The Vigil Magnetometer for Operational Space Weather Services From the Sun‐Earth L5 Point

Severe space weather has the potential to cause significant socio‐economic impact and it is widely accepted that mitigating this risk requires more comprehensive observations of the Sun and heliosphere, enabling more accurate forecasting of significant events with longer lead‐times. In this context, it is now recognized that observations from the L5 Sun‐Earth Lagrange point (both remote and in situ) would offer considerable improvements in our ability to monitor and forecast space weather. Remote sensing from L5 allows for the observation of solar features earlier than at L1, providing early monitoring of active region development, as well as tracking of interplanetary coronal mass ejections through the inner heliosphere. In situ measurements at L5 characterize the solar wind's geoeffectiveness (particularly stream interaction regions), and can also be ingested into heliospheric models, improving their performance. The Vigil space weather mission is part of the ESA Space Safety Program and will provide a real‐time data stream for space weather services from L5 following its anticipated launch in the early 2030s. The interplanetary magnetic field is a key observational parameter, and here we describe the development of the Vigil magnetometer instrument for operational space weather monitoring at the L5 point. We summarize the baseline instrument capabilities, demonstrating how heritage from science missions has been leveraged to develop a low‐risk, high‐heritage instrument concept.


Introduction
The importance of space weather as a threat to infrastructure resilience and associated negative socio-economic impact is now widely established thanks to a variety of studies uncovering and summarizing a wide range of effects (e.g., Cannon et al., 2013;Eastwood, Biffis, et al., 2017;Hapgood et al., 2021;Schrijver et al., 2015).Although still in its infancy, estimations of the economic impact of space weather have largely focused on the loss of power (Eastwood et al., 2018;Oughton et al., 2017Oughton et al., , 2019)), and the risk of such impacts has led to Government agencies being aware for some time that space weather is a risk requiring specific policy development (e.g., Fry, 2012).For example, in the UK severe space weather is included on the national risk register (Cabinet Office, 2023), and is the subject of specific UK Government policy (Department for Business, 2021).
It is generally accepted that for improving our resilience to space weather, improved lead times and more accurate warnings are an essential component.In the context of geomagnetic activity, which is the primary physical pathway for impacts on ground infrastructure such as power grids and pipelines (Eastwood, Nakamura, et al., 2017;Morley, 2020), this requires better knowledge about when geoeffective solar wind structure will impact the Earth (e.g., Vourlidas et al., 2019).
Of particular interest is the Sun and heliosphere as observed from the L5 Sun-Earth Lagrange point (Gibson et al., 2018;Hapgood, 2017;Vourlidas, 2015), which trails the Earth in its orbit with a longitudinal separation of 60°.Images of the Sun taken from L5 allow structure to be identified and monitored for several days before it rotates into the view of Earth, as L5 subtends a point toward the East limb of the Sun as viewed from Earth.Imaging of both the corona and heliosphere from L5 allows Coronal Mass Ejections (CMEs) to be observed remotely as they launch and transition into Interplanetary Coronal Mass Ejections (ICMEs) in the inner heliosphere.Finally, in situ measurements characterize the properties of the solar wind several days before the associated source region rotates onto the Sun-Earth line.The utility of measurements at the L5 point has been demonstrated by STEREO, for example, showing that errors in ICME arrival time can be notably reduced by including data from L5 in modeling (e.g., Colaninno et al., 2013;Möstl et al., 2017;Palmerio et al., 2022;Rodriguez et al., 2020).
Given the wide acceptance of observations away from the Sun-Earth line as being crucial for improved space weather forecasting capabilities, there is a relatively long development history of mission concepts to the L5 point (Akioka et al., 2005;Schmidt & Bothmer, 1996).Considerable further development was stimulated by the success of the STEREO mission (e.g., Bosanac et al., 2018;Gopalswamy et al., 2011;Lavraud et al., 2016;Strugarek et al., 2015;Trichas et al., 2015).In this context, Vigil (initially known as Lagrange) is a space weather mission selected by ESA that would make remote and in situ operational space weather measurements from the L5 Lagrange point (Gibney, 2017;Kraft et al., 2017;Lugaz, 2020).Vigil is part of the ESA Space Safety Program (S2P), and following a Phase 0 mission feasibility study performed in 2015-2016, subsequent mission studies have been carried out to the completion of Phase B1 in 2023.At the conclusion of this phase, the Vigil payload is expected to consist of three remote sensing instruments (Magnetograph, Heliospheric Imager, Coronagraph) and two in situ instruments (Plasma Analyzer and Magnetometer), together with a possible further instrument of opportunity such as an ultra-violet imager.
In this article we describe the development of the Vigil magnetometer instrument (MAG) for measurements at L5, performed in the context of Vigil mission studies completed to date.Section 2 first examines the rationale for in situ observations from L5. Section 3 then describes in more detail the observational requirements, both in terms of the measurements themselves and the availability and timeliness of the data on the ground.Section 4 describes the instrument concept, which is derived from previous science missions, and Section 5 describes the instrument concept of operations.Section 6 reviews the operational context of Vigil, focusing on the availability analysis and magnetic cleanliness, where considerations of data latency strongly drive the requirement for a clean magnetic environment.A summary and outlook is presented in Section 7.

