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Dynamical processes and particle acceleration associated with current sheets, magnetic islands and turbulence-borne structures in space plasmas

Current sheets and 2D magnetic islands or 3D blobs, flux ropes, and plasmoids observed in the solar wind at various scales play a significant role in local particle acceleration to keV-MeV energies. The resulting energetic particle enhancements as well as plasma/magnetic field variations constitute a potentially hazardous condition in the interplanetary and near-Earth space. Current sheets are self-organized structures that are formed ubiquitously in cosmic and laboratory plasmas owing to a change in the magnetic field direction, at strong discontinuities, and as a result of turbulence. Not surprisingly, dynamic processes occurring at current sheets and in their vicinity have a striking similarity in different plasmas. Current sheets experience magnetic reconnection that in turn leads to many subsequent nonlinear effects, triggering the development of a turbulent cascade, the formation of magnetic islands/flux ropes/plasmoids/blobs, and local acceleration of charged particles. These processes are observed from the corona to the outer heliosphere and may often be described by the same equations. They also can be linked physically as some of the structures originating from the corona survive and evolve further in the solar wind. These processes have been studied by different scientific teams in independent ways, but currently there is a tendency to analyze them employing a unified approach.
This interdisciplinary session will bring together specialists from different plasma physics communities, bridging gaps in the understanding of the origin of coherent structures and the development of dynamical processes associated with current sheets. We invite researchers to share recent results of their theoretical studies, modelling and observations. Contributions that discuss and compare different mechanisms of local particle energization that occur in laboratory plasmas, the solar corona, magnetospheres of planets and the heliosphere are especially welcome.

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Dear colleagues,

Welcome to session ST1.6. Dynamical processes and particle acceleration associated with current sheets, magnetic islands and turbulence-borne structures in space plasmas.

We kindly invite you to attend an EGU-related Zoom conference “Dynamical processes and particle acceleration associated with current sheets, magnetic islands and turbulence-borne structures in space plasmas”.
The meeting will be held on Thursday, 29 April 2000 (12:00 – 14:35 and 16:00 – 19:05, CEST). The conference program and other details can be found here: https://drive.google.com/file/d/1y9H4iqOMjU6A35JGElajm7MHZv3lDgpu/view

Convener: Olga Khabarova | Co-conveners: Spiro Antiochos, Roman KislovECSECS, Jakobus le Roux, Olga Malandraki
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Wed, 28 Apr, 09:00–10:30

Chairperson: Olga Khabarova

Mitsuo Oka et al.

In gradual Solar Energetic Particle (SEP) events, shock waves driven by coronal mass ejections (CMEs) play a major role in accelerating particles, and the energetic particle flux enhances substantially when the shock front passes by the observer. Such enhancements are historically referred to as Energetic Storm Particle (ESP) events, but it remains unclear why ESP time profiles vary significantly from event to event. In some cases, energetic protons are not even clearly associated with shocks. Here we report an unusual, short-duration proton event detected on 5 June 2011 in the compressed sheath region bounded by an interplanetary shock and the leading-edge of the interplanetary CME (or ICME) that was driving the shock. While <10 MeV protons were detected already at the shock front, the higher-energy (>30 MeV) protons were detected about four hours after the shock arrival, apparently correlated with a turbulent magnetic cavity embedded in the ICME sheath region.

How to cite: Oka, M., Obara, T., Nitta, N., Yashiro, S., Shiota, D., and Ichimoto, K.: Unusual enhancement of ~30 MeV proton flux in an ICME sheath region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-421, https://doi.org/10.5194/egusphere-egu21-421, 2021.

Helmi Malova et al.

Thin current sheets (TCSs) with thicknesses about ion Larmor radii can play the key role in space; particularly they can store and then explosively release the accumulated free energy. The dynamics of ions moving along quasi-adiabatic trajectories in TCSs is different from one of magnetized electrons following guiding center drift orbits. Due to this property TCSs can be described in a frame of a hybrid approach. The thickness of the super-thin embedded electron sheet remains uncertain because of the scale-free character of magnetized electron motion. We propose a new analytical approach to describe the multilayer TCS and provide the universal expression describing the embedded electron sheet as a function of the cross-sheet transversal coordinate z characterizing TCS. We demonstrated that the unique property of the electron sheet is the nonlinear character of magnetic field profile:  B(z) ~ z 1/3 which conforms excellently with MAVEN observations in the Martian magnetotail. 

