Search

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 through yet-to-be identified mechanisms as they propagate away from the Sun. We analyse magnetic field and plasma data from the Magnetometer and Solar Wind Analyser instruments aboard Solar Orbiter between 10 August and 30 August 2021. During this period, the spacecraft was 0.6 to 0.7 au from the Sun. We identify three instances of reconnection occurring at the trailing edge of magnetic switchbacks, with properties consistent with existing models describing reconnection in the solar wind. Using hodographs and Walen analysis methods, we test for rotational discontinuities (RDs) in the magnetic field and reconnection-associated outflows at the boundaries of the identified switchback structures. Based on these observations, we propose a scenario through which reconnection can erode a switchback and we estimate the timescales over which this occurs. For our events, the erosion timescales are much shorter than the expansion timescale and 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. Hence, our results suggest that the onset of reconnection must occur during transport in the solar wind in our cases. These results suggest that reconnection can contribute to the erosion of switchbacks and may explain the relative rarity of switchback observations at 1 au., Comment: Accepted for publication in Astronomy & Astrophysics 05/05/2023

We investigate the turbulent energy cascade in the near-Sun solar wind by means of a high-resolution fully kinetic simulation initialised with average plasma conditions measured by Parker Solar Probe during its first solar encounter. Our simulation models the electron-scale electric field fluctuations with unprecedented accuracy. This allows us to perform the first detailed analysis of the different terms of the electric field in the generalised Ohm's law (MHD, Hall, and electron pressure terms) at ion and electron scales, both in physical space and in Fourier space. Such analysis suggests rewriting the Ohm’s law in a different form, which disentangles the contribution of different underlying plasma mechanisms in each range of scales. This provides a new insight on how energy in the turbulent electromagnetic fields is transferred through ion and electron scales and seems to favour the role of pressure-balanced structures versus waves. We finally test our assumptions and numerical results by analysing measurements of the magnetic field, electric field, and ion and electron velocity distributions and their moments performed by Solar Orbiter and Parker Solar Probe. Preliminary results show a remarkable agreement with our theoretical expectations inspired by our simulations., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Interplanetary (IP) shocks can be driven in the solar wind by fast coronal mass ejections and by the interaction of fast solar wind with slow streams of plasma. These shocks can be preceded by extended foreshocks where a variety of waves and suprathermal ion populations exist. Shocks characteristics as well as the level of wave activity near them change as they propagate through the heliosphere and this can impact particle acceleration, and modify the ambient solar wind. In this work we study IP shock evolution and the wave modes upstream of them using a multispacrecaft approach with data of Solar Orbiter, STEREO, Parker Solar Probe and Wind. We find that upstream regions can be permeated by whistler waves (f ~ 1 Hz) and/or ultra low frequency (ULF) right-handed waves (f~10-2–10-1 Hz). While whistlers appear to be generated at the shock, the origin of ULF waves is associated with local kinetic ion instabilities. In contrast with planetary bow shocks, most IP shocks have a small Mach number (, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Space plasmas are three-dimensional dynamic entities. Except under very special circumstances, their structure in space and their behavior in time are not related in any simple way. Therefore, single spacecraft in situ measurements cannot unambiguously unravel the full space-time structure of the heliospheric plasmas of interest in the inner heliosphere, in the Geospace environment, or the outer heliosphere. This shortcoming leaves numerous central questions incompletely answered. Deficiencies remain in at least two important subjects, Space Weather and fundamental plasma turbulence theory, due to a lack of a more complete understanding of the space-time structure of dynamic plasmas. Only with multispacecraft measurements over suitable spans of spatial separation and temporal duration can these ambiguities be resolved. We note that these characterizations apply to turbulence across a wide range of scales, and also equally well to shocks, flux ropes, magnetic clouds, current sheets, stream interactions, etc. In the following, we will describe the basic requirements for resolving space-time structure in general, using turbulence' as both an example and a principal target or study. Several types of missions are suggested to resolve space-time structure throughout the Heliosphere., Comment: White Paper submitted to: Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. arXiv admin note: substantial text overlap with arXiv:1903.06890

