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Five images taken by the WISPR telescopes on PSP. The radial range is shown at the top. The yellow and red arrows point to plasmoids ejected along the forming HCS. From Howard et al. (2019).
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The onset of magnetic reconnection in space, astrophysical and laboratory plasmas is reviewed discussing results from theory, numerical simulations and observations. After a brief introduction on magnetic reconnection and approach to the question of onset, we first discuss recent theoretical models and numerical simulations, followed by observation...
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The dynamic evolution of coronal mass ejection (CME) in interplanetary space generates highly turbulent, compressed, and heated shock-sheath. This region furnishes a unique environment to study the turbulent fluctuations at the small scales and serve an opportunity for unfolding the physical mechanisms by which the turbulence is dissipated and plas...
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... This fluctuations' energy is usually injected at large ("fluid") scales and then is nonlinearly transferred towards smaller and smaller scales, until dissipation into heat and non-thermal particles is achieved at the characteristic microscopic ("kinetic") scales of the plasma (i.e., the Larmor radius and/or the inertial length of the particle species). Magnetic reconnection, on the other hand, is a micro-scale process that changes the magnetic-field connectivity of energetically unfavorable configurations by releasing excess magnetic energy (e.g., into bulk flows, waves, and non-thermal particles), and can affect a wide range of scales (e.g., Zweibel and Yamada 2009;Pucci et al. 2020). Reconnection is indeed an intrinsic element of plasma turbulence, as the latter naturally develops tearing-unstable current sheets on a wide range of scales along its cascade, and current-sheet reconnection can convert magnetic energy into fluctuations and structures that feed back into the turbulent cascade (e.g., Carbone et al. 1990;Huang and Bhattacharjee 2016;Cerri and Califano 2017;Franci et al. 2017;Loureiro and Boldyrev 2017;Mallet et al. 2017;Pucci et al. 2017;Comisso et al. 2018;Dong et al. 2018Dong et al. , 2022Papini et al. 2019;Borgogno et al. 2022;Cerri et al. 2022). ...
We perform 3D3V hybrid-Vlasov (fully kinetic ions and fluid electrons) simulations of sub-ion-scale plasma turbulence with quasi-isotropic, compressible injection close to the ion scales to investigate electron-only reconnection and its effect on ion heating. Retaining electron inertia in the generalized Ohm s law provides the mechanism for collisionless magnetic reconnection. Spectral analysis reveals a shift from kinetic Alfv\'{e}n waves (KAW) to inertial kinetic Alfv\'{e}n (IKAW) and inertial whistler waves (IWW) near electron scales. To distinguish the roles of the inertial scale and the gyroradius (d_{\rm{i}} and \rho_{\rm{i}}), three values of ion beta (\beta_{\rm{i}} = 0.25, 1, 4) are studied. Ion-electron decoupling increases with \beta_{\rm{i}}, as ions become less mobile when the injection scale is closer to \rho_{\rm{i}} than to d_{\rm{i}}, highlighting the role of \rho_{\rm{i}} in achieving an electron magnetohydrodynamic (EMHD) regime at sub-ion scales. The EMHD regime promotes electron-only reconnection in turbulence with small-scale injection at \beta_{\rm{i}} \gtrsim 1. Despite this EMHD-favoured dynamics, we still observe significant ion heating also at large \beta_{\rm{i}}, namely Q_{\rm{i}}/\epsilon \approx 69, 91, and 96 at \beta_{\rm{i}} = 0.25, 1, and 4, respectively. Moreover, while ion heating is significantly anisotropic at \beta_{\rm{i}} \leq 1 (namely, T_{\rm i,\perp} > T_{\rm i,\parallel}), it becomes only marginally anisotropic at \beta_{\rm{i}} > 1 (i.e., T_{\rm i,\perp} \gtrsim T_{\rm i,\parallel}). Our results highlight that ion turbulent heating in collisionless plasmas is sensitive to the separation between injection scales \lambda_{\text{inj}} and the ion gyroradius \rho_{\rm{i}}, as well as to \beta_{\rm{i}} and finite-k_{\parallel} effects, requiring an extended investigation of these parameters for accurate modelling.