This work was supported by the Russian Science Foundation (grant # 20-42-04418).


How to cite: Malova, H., Zelenyi, L., Grigorenko, E., Popov, V., and Dubinin, E.: Universal scaling of thin current sheets in space plasma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3521, https://doi.org/10.5194/egusphere-egu21-3521, 2021.

Roman Kislov et al.

High-speed flows from coronal holes are separated from the surrounding solar wind by stream or corotating interaction regions (SIRs/CIRs). The latter have a complex dynamic structure, which is determined by turbulence, the presence of current sheets and magnetic islands/flux ropes/blobs/plasmoids. As the Sun rotates, SIRs along with high-speed flows propagate in the heliosphere. A SIR can be considered as a single large-scale object resembling a magnetic tube with walls of varying thickness. In this case, one can think not only about the speed of the plasma flow inside and near the given object, but also about its movement around the Sun as a whole. Because of this rotation, SIRs can cross the orbits of two separated spacecraft, which may allow one to study the spatial evolution of their structure. We have chosen the events when SIRs were sequentially detected by ACE and one of the STEREO spacecraft. In each case, a position of the Stream Interface (SI) was found, relative to which the position of other structures within the SIR was determined. Using a newly developed method for identifying current sheets [Khabarova et al. 2021], the SIR fine structure and the properties of turbulent plasma flow were studied. The estimates of the angular velocity of rotation SIR around the Sun are given. A model is constructed that describes the motion of SIRs in the heliosphere and their main large-scale properties.

Khabarova O., Sagitov T., Kislov R., Li G. (2021), http://arxiv.org/abs/2101.02804

How to cite: Kislov, R., Sagitov, T., and Malova, H.: Spatial evolution of turbulent regions associated with stream interaction regions in the solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5595, https://doi.org/10.5194/egusphere-egu21-5595, 2021.

Mikhail Fridman

Mid-term prognoses of geomagnetic storms require an improvement since theу are known to have rather low accuracy which does not exceed 40% in solar minimum. We claim that the problem lies in the approach. Current mid-term forecasts are typically built using the same paradigm as short-term ones and suggest an analysis of the solar wind conditions typical for geomagnetic storms. According to this approach, there is a 20-60 minute delay between the arrival of a geoeffective flow/stream to L1 and the arrival of the signal from the spacecraft to Earth, which gives a necessary advance time for a short-term prognosis. For the mid-term forecast with an advance time from 3 hours to 3 days, this is not enough. Therefore, we have suggested finding precursors of geomagnetic storms observed in the solar wind. Such precursors are variations in the solar wind density and the interplanetary magnetic field in the ULF range associated with crossings of magnetic cavities in front of the arriving geoeffective high-speed streams and flows (Khabarova et al., 2015, 2016, 2018; Adhikari et al., 2019). Despite some preliminary studies have shown that this might be a perspective way to create a mid-term prognosis (Khabarova 2007; Khabarova & Yermolaev, 2007), the problem of automatization of the prognosis remained unsolved.

How to cite: Fridman, M.: A neural network-based mid-term prognosis of geomagnetic storms that uses pre-storm effects related to current sheets and magnetic islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8087, https://doi.org/10.5194/egusphere-egu21-8087, 2021.

Timofey Sagitov

We announce the first open access multi-year database of current sheets systematically identified with the one second cadence at 1 AU. The current sheet list comes from an automated method of current sheet identification that suggests a formalization of the long-time experience of observers in the visual finding of CSs based on the analysis of the IMF and plasma parameters that vary sharply at CSs of different origins in the solar wind (Behannon et al. 1981; Blanco et al. 2006; Zhang et al. 2008; Suess et al. 2009; Simunac et al. 2012; Zharkova and Khabarova, 2012, 2015; Khabarova et al. 2015, 2016; Khabarova and Zank 2017; Malova at al. 2017; Adhikari et al. 2019). The main features seen with a resolution not worse than one minute that may characterize a CS crossing are as follows: (i) a decrease in the IMF magnitude B, (ii) a decrease in V_A/V (V_A is the Alfvén speed and V is the solar wind speed), and (iii) an increase in the plasma beta (the ratio of the plasma pressure to the magnetic pressure). Since the automatization of the CS recognition process requires setting the same rules for CSs occurring in different plasmas under different conditions, normalization should be performed. After obtaining B, VA/V, and β with a one second cadence, we calculate their one-second derivatives. Spikes of the derivatives occurring out of the noise level indicate the CS location. Only the spikes that appear simultaneously in dB/dt and any of two other parameters are considered as pointing out the CS location. The database is available at csdb dot izmiran dot ru

How to cite: Sagitov, T.: A new multi-year database of current sheets at 1AU, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8988, https://doi.org/10.5194/egusphere-egu21-8988, 2021.

