Motivated by recent Parker Solar Probe (PSP) observations of "switchbacks" (abrupt, large-amplitude reversals in the radial magnetic field, which exhibit Alfvénic correlations) we examine the dynamics of large-amplitude Alfvén waves in the expanding solar wind. We develop an analytic model which makes several predictions: switchbacks should preferentially occur in regions where the solar wind plasma has undergone a greater expansion, the switchback fraction at radii comparable to PSP should be an increasing function of radius, and switchbacks should have their gradients preferentially perpendicular to the mean magnetic field direction. The expansion of the plasma generates small compressive components as part of the wave's nonlinear evolution: these are maximized when the normalized fluctuation amplitude is comparable to $\sinθ$, where $θ$ is the angle between the propagation direction and the mean magnetic field. These compressive components steepen the primary Alfvénic waveform, keeping the solution in a state of nearly constant magnetic field strength as its normalized amplitude $δB/B$ grows due to expansion. The small fluctuations in the magnetic-field-strength are minimized at a particular $θ$-dependent value of $β$, usually of order unity, and the density and magnetic-field-strength fluctuations can be correlated or anticorrelated depending on $β$ and $θ$. Example solutions of our dynamical equation are presented; some do indeed form magnetic-field reversals. Our predictions appear to match some previously unexplained phenomena in observations and numerical simulations, providing evidence that the observed switchbacks result from the nonlinear evolution of the initially small-amplitude Alfvén waves already known to be present at the coronal base.
Switchbacks -- rapid, large deflections of the solar wind's magnetic field -- have generated significant interest as possible signatures of the key mechanisms that heat the corona and accelerate the solar wind. In this context, an important task for theories of switchback formation and evolution is to understand their observable distinguishing features, allowing them to be assessed in detail using spacecraft data. Here, we work towards this goal by studying the influence of the Parker spiral on the evolution of Alfvénic switchbacks in an expanding plasma. Using simple analytic arguments based …
In the lower solar coronal regions where the magnetic field is dominant, the Alfven speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfvenic, i.e., the wind speed greatly exceeds the Alfven speed. The transition between these regimes is classically described as the "Alfven point" but may in fact occur in a distributed Alfven critical region. NASA's Parker Solar Probe (PSP) mission has entered this region, as it follows a series of orbits that gradually approach more closely to the sun. During its 8th and 9th solar encounters, at a distance of …
We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfvénic wind observed by Parker Solar Probe (PSP), and the morphology of the Alfvénic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfvén radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfvénic wind detected by PSP is an LMB…
{Parker Solar Probe's first solar encounter has revealed the presence of sudden magnetic field deflections that are called switchbacks and are associated with proton velocity enhancements in the slow alfvénic solar wind.} {We study their statistical properties with a special focus on their boundaries.} {Using data from SWEAP and FIELDS we investigate particle and wavefield properties. The magnetic boundaries are analyzed with the minimum variance technique.} {Switchbacks are found to be alfvénic in 73\% of the cases and compressible in 27\%. The correlations between magnetic field magnitude an…
Generation and Expansion-Driven Growth of Switchbacks in the Outer Solar Corona and Solar Wind
Imperial College London（帝国理工学院）；University of California, Los Angeles（加州大学洛杉矶分校）；Auburn University（奥本大学）；Space Sciences Laboratory, University of California, Berkeley（加州大学伯克利分校空间科学实验室）；Istituto per la Scienza e Tecnologia dei Plasmi (ISTP), Consiglio Nazionale delle Ricerche（等离子体科学与技术研究所，意大利国家研究委员会）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）
Recent near-sun solar-wind observations from Parker Solar Probe have found a highly dynamic magnetic environment, permeated by abrupt radial-field reversals, or "switchbacks." We show that many features of the observed turbulence are reproduced by a spectrum of Alfvénic fluctuations advected by a radially expanding flow. Starting from simple superpositions of low-amplitude outward-propagating waves, our expanding-box compressible MHD simulations naturally develop switchbacks because (i) the normalized amplitude of waves grows due to expansion and (ii) fluctuations evolve towards spherical pola…
