Andrew P. Walsh

Aims. Solar Orbiter observations provide an unprecedented opportunity to study solar energetic particle (SEP) events at heliocentric distances ≲1 au. In this work, we compile a catalogue of SEP events, utilising Solar Orbiter measurements. Methods. The catalogue is based on measurements from the High Energy Telescope (HET) of the Energetic Particle Detector (EPD) instrument suite on Solar Orbiter during the ascending phase of solar cycle 25 (2020–2025). We performed a parallel survey of near-relativistic electrons (∼1 MeV) and protons (∼8 MeV), identifying particle enhancements above background levels and registering SEP events. For each SEP event and particle species, we calculated the SEP characteristics (e.g. onset and peak times, peak intensity values, the fluence, and e/p intensity ratios). Our analysis techniques included a velocity dispersion analysis (VDA) and time-shifting analysis (TSA) to infer the SEP release times at the Sun. We compared the SEP release times with solar flare observations from the Spectrometer Telescope for Imaging X-rays (STIX) and X-Ray Sensor (XRS) on Geostationary Operational Environmental Satellites (GOES) and identified the parent solar sources. We also identified the associated coronal mass ejections (CMEs). The SEP events were cross-checked with magnetic field and solar wind data from the Solar Wind Analyser (SWA) and Magnetometer (MAG) and the relevant plasma quantities were tabulated. In addition, the connection angle (CA) for each event was calculated and reported. Results. Our catalogue includes 177 SEP events. The present study focuses on both electron and proton events, with 50 of these events exhibiting high-energy protons (∼50 MeV). A strong correlation is found between peak intensity and fluence for both electrons ( cc = 0.86 ± 0.03) and protons ( cc = 0.95 ± 0.02). Using the CA, events were classified as well-connected (±20°) and poorly connected. The SEP peak fluxes and fluences show weak correlations with flare soft X-ray peak flux ( cc ∼0.17-0.21), while stronger correlations are observed with a CME projected speed for well-connected events ( cc ∼ 0.61 for electrons and ∼0.47 for protons). Across all events with a measurable electron-to-proton (e/p) ratio, the ratio spans more than three orders of magnitude and forms a continuous distribution, with no clear distinction between impulsive and gradual populations. Combined with compositional information, higher e/p ratios are generally associated with ³ He-rich and Fe-rich events and lower values with shock-driven events, although a significant overlap exists. Finally, scaling relations reveal an energy dependence: low-energy protons (∼8-25 MeV) are influenced by both magnetic connectivity and source composition, whereas high-energy protons (∼50 MeV) are primarily governed by source composition ( ³ He and Fe signatures).

Context. Quasi-periodic density structures (PDSs) are quasiperiodic variations in the solar wind density that range from a few minutes to a few hours. They are trains of advected density structures with radial length scales L _R ≈ 100 − 10 000 Mm, and they thus belong to the class of so-called mesoscale structures in the solar wind. Although the precise mechanisms that form PDSs are still debated, cumulative evidence from multiple studies using in situ and remote data supports the view that most PDSs have a solar origin and do not form through dynamics during their propagation in interplanetary space. Low-frequency (< 1 mHz) PDSs have been associated with small-scale flux ropes, which further indicates a solar origin. Aims. We further investigated the origin and properties of PDSs by searching for coherent small-scale helical structures and flux ropes within PDS intervals. Methods. We used an extensive list of PDSs, compiled from Solar Orbiter measurements, and we applied a wavelet analysis technique to obtain the reduced magnetic helicity, cross helicity, and residual energy. Results. Our results indicate that small flux ropes are a constituent of PDS events, while the occurrence probability of helical Alfvénic structures is similar within and outside the PDSs. Conclusions. These results are consistent with the scenario in which PDSs are formed by processes involving magnetic reconnection.

Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 using the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission’s remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRI _EUV ), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRI _EUV , operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter’s top-level science goal: “How do solar eruptions produce energetic particle radiation that fills the heliosphere?” . This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations.