Laxman Adhikari et al.

Recent studies of unusual or atypical energetic particle flux events (AEPEs) observed at 1 au show that another mechanism, different from diffusive shock acceleration, can energize particles locally in the solar wind. The mechanism proposed by Zank et al. is based on the stochastic energization of charged particles in regions filled with numerous small-scale magnetic islands (SMIs) dynamically contracting or merging and experiencing multiple magnetic reconnection in the super-Alfvénic solar wind flow. A first- and second-order Fermi mechanism results from compression-induced changes in the shape of SMIs and their developing dynamics. Charged particles can also be accelerated by the formation of antireconnection electric fields. Observations show that both processes often coexist in the solar wind. The occurrence of SMIs depends on the presence of strong current sheets like the heliospheric current sheet (HCS), and related AEPEs are found to occur within magnetic cavities formed by stream–stream, stream–HCS, or HCS–shock interactions that are filled with SMIs. Previous case studies comparing observations with theoretical predictions were qualitative. Here we present quantitative theoretical predictions of AEPEs based on several events, including a detailed analysis of the corresponding observations. The study illustrates the necessity of accounting for local processes of particle acceleration in the solar wind.

How to cite: Adhikari, L., Zank, G., and Zhao, L.: The Role of Magnetic Reconnection–associated Processes in Local Particle Acceleration in the Solar Wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9328, https://doi.org/10.5194/egusphere-egu21-9328, 2021.

Valentina Zharkova and Qian Xia

We will overview particle motion in 3D Harris-type RCSs without and with magnetic islands using particle-in-cell (PIC) method considering the plasma feedback to electromagnetic fields. We evaluate particle energy gains and pitch angle distributions (PADs) of accelerated particles of both changes in different locations inside current sheets as seen under the different directions by a virtual spacecraft passing through. The RCS parameters are considered comparable to heliosphere and solar wind conditions. 

The energy gains and the PADs of particles are shown to change depending on a topology of magnetic fields.  We report separation of electrons from ions at acceleeration in current sheets with strong guiding fields  and formation of transit and bounced beams from the particles of the same charge. The  transit particles are shown to form  bi-directional energetic electron beams (strahls), while bounced particles are mainly account from driopout fluxes in the heliosphere. In topologies with weak guding field strahls are mainly present inside the magneticislands and located closely above/below the X-nullpoints in the inflow regions. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of current sheets. Mono-directional strahls with PADS along 0 or 180 degrees are found quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature drift acceleration. Meanwhile, high-energy electrons confined inside magnetic islands create PADs about 90◦. 

How to cite: Zharkova, V. and Xia, Q.: Particle acceleration in 3D current sheets with magnetic islands: energy, density and pitch angle distributions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10352, https://doi.org/10.5194/egusphere-egu21-10352, 2021.

Victor Popov et al.

The dynamics of quasi-adiabatic ions in thin current sheets (TCSs) of the planetary magnetotails and solar wind is investigated, when the characteristic scale of the magnetic inhomogeneity is compared with proton gyroradii. A numerical model of TCS is constructed, taking into account the constant normal magnetic component and three kind of the shear magnetic field distributions: 1) constant, 2) bell-shaped relatively equatorial plane and 3) anti-symmetric ones. The Poincaré cross- sections characterizing quasi-adiabatic ion dynamics are considered. The jumps of the quasi-adiabatic invariant of motion are calculated and compared with the case of the absent magnetic shear. It is shown that the presence of constant and bell-shaped magnetic components in the current sheet leads to the asymmetry of particle scattering in the Northern-Southern direction and the peculiarities of the structure of phase space. It is shown that the jumps of the quasi-adiabatic Iz invariant differ are different for plasma flows located in the Northern and Southern hemispheres.  At the same time, for configurations with anti-symmetric shear component, the particle scattering near TCS neutral plane is insignificant and the scattering asymmetry is absent. The results of this study are discussed in terms of their application to explain experimental data.