Utility of In Situ Measurements at L5 for Space Weather
Focusing on in situ measurements more specifically, high speeds streams, causing stream (or sometimes termed corotating) interaction regions (SIRs) can be a source of major geomagnetic storms (Richardson, 2018;Richardson et al., 2006).Observations at L5 provide a characterization of the solar wind that is likely to be observed at Earth when the underlying source regions rotate onto the Sun-Earth line a few days later.The utility of in situ L5 data as provided by STEREO was first explored by comparing SIR observations with L1 observations (Simunac et al., 2009), and subsequent studies have shown that the majority of SIRs seen at L5 are subsequently seen at L1 within a 3 days window (Allen et al., 2020).In situ STEREO-B data from L5 has also been used to establish the performance of Dst prediction, finding performance better than 27-day persistence and utility in predicting the minimum Dst for the next 4 days (Bailey et al., 2020).
More generally, solar wind forecasts using data from L5 provide a level of improvement on persistence models (Thomas et al., 2018), and by using data assimilation techniques in conjunction with solar wind models, contribute to improved longer-term forecasts of the conditions at Earth (Lang et al., 2021;Turner et al., 2023).The availability of multipoint heliospheric data also facilitates the reconstruction of large-scale heliospheric structure, meaning that L5 data and other assets at for example, L1 will provide routine "ground truth" of the global configuration of the interplanetary magnetic field (e.g., Laker et al., 2021).SIRs are also important to better understand how ICMEs propagate through the Heliosphere.Predictions of ICME arrival time show the importance of understanding the nature of the surrounding solar wind (Lee et al., 2013;Mays et al., 2015) including structure such as high speed streams which might complicate the identification and tracking of ICMEs (Wold et al., 2018).This means that L5 measurements provide useful information to better characterize the background nature of the solar wind into which a ICME may be ejected.Connected to this point, the geoeffectiveness of ICMEs and their subsequent impacts depend on the sheath region, the central driver, and the speed of the trailing solar wind (Hietala et al., 2014;Kilpua et al., 2015).The sheath region is "swept up" by the ICME as it transits from the Sun to Earth over several days and can be a significant driver of space weather impacts in its own right (Kilpua et al., 2019).Prior measurement of the solar wind at L5 may therefore provide insight into the likely geoeffectiveness of ICME sheath regions.
Finally, knowledge of the interplanetary magnetic field morphology is crucial for better predictions of Solar Energetic Particles (SEPs) at 1 AU (Marsh et al., 2013).This is particularly important for the extension of space weather forecasting to other systems and assets, particularly at the Moon and Mars (Green et al., 2022).
Magnetic field data from L5 therefore provides an excellent opportunity to more accurately forecast the geoeffectiveness a few days hence of solar wind conditions at the Sun-Earth line, both for direct prediction of SIR-driven storms, and to understand the possible severity of an ICME-driven storm.More generally, magnetic field measurements at L5 constrain models, maximize forecast skill, and are required to maintain a capability that is at least as good as that previously available to end-users when STEREO was in the vicinity of L5.

