC converters of the DPU. It can be seen that the angular coverage is −39◦ to +41◦ which is slightly off target at ±45◦ . The resolution meets the target at 5◦ . However, when the DPU is used, the finite resolution of the DAC converters gives some problems at low energies. A proper value cannot be selected with the DAC and thus the deflection will not be correct. The calibration shows that the ICA instrument cannot measure energies below 16 eV per charge (eV/Q) because of the entrance deflection system. The finite resolution of the digital-to-analogue converter constrains this a bit further to 25 eV/Q. Furthermore only a few energy-angle combinations could be reached for energies up to 300 eV because of the limited resolution of the DAC. In the top energy range the entrance deflection system is also limiting, so the instrument does not reach the highest deflection angles for the highest energies. This is because of the limit of the maximum high voltage available. In the original ICA onboard software such values were masked out (the “bad-HV” mask). In a revised version an entrance deflection value as close to zero as possible is used for energies below 300 eV, i.e. no entrance deflection scanning is performed for these energies. For energies above 15 keV the highest deflection angle which can be achieved is used for the elevation values which cannot reach the nominal value. 687 Counts Elevation, deg RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM D = Udeflector/E, V/eV Figure 7. Elevation deflection angle as a function of D, 7 where D = (Uref − Uref0 )/(E − E0) [mV/eV charge] is the ratio between the angle deflection voltage and the particle energy adjusted by two constants determined from calibration. Blue horizontal lines show the FWHM of the D responses, which is used to estimate the angular resolution. The lower panel shows the width of the original data used to compile the upper panel. 5.2.3. Mass Resolution Properties ICA is an instrument designed to investigate plasma dynamics. It must be able to determine energy and angle of arrival of the ions in addition to their mass. For low energy particles to be detectable by the micro channel plate a post-acceleration is needed. This post-acceleration can be set to different values, which will also affect the mass resolution and the center of mass of an ion beam on the MCP surface. The mass range detectable by ICA will shift with the energy of the detected particles as well as the post-acceleration setting. Some sample results from the calibration are shown in Figure 8. As can be seen in the upper left and middle panels of Figure 8, H+ cannot be measured without sufficient post-acceleration for low energy ions (600 eV). The H+ ions deviate too much in the magnetic field and miss the detector 688 H. NILSSON ET AL. 32 16 2 1 Gl, cm2rad eV/eV Gl, cm2rad eV/eV M/Q Rm Rm Rm Figure 8. Mass resolution properties of the ICA instrument. The three upper panels show the results for relatively low energy ions (600 eV) and the lower three panels the results for 4.7 keV ions. Note that H+ data is missing for the 4.7 keV cases, but the H+ ions would hit the analyzer plate next to the M/Q = 2 (green) line just as in the 600 eV case and thus hit the analyzer plate. The leftmost columns show the result for no post-acceleration, the middle panels for 1.8 keV post-acceleration and the rightmost panels for 3.6 keV post-acceleration. As can be seen the mass-resolution is better at low energies/low post-acceleration, but H+ ions do not hit the analyzer plate. Solid lines show Gaussian fits. The y-axis shows the linear geometrical factor discussed in Section 5.2.4. plate. Note that H+ data is missing for the 4.7 keV cases, but the H+ ions would hit the analyzer plate next to the M/Q = 2 (green) line just as in the 600 eV case and thus hit the analyzer plate. Separation of O+ and He2+ is good as long as both can be measured. Separation of for example O+ and O2+ (M/Q 8, not shown in figure) is possible for low energies and no post-acceleration. Using high post-acceleration (rightmost panels) shifts the H+ beam partially onto the surface of the MCP and allows detection. However this gives much poorer mass resolution for the heavier ions which can no longer be properly distinguished. This is a trade-off typical for this kind of instrument. A decision on the post acceleration setting must be made depending upon the measurement situation. When distributions overlap somewhat it is possible to make fits of expected mass channel distributions and thereby separate two neighboring, somewhat overlapping, mass peaks, i.e. Carlsson et al. (2006). Gaussian fits of the peaks are shown by solid lines in Figure 8. A further test of the mass calibration is shown using solar wind data in Figure 9. The X axis shows the mass channel number, the Y axis the particle energy (eV) and