The complex structure of astrophysical plasmas: turbulence, coherent structures and particle transport

A PhD Thesis in collaboration between UNICAL and KU-Leuven investigating the complexity of space plasma turbulence, exploiting the competencies and research interests addressed by the AIDA project.

Plasma turbulence is a ubiquitous phenomenon that characterizes the universe. From black holes to fusion devices, complex plasma effects are present to a variety of scales. In this PhD thesis, some relevant aspects of astrophysical plasmas will be covered, tackling long-standing problems via synergistic, different approaches. We will make use of (I) plasma theory, (II) numerical simulations, and (III) space data analysis. Theoretical models are necessary to understand the basic ingredients of interesting phenomena. Though sometimes, the mathematical approach becomes either too difficult or requires too strong approximations to be carried on. The invaluable tool of numerical simulations pushed forward our understanding of plasma turbulence, as it is our only way to directly observe it. Simulations also allow preparing ad-hoc experiments to test analytical theories. Finally, observations -- either Earth-based or in-situ -- are the playground where analytical theories and numerical simulations face reality.

In this thesis work, all the methods above were used to approach different plasma turbulence problems at very small (kinetic) and very large (magnetohydrodynamics) scales. At kinetic scales, I investigated the problems of particle diffusion and acceleration. A novel theory that describes diffusion has been derived from the Nonlinear Guiding Center Theory and tested with self-consistent numerical simulations. The results obtained from simulations also allowed studying acceleration phenomena. Conducting a vast campaign of simulations with different techniques set the stage to understand how different algorithms and approximations can affect the physical results at these small scales.

At magnetohydrodynamics (MHD) scales, the focus was on equilibrium and transient coherent structures, which are persistent features in space plasmas. Using well-established theory for MHD equilibria and the detection of small-scale discontinuities, numerical algorithms were used to reveal the texture of the solar wind, finding precise patterns of such structures. Subsequently, a novel technique for the identification of more general equilibrium structures was developed and tested with numerical simulations and in-situ measurements, which led to a deeper understanding of the solar wind.

Last but not least, using this recently developed technique, large scales were coupled back to small scales, finding a close correlation between MHD equilibrium structures and energetic particles using in-situ measurements of the most recent space missions.

astrophysical plasmas

Interaction of a proton with a current sheet in FPIC simulations. The x-y plane is the simulation domain, the z axis represents the time. Strong current sheets are indicated with iso-surfaces (red for positive, and blue for negative jz). Trajectories of some ions (green) and electrons (magenta) are shown. In particular, the blow-up shows a proton passing through an intense current sheet and becomes energized (notice the increase in the orbit radius and in the velocity component parallel to the mean magnetic field in the z direction).

[1] Pecora, F., Pucci, F., Lapenta, G., Greco, A., Servidio, S. “Pair diffusion in isotropic 3D turbulence”. In prep (2021).

[2] Trotta, D., Pecora, F., Settino, A., Perrone, D., Valentini, F., Servidio, S. “On the transmission of turbulent structures across Earth's Bow Shock”. Submitted (2021).

[3] Pecora, F., Servidio, S., Greco, A., Matthaeus, W. H., McComas, D. J., Giacalone, J., Joyce, C. J.,Getachew, T., Cohen, C. M. S., Leske, R. A., Wiedenbeck, M. E., Jr., R. L. M., Hill, M. E., Mitchell, D. G., Christian, E. R., Roelof, E. C., Schwadron, N. A., and Bale, S. D. “Parker Solar Probe Obser-vations of helical structures as boundaries for energetic particles”. In: Monthly Notices of the Royal Astronomical Society (2021 - publishing).

[4] Pezzi, O., Pecora, F., Roux, J. le, Engelbrecht, N. E., Greco, A., Servidio, S., Malova, H. V., Khabarova,O. V., Malandraki, O., Bruno, R., Matthaeus, W. H., Li, G., Zelenyi, L. M., Kislov, R. A., Obridko,V. N., and Kuznetsov, V. D. “Current Sheets, Plasmoids and Flux Ropes in the Heliosphere”. In: Space Science Reviews 217.3 (Mar. 2021), p. 39.issn: 1572-9672. doi:10.1007/s11214-021-00799-7.

[5] Khabarova, O., Malandraki, O., Malova, H., Kislov, R., Greco, A., Bruno, R., Pezzi, O., Servidio, S.,Li,  G.,  Matthaeus,  W.,  Le  Roux,  J.,  Engelbrecht,  N.  E., Pecora, F.,  Zelenyi,  L.,  Obridko,  V.,  and Kuznetsov,  V.  “Current  Sheets,  Plasmoids  and  Flux  Ropes  in  the  Heliosphere”.  In: Space Science Reviews 217.3 (Mar. 2021), p. 38.issn: 1572-9672. doi:10.1007/s11214-021-00814-x.

[6] Pecora, F.,  Servidio,  S.,  Greco,  A.,  and  Matthaeus,  W.  H.  “Identification  of  coherent  structures in  space  plasmas:  The  magnetic  helicity-PVI  method”.  In: Astronomy & Astrophysics (2020). doi:10.1051/0004-6361/202039639.

[7] Chiappetta, F., Pecora, F., Prete, G., Settino, A., Carbone, V., and Riccardi, P. “A bridge between research, education and communication”. In: Nature Astronomy 4.1 (2020), pp. 2–3. doi:10.1038/s41550-019-0997-3

[8] Pecora, F. “Ion Transport and Heating in Simulations of Plasma Turbulence”. In: Il Nuovo Cimento C225.5 (2019). doi:10.1393/ncc/i2019-19225-4.

[9] Pecora, F., Greco, A., Hu, Q., Servidio, S., Chasapis, A. G., and Matthaeus, W. H. “Single-spacecraft Identification of Flux Tubes and Current Sheets in the Solar Wind”. In: The Astrophysical Journal Letters 881.1 (2019), p. L11. doi:10.3847/2041-8213/ab32d9.

[10] Pecora, F., Pucci, F., Lapenta, G., Burgess, D., and Servidio, S. “Statistical analysis of ions in two-dimensional  plasma  turbulence”.In: Solar Physics 294.9 (2019),  p. 114. doi:10.1007/s11207-019-1507-6.

[11] Pecora, F., Servidio, S., Greco, A., Matthaeus, W. H., D. Burgess, C. T. H., Carbone, V., and Veltri,P. “Ion Diffusion and Acceleration in Plasma Turbulence”. In: Journal of Plasma Physics 84.6 (2018), p. 725840601. doi:10.1017/S0022377818000995