In Situ Magnetic Field Measurement Requirements at L5
In considering the measurement requirements for magnetic field observations at L5, the dynamic range requirement can be derived from the reasonable worst-case scenario.For this purpose, we select the 23 July 2012 ICME observed in situ by STEREO-A (Liu et al., 2014;Russell et al., 2013).This ICME did not encounter the Earth, but if it had done so it is predicted that it would have caused a Carrington level event (Baker et al., 2013).
An overview of the magnetic field measured by STEREO-A from 23 to 25 July 2012 is shown in Figure 1.The event consists of two interacting ICMEs corresponding to the two main peaks in the magnetic field strength (Liu et al., 2014).The left vertical line at 2012-07-23 20:55:00 marks the arrival at STEREO of a fast forward shock, and the right vertical line at 2012-07-23 22:55:00 marks the end of the compressed solar wind sheath region and the entry of STEREO-A into the leading ICME structure (Riley et al., 2016;Russell et al., 2013).The peak magnetic field strength was measured to be approximately 109 nT and stated to be "one of the largest interplanetary field strengths on record near 1 AU" (Russell et al., 2013).Liu et al. (2014) also note the existence of a previous event where a field strength of ∼110 nT was observed in the sheath region (D'Uston et al., 1977).This illustrates that strong fields can be observed in both the sheath region and the ICME itself.
Since the magnetic field can assume any orientation, and a safety factor is also desirable, a required maximum of at least ±200 nT on each axis follows directly.A minimum field measurement of 0.1 nT represents a reasonable resolution to diagnose the properties of the ambient solar wind at a distance of 1 AU from the Sun since statistics from the Wind spacecraft suggest the typical field magnitude is ∼ 4-6 nT (Eastwood et al., 2015).This dynamic range is understood to be that required for normal operations at L5.Although not a requirement, the availability of other modes with a larger dynamic range would allow for testing and operation at Earth, discussed further in Sections 4 and 5.
Requirements concerning absolute accuracy are driven by the need to maximize forecast accuracy, which relies on a sufficiently accurate measurement of the field itself.It has been shown that an error of ±1 nT in the north/ south component corresponds to an error of +5/ 6 nT in Dst (Bailey et al., 2020), and so for the purposes of space weather monitoring an absolute accuracy of ±1 nT is considered acceptable.In the solar wind, natural rotations of the solar wind magnetic field can be used to calibrate the instrument (in particular reducing error in the zero-level offsets which are the dominant uncertainties) to the order of ±0.2 nT (Plaschke, 2019).
The required time cadence of the measurement is initially derived from the fact that large-scale ICMEs and SIRs responsible for geomagnetic storms can last for hours or longer, and also from the time-scales of the corresponding geomagnetic activity.A 5-min resolution is a minimum baseline goal for event detectability.However, Figure 2 illustrates the impact of different time resolutions for observing the structure of solar wind drivers, here examining the ICME sheath region.By comparing 1-s, 1-min, and 5-min resolution data, we find that at 5-min cadence, it is difficult to distinguish the precise location of the fast forward shock, and the variability in the sheath region leading to localized strong fields is no longer evident.At 1-min resolution, major features and structure are more apparent.
The time resolution of the data also influences the ability of end-users to diagnose the properties of important features in the data.To illustrate this, Figure 3 shows STEREO-A observations of the fast forward shock between 20:50 and 21:00.At 5-min resolution, the detail of the shock is lost completely.Magnetic field measurements at 1-s resolution have been demonstrated as being sufficient to analyze the properties of the shock and determine its orientation (Riley et al., 2016); this is information which here could be used by forecasters to constrain important questions about the ICME propagation direction.In general, higher time resolution measurements at for example, 1-s resolution will give forecasters a much better understanding of the space weather drivers themselves; for example, to accurately diagnose the properties of the ICME sub-structure (e.g., shock orientation, sheath fluctuations) and SIRs (e.g., turbulence and long-wavelength Alfvén waves, etc.).The goal measurement bandwidth is therefore from DC to 0.5 Hz, assuming a cadence of 1 vector/second.The Mission Requirements Document (ESA, 2022) captures the magnetic field measurement requirements for Vigil and these are reproduced in Table 1; it should be noted that there is no noise requirement.For completeness, project-defined resource requirements are also included in Table 1.We now discuss requirements for latency and availability, which are not typically required in space science missions.Use of the data as actionable information in the context of space weather monitoring and forecasting leads to a latency requirement.Latency is defined here as the time difference between the acquisition of the signal and the delivery of data to users on the ground.This depends on the total system performance consisting of the instrument, the satellite, and the ground segment, and also includes the approximately 8-min light travel time from L5 to Earth.The latency requirement has been set as 60 min for the magnetic field.
Finally, we consider requirements for availability in terms of continuity and interruption.Under quiet conditions, Vigil is required to provide a continuous operational service, with a 0.97 (97%) availability (ESA, 2022).Such a requirement is not typically placed on a science mission, where availability requirements are not typically necessary to meet the science objectives.Within the mission architecture, this flows down to a requirement for 0.99 availability of the magnetic field measurements.Furthermore, to meet its mission objectives, Vigil is required to be able to observe space weather events that themselves represent a hazard to the platform and instrumentation.

Instrument Concept and Technical Implementation
For space applications, measurement of the strength and orientation of the interplanetary magnetic field as required here is typically achieved through a three-axis measurement using a fluxgate magnetometer sensor.The

