Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session JO07: ReconnectionLive Streamed
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Chair: Ari Le, LANL Room: 401 ABC |
Tuesday, October 18, 2022 2:00PM - 2:12PM |
JO07.00001: Enhanced Fermi acceleration in multiscale MHD reconnection Stephen P Majeski, Hantao Ji Reconnection provides myriad mechanisms for the acceleration of plasma particles to high energies. Fermi acceleration within compressing plasmoids in particular has been used to explain energetic particle distributions throughout a range of reconnection parameters. Unfortunately, investigations of this process which focus on the individual particle acceleration are few. We present a detailed investigation of Fermi acceleration in 2D MHD anti-parallel plasmoid reconnection based on the guiding center approximation of test particle orbits. We show that accounting only for the variation in the excursion width of particle orbits around a magnetic island, energization obeys a dtε||~ε||p power law where p=1+2/χ with χ typically between 3 and 3.6. Including further effects of the non-constant E⨉B drift allows for acceleration rates as large as p≥2, with p decreasing as plasmoid size at injection increases. As a result, plasmoid sizes are likely to be coupled to the distributions of energetic particles within. We discuss the implications this has for global energetic particle distributions in multi-island reconnection, as well as the relevance of these results to the collisionless plasmoid instability, where additional particle drifts and non-MHD effects are present. |
Tuesday, October 18, 2022 2:12PM - 2:24PM |
JO07.00002: Magnetic Reconnection Driven by Thermal and Non-thermal Particle Energy Densities Valeria Ricci, Bruno Coppi, Bamandas Basu Magnetic reconnection is usually associated with conversion of magnetic energy into particle acceleration or thermal energy. However, a specific reconnection process driven by plasma pressure gradients was identified in Ref. [1] with a consistent theory of (non-ideal) MHD m0=1 modes in well confined plasmas. This process, based on an Ohm's law contribution to the electron balance equations, remains the basis for the explanation of the observed sawtooth oscillations of the central plasma pressure and for the inferred magnetic field structures due to fishbone oscillations [2] associated with injected high energy particle populations. A novel process [3] expected to have a wide range of applications concerns magnetic reconnection sustained by a significant gradient of the longitudinal electron temperature and based on the electron thermal energy balance equation rather than on an Ohm's law. A comparison is made with the Biermann Battery concept considering that the growth of reconnection depends on the particle density gradient in addition to that (aligned with it) of the electron temperature. |
Tuesday, October 18, 2022 2:24PM - 2:36PM |
JO07.00003: Current sheet equilibrium selection via relaxation and dynamo processes Young Dae Yoon, Deirdre E Wendel, Gunsu Yun From the Harris equilibrium to the force-free equilibrium, there is a continuous spectrum of 1D kinetic current sheet equilibria. However, only a select few solutions, notably those at each end of the spectrum, are readily being used for analyses of magnetized plasma phenomena such as magnetic reconnection. This limited usage is rather unjustified given the infinite number of mixed equilibria, and arises from our lack of understanding of the current sheet equilibrium selection process. Here we show how a particular equilibrium is selected by a given system, by revealing the equilibration process of an initially disequilibrated current sheet with a finite guide field. The initial current sheet equilibrates in a way that entails a dynamo process in which the seed guide field is locally amplified, eventually yielding a mixed equilibrium. The ubiquity of such equilibria is demonstrated by comparing particle-in-cell simulations to spacecraft observations. |
Tuesday, October 18, 2022 2:36PM - 2:48PM |
JO07.00004: Onset of magnetic reconnection in poorly ionized plasmas Elizabeth A Tolman, Matthew W Kunz, James M Stone, Lev A Arzamasskiy In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a nonlinear phase when plasmoids form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. One novel effect in such plasmas is the collisional drag force exerted on the ionized species by the neutral species. We use theory and computation to understand the effect of this drag on tearing-initiated reconnection. Ion-neutral drag causes a CS to sharpen at an increasing rate via ambipolar diffusion. This nonlinear steepening delays the onset of reconnection by decreasing the CS width at which the tearing growth rate becomes larger than the CS formation rate. Once the CS becomes thin enough, however, ions decouple from neutrals and thinning of the CS slows, allowing tearing onset. An eigenmode can be derived for the resulting CS profile, yielding growth rates and wavenumber predictions that compare well with simulation. Later, the system enters a nonlinear phase with a stochastic plasmoid chain and a reconnection rate slowed by ion-neutral drag. The plasmoids are sites of enhanced ionization fraction. |
