The THOR baseline payload includes a suite of instruments to measure electromagnetic fields up to 500k samples/s and to measure electrons and ions (at least H+, He++) at high resolution to resolve the kinetic scale physics of plasma heating and particle acceleration. THOR will use active spacecraft potential control to improve particle and field measurements.

Field Instruments Particle Instruments
MAG fluxgate magnetometer CSW cold solar wind instrument
EFI electric field instrument TEA turbulence electron analyser
SCM search coil magnetometer PPU particle processing unit
FWP fields and waves processor
FAR faraday cup

IMS ion mass spectrometer

EPE energetic particle experiments

Fluxgate Magnetometer (MAG)

PI: R. Nakamura (IWF/OEAW, Graz, Austria)

Co-PI: J. Eastwood (IC, London, UK)

The fluxgate magnetometer (MAG) will measure the background magnetic field and low frequency magnetic field fluctuations. MAG has two sensors that are placed along a solid boom, one at the end of the boom and the second at an intermediate distance along the boom, in order to enable a reliable detection of any residual spacecraft magnetic field. The related front-end electronics including digitization of the magnetic field vectors is on two separate printed circuit boards in the common electronics box of the fields and wave processor (FWP).

Each of the two fluxgate sensors uses only two ring-cores to measure the magnetic field along the required three directions which enables proper sensor miniaturization. The magnetic field is sensed in the X and Y direction via separate ring-cores, while the Z direction is picked-up over both ring-cores. The design of the outboard and inboard sensors is mainly based on Themis and Solar Orbiter heritage, respectively. Both THOR sensors will have the same mounting interface to simplify the boom design. MAG will return magnetic field vectors at up to 128 samples per second, with a noise floor less than 0.01 nT/ √Hz at 1 Hz. MAG provides high quality data with sufficient overlap (1-30 Hz) with the Search Coil Magnetometer (SCM) sensor to allow for an accurate synchronisation and alignment of the two data sets.

Electric Field Instrument (EFI)

PI: Yu. Khotyaintsev (IRF, Uppsala, Sweden)

Co-PI: S. Bale (UCB, Berkeley, USA)
Lead Co-I: H. Rothkaehl (CBK, Warsaw, Poland)

EFI The Electric Field Instrument (EFI) will measure the vector electric field from 0 to 200 kHz. EFI consists of two sets of sensors: Spin-plane Double Probes (EFI-SDP) providing high sensitivity DC electric field in the spacecraft spin plane (2D), and the High-Frequency Antenna (EFI-HFA) providing 3D electric field at frequencies above ~1 kHz. EFI-SDP consists of 4 biased spherical probes extended on 50 m long wire booms, 90 degrees apart in the spin plane, giving a 100 m baseline for each of the two spin-plane electric field components. EFI-HFA consists of 6 x 1.25 m long monopoles, forming 3 dipolar antennas crossed at 90o to each other. In addition to the sensors, EFI contains HFA and SDP pre-amplifiers, as well as bias electronics boards (BEBs) hosted in the man electronics box of the Field and Wave processor (FWP). As THOR spacecraft has a sun-pointing spin axis, EFI-SDP measures the electric field in the plane approximately orthogonal to the sun using long wire booms. The sun-pointing attitude greatly reduces errors due to wake effects and asymmetric photoelectron clouds, enabling the highly accurate in comparison to earlier missions ±0.1 mV/m near-DC electric field measurements. Interferometry using the electric field probes can be used to infer wavelengths and scale sizes at the smallest scales in the plasma.

EFI also measures the floating potential of the satellite, which can be used to estimate the plasma density at very high time resolution (up to a few hundred Hz). The sun-pointing attitude greatly reduces changes in the illuminated area, and hence the associated spin-dependent errors. In combination with densities derived from the observed plasma frequency emission line, EFI monitors the plasma density from DC to a few hundred Hz.

