What determines the departure from equipartition of energy in Alfvénic fluctuations in solar wind streams? Insights from Solar Orbiter observations

2025-03-14

The very first observations by Mariner 5 highlighted the presence of Alfvénic fluctuations in the solar wind identified as nearly incompressible fluctuations accompanied by large correlations between velocity and magnetic field components as predicted by the magnetohydrodynamics (MHD) theory. Since then, Alfvénic fluctuations have been observed to be ubiquitous especially in high-speed solar wind streams, but are also in some cases in slow wind streams, which may in turn exhibit a strong Alfvénic character. The so-called Alfvénic slow wind resembles the fast wind in many aspects, but may also differ from it. Indeed, recent observations performed by Solar Orbiter have shown that the fast wind may display a strong Alfvénic content of the fluctuations than the one observed in the Alfvénic slow wind, especially closer to the Sun.In this context, Solar Orbiter offers a unique opportunity to study the origin and radial evolution of the Alfvénic solar wind. In this particular study, we present a comparative study between different Alfvénic streams, both fast and slow, at different heliocentric distances, focusing on the characterization of Alfvénicity of different streams with particular reference to the energy balance of the fluctuations.The aim of this work is to deepen our understanding of what are the mechanisms responsible for the evolution of Alfvénicity in solar wind fluctuations and to understand better to what extent the two solar wind regimes show different Alfvénic content of the fluctuations and eventually evolve in a different way.

Parker Solar Probe at 9.86 solar radii: magnetic field structure, trends, and connectivity

2025-03-14

Parker Solar Probe had its lowest-ever perihelion of 9.86 solar radii on December 24, 2024 ('Encounter 22') and another on March 22, 2025 (Encounter 23).  We will present data from these Encounters, focusing on magnetic field measurements,  solar connectivity, and Heliospheric Current Sheet (HCS) crossings as the spacecraft crossed nearly halfway around the Sun in just 4 days.  We also compare E22/E23 measurements of the radial magnetic field and field magnitude to trends from earlier Encounters showing increasing radially-normalized magnetic flux with altitude.  We will review highlights of previous PSP solar encounters and compare to the latest encounters at the lowest-ever perihelion.

Switchback Patches Evolve into Microstreams via Magnetic Relaxation

arXiv (Cornell University) · 2024-02-21

Magnetic switchbacks are distinct magnetic structures characterized by their abrupt reversal in the radial component of the magnetic field within the pristine solar wind. Switchbacks are believed to lose magnetic energy with heliocentric distance. To investigate this switchbacks originating from similar solar source regions are identified during a radial alignment of the Parker Solar Probe (PSP; 25.8 solar radii) and Solar Orbiter (SolO; 152 solar radii). We found that 1) the dynamic and thermal pressures decrease at the switchback boundaries by up to 20% at PSP and relatively unchanged at SolO and magnetic pressure jump across the boundary remains negligible at both distances, and 2) bundles of switchbacks are often observed in switchback patches near the Sun, and in microstreams farther away. Background proton velocity (vp) is 10% greater than the pristine solar wind (vsw) in microstreams, whereas vp ~ vsw in switchback patches. Microstreams contain an average of 30% fewer switchbacks than switchback patches. It is concluded that switchbacks likely relax magnetically and equilibrate their plasma with the surrounding environment with heliocentric distance. Switchback relaxation can, in turn, accelerate the surrounding plasma. Therefore, it is hypothesized that magnetic relaxation of switchbacks may cause switchback patches to evolve into microstreams with heliocentric distance. Statistical analysis of PSP and SolO switchbacks is underway to further test our hypothesis.

Scale-Dependent Dynamic Alignment in MHD Turbulence: Insights into Intermittency, Compressibility, and Imbalance Effects

arXiv (Cornell University) · 2024-07-04

Scale-Dependent Dynamic Alignment (SDDA) in Elsässer field fluctuations is theorized to suppress nonlinearities and modulate the energy spectrum. Limited empirical evidence exists for SDDA within the solar wind turbulence's inertial range. We analyzed data from the WIND mission to assess the effects of compressibility, intermittency, and imbalance on SDDA. SDDA consistently appears at energy-containing scales, with a trend toward misalignment at inertial scales. Compressible fluctuations show no increased alignment; however, their impact on SDDA's overall behavior is minimal. The alignment angles inversely correlate with field gradient intensity, likely due to "anomalous" or "counterpropagating" wave packet interactions. This suggests that SDDA originates from mutual shearing of Elsässer fields during imbalanced ($δ\boldsymbol{z}^{\pm} \gg δ\boldsymbol{z}^{\mp}$) interactions. Rigorous thresholding on field gradient intensity reveals SDDA signatures across much of the inertial range. The scaling of Elsässer increments' alignment angle, $Θ^{z}$, steepens with increasing global Alfvénic imbalance, while the angle between magnetic and velocity field increments, $Θ^{ub}$, becomes shallower. $Θ^{ub}$ only correlates with global Elsässer imbalance, steepening as the imbalance increases. Furthermore, increasing alignment in $Θ^{ub}$ persists deep into the inertial range of balanced intervals but collapses at large scales for imbalanced ones. Simplified theoretical analysis and modeling of high-frequency, low-amplitude noise in the velocity field indicate significant impacts on alignment angle measurements even at very low frequencies, with effects growing as global imbalance increases.