... The vertical CP carries a total plasma current I CP and is sustained by a DC voltage V e , applied between electrodes placed inside the vacuum vessel; a confined ST carries a total toroidal plasma current I ST and is formed by self-organization around the CP: the lines of force that wind around the centerpost get broken and reconnected into lines of force winding along the torus [2]. The self-organization phenomenon is based on magnetic reconnections [4], which are associated with non-ideal MHD plasma dissipative effects (such as finite resistivity) and are ubiquitous in astrophysical plasmas, from the terrestrial magnetosphere, to the solar corona, to the PWN where the vertical centerpost plasma, carrying a total plasma current I CP , is surrounded by a plasma torus, carrying a total toroidal plasma current I ST . The plasma is unique, but this image is composed by 3 independent images: the anodic centerpost plasma (top), impinging on a continuous annular gas-puffed anode; the equatorial centerpost + torus plasma (middle); the cathodic centerpost plasma (bottom), emitted by 54 hot tungsten filaments. ...
This paper was presented as an Invited Paper at the ECPD 2023 in Rethymno (Crete), on April 25 2023. Its main purpose is to try and compare plasmas which have similar morphology in Magnetic Confinement Research and in Astrophysics observations. The jet+torus plasma morphology is quite diffused in Astrophysics and this paper illustrates a laboratory experiment (PROTO-SPHERA) that reproduces, as much as possible such a morphology. While in Astrophysics the appearance of jet + torus is a spontaneous phenomenon lasting for enormous times, in the case of the laboratory experiment the appearance and the sustainment of a plasma torus around a central plasma jet was an unexplored phenomenon and therefore a clear bet on the self-organisation properties of a plasma. However such a bet was based on two plasma physics concepts that have had, since more than 60 years, and still have quite a relevance in magnetic confinement research: force-free fields ad magnetic helicity injection. The experiment has been successful and the jet + torus configuration can be routinely formed and sustained in the laboratory for as long as the central plasma jet is maintained by power supplies: 1 sec, a time-scale limited by inertial (passive) power removal, a choice due to the necessity of building a low-cost explorative experiment, without engineering complications connected to active power removal. The duration of 1 sec is nevertheless much longer than both relevant plasma time scales: the Alfvén time, which is 1/2 µs, and also the current diffusion time, which is at the moment 10 ms. So, from the point of view of plasma physics alone, 1 sec is (in this case) perfectly equivalent to maintenance of the jet+torus plasma in steady-state. As the ECPD 2023 was a Plasma Diagnostic conference, the observations of the torus in the experiment presented in this paper is mainly relying on an innovative 3D tomography method in visible light: the simply connected geometry of the plasma and the perfectly transparent vacuum vessel of PROTO-SPHERA allows for 3D tomography of visible light emission, an absolute novelty with respect to the toroidal doubly connected geometry and the metal vacuum vessel of most magnetic confinement experiments. The final sections of this paper deal with application of force-free fields to Pulsar Wind Nebulae morphology and are able to present an extension of the well-know split-dipole model for Pulsar Wind Nebulae; the extension provides a natural description for the presence of tori around the Pulsar plasma jets. The paper concludes by underlining the similarities but also the differences between jet+torus configurations in Astrophysics and in laboratory experiments.
... The vertical CP carries a total plasma current I CP and is sustained by a DC voltage V e , applied between electrodes placed inside the vacuum vessel; a confined ST carries a total toroidal plasma current I ST and is formed by self-organization around the CP: the lines of force that wind around the centerpost get broken and reconnected into lines of force winding along the torus [2]. The self-organization phenomenon is based on magnetic reconnections [4], which are associated with non-ideal MHD plasma dissipative effects (such as finite resistivity) and are ubiquitous in astrophysical plasmas, from the terrestrial magnetosphere, to the solar corona, to the PWN where the vertical centerpost plasma, carrying a total plasma current I CP , is surrounded by a plasma torus, carrying a total toroidal plasma current I ST . The plasma is unique, but this image is composed by 3 independent images: the anodic centerpost plasma (top), impinging on a continuous annular gas-puffed anode; the equatorial centerpost + torus plasma (middle); the cathodic centerpost plasma (bottom), emitted by 54 hot tungsten filaments. ...
