Long-lived Magnetic Switchbacks Tracked across 0.32 au through BepiColombo-Solar Orbiter Radial Alignment.

(Solar Orbiter Nugget #84 by M. Stumpo¹, S. Benella¹, P. P. Di Bartolomeo¹, A. Larosa² , O. Pezzi^1,2, G. Nicolaou³, D. Trotta⁴, T. Alberti⁵, R. D’Amicis¹, M. Laurenza¹, A. Verdini⁶, A. Milillo¹, D. Heyner⁷, A. Aronica¹, L. Biasiotti⁸, A. Brin¹, L. Colasanti¹, E. De Angelis¹, S. Ivanovski⁸, A. Kazakov¹, V. Mangano¹, S. Massetti¹, M. Moroni¹, A. Mura¹, C. Plainaki^9,10, G. Richards¹, R. Rispoli¹, R. Sordini¹, H. U. Auster⁷, D. Constantinescu⁷, D. Fischer¹¹ and I. Richter⁷)

Introduction

Switchbacks are large-amplitude, impulsive rotations of the interplanetary magnetic field, accompanied by correlated enhancements in solar wind velocity and temporary reversals of the radial magnetic field component. While early Ulysses observations reported them as sporadic features [1,2], recent measurements by Parker Solar Probe and Solar Orbiter have shown that switchbacks are ubiquitous in the young solar wind, particularly within Alfvénic streams emerging from coronal holes [3,4].

Despite their prevalence, the physical origin of switchbacks remains debated. One class of models proposes that switchbacks are formed close to the Sun, for instance through interchange magnetic reconnection in the low corona, and are subsequently advected outward by the solar wind as small-scale flux ropes [5,6]. Alternative scenarios invoke local generation mechanisms, such as shear-driven magnetohydrodynamic instabilities developing within the solar wind itself [7]. Importantly, these competing models predict similar magnetic topologies, making it difficult to discriminate between them using single-point measurements.

A key discriminant between coronal-origin and in-situ generation scenarios is the expected lifetime of switchbacks. Structures formed close to the Sun are expected to remain coherent over extended radial distances, whereas switchbacks generated locally by shear-driven instabilities are predicted to dissipate on timescales comparable to the eddy turnover time, typically of the order of one to a few hours [8,9]. Observationally testing this prediction requires tracking the same plasma parcels at different heliocentric distances, a condition that is rarely satisfied [10,11].

In October 2021, a favorable spacecraft configuration enabled such a test. Solar Orbiter and BepiColombo were radially aligned and magnetically connected to the boundary of the same equatorial coronal hole, allowing the two spacecraft to sample the same solar wind stream at heliocentric distances of 0.67 au and 0.35 au, respectively [12]. We show coordinated in-situ observations from this interval that allow us to assess the persistence and structural evolution of individual switchbacks across a radial separation of 0.32 au, providing new constraints on their formation mechanisms.
 

Results

During the selected interval in 6-8th October 2021, Solar Orbiter and BepiColombo were magnetically connected to the boundary of the same equatorial coronal hole, as inferred from Potential Field Source Surface (PFSS) extrapolation. At that time (t₀= 2021-10-06 09:30 UT), BepiColombo and Solar Orbiter were located at heliocentric distances of approximately 0.35 au and 0.67 au, and were separated by 6.56 degrees in heliospheric longitude (Figure 1). 

Figure 1. Left panel: Ephemeris data in the heliocentric inertial reference frame. Scatters represent the positions of the spacecraft at t₀ = 2021 October 6 09:30 UTC, while lines plot represent the spacecraft trajectory over time. Dotted lines are circles centered on the Sun, while dashed lines are the radial vectors from the spacecraft to the Sun. (Right panel) Sketch of a plasma parcel on a non radial trajectory crossing both BepiColombo at t₀ and Solar Orbiter at t₁. The angles ϕ=6.56 deg and ϕ'=3.45 deg are the longitudinal separations of Solar Orbiter at t₀ and t₁, respectively. 

Both spacecraft observed a well-defined cluster of magnetic switchbacks embedded in a Alfvénic solar wind stream which was clearly distinct from the background solar wind. BepiColombo and Solar Orbiter observed this highly structured stream in the interval from 2021-10-06 09:30 UT to 2021-10-07 12:00 UT and from 2021-10-07 23:00 UT to 2021-10-08 20:00 UT, respectively (Figure 2). Provided the BepiColombo position at t₀ as initial condition, forward ballistic propagation of the plasma parcel resulted in a residual distance of 0.002 au to Solar Orbiter’s position at t₁= 2021-10-07 23:00 UT. Therefore, this spacecraft configuration allows us to track those switchbacks that are larger than 0.002 au across different heliocentric distances.