the Z axis the particle count. A red line shows the lower mass channel number of H+ as a function of energy, according to calibration results. Two white lines show the upper and lower mass channel number limits for He2+ . As can be seen RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 689 3000 2500 Counts 2000 1500 1000 500 0 10000 30 1000 20 10 100 Energy [eV] 0 Mass channel number Figure 9. Sample data from 23 September 2004, taken during the high voltage commissioning of ICA. The instrument was run in HAR mode, and the post-acceleration reference level was 6 (on a scale 0–7), which is the default value. The X -axis shows the mass channel number, the Y -axis the energy (eV) and the Z -axis the number of counts. The red line shows the lower mass channel limit for H+ and the two white lines the upper and lower mass channel limits for He2+ . the mass calibration agrees very well with measurements for these sample masses and energies. The two distributions are well separated in both energy and mass. 5.2.4. Instrument Geometrical Factor The effective geometric factor varies somewhat for different masses, energies and post-acceleration levels. This has mainly to do with the geometry of where and to what extent the ion beam hits the MCP detector surface. The calibration results are shown in Figure 10. The figure shows the linear geometrical factor (cm2 srad eV/eV) for different ion masses and post-acceleration settings as a function of particle energy/charge. This geometrical factor is calculated by an integration of the effective aperture of the sensor along E (energy/charge) and polar angle but not along azimuth angle (sector). Given GI corresponds to the center of an azimuthal sector (where GI reaches its maximum). To get the full geometrical factor of one sector one must integrate over the sector response which is a triangle with a base of 32◦ . Figure 8 shows the same thing, but for a single energy and as a function of mass anode number. 5.2.5. Instrument Noise Levels For nominal conditions and low post-acceleration the noise level of the instrument is low. Experience from ICA’s twin, the Ion Mass Analyzer (part of the ASPERA-3 690 Gl, cm2rad eV/eV H. NILSSON ET AL. PACC = 0 1 2 16 32 Gl, cm2rad eV/eV Gl, cm2rad eV/eV PACC = -1800 V PACC = -3600V E/Q, eV Figure 10. The linear geometrical factor for ICA for different ion species and post acceleration voltage as a function of energy/charge (cm2 srad eV/eV). The blue dots and lines corresponds to H+ and one can clearly see the cut-off when the H+ ion trajectories go outside the detector plate. See text for details. RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 691 instrument on Mars Express), shows that noise levels are sometimes elevated when the instrument is in practical use. Mars Express IMA instrument data clearly shows that a varying amount of background counts occur in the instrument. These are in principle of two types, independent of energy and elevation angle, but may be different for different masses and sectors (i.e. different locations on the detector MCP) and energy dependent. The former can relatively easily be subtracted using a statistical amount of full resolution data. The latter occurs as an increase in neighboring mass channels when many counts are seen in a few mass channels of the same sector. An example is shown in Figure 9, where the noise level is somewhat increased for the mass channels corresponding to the energy of peak H+ flux. In Figure 9 a simple background subtraction has been performed, which is easy to do when there is only one major mass peak at a given energy in the distribution. Noise levels increase during very active conditions and will increase as the instrument temperature increase closer to the sun. Therefore the actual noise performance can only be judged in-situ during the mission, and methods to subtract the background must be continuously developed and updated during the different mission phases. One important disadvantage of the noise will be the reduced efficiency of the loss-less compression. It is possible to make a simple background subtraction on board to improve the efficiency of the data compression but this effectively reduces the sensitivity of the instrument. There is also one serious background problem which occurs when the sensor temperature exceeds about 35◦ C: the background level of some mass channels (anodes) increases drastically for all sectors. As mass channels are regularly binned together to reduce the data quantity this spurious signal could also destroy the data from neighboring mass channels or entire mass intervals in the high reduction modes. Therefore a patch to the onboard software has been made, which allows the problematic mass channels to be masked out. The problematic mass channels are numbers 0, 8, 24 and 25 counting from the center of the MCP (and thus high numbers correspond to low mass/low energy). 