Displays

We employ 2D and 3D high-resolution hybrid kinetic simulations of plasma turbulence to explore the physical conditions encountered by the Parker Solar Probe (PSP) spacecraft during its first two orbits, modelling the turbulent cascade self-consistently from large fluid scales down to kinetic scales.
By varying key parameters (e.g., the ion and electron plasma beta, the level of fluctuations with respect to the ambient magnetic field, the injection scale), we explore different plasma conditions. We identify a new kinetic-scale regime with respect to what has previously been found in both hybrid simulations and spacecraft observations of the solar wind and of the near-Earth environment, characterized among other things by a steeper magnetic field spectrum. Our simulations reproduce PSP observations and thus offer the opportunity to investigate the physical mechanism(s) behind such change in the turbulent cascade properties. We discuss our results in the framework of theoretical models of the nonlinear interaction of dispersive wave modes, field-particle interactions, and magnetic reconnection in low-beta plasmas.
We also analyse intermittency, magnetic compressibility, polarization of wave-like fluctuations, and statistics of magnetic reconnection events by means of iterative filters, a new method for the analysis of nonlinear nonstationary signals.
Together with our previous numerical results in quantitative agreement with MMS observations in the Earth’s magnetosheath, our new findings confirm the ability of the hybrid approach to model in-situ observations, which is fundamental for interpreting observational results and for planning future spacecraft missions.

How to cite: Franci, L., Giroul, A., Burgess, D., Papini, E., Chen, C., Del Sarto, D., Landi, S., Verdini, A., and Hellinger, P.: Modelling plasma turbulence observed by Parker Solar Probe during its first two orbits with hybrid-kinetic simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18202, https://doi.org/10.5194/egusphere-egu2020-18202, 2020.

Similarly to the power density spectrum of magnetic field fluctuations in the solar wind, the spectrum of density fluctuations also shows multiple spectral slopes. Both of them present a spectral index varying between –3/2 and –5/3 in the inertial range and close to –2.8 between the proton and electron gyrofrequencies.

Despite these similarities, the spectrum of density fluctuations has a significant difference with respect to the magnetic and velocity fluctuations spectra: it shows a transition region between the inertial and the kinetic ranges with spectral index typically around –1.

We have combined the results of compressible Hall-MHD numerical simulations and measurements of the BMSW instrument onboard Spektr-R satellite to study the possible causes of the flattening in the density spectrum. Both numerical and experimental approaches point towards an important role played by Kinetic Alfvén Waves.

How to cite: Montagud-Camps, V., Němec, F., Šafránková, J., Němeček, Z., Grappin, R., Verdini, A., and Pitňa, A.: What can Hall-MHD simulations tell us about the transition region in the solar wind proton density spectrum?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2440, https://doi.org/10.5194/egusphere-egu2020-2440, 2020.

By analyzing the turbulent magnetic field data from PSP, we find that: the solar wind turbulence in the inner heliosphere close to the Sun has formed the transition from multifractal intermittency at MHD scales to monofractal intermittency at kinetic scales. The order-dependent scaling exponent of the multi-order structure function shows a concave profile indicating the multifractal property at MHD scales, while its counterpart at kinetic scales shows a linear trend suggesting the monofractal property. We also find that, the closer to the sun, the more obvious the concave profile of the scaling exponent in the inertial range, which indicates that the multifractal characteristic of the magnetic field turbulence intermittency is also more evident when getting closer to the Sun.

Based on the Castaing description of the probability distribution function(PDF) of the disturbance difference, the key parameters(μ & λ^2) of the Castaing function are estimated as a function of scale. We find that: (1) when close to the sun (R~0.17 AU), the break point of μ is about 0.2 second, and the peak point of λ^2 is about 0.6 second, the two of which are about three times different in scale; (2) when far from the sun (R~0.8 AU), the break point of μ is about 1 second and the peak point of λ^2 is about 3 seconds, the two of which are also about three times different in scale. We also point out that the profiles (including the break/peak position) of both the parameters (μ & λ^2) along with the scale together determine the profile (including the spectral breaks) of the power spectrum.

Following the PP98 model function of incompressible MHD turbulent cascade rate (εZ), we first compared the cascade rate εZ with εB=<δB^3>/τ at the distance close to the sun, we find that the two trends over scales are in good agreement with one another. We therefore suggest that, to some extent (e.g. in the inertial region), εB=<δB^3>/τ can be used as a proxy of the cascade rate εZ. For the first time, by statistical analysis, we obtained that εB satisfies the following relation with the scale and the heliocentric distance: εB=((τ/τ0)^α)((r/r0)^β). In the inertial range, α changes from about -0.5 to about 0.5 as r increases from 0.17 AU to 0.81 AU, and β is about 6.4; in the kenetic range, when r increases from 0.17 AU to 0.25 AU, α keeps at about 2, and β is about 12.8. The εB(τ,r) expression given in this work, is believed to help understanding the transport and cascade processes of solar wind turbulence in the inner heliosphere. 

Corresponding author:
Jiansen HE, jshept@pku.edu.cn

Acknowledgements:
We would like to thank the PSP team for providing the data of PSP to the public.