The PROTO-SPHERA experiment, built at the CR-ENEA laboratory in Frascati, was in part inspired by the jet + torus astrophysical plasmas, a rather common morphology in Astrophysics. This paper illustrates how the said plasma morphology can be reproduced in a laboratory with the setup of the PROTO-SPHERA experiment. The experiment as such displayed the appearance and sustainment of a plasma torus around an internal magnetized plasma centerpost (jet) by self-organisation; an entirely unexplored phenomenon to date. The remarkable ideal MHD stability of the PROTO-SPHERA plasma is extremely significant, as it is obtained in a simply connected geometry, inside a perfectly insulating vacuum vessel, and without the need of a nearby stabilizing conducting shell. The concluding sections of this paper deal with application of force-free fields to the Pulsar Wind Nebulae morphology and present an extension of the well-known split-dipole model. Such an extension provides a natural description of the presence of tori around the Pulsar plasma jets. In addition, similarities and differences between the laboratory and the astrophysical jet + torus plasmas are detailed.
... These instabilities are purportedly responsible for thinning the current sheet down to the kinetic scale, leading to the growth of electron tearing and reconnection onset. The reader is directed to the recent reviews of Sitnov et al. (2019a) and Pucci et al. (2020) for further information. ...
We analyze the local dynamics of magnetotail reconnection onset using Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic dynamics of this event were captured by a number of other ground and space‐based observatories, as is reported in a companion paper. We find that the local dynamics of the onset were characterized by the rapid thinning of the cross‐tail current sheet below the ion inertial scale, accompanied by the growth of flapping waves and the subsequent onset of electron tearing. Multiple kinetic‐scale magnetic islands were detected coincident with the growth of an initially sub‐Alfvénic, demagnetized tailward ion exhaust. The onset and rapid enhancement of parallel electron inflow at the exhaust boundary was a remote signature of the intensification of reconnection Earthward of the spacecraft. Two secondary reconnection sites are found embedded within the exhaust from a primary X‐line. The primary X‐line was designated as such on the basis that (a) while multiple jet reversals were observed in the current sheet, only one reversal of the electron inflow was observed at the high‐latitude exhaust boundary, (b) the reconnection electric field was roughly five times larger at the primary X‐line than the secondary X‐lines, and (c) energetic electron fluxes increased and transitioned from anti‐field‐aligned to isotropic during the primary X‐line crossing, indicating a change in magnetic topology. The results are consistent with the idea that a primary X‐line mediates the reconnection of lobe magnetic field lines and accelerates electrons more efficiently than its secondary X‐line counterparts.
... The plasmoid instability is a fast instability that breaks down current sheets into multiple magnetic islands, which later undergo nonlinear evolution. We refer the reader to Loureiro & Uzdensky (2015) and Pucci et al. (2020) for a review of recent developments in plasmoid instability research both in the collisional and collisionless regimes. The plasmoid instability results in fast, spontaneous magnetic reconnection in plasmas and, as such, is deeply connected to the fundamental topic of particle heating and acceleration in space and astrophysical plasmas. ...
The growing amount of data produced by simulations and observations of space physics processes encourages the use of methods rooted in machine learning for data analysis and physical discovery. We apply a clustering method based on self-organizing maps to fully kinetic simulations of plasmoid instability, with the aim of assessing their suitability as a reliable analysis tool for both simulated and observed data. We obtain clusters that map well, a posteriori , to our knowledge of the process; the clusters clearly identify the inflow region, the inner plasmoid region, the separatrices and regions associated with plasmoid merging. Self-organizing map-specific analysis tools, such as feature maps and the unified distance matrix, provide us with valuable insights into both the physics at work and specific spatial regions of interest. The method appears as a promising option for the analysis of data, both from simulations and from observations, and could also potentially be used to trigger the switch to different simulation models or resolution in coupled codes for space simulations.
... The plasmoid instability is a fast instability that breaks down current sheets into multiple magnetic islands, which later undergo non-linear evolution. We refer the reader to Loureiro & Uzdensky (2015); Pucci et al. (2020) for a review of recent developments in plasmoid instability research both in the collisional and collisionless regimes. The plasmoid instability results in fast, spontaneous magnetic reconnection in plasmas and, as such, is deeply connected with the fundamental topic of particle heating and acceleration in space and astrophysical plasmas. ...