5.3. S AMPLE DATA At the time of writing ICA has produced data from the solar wind obtained during the high-voltage commissioning phase in September 2004 and during the early phase of the Earth flyby in March 2005. A sample mass-energy spectrum is shown in Figure 9. In Figure 11 we show a sequence of energy spectrograms, summed over all sectors (azimuth angles) from the early part of the Earth flyby on March 1, 2005. Post-acceleration reference level for the shown data was 2 (low level). Therefore the absence of H+ data in the lower energy range below 1 keV is an instrument effect (see Figure 10). Background subtraction has been performed and the data looks very good. This represents the first data of more energetic ions. Rosetta was presumably in the deep tail low latitude boundary layer. The X distance was about 692 H. NILSSON ET AL. Figure 11. Sample data from 1 March 2005, taken during the early part of the Earth flyby. The instrument was run in NRM mode. Energy spectrograms summed over all azimuth angles. The upper panel shows He2+ data and the lower panel H+ data. There were no significant O+ counts during this event. The y-axis shows the time in UT along with GSE coordinates in earth radii (R E ). 200 R E and only H+ and He2+ fluxes were significant. Note that due to the very limited telemetry the data reduction regularly steps down to only 2 masses (Table VI) and therefore He2+ data is missing. Only the H+ fluxes are significant at the higher energies of several keV. He2+ fluxes are present at solar wind-like energies. Part of the He2+ data may actually be cross-talk from H+ . During the comet mission full mass resolution data must be used for significant amounts of data to determine how much this may be and to compensate for it. However as the energy resolution is always good one may compare the energy spectra, and the fact that the He2+ energy spectrum is much narrower than that for H+ indicates that most of the He2+ counts are real. Cross-talk counts gives the same shape of the energy spectrum as in H+ , just at lower count rates. 5.4. M EASUREMENTS OF PICK-UP I ONS The real measurements from the comet will almost certainly contain numerous interesting surprises, but almost certainly also many measurements of picked-up cometary origin ions forming shells in velocity space (e.g. Mukai et al., 1986). In short, when a comet origin atom or molecule is ionized inside the solar wind it will initially have a velocity in the solar wind frame of approximately the same velocity as the solar wind, directed towards the sun (assuming the comet orbital velocity is negligible compared to the solar wind velocity). The ion will start to gyrate around the interplanetary magnetic field with a gyration velocity corresponding to the field- RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 693 Figure 12. Figures showing how the finite resolution of ICA affects the instrument’s ability to resolve shell distributions of pick-up ions. (a) A cut through the X–Z plane of the original shell distribution of picked-up water ions (18 AMU) in an arbitrary logarithmic particle flux scale. (b) The same distribution sampled by ICA where X–Z is the detector plane and the elevation of the measurements out of this plane is set to 0. (c) The sampling of the distribution when all different elevation angles out of the X–Z plane have been added due to limited telemetry. (d) Same as c except that we show the worst case, a torus around a magnetic field in the X–Z plane. perpendicular component of this velocity. For a stationary observer the gyrating ions will form a ring (torus) in velocity space with velocities from 0 up to two times the solar wind velocity (depending on the direction of the interplanetary magnetic field). The torus shape is unstable and quickly evolves into a shell in velocity space through pitch-angle scattering. We have performed some calculations indicating how the limited resolution of ICA will affect the measurements of shell distributions, in particular for the high reduction modes. Figure 12 shows in four panels the results of our calculations. Panel (a) shows a cut through the X–Z plane of the original shell distribution of picked-up water ions (18 AMU) in arbitrary logarithmic particle flux units. Panel (b) shows the same distribution sampled by ICA, where the X–Z plane is the detector plane (6) and the elevation of the measurements out of this plane is 0. The solar wind velocity was assumed to be 350 km s−1 and the magnetic field perpendicular to the velocity. The shell distribution can easily be resolved. In panel (c) we show the result for the case when all elevation angles out of the X–Z plane (±40◦ ) have been added due to limited telemetry. This corresponds to only one elevation bin in Tables III, IV, V. As can be seen the