We provide a simple geometric explanation for the source of switchbacks and associated large and one-sided transverse flows in the solar wind observed by Parker Solar Probe. The more radial, Sub-Parker Spiral structure of the heliospheric magnetic field observed previously by Ulysses, ACE, and STEREO is created within rarefaction regions where footpoint motion from the source of fast into slow wind at the Sun creates a magnetic field line connection across solar wind speed shear. Conversely, when foot-points move from the source of slow wind into faster wind, a Super-Parker Spiral field struct…
One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfvénic magnetic field reversals termed 'switchbacks'. These $δB_R/B \sim \mathcal{O}(1$) 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, …
Averaging Effects on the Solar Wind Alfven Mach Number: Implications for Switchbacks and Alfven Transition
University of Delaware（特拉华大学）；Princeton University（普林斯顿大学）；University of California, Berkeley（加州大学伯克利分校）；Mahidol University（玛希隆大学）；University of Orléans（奥尔良大学）；Kasetsart University（泰国农业大学）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）
In this letter we report observations of magnetic switchback (SB) features near 1 au using data from the \emph{Wind} spacecraft. These features appear to be strikingly similar to the ones observed by the Parker Solar Probe mission (PSP) closer to the Sun: namely, one-sided spikes (or enhancements) in the solar-wind bulk speed $V$ that correlate/anti-correlate with the spikes seen in the radial-field component $B_R$. In the solar-wind streams that we analyzed, these specific SB features near 1 au are associated with large-amplitude Alfvénic oscillations that propagate outward from the sun along…
Magnetic switchbacks, or sudden reversals in the magnetic field's radial direction, are one of the more striking observations of Parker Solar Probe (PSP) thus far in its mission. While their precise production mechanisms are still unknown, the two main theories are via interchange reconnection events and in-situ generation. In this work density and abundance variations of alpha particles are studied inside and outside individual switchbacks. We find no consistent compositional differences in the alpha particle abundance ratio, $n_{αp}$, inside vs outside, nor do we observe any signature when s…
Magnetic switchbacks are rapid high amplitude reversals of the radial magnetic field in the solar wind that do not involve a heliospheric current sheet crossing. First seen sporadically in the seventies in Mariner and Helios data, switchbacks were later observed by the Ulysses spacecraft beyond 1 au and have been recently identified as a typical component of solar wind fluctuations in the inner heliosphere by the Parker Solar Probe spacecraft. Here we provide a simple yet predictive theory for the formation of these magnetic reversals: the switchbacks are produced by the shear of circularly po…
We analyze magnetic field data from the first six encounters of PSP, three Helios fast streams and two Ulysses south polar passes covering heliocentric distances $0.1\lesssim R\lesssim 3$ au. We use this data set to statistically determine the evolution of switchbacks of different periods and amplitudes with distance from the Sun. We compare the radial evolution of magnetic field variances with that of the mean square amplitudes of switchbacks, and quantify the radial evolution of the cumulative counts of switchbacks per km. We find that the amplitudes of switchbacks decrease faster than the o…
Switchbacks are widely acknowledged phenomena observed by the Parker Solar Probe and appear to occur in patches. Previous studies focused on the fluctuations at the magnetic reversals. However, the nature of the fluctuations inside the switchbacks remains unknown. Here we utilize the magnetic field data and plasma data measured by the Parker Solar Probe in the first four encounters. We investigate the fluctuations in the switchback intervals of 100 s with BR>0 at every instant and compare them to the fluctuations in the nonswitchback intervals of 100 s with theta_RB>160o at every instant. We c…
During Parker Solar Probe's first two orbits there are widespread observations of rapid magnetic field reversals known as switchbacks. These switchbacks are extensively found in the near-Sun solar wind, appear to occur in patches, and have possible links to various phenomena such as magnetic reconnection near the solar surface. As switchbacks are associated with faster plasma flows, we questioned whether they are hotter than the background plasma and whether the microphysics inside a switchback is different to its surroundings. We have studied the reduced distribution functions from the Solar …