How to cite: Wang, Y., He, J., Duan, D., and Zhu, X.: Statistical Research of the Intermittency and Cascade of Solar Wind Turbulence Based on Analysis of PSP Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13249, https://doi.org/10.5194/egusphere-egu2020-13249, 2020.

We study magnetic fluctuations at sub-ion scales and down to sub-electron scales using Helios/SCM measurements in the inner Heliosphere and Cluster/STAFF data at the Earth's orbit. Using these data we test the generality of the kinetic spectrum and we show that it follows the ~k^-8/3exp(-kl_d) law at different radial distances from the Sun (k being a wavenumber). We show as well that the dissipation scale l_d correlates well with the electron Larmor radius ρ_e at 0.3 AU and at 1 AU. Then, in the time domain, at 1 AU, using the wavelet transform, we study the nature of magnetic fluctuations, which form the kinetic spectrum. It appears, that the spectrum is dominated by non-linear coherent structures in the form of magnetic vortices with the smallest resolved scale of the order of ρ_e. Finally, we comparer our results with measurements of the Parker Solar Probe/FIELDS and, hopefully, of the Solar Orbiter/RPW in the inner Heliosphere.

How to cite: Alexandrova, O., Jagarlamudi, V., Maksimovic, M., Hellinger, P., Shprits, Y., and Mangeney, A.: Solar Wind Turbulence at Kinetic Scales in the inner Heliosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18486, https://doi.org/10.5194/egusphere-egu2020-18486, 2020.

Recent observational and theoretical work on solar wind turbulence and dissipation suggests that kinetic-scale fluctuations are both heating and isotropizing the solar wind during transit to 1 AU.  The nature of these fluctuations and associated heating processes are poorly understood. Whatever the dissipative process that links the fields and particles - Landau damping, cyclotron damping, stochastic heating, or energization through coherent structures - heating and acceleration of ions and electrons occurs because of electric field fluctuations. The dissipation due to the fluctuations depends intimately upon the temporal and spatial variations of those fluctuations in the plasma frame.  In order to derive that distribution in the plasma frame, one must also use magnetic field and density fluctuations, in addition to electric field fluctuations, as measured in the spacecraft frame (s/c) to help constrain the type of fluctuation and dissipation mechanisms that are at play.

We present here an analysis of electromagnetic fluctuations in the solar wind from MHD scales down to electron scales based on data from the Artemis spacecraft at 1 AU. We focus on a few time intervals of pristine solar wind, covering a reasonable range of solar wind properties (temperature ratios and anisotropies; plasma beta; and solar wind speed). We analyze magnetic, electric field, and density fluctuations from the 0.01 Hz (well in the inertial range) up to 1 kHz. We compute parameters such as the electric to magnetic field ratio, the magnetic compressibility, magnetic helicity, compressibility and other relevant quantities in order to diagnose the nature of the fluctuations at those scales between the ion and electron cyclotron frequencies, extracting information on the dominant modes composing the fluctuations. We also use the linear Vlasov-Maxwell solver, PLUME, to determine the various relevant modes of the plasma with parameters from the observed solar wind intervals. We discuss the results and the relevant modes as well as the major differences between our results in the solar wind and results in the magnetosheath.

How to cite: Salem, C., Bonnell, J., Huang, J., Hanson, E., Chaston, C., Klein, K., Franci, L., Verscharen, D., and Roytershteyn, V.: Electric Field Turbulence in the Solar Wind from MHD down to Electron Scales: Artemis Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18261, https://doi.org/10.5194/egusphere-egu2020-18261, 2020.

Compressible plasma turbulence is investigated at sub ion scales using both the Fast Plasma Investigation instrument on the Magnetospheric MultiScale mission as well as using calibrated spacecraft potential. The data from FPI allow inertial and a small region of sub-ion scales to be investigated before the instrumental noise becomes significant near 3Hz. In this work we give a detailed description of the spacecraft potential and how it is calibrated such that it can be used the measure the electron density. The key advantage of using the calibrated spacecraft potential is that a much higher time resolution is possible when compared to the direct measurement. This allows a measurement down to 40Hz for a measurement of the electron density. This is an improvement of an additional decade in scale. Using a one hour interval of solar wind burst mode data the power spectrum of the density fluctuations is measured from the inertial range to the sub ion range. At inertial scales the density spectrum shows similarities with the magnetic field power spectrum with a characteristic Kolmogorov like power law. In between the ion inertial and kinetic scales there is a brief flattening in the spectra before steepening in the sub ion range to a spectral index comparable to the trace magnetic field fluctuations. The morphology if the density spectra can be explained by either a cascade of Alfv\'en waves and slow waves at large scales and kinetic Alfv\'en waves at sub ion scales, or by the presence of the hall effect. Using electric field measurements the two hypotheses are tested.

How to cite: Roberts, O., Nakamura, R., Narita, Y., Holmes, J., Voros, Z., Lhotka, C., and Thwaites, J.: Sub-ion scale measurements of compressible turbulence in the solar wind MMS Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6815, https://doi.org/10.5194/egusphere-egu2020-6815, 2020.