Magnetic Reconnection as the Driver of the Solar Wind

2023-02-26 · 1 citations

We present an overview of EUV solar observations showing evidence for ubiquitous small-scale jetting activity (i.e., a.k.a. jetlets) driven by magnetic reconnection that might be the primary driver of the solar wind at its source. The jetlets, like the solar wind and the heating of the coronal plasma, are omnipresent throughout the solar cycle. Each event arises from small-scale reconnection of opposite polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of the jetlets leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.

Supporting files for "Interchange reconnection as the source of the fast solar wind within coronal holes"

Zenodo (CERN European Organization for Nuclear Research) · 2023-01-23

IDL data files and code for generating the plots from the PIC simulations in the paper "Interchange reconnection as the source of the fast solar wind within coronal holes". The full data set from the simulations is too large to upload, so only relevant snapshots are included here. The complete data set is stored at the supercomputing centers at which the runs were performed and access can be arranged upon request.

Kinetic Instabilities from Ion Beams and Differential Streaming in the Close-Sun Solar Wind: Hybrid Expanding Simulations

2022-03-28

<p>Parker Solar Probe observations in the inner heliosphere have demonstrated that non-thermal features in solar wind ion distributions are particularly enhanced and dominant in the close-Sun environment. Proton beams and large differential flows of alpha particles are ubiquitously observed, also in slow, though Alfvénic, streams, qualitatively at odds with typical observations at 1AU, where non-Maxwellian features are usually less apparent in the slow solar wind. All this reinforces the idea, also supported by past Helios and Ulysses explorations, that preferential ion heating and acceleration take place already in the Corona and signatures of the kinetic processes involved are gradually washed out during expansion. To explore further properties of ion differential streaming during expansion, as well as associated kinetic instabilities and their possible role in plasma heating, we perform expanding box hybrid simulations of a multi-species solar wind composed by proton core, beam and alpha particles, focussing on the role of wave-particle interactions in shaping distribution functions and controlling relative drifts. Radial trends and typical distributions found in simulations are then compared with PSP and Solar Orbiter observations in the inner Heliosphere.</p>

Investigating Alfvénic Turbulence in Fast and Slow Solar Wind Streams

Universe · 2022-06-27 · 13 citations

Solar wind turbulence dominated by large-amplitude Alfvénic fluctuations, mainly propagating away from the Sun, is ubiquitous in high-speed solar wind streams. Recent observations performed in the inner heliosphere (from 1 AU down to tens of solar radii) have proved that also slow wind streams show sometimes strong Alfvénic signatures. Within this context, the present paper focuses on a comparative study on the characterization of Alfvénic turbulence in fast and slow solar wind intervals observed at 1 AU where degradation of Alfvénic correlations is expected. In particular, we compared the behavior of different parameters to characterize the Alfvénic content of the fluctuations, using also the Elsässer variables to derive the spectral behavior of the normalized cross-helicity and residual energy. This study confirms that the Alfvénic slow wind stream resembles, in many respects, a fast wind stream. The velocity-magnetic field (v-b) correlation coefficient is similar in the two cases as well as the amplitude of the fluctuations although it is not clear to what extent the condition of incompressibility holds. Moreover, the spectral analysis shows that fast wind and Alfvénic slow wind have similar normalized cross-helicity values but in general the fast wind streams are closer to energy equipartition. Despite the overall similarities between the two solar wind regimes, each stream shows also peculiar features, that could be linked to the intrinsic evolution history that each of them has experienced and that should be taken into account to investigate how and why Alfvénicity evolves in the inner heliosphere.

Non-thermal features in proton and alpha velocity distribution functions in the solar wind in the inner heliosphere

2022-03-28

<p>The solar wind is a highly variable, weakly collisional plasma originating from the Sun. The recent launches of PSP and Solar Orbiter have opened the way for the exploration of the innermost regions of our solar system and will greatly advance our understanding of several plasma phenomena occurring in the near-Sun environment, such as the heating and the acceleration of the solar wind. </p><p>Plasma waves and wave-particle interactions play a relevant role in such phenomena, determining significant deviations of the Velocity Distribution Function (VDF) from the Local Thermodynamic Equilibrium. These deviations retain information of the interaction of particles with the turbulent electromagnetic fields and can be identified as thermal anisotropy, or non-thermal ion beams and heavy ion differential streaming in the ion component of the solar wind. This study will cover these topics with particular reference to new in-situ data from Solar Orbiter, PSP and with observations at L1  (e.g. Wind), with a focus on the central role Alfvénic fluctuations play in the evolution of the VDF features mentioned above.</p>

Simulating the FIP effect in coronal loops using a multi-species kinetic-fluid model.

2022-03-27

<p>We investigate abundance variations of heavy ions in coronal loops. We develop and exploit a multi-species model of the solar atmosphere (called IRAP’s Solar Atmospheric Model: ISAM) that solves for the transport of neutral and charged particles from the chromosphere to the corona. We investigate the effect of different mechanisms that could produce the First Ionization Potential (FIP) effect. We compare the effects of the thermal, friction and ponderomotive force. The propagation, reflection and dissipation of Alfvén waves is solved using two distinct models, the first one from <em>Chandran et al. (2011)</em> and the second one that is a more sophisticated turbulence model called Shell-ATM. ISAM solves a set of 16-moment transport equations for both neutrals and charged particles. Protons and electrons are heated by Alfvén waves, which then heat up the heavy ions via collision processes. We show comparisons of our results with other models and observations, with an emphasis on FIP biases. This work was funded by the European Research Council through the project SLOW_SOURCE - DLV-819189.</p>