This work is supported by RFBR grant 19-02-00957.

How to cite: Popov, V., Malova, H., and Belyalova, M.: Quasi-adiabtic particle dynamics in thin current sheets with a magnetic shear, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10417, https://doi.org/10.5194/egusphere-egu21-10417, 2021.

Alexander Khokhlachev et al.

Helium is the second most abundant ion component of the solar wind. The relative abundance of helium can differ significantly in various large-scale structures of the solar wind generated by the nonstationarity and inhomogeneity of the solar corona. For example the helium abundance is ~3% in slow streams and ~4% in fast streams. The maximum helium abundance is usually observed inside magnetic clouds and can reach >10%. The relative abundance of helium can also dynamically vary inside large-scale structures, which can be the result of local processes in plasma.

 In magnetic clouds, the distribution of the helium abundance has an axisymmetric peak with a maximum in the central region of the magnetic cloud, where the ion current flows [Yermolaev et al., 2020]. This research examines the different-scale dynamics of the relative abundance of helium in magnetic clouds. For this purpose, the dependences of the helium abundance on some plasma parameters were studied on different datasets of the OMNI database from 1976 to 2018. It is shown that the helium abundance increases with an increase in the modulus of the interplanetary magnetic field B and with a decrease in the proton plasma parameter β in the center of the magnetic cloud. The scale of this region is ~1 million kilometers. Similar relations of the helium abundance to interplanetary magnetic field direction angles and other solar wind parameters were studied.

In addition, the work studied intermediate-scale changes (at scale <1 hour) in helium abundance inside magnetic clouds and compression regions in front of them in comparison with other large-scale wind types. For this aim, a correlation analysis of the time series of density and relative abundance of helium was carried out on base of measurements on SPEKTR-R and WIND spacecraft located at a considerable distance from each other. The dependences of the local correlation coefficients (at scale ~1 hour or less) between measurements at two points on the solar wind plasma parameters are considered. Meanwhile these dependencies are compared with the same for other types of solar wind. It is shown that the median values of the local correlation coefficient in the regions of compressed plasma ahead of magnetic clouds exceed the values in other types of wind by about 15%. In addition, the local correlation coefficient increases with an increase in the amplitude of fluctuations of the investigated parameter and the proton velocity. Thus, intermediate-scale fluctuations in the relative helium abundance observed in these structures are quite stable and apparently are formed in the corona acceleration region and then propagate without changes.
The work is supported by RFBR grant № 19-02-00177a.

Yermolaev, Y.I. et al., Dynamics of large-scale solar-wind streams obtained by the double superposed epoch analysis. 4. Helium abundance, Journal of Geophysical Research, 125 (7) DOI: 10.1029/2020JA027878

How to cite: Khokhlachev, A., Riazantseva, M., Rakhmanova, L., Yermolaev, Y., and Lodkina, I.: Dynamics of helium abundance in magnetic clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11947, https://doi.org/10.5194/egusphere-egu21-11947, 2021.

Patricio A. Munoz et al.

Turbulence is ubiquitous in solar system plasmas like those of the solar wind and Earth's magnetosheath. Current sheets can be formed out of this turbulence, and eventually magnetic reconnection can take place in them, a process that converts magnetic into particle kinetic energy. This interplay between turbulence and current sheet formation has been extensively analyzed with MHD and hybrid-kinetic models. Those models cover all the range between large Alfvénic scales down to ion-kinetic scales. The consequences of current sheet formation in plasma turbulence that includes electron dynamics has, however, received comparatively less attention. For this sake we carry out 2.5D fully kinetic Particle-in-Cell simulations of kinetic plasma turbulence including both ion and electron spectral ranges. In order to further assess the electron kinetic effects, we also compare our results with hybrid-kinetic simulations including electron inertia in the generalized Ohm's law. We analyze and discuss the electron and ion energization processes in the current sheets and magnetic islands formed in the turbulence. We focus on the electron and ion distribution functions formed in and around those current sheets and their stability properties that are relevant for the micro-instabilities feeding back into the turbulence cascade. We also compare pitch angle distributions and non-Maxwellian features such as heat fluxes with recent in-situ solar wind observations, which demonstrated local particle acceleration processes in reconnecting solar wind current sheets [Khabarova et al., ApJ, 2020].