shell appears thicker (hotter) than it really is but one can still resolve that it is a shell or a torus. It may be difficult to distinguish between a shell and a torus, but a torus is always oriented after the 694 H. NILSSON ET AL. magnetic field which is measured by the RPC-MAG instrument so this should not be a problem. Panel (d) shows a worst case when a torus distribution is sampled for a magnetic field entirely in the detector plane. Then most of the information about the torus shape is not contained in the X–Z plane. One maximum occurs in the flux distribution, but if enough counts are obtained at low energy, and the particle distribution can be reconstructed, two equal maxima will be found where the X–Z plane cuts through the torus. The magnetic field is orthogonal to the line between these two maxima and once again this can be checked with the RPC-MAG instrument. Therefore ICA should be well able to measure and resolve shell and torus distributions also in the reduced resolution modes. The upper energy limit of ICA has been extended from 30 keV for Mars Express IMA to 40 keV in order to cover the high energy part of O+ and H2 O+ pick up ion distributions and this will be enough most of the time as the solar wind is often around 400 km s−1 and the magnetic field in the vicinity of the comet is frequently not orthogonal to the solar wind velocity. Extrapolation will also be straightforward when the highest energy particles are outside the instrument’s energy range as long as the ions form a well-behaved shell or torus distribution. 6. Conclusions ICA is a development of several previous versions of similar instruments (TICS, IMIS, IMI and ASPERA-3 IMA). The ASPERA-3 IMA is almost identical to ICA and has worked very well for more than a year on the Mars Express mission. The design is simple due to the use of permanent magnets for mass separation, and this simplicity provides the dependability necessary for the Rosetta mission. Calibration results, initial commissioning results as well as the experience from ASPERA-3 IMA indicate that ICA will be able to contribute to the RPC and Rosetta mission science goals. Acknowledgements ICA and the Rosetta mission are supported by the Swedish National Space Board, European Space Agency and the funding agencies of our co-investigators. We thank the many engineers, technicians and other staff at the Swedish Institute of Space Physics in Kiruna and all our co-investigators whose hard work has made our participation in the Rosetta mission possible. References Balsiger, H. K., Altwegg, K., Bühler, F., Geiss, J., Ghielmetti, A. G., Goldstein, R., et al.: 1986, Nature 321, 330. Barabash, S. and The ASPERA-4 Team: 2006, Plan Space Sci., submitted. RPC-ICA: THE ION COMPOSITION ANALYZER OF THE ROSETTA PLASMA CONSORTIUM 695 Burch, J. L., Goldstein, R., Cravens, T. E., Gibson, W. C., Lundin, R. N., Pollock, C. J., et al.: 2006, Space Sci. Rev., this issue. Carlsson, E., Fedorov, A., Barabash, S., Budnik, E., Grigoriev, A., Gunell, H., et al.: 2006, Icarus 182, 320. Carr, C.: 2006, Space Sci. Rev., this issue. Eliasson, L., Norberg, O., Lundin, R., Lundin, K., Olsen, S., Borg, H., et al.: 1994, Space Sci. Rev. 70, 563. Johnstone, A., Coates, A., Kellock, S., Wilken, B., Jockers, K., Rosenbauer, H., et al.: 1986, Nature 321, 344. Korth, A., Richter, A. K., Loidl, A., Anderson, K. A., Carlson, C. W., Curtis, D. W., et al.: 1986, Nature 321, 335. Luhmann, J. G. and Schwingenschuh: 1990, J. Geophys. Res. 95(A2), 939. Lundin, R. and Barabash, S.: 2004, Plan Space Sci. 52, 1059. Lundin, R. and Dubinin, E. M.: 1992, Adv. Space Res. 12(9), 255. Lundin, R. and Nilsson, H.: 2004, In: Colangeli, L., Mazzotta Epifani, E., and Palumbo, P. (eds.), ASSL Vol. 311: The New Rosetta Targets. Observations, Simulations and Instrument Performances, pp. 197. Lundin, R., Hultqvist, B., Olsen, S., Zakharov, A., Pissarenko, N., Dubinin, E., et al.: 1989, Solar System Plasma Physics, American Geophysical Union, Washington DC, chap The ASPERA experiment on the Soviet PHOBOS spacecraft, pp. 417–424. Geophysical Monograph Series vol. 40. Lundin, R., Dubinin, E. M., Koskinen, H., Norberg, O., Pissarenko, N., and Barabash, S. W.: 1991, Geophys. Res. Lett. 18(6), 1059. Lundin, R., Barabash, S., Andersson, H., Holmström, H., Grigoriev, A., Yamauchi, M., et al.: 2004, Science 305, 1933. Mukai, T., Miyake, W., Terasawa, T., Kitayama, M., and Hirao, K.: 1986, Nature 299. Norberg, O., Yamauchi, M., Lundin, R., Olsen, S., Borg, H., Barabash, S., et al.: 1998, Earth Planets Space 50, 199. Pérez-de Tejada, H.: 1987, J. Geophys. Res. 92, 4713. Rice, R. F., Yeh, P. S., and Miller, W.: 1993, In: Proceedings of the AIAA Computing in Aerospace 9 Conference, San Diego, CA. Trotignon, J. G., Boström, R., Burch, J. L., Glassmeier, K. H., Lundin, R., Norberg, O., et al.: 1999, Adv. Space Res. 24, 1149.