Space Weather
10.1029/2024SW003867 principle of measurement is long established (e.g., Acuña, 2002) and at the most basic level, the output of each sensor is a set of three voltages.These voltages are a function of the magnetic field vector as projected along three sensing directions that span three-dimensional real space.In addition to the magnetometer sensor itself and its associated conditioning electronics, the instrument may also be required to command and control the magnetometer sensor, undertake data processing, deliver appropriately packetized housekeeping and science telemetry, and to include sub-systems to appropriately use and distribute power delivered by the spacecraft.
The magnetic field requirements described in Section 3 can be met or surpassed by designs based on instrumentation previously developed for science missions, and the Vigil magnetometer instrument design, MAG, is as far as possible a rebuild of the J-MAG instrument developed for the ESA JUICE mission (Grasset et al., 2013).The J-MAG instrument itself has significant flight heritage from projects such as Cassini, Double Star, Rosetta (for electronics), and Solar Orbiter.J-MAG provides an appropriate starting point for the Vigil instrument for two reasons.The first is that the instrument architecture and design choices (e.g., redundancy and cross-strapping) are aligned with the Vigil requirements.The second is that J-MAG is designed to survive the harsh Jovian radiation environment, and radiation assurance requirements met by the J-MAG design through electronics component and sensor materials selection provide a strong basis for the necessary radiation hardness assurance on Vigil (discussed further in Section 6.1).
The MAG Instrument on Vigil consists of a dual-fluxgate design with total mass 5 kg and nominal total power of 8.36 W. Two identical fluxgate sensors are mounted on a dedicated magnetometer boom (discussed further in Section 6) and the associated sensor Front End Electronics (FEEs) reside inside the Electronics Box on the main platform.The Electronics box consists of the FEEs as well as a Direct Current -Direct Current Power Converter Unit (PCU-MEZ), Instrument Controller Unit (ICU), mechanical housing and cable tree.As well as maximizing heritage, the dual sensor design is a consequence of the requirement for availability, and the two sensors are identical, being held to the same operational and measurement requirements.A block diagram of MAG is shown in Figure 4.
The implementation of MAG on Vigil requires some modification of the Technology Readiness Level (TRL) 9 J-MAG which consists of three magnetometer sensors (two fluxgate sensors built by Imperial College London and Technische Universität Braunschweig (TUBS), respectively, and one absolute scalar sensor built by the Space Research Institute, Austrian Academy of Sciences, Graz) connected by harness to a platform-mounted electronics  box.Discussed in more detail below, these modifications are necessary to ensure compatibility with Vigil interface requirements and to accommodate the removal of the scalar sensor and its associated front-end electronics.In contrast to J-MAG, on Vigil the two magnetometer sensors will be almost identical to the J-MAG outboard fluxgate sensor built by Imperial College London.Figure 5 shows an illustration of the proposed Vigil MAG flight hardware, and the main constituent sub-units are summarized in Table 2.This table also shows the sub-unit TRL and associated information in the context of the Vigil mission implementation.In the remainder of this section we discuss these different instrument sub-units in more detail.

Fluxgate Magnetometer Sensor
The MAG instrument will consist of two identical fluxgate sensors built by Imperial College, with direct heritage from JUICE, building on the development of instruments for Solar Orbiter (Horbury et al., 2020), Cassini (Dougherty et al., 2004), and DoubleStar (Carr et al., 2005).These sensors will be mounted on a dedicated boom at different distances from the spacecraft and are designated as the magnetometer outboard sensor (MAGOBS) and magnetometer inboard sensor (MAGIBS) accordingly.Each sensor consists of two Permalloy ring cores, orthogonally mounted to a ceramic sensor block.Each ring core has one drive and two orthogonal sense windings, ensuring each ring core is sensitive to the magnetic field in two directions, and so only two cores are needed to realize a vector measurement.The sensor ceramic block is mounted to the boom via an insulating standoff and titanium baseplate, using a pin-dowel and slot mounting to ensure the alignment error from boom mounting is maintained within a known tolerance.Although the insulating standoff is not strictly necessary for Vigil, it is retained in the Vigil design to maximize the heritage of the design and ensure operational reliability.Finally the mirror cubes previously accommodated on the J-MAG sensor baseplate for optical alignment are removed for Vigil.
Thermistors and heaters (both operational and survival) are typically included as part of the associated electronics within the sensor unit.These provide measurement and control of the sensor temperature.However, whereas operational and survival heaters are included in the J-MAG design, these are not expected to be required for Vigil and so would not be fitted.At least two non-magnetic PT1000 Resistance Temperature Detectors (RTDs) are included in each sensor unit, and the sensors are covered with MLI.Electrical connection to the sensor is achieved via a sensor pigtail cable which mates via a connector to the MAG Inter Experiment Harness (IEH) that is mounted on the boom and platform.This harness is routed to the MAG electronics box (MAGELB).

MAG Electronics Box
The electronics box houses two Front End Electronics (FEE) cards that control the two sensors, an ICU, and a power controller unit (PCU-MEZ).These elements are housed in four individual frames, or slices.The PCU contains main and redundant sections hosted on the same Printed Circuit Board (PCB), together with a mezzanine board (MEZ).The ICU is hosted on the adjacent PCB, again including main and redundant sections.The third slice contains two half-size FEE PCBs, one each for the outboard and inboard magnetometer sensors.The fourth slice contains a dummy FEE (which replicates the mass load from the J-MAG scalar magnetometer FEE card that is not required for Vigil).This design decision was identified as the most optimal way to maximize the heritage of the design and minimize non-recurrent engineering that would significantly impact the project delivery timeline.The four slices bolt together to form a rectangular box as illustrated in Figure 6 which shows a CAD drawing of the complete MAGELB unit.Electrical connections between each slice are made using an external cable tree, which can be seen on top of the box.The connectors on the side of the box connect to the spacecraft, and to the sensors via the MAG IEH.