Impulsive solar energetic particle (SEP) events are typically associated with solar flares but the related particle injection and acceleration processes are still not well understood. We use in-situ and remote-sensing data from Solar Orbiter to establish a plausible link between a series of eruptions in a flaring region and a sequence of four SEP events measured at 0.5 AU between 5 and 6 March 2022. The direct comparison between these four events from the same source region allows to study the variability of the injected SEPs during an extended period of magnetic connectivity between Solar Orbiter and the flaring active region. In this study we analyze energetic electron, proton, and heavy ion data provided by the Energetic Particle Detector (EPD) suite onboard Solar Orbiter. Via a velocity dispersion analysis (VDA) of all measured particle species we estimate the solar event onset times which coincide with a series of solar eruptions that is observed by the Extreme Ultraviolet Imager (EUI) and the Spectrometer Telescope for Imaging X-rays (STIX) onboard Solar Orbiter. Further high-time-resolution EUV images and photospheric magnetic field information of the related active region is given by the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. Solar Orbiter and Earth were nearly perfectly radially aligned at this time which enabled this additional remote sensing by SDO. We find that the energy spectra of the heavy ion in-situ measurements show significant differences between the four investigated SEP events in terms of overall particle intensity, spectral slope, and 3He / 4He abundances. By comparison with the remote-sensing observations we find that the two stronger SEP events (with higher 3He / 4He ratios) are related to solar eruptions with a more complex eruption pattern leading to extended brightening and restructuring of coronal loop structures. These new detailed observations can be used as starting point for quantitative modelling of flare-associated energetic particle acceleration and release in active regions.This work has been funded by the Spanish Ministerio de Ciencia, Innovación y Universidades project PID2019-104863RBI00/AEI/10.13039/501100011033.

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.

Energy dissipation in collisionless plasmas is a long-standing fundamental physics problem. Although it is well known that magnetic reconnection and turbulence are coupled and transport energy from system-size scales to subproton scales, the details of the energy distribution and energy dissipation channels remain poorly understood. Especially, the energy transfer and transport associated with 3D small-scale reconnection that occurs as a consequence of a turbulent cascade is unknown. We use an explicit fully kinetic particle-in-cell code to simulate 3D small-scale magnetic reconnection events forming in anisotropic and decaying Alfvénic turbulence. We identify a highly dynamic and asymmetric reconnection event that involves two reconnecting flux ropes. We use a two-fluid approach based on the Boltzmann equation to study the spatial energy transfer associated with the reconnection event and compare the power density terms in the two-fluid energy equations with standard energy-based damping, heating, and dissipation proxies. Our findings suggest that the electron bulk flow transports thermal energy density more efficiently than kinetic energy density. Moreover, in our turbulent reconnection event, the energy density transfer is dominated by plasma compression. This is consistent with turbulent current sheets and turbulent reconnection events, but not with laminar reconnection.

Turbulence and magnetic reconnection are at the core of the long-standing problem of energy dissipation in collisionless plasmas. More than two decades of research on magnetic reconnection have led us to understand the characteristic plasma flows and particle agyrotropy patterns present in collisionless reconnection events. However, it is still not clear what the agyrotropy patterns associated with reconnection events are that form in a turbulent cascade. In this work, we use an explicit fully kinetic particle-in-cell code to study the plasma particles’ agyrotropy associated with three-dimensional small-scale magnetic reconnection events generated by anisotropic and Alfvénic decaying turbulence. We select one reconnection event involving two reconnecting flux ropes. Although we observe similarities with agyrotropy patterns known from two-dimensional steady-state reconnection events, the agyrotropy patterns in our event are more complex. This has further implications for the energy transfer channels available in three-dimensional turbulent reconnection. 

Context.Solar Orbiter strives to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been in operation, providing excellent data, for just over a year.Aims.EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV nuc⁻¹for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. In addition, we present areas where EPD excels and provides opportunities for significant scientific progress in understanding how our Sun shapes the heliosphere.Methods.We used the solar particle events observed by Solar Orbiter on 21 July and between 10 and 11 December 2020 to discuss the capabilities, along with updates and open issues related to EPD on Solar Orbiter. We also give some words of caution and caveats related to the use of EPD-derived data.Results.During this first year of operations of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10 and 11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as³He, for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10 and 11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and we present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided.