A.

Overview of optical instrumentation

The optical instrumentation is used to monitor the Sun and the heliospheric region between Sun and Earth. For this purpose, four instruments are required. The composite FOV covered by the four instruments is different for the two vantage points L1 and L5 as illustrated in Fig.2 and 3, respectively.

Fig. 2:

Remote-Sensing FoVs in L1 (goal). Red: Heliospheric Imager; blue: Coronagraph; black (centre): Magnetograph and EUV Imager, slightly exceeding the solar disk, which is not explicitly indicated. Note the FoV-gap between ~1.2 and 2.5 Rs (white colour), which is not covered by the current remote sensing payload baselined for the L1 mission.

Fig. 3:

Remote-Sensing FoVs for L5. Red: Heliospheric Imager, blue: Coronagraph, black (centre): Magnetograph and EUV Imager slightly exceeding the solar disk, which is not explicitly indicated. Note the FoV-gap between ~1.2 and 2.5 Rs (white colour), which is not covered by the current remote sensing payload baselined for the L5 mission.

North, south, east, and west in Fig.1 refer to the angular offset from the Sun-spacecraft line. In terms of the Heliospheric Imager (HI), the goal FoV (which is depicted) will most likely require a twin-camera solution, as is the case for STEREO/HI. The current HI L1 goal and L5 FoVs extend out to 60° (red colour), enabling imaging of Earth-directed events most of the way to Earth from the L1 vantage point and all of the way to Earth from L5. The HI inner camera covers 30° x 30°, centred in the ecliptic plane at 19° elongation. The HI outer camera covers 50° x 50° degrees, centred in ecliptic plane at 35° elongation.

Before discussing the individual aspects of the instruments, the related technologies and expected developments, we give an overview of the main features relevant for understanding the configuration and measurement objectives, and the differences and commonalities. It must be noted that although most of the parameters are relatively mature due to the existing heritage and experience, it is expected that the configurations will evolve in the next development phases.

B.

Coronagraph

The most destructive space weather effects at Earth are associated with CMEs, both through direct CME impact and via the Solar Energetic Particles (SEPs) generated at any accompanying CME shock front. As is becoming increasingly clear, the space weather potential of CMEs is magnified when consecutive CMEs interact or when CMEs interact with Stream Interaction Regions (SIRs). Moreover, if a CME propagates through a rarefied high-speed stream region, it is likely to dissipate less energy (as drag on it will be reduced) and will hence impact Earth at higher speed. Through Thomson scattering of photospheric light off free electrons, coronagraphs image the near-Sun solar wind (the solar corona) and the CMEs therein with, in general, full 360° azimuthal position angle coverage around the Sun in broad-band, visible light. Coronagraphs provide definitive, and clear, evidence that a CME has been launched and are hence fundamental to any space weather monitoring solution.

While coronagraphy in L5 provides a side-on view of Earth-directed CMEs, these CMEs exhibit a characteristic “halo” appearance in coronagraph imagery when viewed from a location close to the Sun-Earth line (e.g. L1), in that they appear to expand outwards from the Sun over the full 360° of azimuth. By providing images of such halo CMEs, a coronagraph situated near the Sun-Earth line can provide near-conclusive and timely proof of impending CME impact at Earth (ambiguity between front- and back sided CMEs can potentially be resolved through examination of complementary on-disk datasets). The C2 and C3 LASCO coronagraphs on the L1 SOHO spacecraft have underpinned space weather prediction services over much of the last two decades. For coronagraphic imaging, an L1 vantage point is preferable to Earth orbit ─ particularly for space weather monitoring ─ as the stray light and particle environments are much less challenging, and L1 is not subject to the Earth and lunar eclipses that beset many Earth orbits.

Advances in coronagraphy may in future prove more definitively the benefit of polarization measurements in the localization of CMEs [1], or the potential benefit of near-Sun coronal imaging (i.e. within 2.5 sun radii), for example for the provision of SEP predictions. Coronal imaging can also be performed within specific wavebands, in particular Lyman-alpha, which has the potential to yield unprecedented information on coronal magnetic fields [2]. However, these features are currently not considered in the baseline configurations. Currently initiated developments by ESA investigate the feasibility and target the demonstration of a new coronagraph design that should lead to a more compact instrument, which would be beneficial for the future mission implementations. The concept was first investigated by [3], and a first demonstration of the efficacy of this design has been carried out by NOAA [4]. The concept seems to be very promising and is currently considered as the baseline for the Lagrange missions. If the demonstration of the concept is not successful, then the traditional design used by LASCO C2 and C3 on SOHO will need to be revived resulting in larger resource demands, or alternatively other concepts with European heritage can be considered [5].

C.