The multiphase nature of astrophysical environment and diversity of driving mechanisms give rise to spatial variation of turbulence properties. Nevertheless, the employed model of magneto-hydrodynamic turbulence is often oversimplified being assumed to be only Alfvenic due to a lack of observational evidence. Here we report the employment of our novel method, the signature from polarization analysis (SPA), on unveiling the plasma modes in interstellar turbulence. The method is based on the statistical properties of the Stokes parameters (I,Q,U) of the synchrotron radiation polarization. The application of SPA on the synchrotron polarization data from the Galactic medium has for the first time revealed that interstellar turbulence is magnetized with different plasma modes composition, pinpointing the necessity to account for plasma property of turbulence, which is neither hydrodynamic nor purely Alfvenic, but depends on local physical conditions, particularly the driving process. A highly promising research field is foreseen to unroll with ample results anticipated from the advanced analysis of high resolution synchrotron polarization data and multiple wavelength comparison, that will shed light on the role of turbulence in various physical processes.

How to cite: Yan, H., Zhang, H., Chepurnov, A., and Makwana, K.: Identification of magnetosonic modes in Galactic turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6959, https://doi.org/10.5194/egusphere-egu2020-6959, 2020.

How to cite: Hu, A., Teunissen, J., Sisti, M., Califano, F., Dargent, J., Pedrazzi, G., and Ponti, F. D.: Using machine learning to identify magnetic reconnection in two-dimensional simulations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12160, https://doi.org/10.5194/egusphere-egu2020-12160, 2020.

We report detailed numerical studies of magnetic reconnection in high-Lundquist-number, turbulent plasma by means of a three-dimensional (3D) resistive magnetohydrodynamics model. It is found that although turbulence is pre-existing, magnetic fields still restructure themselves to shape many X-points with evident mean inflow/outflow as well as the hierarchically generated magnetic flux ropes (plasmoids in 2D) with twist field lines. Moreover, the turbulence facilitates magnetic reconnections, and makes the normalized global reconnection rate reach ∼ 0.02 − 0.1, corresponding to turbulence level from very low to high and magnetic energy release from feeble to violent. The rate is nearly independent on the Lundquist number, and thus the fast turbulent reconnection occurs. A stochastic separation of the reconnected magnetic field lines with large opening angles follows a super-diffusion, indicating the broadening of outflow regions owing to the turbulence. These findings manifest that with the high Lundquist numbers (S ≥ 10^4), the 3D reconnection is turbulent and fast.

How to cite: Yang, L., Li, H., Guo, F., Li, X., Li, S., Zhang, L., He, J., and Feng, X.: Fast Magnetic Reconnection by Turbulence with High Landquist Number, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6883, https://doi.org/10.5194/egusphere-egu2020-6883, 2020.

Magnetic reconnection leads to fast streaming of electrons and ions away from the reconnection site. We have used an implicit particle-in-cell simulation (iPic3D) embedded within a global MHD simulation of the solar wind and magnetosphere interaction to investigate the evolution of electrons and ion flows in the magnetotail. We first ran the MHD simulation driven by solar wind observations and then used the MHD results to set the initial and boundary conditions for the PIC simulation. Then we let the PIC state evolve and investigated the electron and ion motion. Within a few seconds of the onset of reconnection, electrons near the reconnection site stream earthward at 500-700km/s while the ions move at less than 100 km/s. For electrons, magnetic trapping occurs very close to the reconnection site and they move mostly in the X_GSM direction at the E×B/B² velocity.  Ion trapping occurs several Earth radii from the reconnection site about 100 s after the start of reconnection where both the electrons and ions move together at ~E×B/B² velocity. Although the particles are moving at the E × B/B² velocity, they are in a state defined by the kinetic physics not the state that exists in the MHD simulation.

How to cite: Walker, R., Lapenta, G., El-Alaoui, M., Berchem, J., Richard, R., and Schriver, D.: Flows in the Magnetotail, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3234, https://doi.org/10.5194/egusphere-egu2020-3234, 2020.

Observed particle distributions in space plasmas usually exhibit a variety of non-equilibrium features in the form of temperature anisotropies, suprathermal tails, field aligned beams, etc. The departure from thermal equilibrium provides a source for spontaneous emissions of electromagnetic fluctuations, such as whistler fluctuations at the electron scales. Analysis of these fluctuations provides relevant information about the plasma state and its macroscopic properties. Here we present a comparative analysis of spontaneous fluctuations in plasmas composed by thermal and non-thermal electron distributions. We compare 1.5D PIC simulations of a finite temperature isotropic magnetized electron–proton plasma modeled with Maxwellian and different kappa velocity distributions. Our results suggest a strong dependence between the shape of the velocity distribution function and the spontaneous magnetic fluctuations wave spectrum. This feature may be used as a proxy to identify the nature of electron populations in space plasmas  at locations where direct in-situ measurements of particle fluxes are not available.

How to cite: Moya, P. S., Hermosilla, D., López, R., Lazar, M., and Poedts, S.: Spontaneous whistler-cyclotron fluctuations of thermal and non-thermal electron distributions., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22174, https://doi.org/10.5194/egusphere-egu2020-22174, 2020.

