Slow Solar Wind Connection Science during Solar Orbiter’s First Close Perihelion Passage

The solar wind is the continuous stream of hot, tenuous plasma that flows away from the Sun, carrying magnetic flux into the heliosphere. Historical observations have shown that the solar wind generally exists in two relatively different regimes: slow ($\lesssim$ 500 km s^-1) and fast ($>$ 500 km s^-1) wind streams. However, the distinction between these two different solar wind streams should not be made based on speed alone.

Despite several decades of remote sensing observations and in situ measurements made by a multitude of spacecraft, there is still no consensus on the origins of the slow solar wind. It is well known that the source of the fast solar wind is open magnetic field that is rooted in dark regions of the solar atmosphere seen in EUV known as coronal holes (CH; Wilcox 1968; Krieger et al. 1973; Zirker 1977; Cranmer 2009). However, there are still many open questions regarding the formation of the slow solar wind including where does the slow solar wind originate, and how is it heated, released, and accelerated into the heliosphere? For recent reviews on this topic see Abbo et al. 2016; Cranmer et al. 2017; Viall & Borovsky 2020.

While the slow solar wind is generally characterised by properties such as low proton speeds, high proton temperatures, dense plasma, large mass flux, greater variability, and high values of first ionisation potential (FIP) bias (Abbo et al., 2016), slow wind can at times exhibit properties similar to the fast wind including high Alfvénicity (high-degree of correlation between magnetic field and velocity components) or a low FIP bias. These varying properties suggest that slow wind likely originates from a multitude of source regions with differing properties, multiple release, and acceleration mechanisms.

One promising candidate that has recently been proposed as a source of the slow solar wind are upflows at the boundaries of active regions (ARs). Spectroscopic observations taken by the Extreme ultraviolet Imaging Spectrometer (EIS; Culhane et al. 2007) on board the Hinode spacecraft (Kosugi et al., 2007) have revealed persistent blue shifts in coronal lines such as Fe XII 195.12 Å indicating the presence of upflowing plasma (Sakao et al. 2007; Harra et al. 2008; Baker et al. 2009; Brooks et al. 2015; Hinode Review Team et al. 2019; Stansby et al. 2021; Tian et al. 2021; Yardley et al. 2021; Brooks et al. 2022 and references therein). These upflows are present during the entire lifetime of active regions (e.g. Démoulin et al. 2013; Baker et al. 2017; Brooks et al. 2021). If these upflows are associated with open field lines, then this plasma is able to escape outwards and into the heliosphere as the solar wind. Remote sensing and in situ abundance measurements have also demonstrated the contribution of plasma originating from active regions to the solar wind (Harra et al., 2008; Wang et al., 2010; Brooks & Warren, 2011; van Driel-Gesztelyi et al., 2012; Brooks et al., 2015; Macneil et al., 2019; Stansby et al., 2020).

Not all active regions that exhibit upflows are associated with open magnetic field. In these cases, upflows can become outflows if the active region is in close proximity to a coronal hole (van Driel-Gesztelyi et al., 2012; Edwards et al., 2016). Alternatively, upflowing plasma could escape indirectly through interchange reconnection between the closed magnetic field loops of an AR and adjacent open magnetic field (Crooker & Owens, 2012; Mandrini et al., 2014; Owens et al., 2020; Brooks & Yardley, 2021).

Another source that can also contribute to the slow solar wind are coronal holes (CHs), although these are mainly responsible for fast solar wind streams. Two ways that coronal holes can contribute is through interchange reconnection between the CH boundary and neighbouring magnetic flux (Fisk & Schwadron, 2001; Antiochos et al., 2011) or through rapidly diverging magnetic field present inside small CHs, or at the boundary of large CHs (Cranmer, 2009). The latter is based on the inverse relationship between the solar wind speed at 1 au and expansion rate of magnetic flux tubes, typically determined at the source surface height of 2.5 R_⊙ using coronal magnetic field extrapolations (e.g. Wang & Sheeley 1990). Recent analysis by Stansby et al. (2019) found that slow wind with high Alfvénicity detected between 0.3 and 0.4 au supports this scenario, as the high Alfvénicity could indicate outflows along open magnetic field. Alfvénic slow wind has also been detected at 1 au originating from CH boundaries or small equatorial CHs (D’Amicis & Bruno, 2015; D’Amicis et al., 2019; Wang & Ko, 2019). Finally, another contributor to the slow solar wind are small blobs that are visible in coronagraph observations and are continuously released and travel outwards from the cusps of helmet streamers, known as streamer blobs (e.g. Sheeley et al. 2009; Rouillard et al. 2010a, b, 2011).