Figure 2. Overview of magnetic field and plasma parameters as observed by Solar Orbiter: solar wind radial (a) and tangential (b) velocity, Alfvén speed (c), magnetic field magnitude and radial component (d), plasma density and temperature (e), and >60 eV electron pitch angle distribution (f). Vertical lines mark four different regions: slow wind, compression region between forward shock and stream interface (region1), compression region between the stream interface and the reverse shock (region2), and fast wind. The shaded yellow area indicates the switchbacks cluster interval.

A direct comparison between Solar Orbiter and BepiColombo measurements of the solar wind stream reveals significant similarities (Figure 3a-b). To compare switchbacks length scale and morphology we normalized the time axis by the spacecraft cutting-angle. Thus,the signals in Figure 3a and 3b present generally a different time-scale due to the fact that BepiColombo and Solar Orbiter sample the structures at different angles. Figure 3d-i displays the comparison of two switchbacks as seen by BepiColombo and Solar Orbiter confirming that the switchbacks morphology and their length scale remain remarkably similar despite the significant radial separation of 0.32 au (and temporal separation of 38 hr). This, in particular, strongly disfavors formation by shear-driven instabilities, because in this case a lifetime of 1-2 hr would be expected. 

Figure 3. A close up of the transition region between the stream interface and the fast wind: (a), (b) Magnetic field radial component (yellow) and magnitude (purple) from BepiColombo (a) and Solar Orbiter (b). The black line in (b) depicts the radial velocity observed at Solar Orbiter. (c) Normalized cross-helicity spectrogram from Solar Orbiter. Vertical lines mark two structures exhibiting large-scale polarity inversions (see text for details). (d)-(i) Comparison of magnetic field components of BepiColombo (yellow) and Solar Orbiter (blue) within the two structures. Dashed lines show the best-fit of the Lundquist flux-rope model through the data for BepiColombo (dark yellow) and Solar Orbiter (dark blue). BepiColombo's time axes are normalized here by the spacecraft cutting-angle effect, i.e., 0.89 and 0.38 for SB1 and SB2 respectively. The double x-axis in (d)-(i) indicates the time at BepiColombo (top) and Solar Orbiter (bottom) data.

Conclusions

Solar Orbiter and BepiColombo observed a cluster of switchbacks embedded within a stream that originated from an equatorial coronal hole. A key point of this observation is that the spacecraft observed the same plasma parcels, not merely sampling the same source region, allowing to investigate the likelihood of shear-driven formation mechanisms. We found that properties of these structures, such as magnetic field rotation, and characteristic scale, remain remarkably consistent as they propagate outward. This coherence strongly suggests that large switchbacks are not merely transient, locally generated fluctuations, but rather robust magnetic structures capable of surviving over large radial distances. From a shear-driven scenario, such remarkable stability is not expected, being the lifetime limited by the eddy-turnover timescale (~1-2hr in this case) [8,9].

These findings lend support to scenarios in which switchbacks originate close to the Sun, possibly linked to interchange reconnection processes, and are subsequently advected by the solar wind. More broadly, this study illustrates the scientific power of coordinated multi-spacecraft observations, a cornerstone of the Solar Orbiter mission concept.

As Solar Orbiter continues its journey, additional conjunctions with other spacecraft become available, so similar approaches will play a crucial role in unraveling the origin and evolution of switchbacks and their role in heliospheric plasma dynamics.

This nugget is based on Stumpo et al., ApJL, 996 L1 (2026).

Affiliations

(1)    INAF—Istituto di Astrofisica e Planetologia Spaziali, I-00133 Roma, Italy
(2)    Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Via Amendola 122/D, I-70126 Bari, Italy
(3)    Mullard Space Science Laboratory, University College London, London, UK
(4)    European Space Agency (ESA), European Space Astronomy Centre (ESAC), Madrid, Spain
(5)    Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, I-00143 Rome, Italy
(6)    Dipartimento di Fisica e Astronomia, Universitá di Firenze, Firenze, Italy
(7)    Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, Braunschweig, Germany
(8)    INAF—Osservatorio Astronomico di Trieste, Trieste, Italy
(9)    Max Planck Institute for Solar System Research (MPS), Germany
(10)  Agenzia Spaziale Italiana, I-00133 Rome, Italy
(11)    IWF, Graz
 

Acknowledgements

This work has been funded by the grant “Investigating the Universal Nature of Magnetic Field Fluctuations in Solar Wind Turbulence” funded by the Italian National Institute for Astrophysics under the call “Fundamental Research 2023.” The authors acknowledge the SERENA ASI-INAF agreement No. 024-66-HH.0 “Attivitá scientifiche per il Payload SERENA su BepiColombo, relative alla fine della fase di crociera e fase operativa.”

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