The Kelvin-Helmholtz instability (KHI) is a nonlinear shear-driven instability that develops at the interface between shear flows in plasmas. KHI has been inferred in various astrophysical plasmas and has been observed in situ at the magnetospheric boundaries of solar-system planets and through remote sensing at the boundaries of coronal mass ejections. While it was hypothesized to play an important role in the mixing of plasmas and in triggering solar wind fluctuations, its direct and unambiguous observation in the solar wind was still lacking. We report in-situ observations of ongoing KHI in the solar wind using Solar Orbiter during its cruise phase. The KHI is found in a shear layer in the slow solar wind in the close vicinity of the Heliospheric Current Sheet, with properties satisfying linear theory for its development. An analysis is performed to derive the local configuration of the KHI. A 2-D MHD simulation is also set up with empirical values to test the stability of the shear layer. In addition, magnetic spectra of the KHI event are analyzed. We find that the observed conditions satisfy the KHI onset criterion from the linear theory analysis, and its development is further confirmed by the simulation. The current sheet geometry analyses are found to be consistent with KHI development. Additionally, we report observations of an ion jet consistent with magnetic reconnection at a compressed current sheet within the KHI interval. The KHI is found to excite magnetic and velocity fluctuations with power-law scalings that approximately follow $k^{-5/3}$ and $k^{-2.8}$ in the inertial and dissipation ranges, respectively. These observations provide robust evidence of KHI development in the solar wind. This sheds new light on the process of shear-driven turbulence as mediated by the KHI with implications for the driving of solar wind fluctuations.

Context. Solar Orbiter and PSP jointly observed the solar wind for the first time in June 2020, capturing data from very different solar wind streams, calm and Alfv\'enic wind as well as many dynamic structures. Aims. The aim here is to understand the origin and characteristics of the highly dynamic solar wind observed by the two probes, in particular in the vicinity of the heliospheric current sheet (HCS). Methods. We analyse the plasma data obtained by PSP and Solar Orbiter in situ during the month of June 2020. We use the Alfv\'en-wave turbulence MHD solar wind model WindPredict-AW, and perform two 3D simulations based on ADAPT solar magnetograms for this period. Results. We show that the dynamic regions measured by both spacecraft are pervaded with flux ropes close to the HCS. These flux ropes are also present in the simulations, forming at the tip of helmet streamers, i.e. at the base of the heliospheric current sheet. The formation mechanism involves a pressure driven instability followed by a fast tearing reconnection process, consistent with the picture of R\'eville et al. (2020a). We further characterize the 3D spatial structure of helmet streamer born flux ropes, which seems, in the simulations, to be related to the network of quasi-separatrices., Comment: 14 pages, 12 Figures, accepted for publication in Astronomy and Astrophysics

One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfv��nic magnetic field reversals termed 'switchbacks'. These $��B_R/B \sim \mathcal{O}(1$) fluctuations occur on a range of timescales and in {\em patches} separated by intervals of quiet, radial magnetic field. We use measurements from PSP to demonstrate that patches of switchbacks are localized within the extensions of plasma structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma $��$ and pressure, and by depletions in the magnetic field magnitude and electron temperature. These intervals are in pressure-balance, implying stationary spatial structure, and the field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in Carrington longitude with a steeper leading edge and a small ($\sim$1$^\circ$) edge of hotter plasma and enhanced magnetic field fluctuations. Some structures contain suprathermal ions to $\sim$85 keV that we argue are the energetic tail of the solar wind alpha population. The structures are separated in longitude by angular scales associated with supergranulation. This suggests that these switchbacks originate near the leading edge of the diverging magnetic field funnels associated with the network magnetic field - the primary wind sources. We propose an origin of the magnetic field switchbacks, hot plasma and suprathermals, alpha particles in interchange reconnection events just above the solar transition region and our measurements represent the extended regions of a turbulent outflow exhaust., 18 pages, 9 figures, submitted to Astrophys. J

A multi-institutional, multi-national science team will soon submit a NASA proposal to build a constellation of spacecraft to fly into the near-Earth solar wind in a swarm spanning a multitude of scales in order to obtain critically needed measurements that will reveal the underlying dynamics of magnetized turbulence. This white paper, submitted to the Plasma 2020 Decadal Survey Committee, provides a brief overview of turbulent systems that constitute an area of compelling plasma physics research, including why this mission is needed, and how this mission will achieve the goal of revealing how energy is transferred across scales and boundaries in plasmas throughout the universe.

