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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
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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
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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
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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
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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.
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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)
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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
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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
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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
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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.
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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
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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).
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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.
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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
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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.
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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
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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
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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
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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.
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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
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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-
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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
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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.
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