Cold Solar Wind (CSW) Instrument

PI: B. Lavraud (IRAP, Toulouse, France)
Co-PI: J. DeKeyser (BIRA, Brussels, Belgium)

The CSW instrument will measure the three dimensional velocity distribution functions (VDFs) of the cold solar wind ions with high energy (7%) and angular resolutions (1.5°) at a typical cadence of 150 ms, i.e., time and angular resolutions never achieved before in this region. Some modes with lower angular coverage and resolution will be devised at time cadences better than 150 ms (e.g., down to 50 ms). Thanks to these resolutions, CSW is primarily devoted to the study of solar wind turbulence at the ion scale. It will provide measurements complementary to the Faraday Cups (FCs), which have higher cadence but cannot provide full 3D VDFs.

CSW can be divided into two main units: the detector unit and the electronics unit. The detector unit first comprises entrance deflectors which allow to sweep over look angles ±24° in elevation out of the main detection plane, with 1.5° angular binning. A collimator is then used to provide the required angular resolution in elevation angles. Deflected and collimated ions are then subject to energy-per-charge (E/Q) selection through a classic top-hat electrostatic analyser. Through this analyser the E/Q selected ions are focused onto the main detection plane which comprises 32 channel electron multipliers (CEMs). These perform a 10^7 gain in charge collection (thanks to electron avalanching following the impact of ions on the entrance of CEMs) on anodes with a 1.5° resolution in azimuth over an angular range of ±24° as well.

Turbulence Electron Analyser (TEA)

PI: A. Fazakerley (MSSL/UCL, U.K.)
Co-PI: T. Moore (NASA/GSFC, U.S.A.)

The TEA instrument will measure the three dimensional velocity distribution functions (VDFs) in the solar wind and magnetosheath with time resolution faster than any previous instrument, with the capability to sample a 3D VDF in 5 ms, sufficient to resolve sub-ion scale and electron scale phenomena. The VDF will typically have 32 energy bins, and be sampled in polar and azimuth at 11.25° angular resolution. Such measurements will enable moments of the VDF to be calculated at comparable cadence from which frequency spectra with a Nyquist frequency of 100 Hz can be generated. The capability to sample 2D VDFs optimised to capture the electron pitch angle distributions is also planned.

Phase A studies in progress now are investigating the possibility of returning detailed timing information related to individual electron measurements (for intervals ~ 125 ms) for use with complementary high resolution wave data in wave-particle correlation studies.

The TEA instrument will use several dual-electron- analyser (DEA) units. Each unit will consist of a main structure and electronics box provided by MSSL/UCL, and a pair of analyser heads of the same design as the MMS=FPI instrument. Each DEA unit is connected to the PPU which provides control, power and data interfaces to and from the spacecraft.

Particle Processing Unit (PPU)

PI: M.F. Marcucci (INAF-IAPS, Rome, Italy)

Particle Processing Unit (PPU) is composed by the Central Processing Unit (CPU), the Compression & Scientific Processing (CSP) units and the Power Condition & Distribution Module (PCDM) in a fully redundant configuration. It provides a single point digital interface to the spacecraft and to all particle instruments (TEA, IMS, CSW and EPE).

PPU is designed to control and manage the instrument functions: receive commands from the spacecraft via the SpW link and route them to the instruments, manage the time synchronization, perform moments computation on the collected data distributions, compress and transmit scientific data to the spacecraft. PCDM is responsible of filtering, monitoring and switching the spacecraft +28V primary power to the instruments and the PPU itself. Additionally the PPU shares information with the FAR and FWP instruments through a direct digital link.

The PPU electronics box contains two CPU boards, based on the dual-core LEON3FT processor and two groups of 3 CSP boards based on FPGAs. Each CSP will be provided with two FPGAs and dedicated input (raw scientific data) and output (data resulting from processing) buffers for each sensors. The input buffers will be based on 256Mbyte SDRAMs while the output ones on 4Mbyte SRAMs. The parallel processing capabilities of the CSP, together with high data rate point-to-point links between the CSP and the CPU, can ensure very good performances also in presence of a considerable number of sensors. Such architecture comprises also two PCDMs for primary power distribution and one Backplane and SpaceWire Repeaters/ Distributors (BSR).

Faraday Cup (FAR) Instrument

PI: Z. Nemecek (Charles University, Prague, Czech Republic)
Co-PI: Y. Yermolaev (IKI, Moscow, Russia)

FAR instrument is designed to measure the integrated energy distribution of solar wind ions. The instrument will provide the basic moments of the energy distribution (density, velocity and temperature) with the time resolution up to 16 ms as well as a full distribution of protons and alpha particles with the 3 s resolution. The primary task of FAR is investigations of (1) solar wind turbulence at both inertial and ion kinetic scales, and (2) a study of fine structure of significant disturbances in the solar wind (e.g., interplanetary shocks, currents sheets). The second task is to provide a proxy of the solar wind velocity vector to other plasma instruments in order to set appropriate ranges for scanning (e.g., CSW).

Six identical FAR Faraday cups (FCs) are oriented in the solar direction in order to integrate a full solar wind ion flux and to facilitate in-flight calibration. FC collectors are split to two halves for a determination of the ion flux direction. The energy distribution is measured by sweeping of the voltage applied onto the control grid; a special feedback loop is used for an estimation of the speed and temperature without scanning of the full distribution. A set of FCs is divided into three identical sections that can be interchanged. Generally, the first section serves for a determination of the ion flux magnitude and direction, the second section provides the proton speed and temperature, and the last one scans the full ion energy distribution and details of the distribution of alpha particles.

Ion Mass Spectrometer (IMS)

PI: A. Retinò (LPP, Paris, France) 
Co-PI: L. M. Kistler (UNH, Durham, New Hampshire, USA)
Lead Co-Is: M. Fraenz (MPS, Göttingen, Germany), Y. Saito (ISAS-JAXA, Sagamihara, Japan), P. Wurz (University of Bern, Switzerland)

The IMS instrument will measure three-dimensional velocity distribution functions (VDFs) of mass-resolved hot ions (30 eV – 30 keV) in foreshock, shock and magnetosheath at the highest time resolution ever achieved in these regions: 150 ms for H+, 300 ms for He++, ~ 1 s for O+. The energy resolution will be 10%, the angular resolution 11.25° and the mass resolution M/M 8 for H+ and He++. With these performance parameters IMS will be able to study ion heating and acceleration by turbulent fluctuations at ion scales and to assess energy partition between different ion species at these scales.

The IMS instrument is composed of four identical units. Each of these units has three main components: the entrance optics, the detector and the electronics box. The optics combines energy (E) per charge (Q) selection by a top-hat electrostatic analyzer with a 6 cm long time-of-flight (TOF) section to determine the three-dimensional VDFs. The high cadence of the VDF measurements will be obtained by mounting the four sensors phased by 90° in the spacecraft spin plane and by using electrostatic deflection over 45° at each sensor. In this way, each sensor will cover a 45°× 360° degree field-of-view (FOV) and the full 4 steradians coverage will be provided independently of the spacecraft spin. The detector uses Micro-Channel Plates (MCPs) to create start and stop signals, an anode pixelized into 32 sectors of 11.25° to determine the position of incoming ions, and front-end electronics (ASICs combined with discrete components) to accurately determine the ion time-of-flight with a precision of ~ 1 ns. The electronics box includes High Voltage Power Supplies (HVPS), Low Voltage Power Supplies (LVPS) and digital boards used for internal data processing. Such digital boards interface with the PPU instrument which provides instrument control, power and data interfaces to and from the spacecraft.

Search Coil Magnetometer (SCM)

PI: F. Sahraoui (LPP, Palaiseau, France)
Co-PI : J. L. Pinçon (LPC2E, Orléans, France)

The SCM is a tri-axial dual-band search coil magnetometer. It is intended to measure three components of the magnetic field in the frequency range [0.1 Hz, 200 kHz]. Associated to the Field and Wave Processor (FWP), it provides spectral information over that frequency range and, in addition, delivers waveform measurements sampled at frequencies up to 524 kHz. The SCM will be located at the end of a solid boom.