In order to provide a complete picture of the solar wind including its origin, heating and acceleration mechanisms, transport processes, and composition, the joint ESA/NASA Solar Orbiter mission (Müller et al., 2020; Garc´ıa Marirrodriga et al., 2021) was launched from Cape Canaveral, Florida on 10 February 2020. Solar Orbiter hosts a payload consisting of six remote sensing (Auchère et al., 2020) and four in situ instruments (Walsh et al., 2020), which are now working together to answer open questions regarding the origin and formation of the slow solar wind by connecting slow solar wind measured by in situ instruments with remote sensing observations of its source regions. Given the limited spatial field of view and temporal observing windows due to its unique orbit, remote sensing observations taken by Solar Orbiter need to be planned and coordinated meticulously in advance. To achieve Solar Orbiter’s science goals, Solar Orbiter Observing Plans (SOOPs) have been designed. For details regarding Solar Orbiter’s Science Activity Plan, mission planning and the individual SOOPs refer to Zouganelis et al. (2020). The coordination of the science operations for the remote-sensing and in situ instruments are governed by the Remote Sensing Working Group (Auchère et al., 2020) and the In situ Working Group (Walsh et al., 2020).

In this paper, we describe the planning, observations and preliminary results of the Slow Solar Wind SOOP that operated during Solar Orbiter’s first perihelion in March 2022.

The Slow Wind SOOP or L_SMALL_HRES_HCAD_Slow-Wind-Connection (Section 4.14 of Zouganelis et al. 2020) has been designed to identify the sources of the slow solar wind and address the top-level science goal of Solar Orbiter: “What drives the solar wind and where does the coronal magnetic field originate from?” More specifically, the SOOP aims to:

• •

  link plasma observed in situ to specific coronal source regions,

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  observe the magnetic field configuration and its evolution at open-closed boundaries,

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  identify possible signatures of interchange reconnection at open-closed boundaries.

The primary targets of the Slow Wind SOOP are therefore either the periphery of an active region or the boundary of a coronal hole.

While the in situ instruments on board Solar Orbiter can take measurements almost continuously, the remote sensing instruments take full disk synoptics throughout the orbit but high-resolution observations only during three $\sim$10-day windows, that during low-inclination orbits are typically centered around the closest approach to the Sun. The Slow Wind SOOP operated at the beginning of the first and second remote-sensing windows (RSWs 1 and 2), prior to Solar Orbiter’s first close perihelion passage on 2022 March 26 (see Figure 1 a, adapted from the Solar Orbiter website¹¹1https://issues.cosmos.esa.int/solarorbiterwiki/display/%****␣main.tex␣Line␣375␣****SOSP/LTP06+Q1-2022).

The time periods within which high-resolution remote-sensing observations were taken for the Slow Wind SOOP ran from 2022 March 3 at 06:30 UT until 2022 March 6 at 18:30 UT (RSW1, 3.5 days in duration) and from 2022 March 17 06:00 UT to 2022 March 22 00:00 UT (RSW2, 5 days) when Solar Orbiter was at a distance from the Sun of 0.55–0.51 and 0.38–0.34 au, respectively. Solar Orbiter was close to the Sun-Earth line for the first instance and had a separation angle of 27-48^∘ from Earth for the second. Observations were taken using all ten instruments on board. The various studies and modes of the instruments are shown in Figure 1 b and are described below. These can mainly be split into continuous, synoptic and high-resolution observations. The SPectral Imaging of the Coronal Environment (SPICE; Spice Consortium et al. 2020; Fludra et al. 2021) instrument is central to achieving the SOOP science goal and so this instrument and its observing modes are described first, followed by the rest of the remote sensing, and finally the in situ instruments.