Many solar coronal jets result from erupting miniature-filament ("minifilament") magnetic flux ropes that reconnect with encountered surrounding far-reaching field. Many of those minifilament flux ropes are apparently built and triggered to erupt by magnetic flux cancelation. If that cancelation (or some other process) results in the flux rope's field having twist, then the reconnection with the far-reaching field transfers much of that twist to that reconnected far-reaching field. In cases where that surrounding field is open, the twist can propagate to far distances from the Sun as a magneti…
We use \emph{Parker Solar Probe} (\emph{PSP}) in-situ measurements to analyze the characteristics of solar wind turbulence during the first solar encounter covering radial distances between $35.7R_\odot$ and $41.7R_\odot$. In our analysis we isolate so-called switchback (SB) intervals (folded magnetic field lines) from non-switchback (NSB) intervals, which mainly follow the Parker spiral field. Using a technique based on conditioned correlation functions, we estimate the power spectra of Elsasser, magnetic and bulk velocity fields separately in the SB and NSB intervals. In comparing the turbul…
Switchbacks (rotations of the magnetic field) are observed on the Parker Solar Probe. Their evolution, content, and plasma effects are studied in this paper. The solar wind does not receive a net acceleration from switchbacks that it encountered upstream of the observation point. The typical switchback rotation angle increased with radial distance. Significant Poynting fluxes existed inside, but not outside, switchbacks and they are related to the increased EXB/B2 flow caused by the magnetic field rotating to become more perpendicular to the flow direction. (Outside the switchbacks, the magnet…
Parker Solar Probe (PSP) observations during its first encounter at 35.7 $R_\odot$ have shown the presence of magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. Their counterparts in the solar wind velocity field are local enhancements in the radial speed, or jets, displaying (in all components) the velocity-magnetic field correlation typical of large amplitude Alfvén waves propagating away from the Sun. Switchbacks and radial jets have previously been observed over a wide range of heliocentric d…
Parker Solar Probe (PSP) has shown that the solar wind in the inner heliosphere is characterized by the quasi omni-presence of magnetic switchbacks ("switchback" hereinafter), local backward-bends of magnetic field lines. Switchbacks also tend to come in patches, with a large-scale modulation that appears to have a spatial scale size comparable to supergranulation on the Sun. Here we inspect data from the first ten encounters of PSP focusing on different time intervals when clear switchback patches were observed by PSP. We show that the switchbacks modulation, on a timescale of several hours, …
Magnetic switchback formation: a review of proposed mechanisms
Durham University（杜伦大学）；University of Otago（奥塔哥大学）；CNRS, Instituto de Astrofísica de Canarias（法国国家科学研究中心，加那利天体物理学研究所）；Sorbonne Université, École polytechnique, Institut Polytechnique de Paris, Université Paris Saclay, Observatoire de Paris-PSL, CNRS, Laboratoire de Physique des Plasmas（索邦大学，巴黎综合理工学院，巴黎理工学院，巴黎萨克雷大学，巴黎天文台-PSL，法国国家科学研究中心，等离子体物理实验室）；SSL, University of California Berkeley（空间科学实验室，加州大学伯克利分校）；University of Maryland（马里兰大学）；Starion Group S.A.（星团集团）；Bartol Research Institute and Department of Physics and Astronomy, University of Delaware（巴托尔研究所和物理与天文系，特拉华大学）；Imperial College London（帝国理工学院）；Mahidol University（玛希隆大学）
Conversion Layer Controls the Evolution of Magnetic Deflections Near the Alfven Surface
University of Michigan（密歇根大学）；University of Delaware（特拉华大学）；Center for Astrophysics, Harvard & Smithsonian（哈佛-史密森尼天体物理中心）；University of Alabama Huntsville（阿拉巴马大学亨茨维尔分校）；Auburn University（奥本大学）；University of California Berkeley Space Sciences Laboratory（加州大学伯克利分校空间科学实验室）
Properties of Magnetic Switchbacks in the Near-Sun Solar Wind
Center for Astrophysics, Harvard & Smithsonian（哈佛-史密森天体物理中心）；Institut de Recherche en Astrophysique et Planétologie, CNES, CNRS, Université de Toulouse（天体物理学与行星学研究所，法国国家空间研究中心，法国国家科学研究中心，图卢兹大学）；Department of Physics, Imperial College London（帝国理工学院物理系）；Space Sciences Laboratory, University of California, Berkeley（加州大学伯克利分校空间科学实验室）；Astronomy and Space Physics Department, National Taras Shevchenko University of Kyiv（基辅塔拉斯舍甫琴科国立大学天文与空间物理系）；Department of Climate and Space Sciences and Engineering, University of Michigan（密歇根大学气候与空间科学与工程系）；LPC2E, OSUC, Univ Orleans, CNRS, CNES（奥尔良大学地球与环境过程实验室，观测与研究宇宙学与气候系统观测站，法国国家科学研究中心，法国国家空间研究中心）；International Space Science Institute（国际空间科学研究所）