The growing amount of data produced by simulations and observations of space physics processes encourages the use of methods rooted in Machine Learning for data analysis and physical discovery. We apply a clustering method based on Self-Organizing Maps (SOM) to fully kinetic simulations of plasmoid instability, with the aim of assessing its suitability as a reliable analysis tool for both simulated and observed data. We obtain clusters that map well, a posteriori, to our knowledge of the process: the clusters clearly identify the inflow region, the inner plasmoid region, the separatrices, and regions associated with plasmoid merging. SOM-specific analysis tools, such as feature maps and Unified Distance Matrix, provide one with valuable insights into both the physics at work and specific spatial regions of interest. The method appears as a promising option for the analysis of data, both from simulations and from observations, and could also potentially be used to trigger the switch to different simulation models or resolution in coupled codes for space simulations.
... In solar chromospheric conditions, and also in other astrophysical environments (Gosling 2007;Pucci et al. 2020b), MR processes mostly evolve in high-Lundquistnumber regimes. In this regime, current sheets become rapidly unstable to resistive tearing instabilities, which produces plasmoids (e.g., Bhattacharjee et al. 2009Bhattacharjee et al. , 2010Pucci et al. 2020b). ...
... In solar chromospheric conditions, and also in other astrophysical environments (Gosling 2007;Pucci et al. 2020b), MR processes mostly evolve in high-Lundquistnumber regimes. In this regime, current sheets become rapidly unstable to resistive tearing instabilities, which produces plasmoids (e.g., Bhattacharjee et al. 2009Bhattacharjee et al. , 2010Pucci et al. 2020b). Plasmoids are believed to play a major role in speeding up reconnection by having an influence on the variation of the current sheet size, as shown by Murtas et al. (2021). ...
Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion–neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between the spectral profiles and synthetic observations of reconnection events suggests that current MHD approaches appear to be inconsistent with observations. First, collisions and multithermal aspects of the plasma play a role in these regions. Second, hydrogen and helium ionization effects are relevant to the energy balance of the chromosphere. This work investigates the multifluid multispecies (MFMS) effects on MR in conditions representative of the upper chromosphere using the multifluid Ebysus code. We compare an MFMS approach based on a helium–hydrogen mixture with a two-fluid MHD model based on hydrogen only. The simulations of MR are performed in a Lundquist number regime high enough to develop plasmoids and instabilities. We study the evolution of the MR and compare the two approaches including the structure of the current sheet and plasmoids, the decoupling of the particles, the evolution of the heating mechanisms, and the composition. The presence of helium species leads to more efficient heating mechanisms than the two-fluid case. This scenario, which is out of reach of the two-fluid or single-fluid models, can reach transition region temperatures starting from upper-chromospheric thermodynamic conditions, representative of a quiet Sun scenario. The different dynamics between helium and hydrogen species could lead to chemical fractionation and, under certain conditions, enrichment of helium in the strongest outflows. This could be of significance for recent observations of helium enrichment in the solar wind in switchbacks and coronal mass ejections.
... Tajima & Shibata 1997;Tanuma et al. 2001;Loureiro et al. 2005;Daughton & Scudder 2006;Drake et al. 2006;Loureiro et al. 2007;Bhattacharjee et al. 2009;Landi et al. 2015;Tenerani et al. 2015;Del Sarto et al. 2016;Del Sarto & Ottaviani 2017;Papini, Landi & Del Zanna 2019b;Singh et al. 2019) and to reconnection in turbulence (see, e.g. Pucci & Velli 2014;Loureiro & Uzdensky 2016;Tenerani et al. 2016;Comisso et al. 2018;Papini et al. 2019a;Betar et al. 2020;Kowal et al. 2020;Pucci et al. 2020;Schekochihin 2020;Franci et al. 2022) in the context of the so-called 'turbulent (or turbulence-mediated or turbulence-driven) reconnection' scenario (Matthaeus & Lamkin 1986;Strauss 1988;Loureiro et al. 2009;Matthaeus & Velli 2011; Schekochihin 2020) -not to be confused with the study of 'turbulent-driven magnetic island' in tokamaks (cf. end of § 2.1). ...