SPICE is an EUV imaging spectrometer that operates in two wavebands: 704–790 Å (short wavelengths), and 973–1049 Å (long wavelengths), providing coverage from the upper chromosphere to the low corona (20 000 K to 2 MK) and two lines that cover the flaring corona (up to 10 MK). The SPICE instrument provides intensities and line widths. SPICE also allows for the investigation of plasma composition through the derivation of elemental abundances. It produces rasters, created by scanning the solar disk from west to east (right to left), acquiring either a full spectrum or a limited wavelength range over the course of a few hours. For the slow wind SOOP, SPICE operated a combination of the following three studies each day:

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  Spectral Atlas. FOV 2’x11’, 4” steps, 60 s exposure, 53 min duration, 1 per day, full spectrum,

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  Composition Map. FOV 10.6’x11’, 4” steps, 60 s exposure, 2.7 hr duration, which includes the following lines: O III 703 Å multiplet, Mg IX 706 Å, S IV 750 Å, N IV 765 Å, Ne VIII 770 Å, Mg VIII 772 Å, S V 78.6 Å, O IV 787 Å, C III 977 Å, O VI 1032 Å,

• •

  Slow Dynamics. Synoptic study, FOV 12.8’x11’, 4” steps, 30 s exposure, 1.6 hr duration for each repetition (5-6 times), which includes the following lines: O III 70.3 Å multiplet, Mg IX 706 Å, O II 718 Å, O V 760 Å multiplet, N IV 765 Å, Ne VIII 770 Å, S V 786 Å, O IV 787 Å, C III 977 Å, H I 1025 Å, O VI 1032 Å.

The observation IDs for SPICE data taken during RSW1 and RSW2 are 100663695 to 100663711, and, 100663837 to 100663864, respectively.

The observations taken by SPICE were complemented by both synoptic and high-resolution images from the Extreme Ultraviolet Imager (EUI; Rochus et al. 2020) and the Polarimetric and Helioseismic Imager (PHI; Solanki et al. 2020). The EUI instrument has three different telescopes; one Full Sun Imager (FSI) providing images in two passbands (174 Å  and 304 Å ), and two high-resolution imagers that capture the Sun in 1216 Å (H ILyman-$\alpha$) and 174 Å. The EUI therefore covers from the upper chromosphere up to the low corona. Both HRIs take 2048 $\times$ 2048 pixel images with a temporal resolution of 2-30 s. The imagers, that have a FOV of 17′$\times$ 17′corresponding to 0.28 R${}_{\odot}^{2}$ when the spacecraft is at perihelion, capture the small-scale structure of the slow wind source regions. During the two slow wind SOOP windows, FSI synoptics were taken with a 900 s cadence and high-resolution images were taken with a cadence of 6 s over a period of 1 hr per day. The EUI observations taken for all SOOPs that operated during the first close perihelion passage are described in Berghmans et al. (2023).

The PHI instrument consists of two telescopes: one that images the full Sun, known as the Full Disc Telescope (FDT), which has a FOV of 2^∘ $\times$ 2^∘ and an angular resolution of 3.75” per pixel, and the high resolution telescope (HRT) with a FOV of 0.28^∘ $\times$ 0.28^∘ with an angular resolution of 0.5” per pixel (Gandorfer et al., 2018). PHI scans the Fe I 6173 Å spectral line to produce filtergrams at 6 different wavelength positions in all 4 Stokes parameters (i.e. full polarimetry). Raw data can be compressed and downlinked directly or the inversion can be carried out on board. PHI provides the continuum intensity, line-of-sight (LOS) velocity, LOS magnetic field, and the vector magnetic field. PHI observations can be used to understand the evolution of the source region’s magnetic field, and, along with EUI, to determine whether interchange reconnection is taking place at the CH or AR boundary. Extrapolations of the photospheric field can also be constructed that provide the coronal magnetic field structure across the boundary of the source region. The FDT only operated during the second observing window, providing full disk images with a 360 min cadence. Conversely, the HRT operated with a 3 and 5 min cadence for 1 hr per day, during the first and second remote sensing windows, respectively, where these 1 hr high-resolution observation time windows coincide with the observations taken by the EUI/HRI telescopes. The aberrations introduced by the non-radial temperature gradient in the entrance window have been removed as described by Kahil et al. (2023, submitted to A&A).

Metis (Antonucci et al., 2020) is a solar coronagraph that simultaneously images the outer solar corona in visible and UV wavelengths. As we observe on-disk targets and Metis is used for off-limb observations of the solar corona, we did not require Metis for these particular runs of the SOOP. Therefore, Metis took generic observations during the first window of the SOOP, but was closed during the second window. Finally, the Spectrometer Telescope for Imaging X-rays (STIX; Krucker et al. 2020), that observes solar flare X-ray emission, and the Solar Orbiter Heliospheric Imager (SoloHI; Howard et al. 2020), that provides white-light images of the outer solar corona, operated in normal modes.

In coordination with the high-resolution remote sensing observations of the slow solar wind source regions, measurements of the solar wind plasma were made by the in situ payload comprising four instruments. Measurements of electrons, protons, alpha particles and heavy ions are made using the Solar Wind Analyser (SWA; Owen et al. 2020). SWA consists of a suite of three sensors: the Electron Analyser System (EAS), the Proton-Alpha Sensor (PAS) and the Heavy Ion Sensor (HIS) which provide high-resolution, full 3D velocity distribution functions and bulk plasma parameters of electrons, major and minor ion species along with elemental abundances and charge states of heavy ions. PAS and EAS ran continuously in normal mode with short periods of burst mode having a cadence of 4 s and $<$ 1 s, respectively. HIS ran in normal mode with a cadence of 5 min with burst periods with a cadence of $<$ 30 s. The Solar Orbiter Magnetometer (MAG; Horbury et al. 2020) ran in burst mode during the SOOP providing interplanetary field measurements with 64 vectors s^-1, which are made available in radial-tangential-normal (RTN) and spacecraft (SRF) coordinates. The magnetic field, particularly the radial component, is essential in tracing the connectivity of the spacecraft back to the source region on the Sun. The Energetic Particle Detector (EPD; Rodr´ıguez-Pacheco et al. 2020) consists of a suite of sensors including: the Supra Thermal Electron and Proton (STEP) sensor, the Electron-Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) in order to record the parameters of different populations of energetic particles. A summary of the measurement capabilities of EPD can be found in Table 2 of Rodr´ıguez-Pacheco et al. (2020). EPD operated continuously in close burst and normal modes as Solar Orbiter was within 0.7 au, with 15 min per day allocated to burst mode. Finally, the Radio and Plasma Waves (RPW; Maksimovic et al. 2020) instrument measures waves along with electric and magnetic fields, observing radio emissions up to 16 MHz. RPW operated in normal high mode with 720 min a day allocated to burst mode measurements.

Tracing solar wind plasma measured in situ by Solar Orbiter back to the Sun and estimating the location of its source region requires modelling the magnetic connectivity of the spacecraft, which can be approximated using ballistic backmapping. In the first instance we assume the magnetic field structure to be constant over the time of flight of particles from the solar wind source to the satellite. In this case, particle trajectories are field-aligned and this will allow us to identify, in broad terms, areas in which the solar wind arriving at the spacecraft originated. However, in general, field lines will move and the topology will evolve during the particle transit times. This means that although particles move along field lines, their trajectories are not field-aligned and will give rise to dispersive features in both energy and pitch angle. If there is a single reconnection location, these dispersions could be exploited to give much more detailed information from the particle measurements. An example of the application of such ideas has been in the cusp regions of Earth’s magnetosphere, which are the outflow regions from the dayside magnetopause reconnection site: much has been deduced from these dispersions from the length and field strength distribution along the magnetic field lines (i.e., knowledge of the magnetic connectivity). For example, from the cusp ions of longest flight time, the conditions at the reconnection site and the reconnection rate have been derived (Lockwood, 1995; Lockwood et al., 1995; Lockwood, 1997; Lockwood & Hapgood, 1998). Application in the solar corona is more complicated due to the more complex field topology and because field lines may undergo a number of reconnection events, rather than at a single reconnection site giving rise to a distinct and identifiable particle population. Nevertheless, initial studies have already made deductions using particle dispersions of suprathermal electrons in interplanetary space (Larson et al., 1997; Owens, 2009).

To model the connectivity of Solar Orbiter, the MADAWG connectivity tool²²2http://connect-tool.irap.omp.eu/ (Rouillard et al., 2020a) is used. The connectivity tool combines different models of the coronal and interplanetary magnetic field to either forecast the magnetic connectivity of the spacecraft in advance, or to aid with post-observation analysis. Currently, by default, the tool reconstructs the coronal magnetic field using the potential field source surface (PFSS) model. The corona is then extended into interplanetary space by assuming a Parker spiral. In this case, the connectivity tool is used for both forecasting the magnetic connectivity of Solar Orbiter to determine the target pointing for the slow wind SOOP and also to aid with the connectivity analysis post-observation.