We investigate dispersive and kinetic effects on the evolution of a two-dimensional kinked Alfvén wave packet by comparing results from MHD, Hall-MHD and hybrid simulations of a low-$β$ plasma. We find that the Hall term determines the overall evolution of the wave packet over a characteristic time $τ^*=τ_a\ell/d_i$ in both fluid and hybrid models. Dispersion of the wave packet leads to the conversion of the wave energy into internal plasma energy. When kinetic protons are considered, the proton internal energy increase has contributions from both plasma compressions and phase space mixing. Th…
We consider 2D joint distributions of normalised residual energy $σ_r(s,t)$ and cross helicity $σ_c(s,t)$ during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale $s$. The broad features of the distributions are similar to previous observations made by HELIOS in slow solar wind, namely well correlated and fairly Alfvénic, except for a population with negative cross helicity which is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, brief periods where the magnetic field polarity reverses. Such switc…
The Parker Solar Probe (PSP) has observed copious rapid magnetic field direction changes in the near-Sun solar wind. These features have been called "switchbacks," and their origin is a mystery. But their widespread nature suggests that they may be generated by a frequently occurring process in the Sun's atmosphere. We examine the possibility that the switchbacks originate from coronal jets. Recent work suggests that many coronal jets result when photospheric magnetic flux cancels, and forms a small-scale "minifilament" flux rope that erupts and reconnects with coronal field. We argue that the…
The solar wind is found by Parker Solar Probe (PSP) to be abundant with Alfvénic velocity spikes and magnetic field kinks. Temperature enhancement is another remarkable feature associated with the Alfvénic spikes. How the prototype of these coincident phenomena is generated intermittently in the source region becomes a hot topic of wide concerns. Here we propose a new model introducing guide-field discontinuity into the interchange magnetic reconnection between open funnels and closed loops with different magnetic helicities. The modified interchange reconnection model not only can accelerate …
We carry out two-dimensional magnetohydrodynamic (MHD) simulations of an ensemble of Alfvénic fluctuations propagating in a structured, expanding solar wind including the presence of fast and slow solar wind streams. Using an appropriate expanding box model, the simulations incorporate the effects of fast-slow stream shear and compression and rarefaction self-consistently. We investigate the radial and longitudinal evolution of the cross-helicity, the total and residual energies and the power spectra of outward and inward Alfvénic fluctuations. The stream interaction is found to strongly affec…
Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic ``flocculated'' fluctuations, we propose that the dynamics of the inner solar wind just outside the Alfvén critical zone, and in the vicinity of the first $β=1$ surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfvén critical zone, as suggested by global magnetohydrodynam…
The long-term evolution of large-amplitude Alfven waves propagating in the solar wind is investigated by performing two-dimensional MHD simulations within the expanding box model. The linear and nonlinear phases of the parametric decay instability are studied for both circularly polarized waves in parallel propagation and for arc-polarized waves in oblique propagation. The non-monochromatic case is also considered. In the oblique case, the direct excitation of daughter modes transverse to the local background field is found for the first time in an expanding environment, and this transverse ca…
Alfvénic fluctuations in the solar wind display many properties reflecting an ongoing nonlinear cascade, e.g. a well-defined spectrum in frequency, together with some characteristics more commonly associated with the linear propagation of waves from the Sun, such as the variation of fluctuation amplitude with distance, dominated by solar wind expansion effects. Therefore both nonlinearities and expansion must be included simultaneously in any successful model of solar wind turbulence evolution. Because of the disparate spatial scales involved, direct numerical simulations of turbulence in the …
\textit{Parker Solar Probe} has shown the ubiquitous presence of strong magnetic field deflections, namely switchbacks, during its first perihelion where it was embedded in a highly Alfvénic slow stream. Here, we study the turbulent magnetic fluctuations around ion scales in three intervals characterized by a different switchback activity, identified by the behaviour of the magnetic field radial component, $B_r$. \textit{Quiet} ($B_r$ does not show significant fluctuations), \textit{weak} ($B_r$ has strong fluctuations but no reversals) and \textit{strong} ($B_r$ has full reversals) periods sh…