This white paper submitted for 2020 Decadal Assessment of Plasma Science concerns the importance of multi-spacecraft missions to address fundamental questions concerning plasma turbulence. Plasma turbulence is ubiquitous in the universe, and it is responsible for the transport of mass, momentum, and energy in such diverse systems as the solar corona and wind, accretion discs, planet formation, and laboratory fusion devices. Turbulence is an inherently multi-scale and multi-process phenomenon, coupling the largest scales of a system to sub-electron scales via a cascade of energy, while simultaneously generating reconnecting current layers, shocks, and a myriad of instabilities and waves. The solar wind is humankind's best resource for studying the naturally occurring turbulent plasmas that permeate the universe. Since launching our first major scientific spacecraft mission, Explorer 1, in 1958, we have made significant progress characterizing solar wind turbulence. Yet, due to the severe limitations imposed by single point measurements, we are unable to characterize sufficiently the spatial and temporal properties of the solar wind, leaving many fundamental questions about plasma turbulence unanswered. Therefore, the time has now come wherein making significant additional progress to determine the dynamical nature of solar wind turbulence requires multi-spacecraft missions spanning a wide range of scales simultaneously. A dedicated multi-spacecraft mission concurrently covering a wide range of scales in the solar wind would not only allow us to directly determine the spatial and temporal structure of plasma turbulence, but it would also mitigate the limitations that current multi-spacecraft missions face, such as non-ideal orbits for observing solar wind turbulence. Some of the fundamentally important questions that can only be addressed by in situ multipoint measurements are discussed.

Although the origins of slow solar wind are unclear, there is increasing evidence that at least some of it is released in a steady state on over-expanded coronal hole magnetic field lines. This type of slow wind has similar properties to the fast solar wind, including a high degree of Alfv��nicity. In this study a combination of proton, alpha particle, and electron measurements are used to investigate the kinetic properties of a single interval of slow Alfv��nic wind at 0.35 AU. It is shown that this slow Alfv��nic interval is characterised by high alpha particle abundances, pronounced alpha-proton differential streaming, strong proton beams, and large alpha to proton temperature ratios. These are all features observed consistently in the fast solar wind, adding evidence that at least some Alfv��nic slow solar wind also originates in coronal holes. Observed differences between speed, mass flux, and electron temperature between slow Alfv��nic and fast winds are explained by differing magnetic field geometry in the lower corona.

We measure the local anisotropy of numerically simulated strong Alfv��nic turbulence with respect to two local, physically relevant directions: along the local mean magnetic field and along the local direction of one of the fluctuating Elsasser fields. We find significant scaling anisotropy with respect to both these directions: the fluctuations are "ribbon-like" --- statistically, they are elongated along both the mean magnetic field and the fluctuating field. The latter form of anisotropy is due to scale-dependent alignment of the fluctuating fields. The intermittent scalings of the $n$th-order conditional structure functions in the direction perpendicular to both the local mean field and the fluctuations agree well with the theory of Chandran et al. 2015, while the parallel scalings are consistent with those implied by the critical-balance conjecture. We quantify the relationship between the perpendicular scalings and those in the fluctuation and parallel directions, and find that the scaling exponent of the perpendicular anisotropy (i.e., of the aspect ratio of the Alfv��nic structures in the plane perpendicular to the mean magnetic field) depends on the amplitude of the fluctuations. This is shown to be equivalent to the anticorrelation of fluctuation amplitude and alignment at each scale. The dependence of the anisotropy on amplitude is shown to be more significant for the anisotropy between the perpendicular and fluctuation-direction scales than it is between the perpendicular and parallel scales., 9 pages, 5 figures. Submitted to MNRAS