Immediately prior to the SOOP observing window, and throughout it, the SOOP coordinators used the tool to predict in advance the source region of the solar wind that will be detected at Solar Orbiter a few days later (i.e. accounting for Sun to spacecraft, Solar Wind lag), the coronal magnetic field is constructed using time-evolved synchronic ADAPT³³3https://www.nso.edu/data/nisp-data/adapt-maps/ (Arge et al., 2010; Hickmann et al., 2015) magnetic field maps as the boundary condition. The tool then assumes two extreme values for the solar wind speed: 300 km s^-1 for slow and 800 km s^-1 for fast wind. These two speeds are used to construct the shape of the corresponding Parker spirals and the tool then computes the magnetic connectivity points at the source surface, and then at the solar surface, by tracing the coronal field. The tool provides a distribution of connectivity points, rather than a single point, to take into account uncertainties in the magnetic field tracing.

When applying the tool to aid with post-observation analysis to trace solar wind plasma, measured in situ at the spacecraft, back to its source on the Sun (spacecraft to Sun, SW lag) the propagation time needs to be calculated using observed speeds. Once the tool has calculated the propagation time of the solar wind parcel, the appropriate magnetogram that should be used as the boundary condition to reconstruct the coronal field is determined. The propagation time also defines the azimuthal drift of the connection points to the Sun, that keeps rotated while the solar wind plasma propagates. The Parker spiral is constructed using the solar wind speed measured by Solar Orbiter that has been retrieved either from low latency or fully calibrated science data from SWA/PAS. If the measured solar wind speed is unavailable, then the two extreme values (300, 800 km s^-1) are used and the connectivity points are determined as described previously.

Coordinated observations were taken with Hinode and the Interface Region Imaging Spectrometer (IRIS; De Pontieu et al. 2014, 2021) via IHOPs 433 and 434 during RSW1 and RSW2 of the Slow Wind SOOP. Coordinates and timings for the targets were provided for Hinode and IRIS using the Solar Orbiter planning website⁴⁴4https://umbra.nascom.nasa.gov/solar_orbiter/planning/.

The Slow Solar Wind SOOP has been designed in order to understand the origins and formation mechanisms of the slow solar wind. The targets of the Slow Wind SOOP are open-closed magnetic field boundaries such as the edges of coronal holes and the peripheries of active regions where plasma can escape directly along open magnetic field or indirectly i.e. due to interchange reconnection. Solar Orbiter’s extensive suite of instruments and unique orbit provides the first opportunity to trace solar wind plasma measured in situ back to its source region on the Sun using data from a single space-based observatory.

The SOOP operated for the first time during Solar Orbiter’s first close perihelion passage in March 2022. High-resolution observations were taken during two remote sensing windows, which took place between 2022 March 3 06:00 UT and 2022 March 6 at 18:30 UT (RSW1), and 2022 March 17 06:00 UT until 2022 March 22 (RSW2). Solar Orbiter was at a Heliocentric distance of 0.55-0.51 and 0.38-0.34 au, respectively, during these time periods.

The targets for the high-resolution observations during both RSWs were chosen roughly three days in advance during pointing decision meetings based upon the predictions provided by the connectivity tool, low latency Solar Orbiter observations and supporting space-based observatories such as SDO and STEREO-A.

A target for RSW1 was easily identified prior to the commencement of the remote-sensing observations on 2022 March 3 as Solar Orbiter was predicted to be magnetically connected to a negative polarity of an AR complex in the southern hemisphere for the entire observation window. The predicted connectivity and its evolution was also very similar during the previous solar rotation. Therefore, high-resolution remote sensing observations were taken of the AR complex including ARs 12957 and 12961. Post observation, the results from the connectivity tool show that the connectivity of Solar Orbiter evolves as follows. Solar Orbiter was connected to the positive polarity of AR 12954 in the northern hemisphere prior to RSW1, crossed the HCS to the negative polarity of AR 12955, traced the equatorial coronal hole boundary, before being connected to AR 12961 then 12957 during the entirety of RSW1. The in situ-data from Solar Orbiter supports this analysis. Firstly, inward-directed (negative) magnetic field is measured by MAG throughout and after RSW1. Secondly, the range of solar wind speeds between 670 and 260 km s^-1 measured by SWA/PAS. Initially the arrival of moderately fast solar wind is observed followed by a gradual decrease to speeds typical of slow solar wind. These results, along with the connectivity tool, suggest that the solar wind originates from the equatorial coronal hole arrived at Solar Orbiter first, followed by the boundary of AR 12961 then 12957. A more detailed analysis is required using additional datasets taken by Solar Orbiter during this period to confirm the connection to this region. In particular, it would be important to trace abundance measurements made by SWA/HIS to coronal abundance measurements made using the SPICE composition studies. Oxygen and carbon charge states measured by SWA/HIS can also help to characterise the origin of the solar wind (Yardley et al. 2023, in preparation).