Wave-particle interaction plays a critical role in producing the newborn waves/turbulence in the foreshock region in front of supercritical shock, which is prevalent in the heliosphere. It has been a long-lasting goal to catch and witness the excitation and growth of waves/turbulence by identifying the ongoing process of wave-particle interaction. This goal cannot be fulfilled until the arrival of the MMS’s era, during which we can simultaneously measure the electromagnetic fields and particle phase space densities with the unprecedented data quality. By surveying the data of burst mode, we are lucky to find some good examples illustrating the clear signals of wave activities in front of the shock. The active waves are diagnosed to be right-handed cyclotron waves, being highly circularly polarized and rotating right-handed about the background magnetic field vector. The waves are large amplitude with dB being greatly dominant over B0, or in other words, almost the whole magnetic field vector is involved in the circular rotation. Furthermore, we investigate the growth evolution of the large-amplitude cyclotron waves by calculating the spectrum of dJ.dE and its ratio to the electromagnetic energy spectrum. As far as we know, it is the first time to provide the spectrum of growth rate from in-situ measurements. Interestingly, we find that the contribution to the growth rate spectrum mainly comes from dJ_e,perp·dE_perp rather than dJ_e,para·dE_para or dJ_i·dE. Although the eigen mode to couple the oscillating electromagnetic field is the electron bulk oscillation, the ultimate free energy to make the eigen mode unstable comes from the ion beams, which are reflected from the shock. The dynamics of 3D phase space densities for both ion and electron species are also studied in detail together with the fluctuating electromagnetic field, demonstrating the ongoing energy conversion during the wave-particle process.

How to cite: He, J., Hou, C., Zhu, X., Luo, Q., Verscharen, D., and Zhao, J.: Direct Measurement of Excitation and Growth of Large Amplitude Cyclotron Waves by Reflected Ion Beams in Front of Shock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17128, https://doi.org/10.5194/egusphere-egu2020-17128, 2020.

Energetic particles are widely observed in many astrophysical systems, but the physical mechanisms responsible for their acceleration are not yet fully understood. We address the interaction of suprathermal, transrelativistic electrons with plasma turbulence at ion and sub-ion scales using a combination of hybrid particle-in-cell and test particle simulations. First, we present results of simulations with different turbulence amplitude. Two different mechanisms for electron energisation are identified: one is consistent with the picture of stochastic acceleration in turbulence, yielding to moderate electron energisation, while the other one involves electron trapping in turbulent structures, resulting in an efficient and fast electron energisation. The latter is observed to be active only for certain combinations of turbulence amplitude and electron initial energy. Furthermore, varying the injection scale, we explore the importance of the size of turbulent magnetic structures and of the nonlinear time associated to their dynamical evolution on electron acceleration. These results have important implications for electron acceleration in a wide range of space and astrophysical systems.

How to cite: Trotta, D., Franci, L., Burgess, D., Hellinger, P., and Giacalone, J.: The role of turbulence strength on the acceleration of transrelativistic electrons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13180, https://doi.org/10.5194/egusphere-egu2020-13180, 2020.

In the presence of strong compressibility an oblique configuration between the mean density gradient and magnetic field contributes to the electromotive force [1,2]. This effect can be called “magnetoclinicity” and may contribute to the formation of large-scale magnetic-field structure in compressible magnetohydrodynamic (MHD) turbulence. With the aid of the multiple-scale direct-interaction approximation (Multi-Scale DIA), a combination of the DIA and multiple-scale analysis, analytical expressions of the turbulent correlations (turbulent electromotive force, turbulent mass flux, turbulent heat flux, Reynolds stress, turbulent Maxwell  stress, etc.) are obtained for the compressible MHD turbulence. Utilizing these analytical results, a large-scale instability of the strongly compressible MHD turbulence is investigated. An analysis into normal modes of the periodic plane waves is performed to get a dispersion relation of the instability modes [3]. It is shown that, depending on the mean density configuration, the inhomogeneity of the mean density variation coupled with the density variance <ρ'²> (ρ': density fluctuation, <...>: average) leads to a finite growth of the mean magnetic disturbance at large scales. This magnetoclinicity effect counter-balance to the turbulent magnetic diffusivity, and contribute to the formation of large-scale magnetic fields. This magnetoclinicity effect is expected to play essential roles in global structure formation in strongly compressible plasma turbulence.

Reference

[1] N. Yokoi, “Electromotive force in strongly compressible magnetohydrodynamic turbulence,” J. Plasma Physics, 84, 735840501, pp.1-26 (2018).

[2] N. Yokoi, “Mass and internal-energy transports in strongly compressible magnetohydrodynamic turbulence,” J. Plasma Physics, 84, 775840603, pp.1-30 (2018).

[3] S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability (Oxford University Press, 1961).

How to cite: Yokoi, N.: Magnetoclinicity: Density variance effects in large-scale instability in magnetohydrodynamic turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11522, https://doi.org/10.5194/egusphere-egu2020-11522, 2020.