SCM is an inductive magnetic sensor. It is made of a core in a high permeability material (ferrite or permalloy) on which are wound a main coil with several thousand turns and a secondary coil with a fewer turns. The secondary coil is used to create a flux feedback in order to have a flat frequency response on a bandwidth centered on the resonance frequency of the main coil. The induced voltage is raised to a proper level by a preamplifier to allow its transportation to the onboard analyzers.

Each antenna is made of a ferrite core with a first coil to perform the measurements in the LF range [0.1Hz, 4kHz], and a second coil to perform the measurements in HF range [1, 200] kHz. A mutual reducer is inserted to decouple the two windings. The mutual reducer is a cylinder made of a high permeability material (Fig 1-left). Secondary coils are used as a flux feedback, to create a flat frequency response on a bandwidth centred on the resonance frequencies of the two main coils. This active part is potted inside an epoxy tube (400 mm long, external diameter 20 mm). The magnetic sensors are assembled orthogonally in a compact configuration. This mechanical support is made in a nonmagnetic material (PEEK KETRON) and stands for the interface with the satellite. The amplification electronic circuit is made in 3D+ technology (an option would be ASIC technology allowing for additional mass and power savings). It is divided into several Printed Circuit Boards (PCB) that are stacked and molded in an epoxy resin. Tantalum layers are inserted between electronic boards to improve the radiation tolerance. It is composed of 3 HF amplification channels, 3 LF channels, and 1 power supply regulation circuit. The 3D+ module will be housed in the foot of the sensor (close to the antennas) to improve the signal to noise ratio

Fields and Waves Processor (FWP)

PI: J. Soucek (IAP, Prague, Czech Republic) Co-PI: H. Rothkaehl (CBK, Warsaw, Poland)
Lead Co-Is: A.Zaslavsky (LESIA, Meudon, France), M. Balikhin (U. of Sheffield, Sheffield, UK)

The Fields and Waves Processor instrument (FWP) is a common electronics box for electromagnetic field measurements, responsible for digitization and digital processing of data from all THOR electromagnetic field sensors (MAG magnetometers, SCM search-coil and the EFI-SDP and EFI-HFA antennas). The instrument represents a single power and communication interface between all electromagnetic field instruments and the spacecraft and all data from those instruments are transmitted to the spacecraft through FWP. The high integration of the field measurements on THOR enables excellent time synchronization of data from different sensors and allows for efficient on-board processing.

The FWP instrument shall be able to provide the following data products.

  1. Continuous multi-component E/B waveforms at low to medium sampling rates (up to 128 Hz for DC magnetic field, up to 8192 Hz for EFI/SCM signals)

  1. Waveform snapshots: Fragments of multi-component high time resolution waveform collected at a low duty cycle. Up to 10 field components from the above table can be sampled simultaneously at a maximum sampling rate of 524 kHz. 

  1. Spectra and cross-spectral matrices calculated on-board to provide continuous coverage of the entire frequency band (up to 200 kHz) in spectral domain.

  1. Advanced data products derived by on-board analysis of spectra. These include electron density derived from thermal noise spectrum at a high time resolution and absolute electron density derived from the active sounding of EDS.

FWP is an electronic box housing 11 circuit boards with different functionalities interconnected via a backplane. The individual subsystems of FWP, realized as circuit board cards, are:

MAG-IBS and MAG-OBS electronics boards: The FWP box will contain the electronics boards responsible for driving and data acquisition from the MAG sensors (one board for each MAG sensor). The OBS board will implement the front end electronics in an ASIC, the IBS shall use a high TRL design based on Solar Orbiter heritage. Both boards will implement control and data processing logic in an FPGA and provide to FWP DPU digital 20-bit magnetic field waveform. Formally, these boards are considered parts MAG under MAG responsibility.