During Parker Solar Probe's first orbit, the solar wind plasma has been observed in situ closer than ever before, the perihelion on November 6th 2018 revealing a flow that is constantly permeated by large amplitude Alfvénic fluctuations. These include radial magnetic field reversals, or switchbacks, that seem to be a persistent feature of the young solar wind. The measurements also reveal a very strong, unexpected, azimuthal velocity component. In this work, we numerically model the solar corona during this first encounter, solving the MHD equations and accounting for Alfvén wave transport and…
One of the discoveries made by Parker Solar Probe during first encounters with the Sun is the ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were called switchbacks as some of them show up the full reversal of the radial component of the magnetic field and then return to regular conditions. Analyzing the magnetic field and plasma perturbations associated with switchbacks we identify three types of structures with slightly different characteristics: 1. Alfvenic structures, where the variations of the magnetic field compone…
Using one-dimensional hybrid expanding box model we investigate properties of the solar wind in the outer heliosphere. We assume a proton-electron plasma with a strictly transverse ambient magnetic field and, beside the expansion, we take into account influence of a continuous injection of cold pick-up protons through the charge-exchange process between the solar wind protons and hydrogen of interstellar origin. The injected cold pick-up protons form a ring distribution function that rapidly becomes unstable and generate Alfvén cyclotron waves. The Alfvén cyclotron waves scatter pick-up proton…
The decay of the solar wind helium to hydrogen temperature ratio due to Coulomb thermalization can be used to measure how far from the Sun strong preferential ion heating occurs. Previous work has shown that a zone of preferential ion heating, resulting in mass-proportional temperatures, extends about $20-40 R_\odot$ from the Sun on average. Here we look at the motion of the outer boundary of this zone with time and compare it to other physically meaningful distances. We report that the boundary moves in lockstep with the Alfvén point over the solar cycle, contracting and expanding with solar …
The effects of the heliospheric current sheet (HCS) on the evolution of Alfvénic turbulence in the solar wind are studied using MHD simulations incorporating the expanding-box-model (EBM). The simulations show that near the HCS, the Alfvénicity of the turbulence decreases as manifested by lower normalized cross helicity and larger excess of magnetic energy. The numerical results are supported by a superposed-epoch analysis using OMNI data, which shows that the normalized cross helicity decreases inside the plasma sheet surrounding HCS, and the excess of magnetic energy is significantly enhance…
The Parker Solar Probe (PSP) spacecraft is performing the first in situ exploration of the solar wind within 0.2 au of the Sun. Initial observations confirmed the Alfvénic nature of aligned fluctuations of the magnetic field B and velocity V in solar wind plasma close to the Sun, in domains of nearly constant magnetic field magnitude $|{\bf B}|$, i.e., approximate magnetic pressure balance. Such domains are interrupted by particularly strong fluctuations, including but not limited to radial field (polarity) reversals, known as switchbacks. It has been proposed that nonlinear Kelvin-Helmholtz i…
Magnetic flux ropes in space are generally connected to some regions electromagnetically. We consider the whole closed current system of the expanding flux ropes including the electric current associated with them. By combining the theories regarding the self-similar expansion of cylindrical flux ropes and the Alfven wave current system, we examine conditions under which the electric current matches. These matching conditions are satisfied when the time dependence of the current flowing in the closed circuit agrees with that which maintains the expanding flux rope. In consequence, we encounter…
Remote sensing observations of coronal holes show that heavy ions are hotter than protons and their temperature is anisotropic. In-situ observations of fast solar wind streams provide direct evidence for turbulent Alfvén wave spectrum, left-hand polarized ion-cyclotron waves, and He$^{++}$ -- proton drift in the solar wind plasma, which can produce temperature anisotropies by resonant absorption and perpendicular heating of the ions. Furthermore, the solar wind is expected to be inhomogeneous on decreasing scales approaching the Sun. We study the heating of solar wind ions in inhomogeneous pla…