Identifying a target for RSW2 was more difficult compared to RSW1. The connectivity of Solar Orbiter was not only different during the previous solar rotation but it also changed significantly prior to and during RSW2. This was mainly due to the increase in number of active regions on the solar disk during this time period and in particular, the large amount of flux emergence occurring in AR 12965. Significant flux emergence influences the ADAPT magnetic field maps and hence the predicted connectivity. Therefore, the change in connectivity made it very challenging to select a target during the pointing decision meetings for RSW2. Ultimately, two targets were chosen: the boundary of the southern polar coronal hole and the decayed positive polarity of AR 12967. Analysing the connectivity post observation suggests the following evolution in connectivity. Before RSW2 Solar Orbiter was connected to the decayed negative polarity in the south before crossing the HCS to the positive polarity of AR 12965. From 2022 March 19 onwards (suntime, corresponding to an in situ time of March 21 12:00 UT) Solar Orbiter was connected to the decayed positive polarity of AR 12967. The MAG and SWA/PAS data support these conclusions.

The radial magnetic field measured by MAG was positive from midday on 2022 March 17 onwards, apart from the arrival of an ICME on 2022 March 20. The solar wind speed measured is relatively slow throughout the window with initial speeds averaging around 400 km s^-1, gradually decreasing to a minimum of 210 km s^-1. The decrease in speed could be due to the change in connectivity from the large positive polarity of AR 12965 to the small, decayed positive polarity of AR 12967 and the typical decreasing radial velcity profile in the ICME. The in situ data, connectivity, and upflows in Hinode/EIS data is investigated further in Baker et al. (2023).

Unfortunately, for this period we do not have SWA/HIS data as HIS reached its thermal limit and was switched off on 2022 March 13. This will make it more challenging to characterise the solar wind detected in situ with Solar Orbiter. Ngampoopun & et al. (2023) focuses on the first target during RSW2 where a coronal dimming associated with a filament eruption merges with the southern polar coronal hole boundary. The SO/EUI, Hinode/EIS, SDO/AIA and HMI data suggest that there is component reconnection occurring between the boundary of the coronal hole and the coronal dimming.

In Summary, the Slow Wind SOOP operated for 3.5 and 5 days, respectively, during the first two remote sensing windows of Solar Orbiter’s close perihelion passage in March 2022. The SOOP campaign was very successful, given that the targets for the RSWs must be decided in advance and the targets depend upon the predicted magnetic connectivity of the spacecraft during these time periods. During the RSWs Solar Orbiter observed both primary targets i.e. the periphery of an AR and the boundary of a CH, outlined in the Slow Wind SOOP. Complementary data of these targets were also obtained by IRIS and Hinode through coordinated observation campaigns. During and after both remote sensing windows slow solar wind was measured by SWA/PAS. The radial magnetic field measured by MAG along with the connectivity tool suggest that the slow wind detected in situ by Solar Orbiter originated from two out of three of our selected targets, namely the boundaries of the negative polarity of AR 12961 during RSW1 and the periphery of the decayed positive polarity of AR 12967.

The next steps are to validate these results by combining the analysis of a wider range of both remote sensing and in situ data taken from Solar Orbiter during these time periods. As the Slow Wind SOOP was very successful it will operate again during the next perihelion passage in March and April 2023 (LTP11), where the knowledge and experience gained from operating the SOOP during the first close perihelion will be applied.

