Multi-source connectivity drives heliospheric solar wind variability

(Solar Orbiter Nugget #35 by Stephanie L. Yardley^1,2,3,4, David H. Brooks⁵, Raffaella D’Amicis⁶, Christopher J. Owen³, David M. Long⁷, et al. )

Introduction

The solar wind is the continuous outflow of tenuous, magnetised plasma into the heliosphere that originates from multiple sources in the solar corona. The solar wind is generally classified into two different types: relatively fast (>= 500 km/s), smooth and continuous streams from coronal holes, and slow (< 500 km/s), highly variable streams whose origins are still under debate [1,2,3]. One of the main goals of the ESA/NASA Solar Orbiter mission [4] is to determine the origin of solar wind plasma and what drives its heliospheric variability.

Promising source regions of the slow solar wind are the boundaries between open and closed magnetic fields such as (but not limited to) the edges of active regions [5,6] and coronal holes [7,8,9] where interchange reconnection can take place in order to release plasma into the solar wind [10,11,12]. Previous works have used plasma composition diagnostics to trace solar wind plasma detected in situ at 1au by ACE back to observations of outflows at AR boundaries taken by Hinode [13], although small-scale variability is often lost at large distances from the Sun due to transport processes.

Solar Orbiter Remote-sensing Observations

The remote-sensing observations and in situ measurements utilized in this analysis were taken as part of the Slow Solar Wind Connection Science Solar Orbiter Observing Plan (SOOP) (see more details of in [14], and other recent results in [15,16]). The Slow Wind SOOP operated for the first time during Solar Orbiter’s close perihelion passage in March 2022 (from 3 March 2022 06:00 UT until 6 March 2022 18:30 UT), when Solar Orbiter was at a close distance of approximately 0.5au. Figure 1 shows an example of the high-resolution remote-sensing observations taken at the beginning of the window on 3 March 2022.

Figure 1: Solar Orbiter remote-sensing observations taken on 3 March 2022 during the Slow Wind SOOP.  (a) The EUI/FSI 174 Å image with the partial disk EUI/HRI 174 Å and PHI/HRT overlayed, where the green (blue) contours represent positive (negative) polarity magnetic field of the AR complex with a saturation of +(-)500) G. The active region numbers are labelled in white. (b-c) A close-up of (a) with the SPICE Mg VIII/Ne VIII intensity map overplotted. The points represent the connectivity points taken from the magnetic connectivity tool [17] corresponding to the in situ times of 3 March 2022 (b) and 7 March 2022 (c) where the colourbar gives the probability of the connectivity points. R1 and R2 are the regions where the composition was determined by applying spectroscopic techniques to the SPICE data. 

To connect in situ measurements of solar wind plasma back to the remote-sensing observations of its solar origins requires the source location to be estimated by using the magnetic connectivity tool [17]. The magnetic connectivity of Solar Orbiter around the time of the Slow Wind SOOP moves across a dark channel that later merges with the large equatorial coronal hole (visible in SDO/AIA 193Å, not shown here) and the active region complex (from the negative polarities of AR 12961 to AR 12957) as seen in Figure 1(b-c). During this period, continuous flux emergence was observed (visible in SDO/HMI and PHI/HRT) in the region along with large-scale coronal fan loops (as seen in EUI/HRI 174 Å) associated with the negative polarities of the AR complex. By using a potential field extrapolation, we found that the large-scale coronal fan loops were associated with open magnetic fields whereby coronal plasma could potentially escape as the solar wind and be detected in situ by Solar Orbiter a few days later.

The composition of solar wind plasma varies depending on its solar origin. We applied spectroscopic techniques to SPICE data to determine that there was a clear difference in the plasma composition between the areas associated with the two negative polarities (R1 and R2 in Figure 1c) of the AR complex. A larger Mg/Ne abundance ratio suggesting coronal or strong coronal composition was found in the region associated with the negative polarity of AR 12961 (R1) compared to a smaller ratio suggesting photospheric or weak coronal composition in the region associated with the negative polarity of AR 12957 (R2).

In situ measurements

During 1 to 9 March 2022, SWA/PAS detected three fast and two slow-speed solar wind streams originating from the CH-AR complex (Figure 2a,b). MAG measured Sunward directed radial magnetic field (Figure 2c), which was consistent with the sources, namely the negative polarities of the CH and the two active region leading polarities. The VB correlation coefficient, a measure of Alfvénicity, is high during the fast streams and the subsequent Alfvénic slow stream but shows a poor correlation during the final typical (i.e. non-Alfvénic) slow wind stream (Figure 2d).

The electron strahl, which is a field-aligned beam of electrons propagating away from the solar corona, provides a direct probe of the magnetic connectivity and is measured by SWA/EAS (Figure 2e). For most of the period, the electron strahl is streaming outwards, directed anti-parallel to the magnetic field. There are numerous small periods of bidirectional strahl that are associated with the boundaries of the fast solar wind streams originating from the different sections of the CH. There is a long period of asymmetric bidirectional strahl beginning on 5 March 2022, suggesting that the magnetic field was locally connected back to different locations on the Sun, indicative of closed expanding field associated with the emerging AR complex.