How to cite: Munoz, P. A., Büchner, J., and Jain, N.: Fully kinetic PIC simulations of particle acceleration and non-Maxwellian distribution functions due to current sheets in solar wind turbulence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12010, https://doi.org/10.5194/egusphere-egu21-12010, 2021.

Harald Kucharek et al.

While the Earth’s bow shock marks the location at which the solar wind is thermalized, recent publications provided evidence that filamentary structures such as reconnecting current sheets at the shock ramp region may participate in the thermalization process.  Small scale filamentary structures are distinct features that are abundant at the shock and inside the magnetosheath. These structures are not limited to current sheets but include electric and magnetic field enhancements. They may consist of a single or multiple filaments.  They originate from energy dissipation at and downstream of the bow shock, in particular the parallel bow shock. 

We have studied several crossings of the magnetosheath made by the MMS spacecraft, characterising and quantifying the occurrence and consequences of current sheets and field enhancements in terms of local plasma heating and ion acceleration far downstream of the shock. These observations suggest that a combination of current sheet formation, and electric field and magnetic field gradients can contribute to local downstream ion acceleration, and heating. The associated turbulence is likely a consequence of solar wind input parameters. These observations provide evidence that under certain plasma conditions these filamentary structures can play a significant role in thermalizing of the magnetosheath plasma as it propagates further downstream toward the magnetopause, thus augmenting the effect due to the bow shock itself.

How to cite: Kucharek, H., Gingell, I., Schwartz, S., Farrugia, C., and Trattner, K.: Energy Dissipation at Filamentary Structures Downstream of the Earth’s Parallel Bow Shock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12984, https://doi.org/10.5194/egusphere-egu21-12984, 2021.

Zdenek Nemecek et al.

Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind and thus the turbulent behavior of these quantities is intensively studied. However, the temperature is also highly fluctuating quantity but behavior of its variations is studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of the temperature variations requires knowledge of the full velocity distribution with a time resolution and such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board the Spektr-R spacecraft uses the Maxwellian approximation and provides the thermal velocity with 32 ms time resolution. We use these measurements and complement them with 10 Hz magnetic field observations from the Wind spacecraft propagated to the Spektr-R location and analyze factors influencing the shape of the temperature power spectral density. A special attention is devoted to mutual relations of power spectral densities of different quantities like parallel and perpendicular temperature, magnetic field and velocity fluctuations and their evolution in course of solar wind expansion.

How to cite: Nemecek, Z., Šafránková, J., Pitňa, A., and Němec, F.: Spectra of temperature fluctuations in the solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14838, https://doi.org/10.5194/egusphere-egu21-14838, 2021.

Elena Zhukova et al.

The mechanisms of particle acceleration in the CS of the Mercury magnetosphere were investigated. The numerical model is developed that allows evaluating the acceleration of ions H+, He+, O+ in two possible mechanisms of particle acceleration: (1) by multiple dipolarizations during substorm activity passage of fronts; (2) by the turbulent electromagnetic field in the magnetosphere. Our simulation show that all kinds of charged plasma particles can be efficiently accelerated during multiple dipolarizations processes of the type (2) to maximum energies about 100-200keV. The gain of energies of ions under the (2) process of magnetospheric perturbations is about 10% higher than in the second case. The shapes of obtained in the model energy spectra were shown to be in agreement with experimental spectra. We conclude that the role of these mechanisms is more important near Mercury in comparison with plasma processes in the Earth’s magnetosphere.

How to cite: Zhukova, E., Popov, V., Malova, H., and Zelenyi, L.: Сharged particle acceleration in the CS of the Mercury magnetosphere: comparison of different mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14920, https://doi.org/10.5194/egusphere-egu21-14920, 2021.