Heliospheric Imager

The Heliospheric Imager shall provide wide-angle, white-light images of the heliospheric region between Sun and Earth. These images are required to enable tracking of Earth-directed CMEs over much of their propagation path, in order to mitigate deficiencies in arrival times predictions based on near-Sun data. CME morphology and propagation can evolve all the way out to 1 AU, due to mutual interactions and the interaction with the background solar wind (e.g. SIRs and Co-rotating Interaction Regions [CIRs]); such interactions are often poorly modelled. Moreover, heliospheric imagery can also provide information on the background solar wind itself (enabling, for example, prediction of SIR arrival at Earth). Such imagery is virtually impossible from the ground. Continued monitoring of Earth-directed CMEs requires observations from two vantage points to more accurately characterize CME morphology and kinematics. Heliospheric imaging from an L5 vantage point provides a clear, side-on view of Earth-directed CMEs which permits more accurate tracking of their propagation. A vantage point some 40° to 80° from the Sun-Earth line is considered optimal, L5 is preferred also due to other advantages. Together with observations from L1, this stereoscopic view of CMEs enables their properties to be reconstructed and hence the accurate determination of CME arrival time and speed at Earth. Note that an L1 vantage point is preferable to Earth orbit as the stray light and particle environments are much less challenging, and L1 is not subject to the Earth and lunar eclipses that beset many Earth orbits.

The technical challenge with this instrument lies in the very faint signal that is generated by the CMEs, which is much less than the background signal. Any additional stray-light generated by scattering off S/C structures or insufficient baffling could easily result in the inability of the instrument to observe CMEs. However, the concept of heliospheric imaging has proven heritage (STEREO HI). Due to the limited azimuthal coverage of the FoV, observations from L1 require two instruments to observe both east and west of the Sun-Earth line. Although the HI optical camera systems are relatively small, the instrument is dominated by the baffling arrangement and the cooling configuration of the detectors to enable low-noise detection. Further miniaturisation relies therefore on advances in detector technologies, baffle design and coating technologies, including the corresponding modelling capabilities. It is planned to investigate if such instrument can be further optimised to reduce the resource demands and to improve the performance.

It should be noted that future technologies might exploit also here the polarization properties of heliospheric visible-light, which is suggested to be able to greatly improve the localization of CMEs (more so than in the corona). Although proposed for several potential missions [6] a polarized heliospheric imager has not yet been flown, so this capability is unproven and also not considered in the baseline configuration. However, this, and other advancements in heliospheric imaging shall be borne in mind as the instrument and mission design processes progress, or for future next generation implementations.

D.

Magnetograph

Presently, low-resolution (angular resolution ≥5 arcsec) longitudinal magnetograms are, for the majority of observational endeavours, used as the basis on which to model the background solar wind (including SIRs and CIRs) for arrival time prediction of CMEs (and background structures themselves) at Earth. Here we propose a longitudinal magnetograph with 5 arcsec spatial (angular) resolution as the threshold solution. Inspection of low-resolution longitudinal magnetic fields can also provide some basic “watch” warning of developing solar activity, from a magnetic perspective. From L5, such data would allow active regions that may present a threat to Earth to be identified 6 or 7 days in advance (compared to 3 or 4 days feasible using terrestrial or L1 observations).

The high-resolution (≤2 arcsec) vector magnetograph proposed here as the goal solution, instead would provide the complete photospheric vector magnetic field information (field strength, azimuth, inclination) and most of the physical parameters (e.g. distribution of vertical and horizontal magnetic fields, distribution of inclination angles, twist, writhe, helicity, current density, share angles, photospheric magnetic excess energy etc.) for future enhanced space weather applications. Studies on how this data can be utilized and how much it would benefit sophisticated flare and CME forecast models shall be carried out in parallel to the on-going technical activities. The absence of a comparable ground based network perfectly justifies the consideration of the high-resolution vector magnetograph also for the L1 mission. Identical instrumental, observational and timing constraints at both Lagrange points shall allow the perfect merge of high-resolution vector magnetic field information for 67% of the solar surface, one of the important synergies to be expected from a combined L1/L5 mission.

Solar white light images are a low-cost by-product (in terms of telemetry, mass, power) of standard magnetographs and are realized in the form of continuum images, observed at an additional wavelength point in the vicinity of the magnetically sensitive spectral line under investigation. Hα and Ca II IR line core images could be considered as a further optional by-product of the magnetograph, delivering important chromospheric context information on the formation of solar eruptive events. Thus finally, the magnetograph could in principle sequentially scan the magnetically sensitive white-light photospheric line (at 4 wavelength points for magnetograms and at 1 wavelength point for the continuum image), and optionally the chromospheric Hα line (at 1 additional wavelength point at the Hα line core) and/or the chromospheric Ca II IR line (at one additional wavelength point at Ca II IR line core).

