Gabriel Ho Hin Suen

Context. The large-amplitude Alfvénic deflections in the solar wind, called magnetic switchbacks, are a ubiquitous feature of the inner heliosphere, yet their origin and variability remain poorly understood. Aims. We investigate the dependence of these large-amplitude magnetic fluctuations in the inner heliosphere on both the heliocentric distance and global solar activity. Methods. Using Parker Solar Probe observations, we quantified switchback deflections through their normalised deflection angle. We examined both their distribution and the probability of large events, combining radial binning, sunspot-number-based activity classification, and regression analysis. Results. We find a statistically significant but weak dependence of switchback properties on solar activity, with a decrease in large deflections at higher activity levels and a modest increase with heliocentric distance. The weak activity trend suggests that multiple processes act simultaneously. Solar-cycle variations in coronal magnetic topology may modulate switchback generation at the source, while nonlinear in situ effects in the solar wind may partially evolve these signatures. Conclusions. Our results indicate that switchbacks retain only a limited imprint of solar activity, reflecting a coupled interplay between coronal origin and in situ evolution. This study provides a quantitative framework for disentangling activity and radial effects, and highlights the need for multi-parameter and multi-spacecraft analyses to fully understand the origin and evolution of switchbacks.

The Sun continuously releases a stream of charged particles known as the solar wind, threaded by magnetic field lines that occasionally fold back on themselves in sharp reversals called magnetic switchbacks. These structures are frequently observed by spacecraft near the Sun, but their formation mechanism remains an open question. Using measurements from Solar Orbiter at 0.55 AU, we report heavy ion observations within a magnetic switchback that provide direct evidence of its solar source. Elevated charge-state ratios of oxygen and carbon, combined with a depletion of helium relative to protons, indicate that the switchback plasma originated from hot, closed coronal loop structures. A semi-empirical coronal model, constrained by electron temperature and density measurements from the Solar Dynamics Observatory, traces the ion signatures to either a coronal hole boundary or loops within the coronal hole, implicating the source mechanism as interchange reconnection, where closed loops reconnect with open magnetic field and eject plasma. The proton velocity and magnetic field fluctuations of the switchback are in accordance with an outward-propagating Alfvén wave. These findings suggest that interchange reconnection and wave or turbulence mechanisms jointly govern switchback origin and propagation, respectively. Most importantly, we show that switchbacks preserve measurable signatures, carrying information about the physical processes that heat the solar atmosphere and accelerate the solar wind.

Magnetic switchbacks are large-amplitude fluctuations in the interplanetary magnetic field, and appear frequently in the near-Sun solar wind explored recently by Parker Solar Probe: these new observations have prompted many new studies into their properties and origins. Here, we first review what is known about how switchbacks evolve as they travel away from the Sun: both in terms of their expansion-driven growth and their decay due to various processes like turbulence, reconnection, dispersion, parametric instability, and interaction with interplanetary shocks. We then review the current state of knowledge on how switchbacks impact the physics of the solar wind as a whole: in terms of the turbulent cascade, acceleration and heating of the wind, modification of the open solar flux and scattering of energetic particles. Finally, we suggest future studies to further our understanding of switchback evolution and impacts on the heliosphere.

The current sheet stress balance conditions describe the equilibrium between magnetic stresses and plasma pressure across a thin current sheet. We build upon existing work developed in the context of magnetotail reconnection to derive a set of stress balance conditions for reconnection outflows in the solar wind, which are typically characterised by a bifurcated reconnection current sheet (RCS). Applying our framework to a symmetric bifurcated RCS model, we determine the outflow region opening angle and beam population properties, obtaining values consistent with observations of reconnection in the solar wind. We then validate our framework against observations of solar wind reconnection outflows from Solar Orbiter, highlighting one event with properties compatible with our simple symmetric model. For this event, we estimate an outflow region opening angle ranging from 3.4°-8.2°, in line with values reported in previous studies. We also reconstruct the outflow beam distribution functions and find that the predicted beam velocities and temperatures match observations well, although the densities are underestimated. Overall, our stress balance framework captures some of the key features of solar wind reconnection outflow, including current sheet bifurcation and counter-streaming beams. Future work will extend the framework to asymmetric reconnection geometries.

Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into “patches”. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.

Magnetic field reversals, where the radial component of the heliospheric magnetic field changes direction, are frequently observed in the near-Sun region. Theory and numerical simulations regarding these reversals suggest that possible generation mechanisms include interchange reconnection in the solar corona, or solar wind expansion and turbulence. Magnetic field reversals thus provide information about both the solar corona and solar wind acceleration and propagation. Previous observations, including magnetic field structure, He2+- abundance, and the so-called patchy-ness are not conclusive in revealing their origin. In this presentation we discuss in situ observations of protons, electrons, He2+, and heavy ions from Solar Orbiter's Solar Wind Analyser instrument , together with the magnetic field from the MAG instrument, during a long-duration magnetic field reversal. We identify the origin of the reversal using the in situ heavy ion data, magnetic connectivity tools, and plasma emission measurements from the Solar Dynamic Observatory. In addition, we study kinetic properties of the electrons, protons and heavy ions, in order to provide readily employable observational tests to help discern the origin of the magnetic field reversals.

Switchbacks, defined as Alfvénic reversals in magnetic field polarity, can dissipate their magnetic energy with heliocentric distance. To further investigate this, two distinct solar wind parcels tracing back to a similar solar source region were examined during a radial alignment between Parker Solar Probe (@25.8RS) and Solar Orbiter (@152RS). The one caveat was that the two probes were located on opposite sides of the heliospheric current sheet during the alignment. The two parcels contained a multitude of switchbacks—the parcel closer to the Sun was characterized as a switchback patch (SBP), where background proton velocity (v_p ) is comparable to the pristine solar wind (v _sw), while the parcel farther from the Sun showed characteristics attributable to a microstream (MS; v _p > v _sw). It was found that (1) MS contains, on average, 30% fewer switchbacks than SBP, and (2) dynamic and thermal pressures decreased by up to 20% across switchback boundaries in SBP and relatively unchanged in MS. Magnetic relaxation can explain the lower number of switchbacks in MS compared to SBP. Switchback relaxation inside SBP can, in turn, accelerate plasma inside SBP over time and heliocentric distance, thus resulting in v_p > v _sw in MS. Therefore, it is hypothesized that magnetic relaxation of switchbacks may cause SBPs to evolve into MSs over time and heliocentric distance.

The ambient solar wind that fills the heliosphere originates from multiple sources in the solar corona and is highly structured. It is often described as high-speed, relatively homogeneous, plasma streams from coronal holes and slow-speed, highly variable, streams whose source regions are under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify solar wind sources and understand what drives the complexity seen in the heliosphere. By combining magnetic field modelling and spectroscopic techniques with high-resolution observations and measurements, we show that the solar wind variability detected in situ by Solar Orbiter in March 2022 is driven by spatio-temporal changes in the magnetic connectivity to multiple sources in the solar atmosphere. The magnetic field footpoints connected to the spacecraft moved from the boundaries of a coronal hole to one active region (12961) and then across to another region (12957). This is reflected in the in situ measurements, which show the transition from fast to highly Alfvénic then to slow solar wind that is disrupted by the arrival of a coronal mass ejection. Our results describe solar wind variability at 0.5 au but are applicable to near-Earth observatories.

Context. Magnetic switchbacks are localised polarity reversals in the radial component of the heliospheric magnetic field. Observations from Parker Solar Probe (PSP) have shown that they are a prevalent feature of the near-Sun solar wind. However, observations of switchbacks at 1 au and beyond are less frequent, suggesting that these structures evolve and potentially erode as they propagate away from the Sun. The specific mechanisms at play have not been identified thus far. Aims. We search for magnetic switchbacks undergoing magnetic reconnection, characterise them, and evaluate the viability of reconnection as a possible channel for their erosion. Methods. We analysed magnetic field and plasma data from the Magnetometer and Solar Wind Analyser instruments aboard Solar Orbiter collected between 10 August and 30 August 2021. During this period, the spacecraft was 0.6–0.7 au from the Sun. Using hodographs and Walén analysis methods, we tested for rotational discontinuities (RDs) in the magnetic field and reconnection-associated outflows at the boundaries of the identified switchback structures. Results. We identified three instances of reconnection occurring at the trailing edge of magnetic switchbacks, with properties that are consistent with existing models of reconnection in the solar wind. Based on these observations, we propose a scenario through which reconnection can erode a switchback and we estimated the timescales for these occurrences. For our events, the erosion timescales are much shorter than the expansion timescale. Thus, the complete erosion of all three observed switchbacks would occur well before they reach 1 au. Furthermore, we find that the spatial scale of these switchbacks would be considerably larger than is typically observed in the inner heliosphere if the onset of reconnection occurs close to the Sun. Our results suggest that the onset of reconnection must occur during transport in the solar wind in the cases we consider here. These results suggest that reconnection can contribute to the erosion of switchbacks and may explain the relative rarity of switchback observations at 1 au.

The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilize the extensive suite of remote-sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote-sensing and in situ measurements of slow wind originating at open–closed magnetic field boundaries. The SOOP ran just prior to Solar Orbiter’s first close perihelion passage during two remote-sensing windows (RSW1 and RSW2) between 2022 March 3–6 and 2022 March 17–22, while Solar Orbiter was at respective heliocentric distances of 0.55–0.51 and 0.38–0.34 au from the Sun. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low-latency in situ data and full-disk remote-sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Postobservation analysis using the magnetic connectivity tool, along with in situ measurements from MAG and SWA/PAS, showed that slow solar wind originating from two out of three of the target regions arrived at the spacecraft with velocities between ∼210 and 600 km s⁻¹. The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.