Tuesday, October 18, 2022 2:48PM - 3:12PM |
JO07.00005: Laboratory study of the stability of solar-relevant, arched, line-tied magnetic flux ropes Andrew D Alt, Hantao Ji, Jongsoo Yoo, Sayak Bose, Aaron Goodman, Masaaki Yamada Coronal mass ejections occur when long-lived magnetic flux ropes (MFR) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal MHD instability such as the kink or torus instability. These instabilities have long been studied in axisymmetric fusion devices where the instability criteria are given in terms of the edge safety factor and confining magnetic field decay index. Laboratory experiments have been performed in the Magnetic Reconnection Experiment (MRX), where the stability properties of arched, line-tied MFRs were controlled via the external fields. Previous experiments revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These `failed-tori' went through a process like Taylor relaxation where the toroidal current was redistributed before the eruption failed. In more recent experiments, we have investigated this behavior via additional diagnostics that measure the current distribution at the foot points and the energy breakdown before and after an event. Measurements have revealed that the current redistribution during a failed torus event can be explained by ideal MHD without the need for non-ideal effects such as reconnection which is necessary for Taylor relaxation. |
Tuesday, October 18, 2022 3:12PM - 3:36PM |
JO07.00006: Experimental Observation of a Field-Aligned Ion Beam Produced by Magnetic Reconnection of Two Flux Ropes on the LArge Plasma Device Shawn W. Tang, Walter N Gekelman Magnetic reconnection is the process in which magnetic energy is converted into heat or electric fields that can energize the plasma and/or accelerate particles. This is typically observed in space such as in the solar corona, as well as in various laboratory experiments and plasma simulations. In a study of two flux ropes (L=11 m, d=7.6 cm) on the LArge Plasma Device (LAPD), the energization of ions due to magnetic reconnection that is triggered by the collision of the two flux ropes is explored. A retarding field energy analyzer was constructed to measure the local ion energy distribution function of the plasma and a 9 to 15 eV sub-Alfvénic ion beam moving in the direction of the background magnetic field was observed. To the authors' knowledge, this is the first known observation of an ion beam that is field-aligned to the guide field. The beam ions do not appear to be heated and are accelerated in the direction of the induced electric fields (-dAz/dt). In addition, the energy released from magnetic field annihilation appears to be sufficient to account for the increase in the energy of the ions. Evidence suggests that the presence of the beam is correlated with the oscillation of the ropes and could cause ion acoustic waves that propagate along the magnetic field. |
Tuesday, October 18, 2022 3:36PM - 3:48PM |
JO07.00007: Study of the conversion of magnetic energy to plasma kinetic energy during guide field magnetic reconnection in MRX Sayak Bose, William R Fox, Hantao Ji, Jongsoo Yoo, Aaron Goodman, Andrew D Alt, Masaaki Yamada We have studied the conversion of the magnetic energy to the plasma kinetic energy and the associated particle dynamics during guide field reconnection in the laboratory. The upstream reconnecting magnetic field is comparable to the guide field. The electron flow along the separatrices towards the electron diffusion region (EDR) is observed to be faster than the outflow velocity. The parallel electric field E||, energizes inflowing electrons near the separatrices outside the EDR. The majority of the power deposition on electrons occurs in the EDR by E||. The electron dynamics in the ion diffusion region (IDR) sets up a perpendicular electric field E⊥ by the (ve × B) term of Ohm's law. This E⊥ energizes ions in the regions of IDR where plasma density is enhanced. These high-density regions are part of a quadrupolar density structure that pervades the IDR. The spatial extent of the E⊥ structure driving the ion energization is comparable to the ion gyroradius and the ion acceleration is ballistic like. Energy partition calculation shows 40% of the magnetic energy is converted to particle energy, 67% of which is transferred to ions and 33% to electrons. |