Nearly collisionless solar wind plasma originating in the solar corona is a turbulent medium. The energy within large scale fluctuations is continuously transferred into smaller scales and it eventually reaches scales at which it is converted into a random particle motion, thus heating the plasma. Although the processes that take place within this complex system have been studied for decades, many questions remain unresolved. The power spectra of the fluctuating fields of the magnetic field, bulk velocity, and ion density were studied extensively; however, the spectrum of the thermal velocity is seldom reported and/or discussed. In this paper, we address the difficulty of estimating its power spectrum. We analyze high-cadence (31 ms) thermal velocity measurements of the BMSW instrument onboard the Spektr-R spacecraft and the SWE instrument onboard the Wind spacecraft. We discuss the role of the proton temperature anisotropy (parallel/perpendicular) and its influence on the shape of the power spectra in the inertial range of turbulence.

How to cite: Pitňa, A., Šafránkova, J., and Němeček, Z.: Solar wind temperature anisotropy and its influence on the spectrum of turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2738, https://doi.org/10.5194/egusphere-egu2020-2738, 2020.

The analysis of magnetic field and velocity fluctuations in corresponding minimum variance frames revealed that: (1) Minimum variance and mean magnetic field directions would be similar but these two directions are often perpendicular, especially in the high-beta environment, and a number of perpendicular cases decreases with the scale length; (2) Compressibility computed in the minimum variance frame generally increases with frequency but the increase is not monotonic; it exhibits two breaks observed for the magnetic field as well as for velocity fluctuations with approximately the same break frequencies. (3) We suggest that the first break can be connected with a change of pure Alfven to kinetic Alfven modes and the second break approximately coincides with the transition from the inertial to kinetic scales.

How to cite: Šafránková, J., Němeček, Z., Němec, F., Verscharen, D., Ďurovcová, T., and Pitňa, A.: Relations of velocity and magnetic field fluctuations in the minimum variance frames, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2742, https://doi.org/10.5194/egusphere-egu2020-2742, 2020.

Observations of solar wind electron properties, as displayed in the T_perp/T_par vs β_par plane, appear to be constrained both in the T_perp/T_par <1 and in the T_perp/T_par >1 regimes by the electron firehose instability (EFI) and by the whistler instability respectively [Štverák 2008]. The onset mechanism of the EFI is established: solar wind expansion results in an electron thermal anisotropy, which in turns promotes the development of the instability that contributes to limit that same anisotropy [Innocenti 2019a]. However, if this were the only mechanism at work in the expanding solar wind, electron observations would pool at the EFI marginal instability line. Instead, they populate the “stable” interval bound by EFI and whistler marginal instability lines. It is not fully clear which role fully kinetic processes have in lifting the observed data points above the EFI marginal stability line and into the “stable” area. Other competing processes redistributing excess parallel energy into the perpendicular direction, such as collisions, may be at work as well [Yoon 2019].

We investigate this issue with Particle In Cell, Expanding Box Model  simulations [Innocenti 2019b] of EFI developing self consistently in the expanding solar wind. Our results show that after the EFI marginal stability line is reached, further collisionless evolution brings our simulated data points in the “stable” area. We thus demonstrate that, at least under certain circumstances, purely collisionless processes may explain observed solar wind observations, without the need of invoking collisions as a way to channel excess parallel energy into the perpendicular direction.

Štverák, Štěpán, et al. "Electron temperature anisotropy constraints in the solar wind." Journal of Geophysical Research: Space Physics 113.A3 (2008).

Innocenti, Maria Elena, et al. "Onset and Evolution of the Oblique, Resonant Electron Firehose Instability in the Expanding Solar Wind Plasma." The Astrophysical Journal 883.2 (2019): 146.

Yoon, P. H., et al. "Solar Wind Temperature Isotropy." Physical review letters 123.14 (2019): 145101.

Innocenti, Maria Elena, Anna Tenerani, and Marco Velli. "A Semi-implicit Particle-in-cell Expanding Box Model Code for Fully Kinetic Simulations of the Expanding Solar Wind Plasma." The Astrophysical Journal 870.2 (2019): 66.

How to cite: Innocenti, M. E., Boella, E., Tenerani, A., and Velli, M.: Collisionless electron dynamics in the expanding solar wind, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12596, https://doi.org/10.5194/egusphere-egu2020-12596, 2020.

Magnetic power spectra in the solar wind typically exhibit a transition, steepening, on characteristic ion scales. This transition is not yet fully understood. Two basic phenomena are usually suspected: Hall physics and dissipation. We investigate properties of this transition using numerical simulations.  We analyze results of two-dimensional hybrid simulations using a compressible version of von Kármán-Howarth equation for statistically homogeneous Hall MHD turbulence and compare these results to the predictions for the incompressible Hall MHD. The simulation results indicate that the transition between large, MHD and sub-ion scales is related to a combination of the Hall effect and ion heating/energization.

How to cite: Hellinger, P., Verdini, A., Landi, S., Franci, L., Papini, E., and Matteini, L.: Turbulent cascade in the solar wind on ion scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8738, https://doi.org/10.5194/egusphere-egu2020-8738, 2020.