EFI Boom Electronics Boards (BEBs): The BEBs are a set of two boards in the FWP main electronics box (BEB-SDP and BEB-HFA), one for each set of sensors (4xSDP and HFA). The BEBs perform signal conditioning on all preamplifier signals, and feeds the processed signals to the FWP analysers for analogue-to-digital conversion and analysis. Bias settings and setting of the SDP control surfaces are also done on the BEBs. The control surface potentials are adjusted to prevent photoelectrons originating from surfaces outside the probe from reaching the probe. Formally, these boards are considered parts of EFI, under EFI responsibility.

The Thermal noise High frequency Receiver (THR) is a wave analyzer board responsible for digitization and spectral analysis of signals from EFI antennas and SCM in the full frequency range up to 200 kHz. THR will perform on-board spectral processing and thermal noise analysis which allows estimating absolute electron density and temperature from thermal noise spectra in the solar wind, complementing TEA measurements and offering a calibration proxy for particle data. High resolution waveform snapshots will also be captured. Digitization will be performed with 14 or 16 bit ADC’s and oversampling will be applied to further increase dynamics.

The Low Frequency Receiver (LFR) is a wave analyzer board responsible for digitization and processing of multicomponent signals from EFI antennas and SCM in the frequency range up to 20 kHz and waveform acquisition up to 200 kHz. The signal shall be processed by integrated digital logic implemented in an FPGA, performing filtering, decimation and spectral analysis of the signals in order to reduce the telemetry volume. LFR will allow to sample up to 12 signals simultaneously. Digitization will be performed with 14 or 16 bit ADC’s and oversampling will be applied to further increase dynamics.

Electron Density Sounder (EDS) is an active experiment injecting oscillating signal on the shields of the EFI/SDP wire booms and measuring the response of the plasma in electric field. Analysis of plasma resonances then allows obtaining precise absolute measurement of electron density invaluable for particle instrument cross-calibration. Comparing to THR, EDS provides good estimates even in the presence of natural waves, but the active nature of the measurement perturbs other field measurements. EDS has to be run in a low duty cycle.

The Data Processing Unit (DPU) is a central computer dedicated to controlling the units within the FWP box, receiving raw telemetry data from all FWP units, formatting and compressing the science data and transmitting them to the spacecraft. DPU software will also perform numerical calculations for producing spectral matrices. The board will include a powerful fault tolerant CPU (dual-core Leon3-FT), 128MB of memory, SpaceWire interface to the spacecraft and digital interfaces to other FPW subunits. As the FWP DPU represents a critical system for the entire mission, two separate units in cold redundancy will be included in FWP box.

Power Supply Unit (PSU) is a DC-DC power converter providing stabilized low voltages to all subunits of FWP and to the MAG, SCM and EFI sensors. The PSU will include a digitally controlled power distribution unit with the ability to power on or off any subunit independently, secondary current and voltage monitoring as well as overcurrent protection. Two separate units in cold redundancy will be included in FWP box.

Energetic Particle Experiments (EPE)

PI: Robert F. Wimmer-Schweingruber (Institut für Experimentelle und Angewandte Physik, University of Kiel, Germany)
Co-PI: Rami Vainio (Department of Physics and Astronomy, University of Turku, Finland)

The Energetic Particle Experiment (EPE) on-board THOR is a particle instrument measuring the energy spectra and angular distributions of energetic electrons (20-600 keV) and ions (20-8000 keV/n). The instrument has two sensor units, each one measuring with two double-ended telescope pairs in four view cones. Utilizing the spin of the spacecraft, EPE observations cover the full sky. The EPE instrument collects 3D distributions of all particles during half of a spacecraft spin period (every 15 s). The relative orientation of the 8 independent telescopes was conceived to enable good quality sub-spin measurements for most magnetic field configurations. EPE has heritage from the Energetic and Relativistic Nuclei and Electron experiment on SOHO (ERNE), the Solar Electron and Proton Telescope on STEREO (SEPT) and from the Electron Proton Telescope on Solar Orbiter (EPT). EPE also adds composition capabilities not foreseen in SEPT and only to a limited extent in EPT by adding an ultra-thin front detector in the ion channel.