Heavy ion measurements taken by SWA/HIS (Figure 3f,g), which can be linked to the spectroscopic data from SPICE, are also consistent with the magnetic connectivity moving across the CH-AR complex. Initially the Fe/O and O7+/O6+ ratios are low, indicative of plasma with photospheric abundance and temperatures originating from the CH. The Fe/O and O7+/O6+  ratios then increase, indicative of coronal and high temperature plasma from the leading edge of the AR complex. This coincides with the period of high Alfvénicity and bidirectional electron strahl. The Fe/O ratio decreases again while the O7+/O6+ ratio remains increased and variable suggesting that the slow wind arriving at SO originates from the leading polarity of the second AR in the complex.

Conclusions

By combining the magnetic field modelling and spacecraft connectivity, the spectroscopic analysis with the remote-sensing observations and in situ measurements from Solar Orbiter we have shown that Solar Orbiter detects three fast and two solar wind streams originating from the CH-AR complex. These results strongly support the scenario that the variability of the solar wind is driven by the transition of the magnetic connectivity of Solar Orbiter across the CH-AR complex, where the magnetic configuration is also altered due to interchange reconnection occurring between open and closed magnetic fields within the AR complex. We have shown that through coupling high-resolution remote-sensing observations and in situ measurements taken at close distances from the Sun that the solar wind still exhibits the footprints of its multiple solar sources. More cases should be investigated to determine whether similar in situ signatures are observed at close distances from the Sun taking advantage of measurements other close-in missions such as Parker Solar Probe.

This article has been published in Nature Astronomy with DOI: https://doi.org/10.1038/s41550-024-02278-9

Acknowledgments

S.L.Y. is grateful to the Science Technology and Facilities Council for the award of an Ernest Rutherford Fellowship (ST/X003787/1), for funding via the consolidated grant (STFC ST/V000497/1), and NERC for the SWIMMR Aviation Risk Modeling (SWARM) project grant (NE/V002899/1). The work of D.H.B. was performed under contract to the Naval Research Laboratory and was funded by the NASA Hinode program. 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 Solar Orbiter EUI Operations grant number ST/X002012/1 and Hinode Ops Continuation 2022-25 grant number ST/X002063/1. R.M.D. acknowledges support from NASA grant 80NSSC22K0204. N.N. is supported by STFC PhD studentship grant ST/W507891/1 and UCL Studentship. Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. 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 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. The development of SPICE has been funded by ESA member states and ESA. It was built and is operated by a multi-national consortium of research institutes supported by their respective funding agencies: STFC RAL (UKSA, hardware lead), IAS (CNES, operations lead), GSFC (NASA), MPS (DLR), PMOD/WRC (Swiss Space Office), SwRI (NASA), UiO (Norwegian Space Agency). Solar Orbiter Solar Wind Analyser (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 is currently funded under STFC grants ST/W001004/1 and ST/X/002152/1. Solar Orbiter magnetometer operations are funded by the UK Space Agency (grant ST/T001062/1). Funding for SwRI was provided by NASA contract NNG10EK25C. Funding for the University of Michigan was provided through SwRI subcontract A99201MO.

Full list of authors: Stephanie L. Yardley^1,2,3,4, David H. Brooks⁵, Raffaella D’Amicis⁶, Christopher J. Owen³, David M. Long⁷, Deb Baker³, Pascal Démoulin^8,9, Matt Owens², Mike Lockwood², Teodora Mihailescu³, Jesse T. Coburn³, Ryan M. Dewey¹⁰, Daniel Müller¹¹, Gabriel H.H. Suen³, Nawin Ngampoopun³, Philippe Louarn¹², Stefano Livi^13,10, Sue Lepri¹⁰, Andrzej Fludra¹⁴, Margit Haberreiter¹⁵ and Udo Schuehle¹⁶

Affiliations 

1 Department of Mathematics, Physics and Electrical Engineering, Northumbria University, UK

2 Department of Meteorology, University of Reading, UK

3 Department of Space and Climate Physics, UCL Mullard Space Science Laboratory, UK

4 Donostia International Physics Center (DIPC), Spain

5 Department of Physics & Astronomy, George Mason University, USA.

6 INAF-Istituto di Astrofisica e Planetologia Spaziali, Italy

7 School of Physical Sciences, Dublin City University, Ireland

8 LESIA, Observatoire de Paris, Université PSL, CNRS, France

9 Laboratoire Cogitamus, France

10 Climate and Space Sciences and Engineering, University of Michigan, USA

11 European Space Agency, ESTEC, The Netherlands

12 Institut de Recherche en Astrophysique et Planétologie, France

13 Southwest Research Institute, USA

14 RAL Space, UKRI STFC Rutherford Appleton Laboratory, UK

15 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Switzerland

16 Max-Planck-Institut für Sonnensystemforschung, Germany

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