The fluctuations of proton density in the slow solar wind are analyzed by means of joint Empirical Mode Decomposition (EMD) and Mutual Information (MI) analysis. The analysis reveal that, within the turbulent inertial range, the EMD modes associated with nearby scales have their phases correlated, as shown by the large information exchange. This is a qunatitative measure of the information flow occurring in the turbulent cascade. On the other hand, at scales smaller than the ion gyroscale, the information flow is lost, and the mutual information is low, suggesting that in the kinetic range the nonlinear interacions are no longer sustaining a turbulent energy cascade.

How to cite: Sorriso-Valvo, L., Carbone, F., and Telloni, D.: Mutual Iinformation exchange in solar wind density fluctuations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11410, https://doi.org/10.5194/egusphere-egu2020-11410, 2020.

Elsässer Variables z± are widely considered as outward and inward propagating Alfvén waves in the solar wind turbulence study. It is believed that they can interact nonlinearly with each other to generate energy cascade. However, z− variations sometimes show a feature of convective structures or a combination of white noise and pseudo-structures. Here we present the amplitude of z± in σc (normalized cross helicity) - σr (normalized residual energy) plane in order to get some information on the nature of z±. Measurements from the WIND spacecraft in the slow solar wind during 2007-2009 are used for analysis. In each interval with length of 20 min, we calculate σc, σr, and consider the variance of z± as the amplitude of them for the given interval. We find that in the σc-σr plane, the level contours of the average z- amplitude present a feature of nearly horizontal stratification, which means that the amplitude of z- is independent of the value of σc, and is just related to σr. The horizontal-stratification feature suggests that z- could be convective structures. While the level contours of the average amplitude of z+ are approximately concentric semicircles, and the circle with larger radius corresponds to larger z+ amplitude. It indicates that z+ represents Alfvén waves. The nature of z± in the slow wind here will help us to understand more about the cascade process in the solar wind turbulence.

How to cite: Wang, X., Tu, C., and He, J.: Nature of Elsässer Variables in the slow solar wind turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12171, https://doi.org/10.5194/egusphere-egu2020-12171, 2020.

The scaling anisotropy is crucial to interpret the nonlinear interactions in solar wind turbulence. Previous observations provide diverse results and the structure functions analyses are also reported to be an approach to investigate the scaling anisotropy based on a local magnetic field. However, the determination of the sampling angle with respect to the local background magnetic field implicitly assumes that the observed time series are time stationary. If this assumed time-stationarity is compatible with the measurements has not been investigated. Here we utilize the second-order structure function method to study the scaling anisotropy with a time-stationary background field. We analyze 88 fast solar wind intervals each with time durations >=2 days measured by Wind spacecraft in the period 2005-2018. We calculate the local magnetic field as the average of the time series B(t') whose time-stationarity are fulfilled by our criterion φ<10^o (φ is the angle between the two averaged magnetic field after cutting B(t') into two halves). We find for the first time the isotropic scaling feature of the magnetic-trace structure functions with scaling indices -0.63±0.08 and 0.70±0.04 respectively in the local parallel and perpendicular directions. The scaling for the velocity-trace structure functions is also isotropic and the indices are -0.47±0.10  and 0.51±0.09. 

How to cite: Wu, H., Tu, C., Wang, X., He, J., Yang, L., and Wang, L.: Isotropic Scaling Features Measured Locally in the Solar Wind Turbulence with Stationary Background Field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12022, https://doi.org/10.5194/egusphere-egu2020-12022, 2020.

The plasma that permeates the solar wind, the solar corona, the Earth's magnetosheath and several
other space environments is in a turbulent state. The effect of turbulence on the dynamics of such systems 
is very relevant, considering that it is invoked to explain plasma heating, and particle acceleration and transport
in those environments.
From a mathematical point of view, turbulence is a non linear phenomenon whose study, in the kinetic 
description of plasmas, requires the solution of the non linear Vlasov-Maxwell system of equations. 
Due to the complexity of the problem, the solutions are nowadays found mainly by means of numerical simulations. 
The most widely used method for the solution of the Vlasov-Maxwell system is the Particle In Cell (PIC) method.   
PIC methods can be divided into two major classes: explicit and implicit, depending on the algorithm used
for advancing the solution in time.
In this work, we compare two different PIC methods that use an explicit and a semi-implicit algorithm, respectively. 
The explicit method is implemented in the code P3D[1], while the semi-implicit method in code iPic3D[2].
Both methods are fully kinetic, namely they retain the kinetic effects for both ions and electrons. 
The two codes are tested against a classical set up of plasma turbulence in a 2D cartesian 
geometry[3]. The system is initialized with a restricted number of modes at large scale and
evolves in time without forcing. The box size is of several tens of ion inertial length. The 
grid size is of the order of the Debye length for the explicit scheme, to ensure numerical stability,
and is varied across the electron skin depth for the semi-implicit, by performing different simulations.
Several analyses are presented: global energy conversion, magnetic and electric spectra, scale dependent kurtosis, 
temperature anisotropy for both species, proxies of dissipation such as J.E and PiD[4].
The weaknesses and strengths of the two methods in terms of description of the physical dynamics and of 
computational time are presented, along with a convergence study of the semi-implicit to the explicit
method as the resolution of the former is varied. 