Front End Electronics (FEE)
The FEE for both sensors are identical.They are digital magnetometers with field detection being implemented inside a Field Programmable Gate Array on the sensor FEE as on JUICE, which itself is based on the design developed for the Solar Orbiter mission (Horbury et al., 2020).The sensor uses a drive frequency of F = 15.36 kHz to cycle the soft magnetic ring core material between deep positive and negative saturation states.The component of the external magnetic field aligned with the sense axis distorts the symmetry of the magnetic flux, generating a signal at 2F.This signal is processed to create a smoothed "field compensation" signal which is fed back into the sense coil.This offsets the external field within the measurement coil, bringing the resultant magnetic field at the position of the ring core inside the coil down to zero.The actual external magnetic field strength in the sensor direction is therefore proportional to the "field compensation" current in the coil.
The measured value of the external magnetic field is recorded as a digital value in the FPGA and then sent over the communications link to the ICU at a rate of 128 Hz.Communication between the ICU and FEE is implemented via UART.The FEE also contains the resistor network for range control (see Section 5), which scales the field compensating current and the selection of which is commanded through the ICU via the UART link.Although not required, this functionality is retained both to enhance the capabilities of MAG and maximize design heritage.

Instrument Controller Unit (ICU)
The ICU provides the command and data handling interface to the spacecraft bus and the sensor FEEs.It is built around the GR712RC-MS-CQ240 micro-processor ASIC from Cobham-Gaisler.It uses the SpaceWire (SpW) protocol, with the main and redundant links to the spacecraft bus connected via Low Voltage Differential Signaling (LVDS) drivers/receivers to the corresponding ICU.The on-board software is implemented as a Real Time Operating System based on Real Time Executive for Multiprocessor Systems (RTEMS) Version 4.10.2.A key role of the ICU is to filter and decimate the raw 128 Hz data from the two FEEs down to the appropriate rate for the selected operational mode, and to packetize and transfer this data to the spacecraft.The time delay for transmission from the sensor to the spacecraft bus is less than a second and the MAG instrument in itself is not a driver for meeting the latency requirements.The MAG instrument synchronizes its local instrument time with the spacecraft on-board time via Time Distribution Protocol.
The ICU design, based on J-MAG, was selected because it offers a fully redundant implementation, with ICU-1 and ICU-2 forming a cold redundant pair.ICU-1 is powered via PCU-1 from the main spacecraft power supply and ICU-2 is powered via PCU-2 from the redundant spacecraft power supply.The I/O lines for communication to the FEEs are cross-strapped.The SpW outputs are interfaced to the spacecraft bus using dedicated LVDS devices, in order to reduce the risk of damage from electro-static discharge, electro-magnetic interference, or short-circuits occurring external to the unit.The SpW interfaces within the MAG instrument are not crossstrapped that is, each SpW interface has a dedicated ICU processor ASIC that are individually interfaced to the Main and Redundant SpW.The ICU can communicate over the main or redundant SpW interface depending on whether main or redundant power is applied to the instrument.
Each ICU includes a number of internal hardware I/O lines from the PCUs and other ICU that are used for instrument control and instrument Failure Detection Isolation and Recovery (FDIR).These enable monitoring for both overvoltage and current limiting triggering from the PCU.The control lines give the ICU the ability to reset the FEEs by power cycling the corresponding FEE.The ICU-ICU line allows ICU-1 to notify ICU-2 that it is switched on and vice-versa.The ICU utilizes a watchdog requiring predefined reset signals to prevent the ICU software from becoming irrecoverably locked in an undesirable state.It is possible to disable the watchdog in the event of a malfunction.Finally, the ICU also includes the capability to communicate with the PCU for the receipt of housekeeping information, and this uses the built in SPI core of the processor.