Nicole Vilmer and Sophie Musset

Efficient electron (and ion) acceleration is produced in association with solar flares. Energetic particles play a major role in the active Sun since they contain a large amount of the magnetic energy released during flares. Energetic electrons (and ions) interact with the solar atmosphere and produce high-energy X-rays and γ-rays. Energetic electrons also produce radio emission in a large frequency band through gyrosynchrotron emission processes in the magnetic fields of flaring active regions and conversion of plasma waves when e.g. propagating to the high corona towards the interplanetary medium. It is currently admitted that solar flares are powered by magnetic energy previously stored in the coronal magnetic field and that magnetic energy release is likely to occur on coronal currents sheets along regions of strong gradient of magnetic connectivity. However, understanding the connection between particle acceleration processes and the topology of the complex magnetic structures present in the corona is still a challenging issue. In this talk, we shall review some recent results derived from X-ray and radio imaging spectroscopy of solar flares bringing some new observational constraints on the localization of HXR/radio sources with respect to current sheets, termination shocks in the corona derived from EUV observations.

How to cite: Vilmer, N. and Musset, S.: Energetic electrons in solar flares: observational support for acceleration processes linked to magnetic reconnection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15019, https://doi.org/10.5194/egusphere-egu21-15019, 2021.

Philippa Browning et al.

In this study, we inverstigate the acceleration of electrons and ions at current sheets in the flaring solar corona, and their transport into the heliosphere. We consider both generic solar flare models and specific flaring events with a data-driven approach. The aim is to answer two questions: (a) what fraction of particles accelerated in different flares can escape into the heliosphere?; and (b) what are the characteristics of the particle populations propagating towards the chromosphere and into the heliosphere?

We use a combination of data-driven 3D magnetohydrodynamics simulations with drift-kinetic particle simulations to model the evolution of the magnetic field and both thermal and non-thermal plasma and to forward-model observable characteristics. Particles are accelerated in current sheets associated with flaring reconnection. When applied to a specific flare, the model successfully predicts observed features such as the location and relative intensity of hard X-ray sources and helioseismic source locations. This confirms the viability of the approach.

Using these MHD-particle models, we will show how the magnetic field evolution and particle transport processes affect the characteristics of both energetic electrons and ions in the the inner corona and the heliosphere. The implications for interpretation of in situ measurements of energetic particles by Solar Orbiter and Parker Solar Probe will be discussed.



How to cite: Browning, P., Gordovskyy, M., Inoue, S., Kontar, E., Kusano, K., and Vekstein, G.: Transport of energetic particles from reconnecting current sheets in flaring corona to the heliosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15163, https://doi.org/10.5194/egusphere-egu21-15163, 2021.

Xiaozhou Zhao et al.

In an idealized system where four magnetic islands interact in a two-dimensional periodic setting, we follow the detailed evolution of current sheets forming in between the islands, as a result of an enforced large-scale merging by magnetohydrodynamic (MHD) simulation. The large-scale island merging is triggered by a perturbation to the velocity field, which drives one pair of islands move towards each other while the other pair of islands are pushed away from one another. The "X"-point located in the midst of the four islands is locally unstable to the perturbation and collapses, producing a current sheet in between with enhanced current and mass density. Using grid-adaptive resistive magnetohydrodynamic (MHD) simulations, we establish that slow near-steady Sweet-Parker reconnection transits to a chaotic, multi-plasmoid fragmented state, when the Lundquist number exceeds about 3×104, well in the range of previous studies on plasmoid instability. The extreme resolution employed in the MHD study shows significant magnetic island substructures. Turbulent and chaotic flow patters are also observed inside the islands. We set forth to explore how charged particles can be accelerated in embedded mini-islands within larger (monster)-islands on the sheet. We study the motion of the particles in a MHD snapshot at a fixed instant of time by the Test-Particle Module incorporated in AMRVAC (). The planar MHD setting artificially causes the largest acceleration in the ignored third direction, but does allow for full analytic study of all aspects leading to the acceleration and the in-plane, projected trapping of particles within embedded mini-islands. The analytic result uses a decomposition of the test particle velocity in slow and fast changing components, akin to the Reynolds decomposition in turbulence studies. The analytic results allow a complete fit to representative proton test particle simulations, which after initial non-relativistic motion throughout the monster island, show the potential of acceleration within a mini-island beyond (√2/2)c≈0.7c, at which speed the acceleration is at its highest efficiency. Acceleration to several hundreds of GeVs can happen within several tens of seconds, for upward traveling protons in counterclockwise mini-islands of sizes smaller than the proton gyroradius.

How to cite: Zhao, X., Keppens, R., and Bacchini, F.: Particle energization inside plasmoids: numerical and analytical investigations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15488, https://doi.org/10.5194/egusphere-egu21-15488, 2021.

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