Tuesday, October 18, 2022 3:48PM - 4:00PM |
JO07.00008: Experimental Plasma Jet Formation in a Three-dimensional Magnetic Null Point Topology David L Chesny, Mark B Moffett, Norton B Orange, Kaleb W Hatfield We demonstrate the formation of an approximately 15 cm plasma jet resulting from the operation of a coaxial plasma gun (CPG) within a strong (0.7 T) three-dimensional magnetic null point field (3D null point). Circuit control is established to operate a CPG with a gas puff inlet and a 3D null point conducting coil assembly, all within a low pressure environment (under 1 Torr). Two separate multi-kilovolt capacitor banks are configured to discharge simultaneously so that the CPG plasma interacts with the spine axis of the strong 3D null point. High-speed camera imaging elucidates the differences between conventional CPG operation and the CPG operating within the strong magnetic field. Deflagration of the gas puff CPG precedes the snowplow mode of plasma sheath formation as confirmed with Rogowski coil diagnostics. During simultaneous operation, the peak CPG snowplow mode occurs during magnetic field ramp-up and approximately 120 µs before the peak magnetic field strength, which is accompanied by an unambiguous plasma jet forming along the spine axis of the 3D null point. This demonstrated snowplow plasma sheath injection into a 3D null point field represents the first known experimental test of plasma and magnetic field dynamics known to lead to 3D torsional magnetic reconnection. |
Tuesday, October 18, 2022 4:00PM - 4:12PM |
JO07.00009: Dynamics of a guide-field reconnection layer: Pathway to collisionless dissipation Li-jen Chen, Jonathan Ng, Naoki Bessho, James F Drake, Jason Shuster Lower-hybrid-drift (LHD) vortices driving non-gyrotropic electron heating have recently been discovered in the electron-scale reconnection layer with a guide field in the terrestrial magnetotail. The observation presents a new regime of electron interaction with LHD waves. In this talk, a physical pathway of how the vortices facilitate dissipation to smaller spatiotemporal scales is discussed based on measurements from the Magnetospheric Multiscale Mission and particle-in-cell simulations. In the observation, electron distribution functions are modified by the lower-hybrid-wave fields, and unstable to whistler, upper hybrid, and Langmuir waves, presenting a cascade of dissipation to higher frequencies and finer spatial scales. Particle-in-cell simulations produce lower-hybrid-drift vortices, but the wave field is too weak to demagnetize electrons as in the observations. The electrostatic potential along the vortex tube in the PIC simulation exhibits features suggestive of slippage reconnection. The new regime of strong electron-wave interaction, the dissipation cascade, and the possibility of further reconnection within the vortex tubes call for concerted laboratory, simulation, and space investigations. |
Tuesday, October 18, 2022 4:12PM - 4:24PM |
JO07.00010: Dynamics of shear flows in magnetic island coalescence problem using Hall-MHD model Jagannath Mahapatra, Rajaraman Ganesh, Abhijit Sen Shear flows are an important attributes of many plasma environment including laboratory and astrophysical plasmas. Although the effect of external flow shear on magnetic reconnection in Harris current sheet set-up has been investigated extensively [1], its effect has not been explored in the magnetic island systems. In a recent work using incompressible resistive MHD model [2], with an in-plane shear flow having amplitude ranging from sub-Alfvénic to super-Alfvénic scale, coalescence instability is found to be dominant against the MHD-Kelvin-Helmholtz instability and upstream/downstream magnetic and flow field components follow different scaling laws [3] in-comparison to that of Harris set-up. However, the effect of compressibility which can be expected to be important for shear flow dynamics and Hall physics was not addressed. In this work, using a compressible MHD code MPI-AMRVAC [4], we investigate the role of Hall term on the in-plane/out-of-plane shear flow affected current sheet properties, reconnection parameters and overall island time evolution during the coalescence process. Likewise, as the island system size is known to play a major role in the Hall-dominant reconnection [5], shear flow effects on this is another important aspect that will be addressed in this work. |