Power Controller Unit (PCU-MEZ)
The MAG PCU is a fully redundant power supply, and its design is functionally identical to the J-MAG design.It is formed by a redundant pair PCU-1 and PCU-2, which are functionally isolated and not cross-strapped (i.e., the main power cannot be routed through the redundant supply chain and vice versa).These configurations also meet the requirement of prevention of failure propagation from one supply to the other (main and redundant), or from MAG to the spacecraft.ICU-1 and ICU-2 supplies are not cross-strapped, thus PCU-1 will power ICU-1 and PCU-2 will power ICU-2.This also means that communication between the ICU and the PCU as well as the control of power switches does not need to be cross-strapped, reducing complexity and simplifying the overall design.Each PCU has two DC/DC converters (providing separate analog and digital voltage supplies) for the instrument sub-units, to prevent cross-talk of digital loads with sensitive analog circuits.The PCU is implemented as a single card filling one slice inside the MAGELB with an attached mezzanine card (MEZ).The MEZ card houses the power switches and two of the cable tree connectors.
When 28 V power (with minimum of 26 V and maximum of 29 V) is applied to the power connector of either PCU, it will start up and power up the corresponding (i.e., main or redundant) ICU immediately.The ICU then manages switch on of the FEEs via telecommand.The ICU is supplied with secondary voltages featuring additional overvoltage protection.Power switches are included on the secondary lines to the two FEE cards to give the ICU the ability to switch the FEE electronics on individually and switch them off in the event of a fault.
ICU monitoring of the PCU is achieved via two analog-to-digital converters, measuring the levels of supplied voltages, and the combined current levels for each supply, as well as the output voltage of a temperature sensor on the PCB.The PCU also provides hardware over-voltage protection at each output.If any output line exceeds a fixed threshold voltage, immediately all FEEs are powered off, and ICU is signaled that an overvoltage has occurred, which may trigger further FDIR actions.The FEE power switches (within MEZ) include soft-start and over current limiting which signal the ICU if any of the power line switches are limiting the current to either of the FEEs.The ICU can then respond by triggering FDIR actions to power-off the FEE after a certain threshold time, as spurious events (e.g., Single Event Transients, switch on, power glitches) might trigger the limiter for a very short time and should be allowed to do so without causing it to be switched off.

Harness and Cable Tree
The MAG IEH consists of several cable assemblies (one set per MAG sensor) that connect the sensor FEE on the MAGELB to the sensor pigtails on the boom.External cable trees (illustrated in Figure 6) are used to make the interconnections between the electronics boards inside MAGELB.Micro-D Miniature (MDM) connectors are used.The cable tree is designed for ease of use and debugging purposes.

Instrument Concept of Operation
The nominal procedure for acquiring science data is straightforward.Data rates are configurable and can be set for each sensor.The data rates determine the filter and/or decimation factor that the ICU applies to the raw FEE data to create the final data product.24-bit sampling of the magnetic field is performed on the FEE, and this is decimated to 20-bit by a digital filter on the ICU (no compression is applied).The precision is based on the 20-bit digital resolution; this is calculated to be 0.5 pT for a range of ±250 nT and 95.4 pT for a range of ±50,000 nT.Science data can be sent from the instrument at either 128 Hz, 4 Hz, 1 Hz or 1/60 Hz, selected by telecommand, and other rates can be configured in the final design.Any combination of these data rates on the inboard or outboard sensor can be selected.Note that the 128 Hz mode is for ground-based performance validation only.The science packet itself can either be delivered to the bus at 1 Hz or (1/60) Hz.
Conversion of the measured voltage to magnetic field is again well understood.Each voltage is converted into a magnetic field measurement based on the known response of the instrument, which is quantified by gain and offset.The data vector is then converted from the non-orthogonal measurement coordinate system to a known spacecraft coordinate system and then to a heliophysical coordinate system.This calibration process is discussed in more detail in the next section.
The MAG instrument will have at least two operating ranges.Provisional values are shown in Table 3. Range two is primarily designed to facilitate ground testing, but also leaves open the possibility of measurements to be made from the start of the mission in low-Earth orbit should this be considered desirable.Depending on the ambient
field, the MAG instrument automatically selects the most appropriate operating range for each sensor via an autoranging function implemented across the ICU and FEE.All three axes of each fluxgate sensor will always operate in the same range.The magnetometer thus operates largely autonomously and will switch instrument range in response to the magnetic field crossing pre-set thresholds that are configurable by telecommand.However, this functionality can also be disabled and the range commanded manually.In practice it is not anticipated MAG will switch out of the ±250 nT range during the operational phase of the mission at L5.

Availability Analysis
A critical difference between operational and scientific measurements is the need in the former to meet availability requirements.To demonstrate this, a reliability and availability analysis of the MAG instrument as a whole has been performed, which considers the reliability rating of individual parts, and modeling of the environment they will operate in.To do this, the FIDES tool based on Part Count Method was used, and calculations were made using representative boards derived from the JUICE J-MAG magnetometer, assuming identical MAGELB panel thickness.
In completing this work, we examined two mission scenarios where the transit time to L5 is different, but the overall mission lifetime is the same: • Scenario 1: 18,250 hr transfer to L5, 47,450 hr at L5, 65,700 hr total • Scenario 2: 13,870 hr transfer to L5, 51,830 hr at L5, 65,700 hr total Given that the transfer to L5 is essentially the same environment as experienced at L5, both scenarios lead to an availability of 99.9894%.This availability follows from a calculated failure rate for the whole MAG instrument of 0.012,864,586 failures/year, and a nominal outage of 3 days for reconfiguration and recovery.This means that no changes to parts selection or the mechanical design of MAGELB are required to meet the availability requirement.