[1] Zeiler, A., Biskamp, D., Drake, J. F., Rogers, B. N., Shay, M. A., & Scholer, M. (2002). Journal of Geophysical Research: Space Physics, 107(A9), SMP-6.
[2] Markidis, S., Lapenta, G., & Rizwan-uddin (2010). Mathematics and Computers in Simulation, 80(7), 1509-1519.
[3] Parashar, T. N., Matthaeus, W. H., & Shay, M. A. (2018). The Astrophysical Journal Letters, 864(1), L21.
[4] Yang, Y., Matthaeus, W. H., Parashar, T. N., Wu, P., Wan, M., Shi, Y., et al. (2017). Physical Review E, 95(6), 061201.

How to cite: Pucci, F., Parashar, T. N., Matthaeus, W. H., and Lapenta, G.: Comparison of semi-implicit and explicit particle in cell methods for the study of turbulence in space plasmas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9805, https://doi.org/10.5194/egusphere-egu2020-9805, 2020.

A grand challenge in physics is to understand how and where electrons and ions accelerate to high energy, often forming power law distributions. We report the result of a combined kinetic-fluid effort [1]. For Earth, a full kinetic model of sizeable chunks of the magnetosphere (e.g. the magnetopause up to the bow shock, or the magnetotail) of order 10-20 RE in size in each dimension is hosted within a MHD simulation that provides it with initial and boundary conditions. For Mercury, it is possible to treat the whole magnetosphere that in this case is spawn from a state derived from a hybrid code [2].
With this approach, we search for regions of most intense particle heating and acceleration, comparing the full kinetic (i.e. we treat both electrons and ions as particles) evolution with the evolution of the host global simulation: MHD for the Earth and hybrid for Mercury.

The results highlight some significant effects peculiar to kinetic models. First, of all full kinetic models provide a detailed view of the electron role in energy transfer, with a distinct role for the enthalpy, bulk and heat flux. Second, the full kinetic approach, allows for the development of modes and instabilities absent in global fluid or hybrid models [3].

How to cite: Lapenta, G., Berchem, J., Walker, R., El Alaoui, M., Schriver, D., Richard, R., and Travnicek, P.: Particle energisation and energy transport in Multiscale MHD - Hybrid - Kinetic PIC models of magnetospheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10074, https://doi.org/10.5194/egusphere-egu2020-10074, 2020.

It is widely accepted that flux ropes play important roles in the momentum and energy transport in space plasmas. Recent observations found that magnetic reconnection occurs at the interface between two counter flows around the center of flux ropes. In this presentation, we report a novel observation by MMS that reconnection occurs at the edge of a large-scale flux rope, the cross-section of which was about 2.5 Re. The flux rope was observed at the dusk side in Earth’s magnetotail and was highly oblique with its axis proximity along the X_GSM direction. We found an electron-scale current sheet near the edge of this flux rope. The Hall magnetic and electric field, super-Alfvénic electron outflow, parallel electric field and positive energy dissipation were observed associated with the current sheet. All the above signatures indicate that MMS detected a reconnecting current sheet in the presence of a large guide field. Interestingly, ions were not coupled in this reconnection, akin to the electron-only reconnection observed in the magnetosheath turbulence. We suggest that the electron-scale current sheet was caused by the strong magnetic field perturbation inside the flux rope. This result will shed new lights for understanding the multi-scale coupling associated with flux ropes in space plasmas.

How to cite: Man, H., Zhou, M., Yi, Y., Zhong, Z., and Deng, X.: Observations of an Electron-only Magnetic Reconnection within a macroscopic Flux Rope in the Magnetotail, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2841, https://doi.org/10.5194/egusphere-egu2020-2841, 2020.

Plasma waves are one of the important products of the magnetic reconnection process.  Plasma waves can produce particle heating, diffusion, and anomalous effects, which can potentially affect magnetic reconnection. We investigate the evolution and properties of plasma waves during a multiple X-line reconnection event at the magnetopause using measurements from the Magnetospheric Multiscale (MMS) mission. Both whistler waves and large-amplitude electrostatic waves were observed around the reconnecting current sheet. In these regions, the electron velocity distribution functions consist of a combination of a cold beam at low energies with an anisotropic population or a loss-cone at high energies. The electrostatic waves corresponded to regions where the cold beams are accelerated, while the whistlers corresponded to regions with significant anisotropies or loss cones. When the cold beams were accelerated to higher energies, the whistlers disappeared since the anisotropy or loss-cone distributions became less apparent. These results present the detailed evolution of the plasma waves reflecting the electron dynamics during magnetic reconnection.

How to cite: Zhong, Z., Graham, D. B., Khotyaintsev, Y. V., Zhou, M., Tang, R., and Deng, X.: Observations of Plasma Waves in the Multiple X-line Reconnection at the Magnetopause, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8731, https://doi.org/10.5194/egusphere-egu2020-8731, 2020.