We investigate the anisotropy of Alfv��nic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring the anisotropic second-order structure functions in both data sets. In the solar wind, the perpendicular spectral index of the magnetic field is close to -5/3. In the forced simulation, it is close to -5/3 for the velocity and -3/2 for the magnetic field. In the decaying simulation, it is -5/3 for both fields. The spectral index becomes steeper at small angles to the local magnetic field direction in all cases. We also show that when using the global rather than local mean field, the anisotropic scaling of the simulations cannot always be properly measured., 9 pages, 8 figures

The Solar Orbiter mission seeks to make connections between the physical processes occurring at the Sun or in the solar corona and the nature of the solar wind created by those processes which is subsequently observed at the spacecraft. The mission also targets physical processes occurring in the solar wind itself during its journey from its source to the spacecraft. To meet the specific mission science goals, Solar Orbiter will be equipped with both remote-sensing and in-situ instruments which will make unprecedented measurements of the solar atmosphere and the inner heliosphere. A crucial set of measurements will be provided by the Solar Wind Analyser (SWA) suite of instruments. This suite consists of an Electron Analyser System (SWA-EAS), a Proton and Alpha particle Sensor (SWA-PAS), and a Heavy Ion Sensor (SWA-HIS) which are jointly served by a central control and data processing unit (SWA-DPU). Together these sensors will measure and categorise the vast majority of thermal and suprathermal ions and electrons in the solar wind and determine the abundances and charge states of the heavy ion populations. The three sensors in the SWA suite are each based on the top hat electrostatic analyser concept, which has been deployed on numerous space plasma missions. The SWA-EAS uses two such heads, each of which have 360◦ azimuth acceptance angles and ±45◦ aperture deflection plates. Together these two sensors, which are mounted on the end of the boom, will cover a full sky field-of-view (FoV) (except for blockages by the spacecraft and its appendages) and measure the full 3D velocity distribution function (VDF) of solar wind electrons in the energy range of a few eV to ∼5 keV. The SWA-PAS instrument also uses an electrostatic analyser with a more confined FoV (−24◦ to +42◦ × ±22.5◦ around the expected solar wind arrival direction), which nevertheless is capable of measuring the full 3D VDF of the protons and alpha particles arriving at the instrument in the energy range from 200 eV/q to 20 keV/e. Finally, SWA-HIS measures the composition and 3D VDFs of heavy ions in the bulk solar wind as well as those of the major constituents in the suprathermal energy range and those of pick-up ions. The sensor resolves the full 3D VDFs of the prominent heavy ions at a resolution of 5 min in normal mode and 30 s in burst mode. Additionally, SWA-HIS measures 3D VDFs of alpha particles at a 4 s resolution in burst mode. Measurements are over a FoV of −33◦ to +66◦ × ±20◦ around the expected solar wind arrival direction and at energies up to 80 keV/e. The mass resolution (m/∆m) is >5. This paper describes how the three SWA scientific sensors, as delivered to the spacecraft, meet or exceed the performance requirements originally set out to achieve the mission’s science goals. We describe the motivation and specific requirements for each of the three sensors within the SWA suite, their expected science results, their main characteristics, and their operation through the central SWA-DPU. We describe the combined data products that we expect to return from the suite and provide to the Solar Orbiter Archive for use in scientific analyses by members of the wider solar and heliospheric communities. These unique data products will help reveal the nature of the solar wind as a function of both heliocentric distance and solar latitude. Indeed, SWA-HIS measurements of solar wind composition will be the first such measurements made in the inner heliosphere. The SWA data are crucial to efforts to link the in situ measurements of the solar wind made at the spacecraft with remote observations of candidate source regions. This is a novel aspect of the mission which will lead to significant advances in our understanding of the mechanisms accelerating and heating the solar wind, driving eruptions and other transient phenomena on the Sun, and controlling the injection, acceleration, and transport of the energetic particles in the heliosphere.

International audience