We revise in detail and in a pedagogical way the analysis of the boundary layer theory of warm tearing modes in slab, reduced magnetohydrodynamics (MHD), when magnetic reconnection is driven by electron inertia and/or resistivity, and ion-sound Larmor radius effects are included. By comparison with the numerical solution of the corresponding eigenvalue problem, we interpret these results by means of a heuristic approach, which in the warm-electron regime, we show to be in general not feasible without knowledge of the scaling of the gradient of the magnetic flux function, differently from what happens in the cold-electron regimes. We put in evidence for a non-trivial relation between the first derivative of the magnetic flux function and of the velocity parallel to the neutral line, evaluated in its proximity, by thus providing insight to the multiple boundary layer analysis that Pegoraro & Schep ( Plasma Phys. Control. Fusion , vol. 28, 1986, p. 647) first showed to be required in warm-tearing regimes. In this way, we also suggest and justify a general operational definition of the reconnecting layer width and we discuss the linear appearance of microscopic scales related to the gradients of the eigenfunctions of the tearing modes.
... Thereafter, classical expressions for electrical conductivity were developed in various types of non-magnetized plasma such as in turbulent 4,5 , electric arc 6 , non-ideal 7 , dense 8 , strongly magnetized 9 and Lorentz plasma 10 . Further investigations on conductivity have also been carried out to understand phenomena such as non linear response to large electric field 11 , instabilities regulating anisotropy in solar wind 12 , reverse conductivity in dusty plasma 13 , effect of Spitzer resistivity on magnetic reconnection with 14 and without 15 guiding field, including regulation of resistivity 16 , energy dissipation 17 and magnetic flow of currents 18 . ...
... It may be noted that in the absence of magnetic field ( ì 0 = 0), and reduces to 0 given by, 0 = 2 [( 0 /( ℎ )) + ( + /( + + ))] ≈ 0 2 /( ℎ ) and 0 = 0 2 /( ( ℎ − )) respectively, ℎ and + being independent of the direction of collision (see Appendix D.2.). But in the presence of magnetic field, 0+ = + 2 /( + + ) and 0 = 0 2 /( ℎ ) in Eq. (7) - (11), and 0 = 0 2 /( ( ℎ − )) in Eq. (12) - (16). The isotropic conductivity depends upon the direction of collisionality as well. ...
A generalization of electrical conductivity in a plasma confined in a dipole magnetic field, in the presence of temperature anisotropy is presented. The anisotropy governed by the magnetic field distribution is found to be significant in the strong field region, and has a considerable effect on Pedersen and longitudinal conductivity of electrons over Hall conductivity, whereas the effect of temperature anisotropy on Hall conductivity can be observed in the case of ions. The work reveals new features in the conductivity tensor arising due to the temperature anisotropy and bidirectional nature of the dipole field, by incorporating all possible particle drifts, which would be helpful to enhance the understanding of electrical conduction in both laboratory and space dipole plasmas.
... Apparently, the inverse aspect ratio ∕ is a key parameter determining the growth rate . For an arbitrary inverse aspect ratio ∕ ∼ − , we get ∼ − + ( +1) , implying a threshold = ∕( + 1) , at which ∼ (1) is achieved and this is the so-called "ideal tearing" (Pucci & Velli, 2013;Pucci et al., 2020;Tenerani et al., 2016). For the classical tearing = 1∕2 , we have = 1∕3 , that is to say when a macroscopic current thins to ∕ ∼ −1∕3 , Figure 4. Maximum growth rate as a function of for = 1 and varying in log-log scale. ...
One of the most important problems of magnetotail dynamics is the substorm onset and the related instability of the magneotail current sheet. Since the simplest 2D current sheet configuration with monotonic Bz was proven to be stable to the tearing mode, the focus of the instability investigation moved to more specific configurations, for example, kinetic current sheets with strong transient ion currents and current sheets with non‐monotonic Bz (local Bz minima or/and peaks). The stability of the latter current sheet configuration has been studied both within kinetic and fluid approaches, whereas the investigation of the transient ion effects was limited to kinetic models only. This paper aims to provide a detailed analysis of the stability of a multi‐fluid current sheet configuration that mimics current sheets with transient ions. Using the system with two field‐aligned ion flows that mimic the effect of pressure non‐gyrotropy, we construct a 1D current sheet with a finite Bz. This model describes well recent findings of very thin intense magnetotail current sheets. The stability analysis of this two‐ion model confirms the stabilizing effect of finite Bz and shows that the most stable current sheet is the one with exactly counter‐streaming ion flows and zero net flow. Such field‐aligned flows may substitute the contribution of the pressure tensor nongyrotropy to the stress balance but cannot overtake the stabilizing effect of Bz. Obtained results are discussed in the context of magnetotail dynamical models and spacecraft observations.