Magnetic Cleanliness
First considering the MAG instrument itself, as introduced above each sensor measures the component of the magnetic field along three sensing directions that span real space.These directions, S 1 , S 2 , and S 3 , are only nominally orthogonal, as manufacturing limitations prevent perfect orthogonality.The use of feedback means that the response along each sensing direction is highly linear, and so the response along direction S i can be characterized by a gain G i and offset O i .The transformation of these measurements into an orthogonal coordinate system defined by the sensor's mechanical build axes requires knowledge of the elevation angle θ i and azimuth angle ϕ i of S i .Twelve parameters (3 gains, 3 offsets, and 6 angles) are therefore required to convert the sensor measurement into an orthogonal measurement in physical units.If B C represents the true magnetic field in the Cartesian sensor frame, and B S is the measured signal in the non-orthogonal sensor frame, we may write where M is a 3 × 3 matrix derived from θ i and ϕ i (transforming from the cartesian to the sensor frame), G is a 3 × 3 diagonal matrix containing gains G i , and O is a vector containing the three offsets.
In practice, the 12 calibration parameters are determined pre-launch by calibration of the magnetometer (see e.g., Connerney et al. (2017) for a comprehensive discussion of this process).During construction, multiple measurements of sensor offsets will be made as a function of sensor and electronics box temperatures, as well as tests of instrument bandwidth and filter performance.The three-axis coil system at the Conrad magnetic observatory will be used to apply known fields and compare against calibration magnetometers traceable to national standards, establishing the calibration parameters before flight.However, external factors can potentially influence the stability of these parameters, which means that they must be analyzed and tracked during inflight operations.Sensor gains and offsets can drift slowly during flight particularly because of changes in temperature, and so periodic in-flight calibration is required using standard and widely used procedures, for example, the statistical properties of the solar wind (e.g., Plaschke, 2019 and references therein).Since the drift in sensor offset is typically slow (e.g., Alconcel et al., 2014), the nature of the changes means that this can be performed relatively quickly and infrequently (e.g., daily or even less frequently), with little human intervention, and provides an updated set of parameters that can be used for the next day's observations.This is particularly true in the case of Vigil as the thermal environment at the fluxgate sensors should be almost constant once the spacecraft reaches its final orientation at L5. Thus, the latency requirements are fundamentally achievable and realistic since the data processing requires only a series of matrix transformations which can be performed relatively quickly.However, this does not account for contamination of the magnetic field measurement as we now describe.
Contamination of the magnetic field measurement by the spacecraft in the form of remnant, stray, and induced magnetic fields can take several forms (see e.g., Finley et al., 2023): • constant (e.g., sources of permanent magnetization near the sensor); • periodic (e.g., generated by electrical systems); and • aperiodic (e.g., magnetic disturbances associated with operation of the spacecraft such as camera actuation, operation of reaction wheels, etc.) Flight heritage and experience, for example, with Rosetta (Richter et al., 2012), DoubleStar (Carr et al., 2005), Venus Express (Zhang et al., 2008) and Solar Orbiter (Angelini et al., 2022), show that the removal of these types of measurement contamination are not easily automated and typically require human supervision and intervention.For some science investigations a clean measurement is fundamentally required (e.g., on the JUICE mission and the characterization of Ganymede's sub-surface ocean) whereas for others (e.g., Rosetta and Venus Express) there is a trade-off that can be made where the latency, science, and measurement requirements can accommodate a worse magnetic environment.However, this trade is highly non-linear.As soon as any significant human intervention is required, the latency of the data delivery rises well beyond the requirements of the Vigil mission.
For a mission such as Vigil, this represents a significant risk to achieving the mission goals, as there is the potential to dramatically increase the latency of the data and the complexity of the operational data pipeline.In the case of Vigil, there are further unique considerations.For example, an operation on the spacecraft could increase the field strength at the sensor mimicking a ICME shock signature.The end-user would be required to establish whether this signal was natural, which could take several minutes, providing a significant delay and distraction.
Were this to occur regularly, it could be very detrimental to the overall service.
The appropriate technical solution to meet the measurement requirements is therefore to place two sensors on a boom, at different distances from the spacecraft.This provides redundancy, but also the opportunity to perform gradiometry to characterize any magnetic field signatures generated by the spacecraft, should they be detectable (e.g., Georgescu et al., 2008;Ream et al., 2021), as well as more advanced signal processing techniques (e.g., Hoffmann & Moldwin, 2023).This solution has the highest heritage, being the implementation of choice on the vast majority of missions with measurement requirements comparable to those of Vigil.The length of the boom is established through an engineering trade-off between the boom length and the platform Electro-Magnetic Cleanliness (EMC) program.For example, on JUICE, the boom length is 10.6 m.
The use of a long boom does potentially introduce a second source of "contamination" which is knowledge of the orientation of the sensor in a relevant heliophysical coordinate system.The axes of the sensor elements can usually be determined quite precisely on the ground, as can the knowledge of the sensor relative to the spacecraft mounting on the boom.There is then the knowledge of the mounting point orientation relative to the spacecraft body, and the orientation of the spacecraft relative to the heliophysical coordinate system of interest.These effects combine to add a further source of error in the measurement.MAG itself does not require any absolute or relative pointing accuracy: these considerations correspond to the definition of the Absolute Knowledge Error (AKE) of the sensor.The use of a boom increases the AKE of the sensor, but this is a well explored and understood area of study.
In summary, the magnetic field measurement requirements on Vigil are comparable to previously flown science missions, and do not represent any significantly novel challenge to the mission design.The solution with highest heritage, and therefore identified as the correct approach for this mission, is to: • Place two sensors on a boom whose length is likely to be comparable to that used on previous missions in similar environments (In the case of Vigil, the boom is required to place MAGOBS more than 7 m away from the spacecraft, with MAGIBS placed 0.8 times the distance of MAGOBS); • Place the two sensors in a stable thermal environment; • Reserve the boom for the exclusive use of the magnetic field instrument; and • Implement an appropriate EMC program on the spacecraft making full use of heritage and knowledge gained in the development of ESA science missions.