Tuesday, October 18, 2022 4:24PM - 4:36PM |
JO07.00011: Reconnection-Driven Energy Cascade in Magnetohydrodynamic Turbulence Chuanfei Dong, Liang Wang, Luca Comisso, Yi-Min Huang, Timothy A Sandstrom, Amitava Bhattacharjee Magnetohydrodynamic turbulence regulates the transfer of energy from large to small scales in many astrophysical systems, including the solar atmosphere. We performed three-dimensional magnetohydrodynamic simulations with unprecedentedly large magnetic Reynolds number to reveal how rapid reconnection of magnetic field lines changes the classical paradigm of the turbulent energy cascade. By breaking elongated current sheets into chains of small magnetic flux ropes (or plasmoids), magnetic reconnection leads to a new range of energy cascade, where the rate of energy transfer is controlled by the growth rate of the plasmoids. As a consequence, the turbulent energy spectra steepen and attain a spectral index of -2.2 that is accompanied by changes in the anisotropy of turbulence eddies. The omnipresence of plasmoids and their consequences on, e.g., the solar coronal heating, can be further explored with current and future satellites/telescopes. |
Tuesday, October 18, 2022 4:36PM - 4:48PM |
JO07.00012: Capturing kinetic counter-streaming effects in magnetic reconnection using 4-fluid plasma simulations Madox C McGrae-Menge, Jacob R Pierce, Mark Almanza, Jason Chou, Nathaniel Barbour, Noah R Mandell, Alexander Velberg, William D Dorland, Nuno F Loureiro, Frederico Fiuza, E. Paulo Alves The development of fluid plasma models capable of capturing important kinetic microphysical processes is a key component in the design of computationally efficient algorithms of multi-scale plasma dynamics. In the context of collisionless magnetic reconnection, intra-species counter-streaming is an important kinetic feature which we show is associated with breakdown of commonly used fluid closures; traditional 2-fluid simulations cannot resolve this feature. Here, we introduce a method to resolve counter-streaming in fluid simulations by including four overlapping fluids, two for electrons and two for ions. We demonstrate that these four-fluid simulations reproduce counter-streaming and significant aspects of field and plasma structure from fully kinetic simulations of magnetic reconnection, including the reconnection rate. In addition to improving the accuracy of fluid simulations, our 4-fluid approach highlights the prominent role of counter-streaming in magnetic reconnection and offers a new lens into its investigation. |
Tuesday, October 18, 2022 4:48PM - 5:00PM |
JO07.00013: A two-dimensional numerical study of ion-acoustic turbulence and its potential impact on magnetic reconnection onset Zhuo Liu, Ryan White, Lucio Milanese, Nuno F Loureiro During the current sheet formation process that precedes the fast stage of reconnection events, micro-instabilities potentially capable of providing a large effective resistivity can be triggered, which may determine the reconnection onset and properties of the reconnecting system. A prominent example is the ion-acoustic instability (IAI). Ion-acoustic waves are one of the most fundamental oscillation modes in plasmas, and are widely observed in the heliosphere; however, the nonlinear development of the IAI remains poorly understood, partly because of the restriction to one-dimensional simulations. To address this issue, we perform two-dimensional Vlasov-Poisson simulations of this problem for the first time and study the linear and nonlinear evolution of the IAI in a collisionless plasma. We circumvent the problem of the realizability of the initial conditions by driving the system towards the ion-acoustic-unstable state via an imposed, weak external electric field. We demonstrate that two-dimensional effects are significant in the effectiveness of ion-acoustic turbulence. We show that the famous Sagdeev formula of anomalous resistivity overestimates the effective collision frequency under the weak electric field, rather than underestimating it as claimed by many previous studies. As it has been reported in some previous numerical studies, no steady saturated nonlinear state is ever reached in our simulations. After saturation of the instability, the bulk of electrons stays marginally unstable to the IAI; however, a run-away tail that is freely accelerated by the external electric field forms, which may lead the system to become Buneman unstable at later time. We also briefly discuss how these findings may impact the current sheet formation process and the onset of magnetic reconnection. |
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