How to cite: Sisti, M., Califano, F., Faganello, M., Pedrazzi, G., Delli Ponti, F., Hu, A., and Teunissen, J.: Current structures and reconnection events analysis in hybrid-kinetic turbulence simulations using unsupervised machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9476, https://doi.org/10.5194/egusphere-egu2020-9476, 2020.

Magnetic reconnection and turbulence are the two most important energy conversion phenomena in plasma physics. Magnetic reconnection and turbulence are often intertwined. For example, reconnection occurs in thin current layers formed during cascades of turbulence, while reconnection in large-scale current sheet also evolves into turbulence. How energy is dissipated and how particles are accelerated in turbulent magnetic reconnection are outstanding questions in magnetic reconnection and turbulence. Here we report MMS observations of filamentary currents in turbulent outflows in the Earth's magnetotail. We found sub-ion-scale filamentary currents in high-speed outflows that evolved into turbulent states. The normal direction of these current filaments is mainly along the X_GSM direction, which is distinct from the neutral sheet. Some filamentary currents were reconnecting, thereby further dissipating the magnetic energy far from the X line. We notice that turbulent reconnection is more efficient in energizing electrons than laminar reconnection. Coherent structures composed of these filaments may be important in accelerating particles during turbulent reconnection.  

How to cite: Zhou, M., Deng, X., Zhong, Z., and Pang, Y.: Filamentary Currents in Turbulent Magnetic Reconnection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12754, https://doi.org/10.5194/egusphere-egu2020-12754, 2020.

Since early 2D PIC full self-consistent quasi-perpendicular simulations of the foreshock region [Savoini et Lembege, 2001] performed for a supercritical regime, different efforts have been invested later on to analyze the foreshock region. Previous 2D PIC simulations have succeeded in recovering both the local electron distribution [Savoini and Lembege, 2001] and the ion distribution [Savoini et al., 2013] in good agreement with the in-situ experimental data. These studies have retrieved both kinds of distributions and have analyzed in detail how these local distributions vary versus (i) the local angle Θ_Bn to the curved shock (defined between the normal of the shock front and the upstream interplanetary magnetic field) and (ii) the distance from the shock front, in order to identify in detail the different acceleration mechanisms at work at the curved front and supporting these local ion and electron distributions within the foreshock region [Savoini and Lembege, 2001, 2015; Savoini et al, 2013]. This last point can only be accessible to a self-consistent approach (where ion and electron scales are fully included) as in 2D PIC simulations.  

Then, the present work is an extension of the previous analyses listed above for a curved (quasi-perpendicular) shock applied now in a subcritical regime. This work is performed thanks to a new 2D parallel PIC code (SMILEI) which is highly optimized and allows much higher statistics. The main characteristics of the curved front microstructures, its time dynamics, and preliminary results on local distribution functions obtained for both electrons and ions in this new Mach regime will be presented.      

Savoini, P. and B. Lembege, « Two-dimensional simulations of a curved shock: Self-consistent formation of the electron foreshock »,  J. Geophys. Res., Vol. 106, A7, 12975-12992, 2001

Savoini P., B. Lembege and J. Stienlet, « On the origin of the quasi-perpendicular ion foreshock: Full-particle simulations”, J. Geophys. Res., V. 118, 1–14, doi:10.1002/jgra.50158, 2013 

Savoini P. and B. Lembege, “Production of nongyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock region”, J. Geophys. Res., V. 120, 7154–7171, doi: 10.1002/2015JA021018, 2015.

How to cite: Savoini, P. and Lembege, B.: Evidence and analysis of ion/electron foreshocks for a curved shock in Subcritical regime:   2D self-consistent PIC simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6983, https://doi.org/10.5194/egusphere-egu2020-6983, 2020.

Energy conversion via reconnecting current sheets is common in space and astrophysical plasma. Frequently, current sheets disrupt at multiple reconnection sites, leading to the formation of plasmoid structures between the sites, which might affect energy conversion. We present in situ observations of multiple reconnection in the Earth’s magnetotail. The observed highly accelerated proton beams parallel to magnetic field and the ion-scale wave-like fluctuations of the whistler type imply the development of firehose instability between two active reconnection sites. The linear wave dispersion relation estimated for the measured plasma parameters, indicates a positive growth rate of the firehose-related electromagnetic fluctuations. The detailed time-space evolution of the plasmoid is obtained by reconstruction of observations with the 2.5D implicit particle-in-cell simulations. In course of time, plasma on the periphery of the plasmoid becomes anisotropic and as it overcomes the firehose marginal stability threshold, the corresponding magnetic field fluctuations arise. The results of observations and simulations suggest that the firehose instability operating between reconnection sites, converts plasma energy of the proton temperature anisotropy to the energy of magnetic field fluctuations, counteracting with the conversion of magnetic energy to the energy of plasma in reconnection sites.

How to cite: Alexandrova, A., Retinò, A., Divin, A., Matteini, L., Le Contel, O., Breuillard, H., Catapano, F., Cozzani, G., and Deca, J.: In situ evidence of firehose instability in multiple magnetic reconnection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18188, https://doi.org/10.5194/egusphere-egu2020-18188, 2020.