Summary and Outlook
Improved societal resilience to space weather can be achieved in a variety of ways, but arguably the most important is to increase the prediction time for severe events, giving end users more time to prepare and respond.This fundamentally motivates the Vigil mission, which will observe the Sun and solar wind from the L5 point.These observations will include in situ measurements of the solar wind and the interplanetary magnetic field, and here we have described in detail a magnetometer instrument, MAG, that will meet the requirements of Vigil.
The MAG instrument is based on considerable space science mission heritage, meaning that it is mature and relatively low risk.Its design is well-understood and requires relatively little tailoring to meet the requirements of Vigil.By placing the magnetometer sensors on a dedicated boom away from the spacecraft, the design solution ensures that the measurements will require relatively little post-processing ensuring the latency goals are also met.
This illustrates that space weather requirements can be different from science requirements, because the requirements for latency, availability and fault tolerance are more stringent.They are not a priori easier to meet, and this means that space weather missions are not necessarily less complex, even if the measurement requirements might be less stringent than for a science-oriented mission.In particular, we note that requirements for Product Assurance have the potential to be significantly more rigorous, ensuring stringent data availability requirements are achieved, potentially requiring more analysis and more intensive validation and verification processes compared to a science mission.
Vigil will provide operational space weather data, here taken to mean an effectively real-time low-latency feed of calibrated data, from instruments and detectors that offer high reliability, meeting the needs of space weather forecasters and end users (e.g., Morley, 2020).Vigil MAG thus represents the transfer of research to operations.However, even though Vigil is an operational mission, the entire payload, including MAG, has the potential to contribute to a variety of heliospheric science challenges beyond the operational space weather goals of the mission.We anticipate that the Vigil MAG data set will provide new avenues for scientific studies of the heliosphere, enabling the next generation of space weather practitioners to identify both new space weather risks and a deeper understanding of heliophysics more generally.In this way we hope that the MAG instrument and Vigil will inform basic research and contribute to creating a virtuous circle between research and operations.

Figure 1 .
Figure 1.STEREO-A magnetic field observations between 23-25 July 2012, at 5-min resolution.Panel (a) shows the magnetic field strength, and panels (b-d) show the components of the field in the R-T-N coordinate system.Vertical lines mark the arrival of a shock associated with the interplanetary coronal mass ejection (ICME), and the arrival of the ICME itself.

Figure 2 .
Figure 2. Magnetic field strength and R-T-N components observed by STEREO-A from 20:00-24:00 on 23 July 2012.From top to bottom, the data are shown at 1-s (panel a-b), 1-min (panel c-d) and 5-min (panel e-f) resolution.

Figure 3 .
Figure 3. Magnetic field measured by STEREO-A at 1-s (panel a-b), 1-min (panel (c-d) and 5-min (panel e-f) resolution, in the same format as Figure 2. The fast forward shock is marked by the red vertical line.

Figure 4 .
Figure 4. Block diagram of the Vigil MAG instrument.

Figure 5 .
Figure 5. Mock-up of the proposed Vigil MAG instrument, based on a photograph of the JUICE J-MAG flight model electronics box and outboard sensor.The red box marks the duplicated picture of the outboard sensor thus representing the anticipated hardware solution.

Figure 6 .
Figure 6.CAD drawing of the MAGELB Box Chassis.

Table 1
Vigil Magnetic Field Measurement Requirements, and Resource Requirements EASTWOOD ET AL.

Table 2
List of Vigil MAG Sub-Units, Including TRL and Justification EASTWOOD ET AL.

Table 3 EASTWOOD ET AL.