Due to the involvement of European institutes in previous developments of magnetographs in space [7], and in particular for the PHI instrument on Solar Orbiter [8], a high level of expertise and good heritage for the operational instrument development already exists in Europe. However, the PHI instrument is built for the requirements of the Solar Orbiter mission and therefore includes two telescopes, a Full Disk Telescope (FoV=2.1°) and a High Resolution Telescope (FoV= 16.8 arcmin). For the operational space weather monitoring a single telescope is currently presumed to be sufficient. Due to the much closer distance of Solar Orbiter to the Sun, the optical configuration of PHI is not directly suitable and must be redesigned or adapted for the Lagrangian missions (at 1 AU distance to the Sun) in any case. However, most of the technologies, subunits, electronics or instrument control- and data processing software should be reusable. At present it is unclear whether or not LiNbO₃ etalons will be still available or if a potential backup solution will be required. As far as the detector is concerned, it is expected that the detector from PHI, which is a 2048 x 2048 CMOS detector, can be reused. For the preparation of the Lagrangian missions it is foreseen to build an elegant breadboard, focussing on the consolidation of the required instrument performance and the optimisation of the optical design for space weather purposes. If necessary, the development or adaptation of the critical subunits has to be initiated. Identical instruments are foreseen for the L1 and the L5 mission.

E.

EUV Imager

EUV imaging, exploiting a core set of wavelengths corresponding to different atomic transitions from highly ionized ions at a wide range of different temperatures, is used to monitor solar activity in the solar transition region and corona. This allows the imaging and monitoring of active regions, coronal holes and quiet Sun, as well as other solar activity such as flares and prominence eruptions. Thus, images in selected EUV emission lines are valuable for the monitoring of magnetic complexity and impending Earth-affecting solar activity, particularly from L5 on the region of the Sun that is yet to rotate to the longitude of Earth. The different EUV wavelengths provide information on different optical depth layers of the solar atmosphere; whilst one wavelength, utilizing a highly ionized ion of, say, iron, can provide valuable monitoring of coronal structure and activity (threshold scenario). Several lines across a range of temperatures (goal scenario) allow the investigation of a wider range of key phenomena and solar atmospheric heights. In this context, EUV imaging provides a basic measure of approaching complex active regions and the more immediate indicator of potentially Earth-impacting events such as flares and eruptions.

Recent examples of such imagers include the EIT instrument [9] aboard SOHO at L1, the SDO/AIA [10], the Proba-2/SWAP instrument [11] in Earth orbit, the EUI of Solar Orbiter [12] and the EUVI instrument [13] aboard the STEREO spacecraft in solar orbit. Due to identical instrumental, observational and timing constraints at both Lagrange points, the synergies gained from a combined L1/L5 mission will be without existing comparison, allowing the perfect merge of high-resolution high-cadence EUV images for 67% of the solar surface and off solar limb. This will lead to much improved coverage of the development of active regions on the east side of the Sun, yet to rotate to the longitude of Earth, where increasing magnetic activity can finally lead to energy release and the eruption of Earth-directed events. Therefore, the operational follow-up and investigation of chromospheric and low-coronal activity by means of EUV images shall be another cornerstone for enhanced flare and CME forecast in the frame of the combined L1/L5 mission. The mentioned heritage instruments have various configurations. Whereas ESIO is geared towards a small instrument with a detector array of 512 x 512 pixels and a resolution of 12 arcsec, SWAP provides (as part of the SSA service fleet) 3.2 arcsec sampling with a 1k x 1k detector. EUI employs ‘low’ and high resolution channels with 2k x 2k and 3k x 3k detectors. The choice of the (EUV enhanced) detector appears therefore uncritical, and instead it needs to be decided which of the mentioned instruments are considered as the best baseline for the Lagrangian missions. Developments will depend on the choice of the final spatial resolution of the instrument and the number of wavelengths considered necessary. Wavelength selection is certainly a critical aspect for an operational mission with the expectation of high reliability for mechanisms, and the selected components of the used electronics, since none of the heritage instruments are ready to provide four different wavelengths. The selection of the number of wavelengths / filters needs therefore careful consideration.

F.

Optional Remote Sensing Strawman Payload

Within the Phase 0 studies and to support the objectives of the SSA programme, we have investigated the usefulness of a NEO Imager to detect, monitor and characterize Near-Earth Objects (NEOs). Because of the background light from the Sun, optical observations from the Earth are very constrained if objects within 1 AU of the Sun are to be observed. Observations from the Lagrangian point could therefore make use of the shadow of the S/C and be able to scan the inner regions within the Earth orbit. It was confirmed that a scanning instrument would be very efficient and could potentially detect the majority of the otherwise invisible objects, and thereby prevent unexpected impacts on Earth such as the Chelyabinsk meteor or asteroid 2016QA2 that recent passed unexpectedly close to Earth. Some first ideas of the corresponding scanning strategies have been identified. Further investigations are necessary to maximize the overall NEO detection and orbit characterization efficiency, and to allow for early and effective mitigation strategies against objects with high impact probability on Earth. A scanning mechanism and optical configuration with low system impact would need to be developed. In the current configuration, the L1 and L5 mission architectures and preliminary designs show sufficient system margin. For the L5 mission, limitations are rather on the data transmission than on the power and mass. Other instruments could therefore in principle be included in the L5 payload if sufficient funding for the mission is provided and if such instruments can be developed within the current time frame, which foresees the launch in 2023.