S.L.Y. would like to thank STFC via the consolidated grant (STFC ST/V000497/1), NERC via the SWIMMR Aviation Risk Modelling (SWARM) project grant (NE/V002899/1), and the Donostia International Physics Center for their hospitality. D.M.L. is grateful to the Science Technology and Facilities Council for the award of an Ernest Rutherford Fellowship (ST/R003246/1). D.B. is funded under STFC consolidated grant number ST/S000240/1. The work of D.H.B. was performed under contract to the Naval Research Laboratory and was funded by the NASA Hinode program. L.M.G. would like to thank NERC via the SWIMMR Aviation Modelling Risk (SWARM) project (grant no. NE/V002899/1). V.P. acknowledges support from NASA contract NNG09FA40C (IRIS) and NASA grant no. 80NSSC21K0623. L.v.D.G. acknowledges the Hungarian National Research, Development and Innovation Office grant OTKA K-131508. S.P. S.P. acknowledges the funding by CNES through the MEDOC data and operations center. W.T.T. was funded under NASA Grant NNG06EB68C. Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. We are grateful to the ESA SOC and MOC teams for their support. The SO/EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO). The ROB team thanks the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX Programme of the European Space Agency (ESA) under contract numbers 4000134088, 4000112292, 4000136424, and 4000134474. Solar Orbiter magnetometer operations are funded by the UK Space Agency (grant ST/T001062/1). T.H. is supported by STFC grant ST/X002098/1. The German contribution to SO/PHI is funded by the BMWi through DLR and by MPG central funds. The Spanish contribution is funded by FEDER/AEI/MCIU (RTI2018-096886-C5), a “Center of Excellence Severo Ochoa” award to IAA-CSIC (SEV-2017-0709), and a Ramón y Cajal fellowship awarded to DOS. The French contribution is funded by CNES. J.C.T.I. and D.O.S. are supported by the Spanish Ministry of Economy and Competitiveness through projects ESP-2016- 77548-C5-1-R, and by the Spanish Science Ministry “Centro de Excelencia Severo Ochoa” Program under grant SEV-2017- 0709 and project RTI2018-096886-B-C51. D.O.S. also acknowledges financial support from a Ramón y Cajal fellowship. The development of the SPICE instrument was funded by ESA and ESA member states (France, Germany, Norway, Switzerland, United Kingdom). The SPICE hardware consortium was led by Science and Technology Facilities Council (STFC) RAL Space and included Institut d’Astrophysique Spatiale (IAS), Max-Planck-Institut für Sonnensystemforschung (MPS), Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center (PMOD/WRC), Institute of Theoretical Astrophysics (University of Oslo), NASA Goddard Space Flight Center (GSFC) and Southwest Research Institute (SwRI). The in-flight commissioning of SPICE was led by the instrument team at UKRI/STFC RAL Space. AF’s and AG’s research is funded by UKRI STFC. SO/SWA data are derived from scientific sensors which have been designed and created, and are operated under funding provided in numerous contracts from the UK Space Agency (UKSA), the UK Science and Technology Facilities Council (STFC), the Agenzia Spaziale Italiana (ASI), the Centre National d’Etudes Spatiales (CNES,France), the Centre National de la Recherche Scientifique (CNRS, France), the Czech contribution to the ESA PRODEX programme and NASA. Solar Orbiter SWA work at UCL/MSSL was funded under STFC grants ST/T001356/1, ST/S000240/1, ST/X002152/1 and ST/W001004/1. SDO data are courtesy of NASA/SDO and the AIA, EVE, and HMI science teams. IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research Center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre. CHIANTI is a collaborative project involving George Mason University, the University of Michigan (USA), University of Cambridge (UK) and NASA Goddard Space Flight Center (USA). This work also utilises data produced collaboratively between AFRL/ADAPT and NSO/NISP. For the purpose of open access, the author has applied a ‘Creative Commons Attribution (CC BY) licence (where permitted by UKRI, ‘Open Government Licence’ or ‘Creative Commons Attribution No-derivatives (CC BY ND) licence’ may be stated instead) to any Author Accepted Manuscript version arising.

Facilities: Solar Orbiter (SO), Solar Dynamics Observatory (SDO), Hinode, Interface Region Imaging Spectrograph (IRIS).

Data Availability: All data published in this manuscript are publicly available online. Solar Orbiter data is available via the SOAR archive (https://soar.esac.esa.int/soar/), data from the Solar Dynamics Observatory can be downloaded from JSOC (http://jsoc.stanford.edu/), finally data taken by Hinode and IRIS can be retrieved from the OSLO database (http://sdc.uio.no/sdc/).

Software: JHelioviewer (Müller et al., 2017), SunPy (The SunPy Community et al., 2020) v4.1.0 (DOI:10.5281/zenodo.7314636), AstroPy (Astropy Collaboration et al., 2022), SolarSoft IDL PFSS package (Schrijver & De Rosa, 2003).