Abstracts

1) Fluttuazioni magnetiche a scale elettroniche nel vento solare

D. Telloni, V. Carbone, F. Lepreti, A. Vecchio

Mentre le fluttuazioni del plasma a bassa frequenza nello spazio interplanetario sono state ampiamente studiate e descritte con successo nel quadro della turbolenza, le fluttuazioni ad alta frequenza, dove l'energia viene dissipata, rappresentano ancora una sfida per i modelli teorici. Recentemente, è stato proposto un nuovo quadro unificante per le fluttuazioni e la dissipazione alle alte frequenze, basato su un modello Brownian-like (Carbone et al. 2019), che ha permesso di introdurre, in modo predittivo, una relazione di fluttuazione-dissipazione a scale elettroniche. Questa relazione è stata validata osservativamente in magnetosfera, ed è risultata essere compatibile con la presenza del Landau damping a scale elettroniche. Con l'avvento di Solar Orbiter sarà possibile validare nel vento solare il modello predittivo sviluppato da Carbone et al. (2019), e studiarne l'evoluzione radiale, per sondare le scale elettroniche alla ricerca dei più probabili meccanismi di riscaldamento del vento solare.

2) Kinetic processes in the solar wind close to the Sun: departure from thermodynamic equilibrium and plasma heating

F. Valentini, S. Servidio, S. Perri, A. Settino O. Pezzi, F. Pucci, G. Nigro, F. Malara, V. Carbone, P. Veltri

Recent observational and numerical results suggest that the interaction of space plasmas with electromagnetic fluctuations can drive local departure from thermodynamic equilibrium. Fundamental processes develop locally in turbulence, like the generation of current sheets, waves, plasma heating, ion to proton differential heating, electron anisotropy and so on. These regions are characterized by non-Maxwellian features. Measurements of fields and particle velocity distributions, in synergy with multidimensional kinetic simulations, would give significant insights into the identification of the physical processes responsible for heating and dissipation in weakly collisional systems. Moreover, an indicator of departure from Maxwellian could be useful to authomatically identify regions of scientific interest in space.

3) Feedback of kinetic effects on turbuelnt heating in collisionless space and astrophysical plasmas

S. Cerri

Within the turbulent solar wind (SW), we have a unique opportunity to study turbulence in nearly collisionless plasmas and how kinetic effects feed back into the process of turbulent heating. This, of course, has long-reaching implications for modeling a wide variety of distant astrophysical environments and their consequent radiative emission. In this context, Solar Orbiter would provide significant insights and constraints on various processes taking place in the SW plasma. The main idea is to compare in situ data with existing, as well as new appropiately planned, kinetic simulations in order to address the general issue of turbulent heating. Specifically, this includes: (i) observing the signatures of (resonant and non-resonant) wave-particle interactions and determining their role in determining the heating partition among species wihtin different plasma regimes/conditions, (ii) understanding the anisotropic heating of the plasma and the consequent possible generation of favorable conditions for the onset of various instabilities, (iii) exploring the emergence of features in the velocity distribution functions, such as the presence of the so-called velocity space turbulent cascades, and (iv) disentangling the spatio-temporal relations between the above-mentioned mechanisms and the presence of coherent structures (as well as elucidating their role in the kinetic-scale turbulent cascade).

4) Enhancement of inter-particle collisions in turbulent weakly-collisional plasmas

O. Pezzi S. Servidio, F. Valentini, L. Sorriso-Valvo, P. Veltri

Space plasmas are often observed in non-equilibrium thermodynamic states, characterized by the presence of fine structures in the particle velocity distribution function. Although the effect of inter-particle collisions is routinely neglected in the description of such systems, recent numerical results have indicated that these velocity-space structures locally increases the role of collisions. The combined adoption of high-resolution in-situ measurements of Solar Orbiter and numerical simulations would shed light on the role of collisions as one of the viable mechanisms to irreversibly dissipate energy, by highlighting potential correlations of the enhancement of collisional effects with other dynamical signatures of plasma turbulence and magnetic reconnection. The intensification of collisional effects may be also assessed through a parameter, such as the collisional dissipation rate, that - by also considering fine velocity-space structures- helps to identify regions where collisions are effective.

5) Simulazioni cinetiche di turbolenza, modello Vlasov - LF, modello full Vlasov in approssimazione Darwin

F. Califano e il gruppo di plasmi spaziali di Pisa

Utilizzando un nuovo modello numerico con ioni cinetici (Vlasov) e elettroni con chiusura “Landau-fluid” si intendono studiare i processi fondamentali in gioco nello sviluppo della turbolenza nel passaggio attraverso la frequenza di ciclotrone degli ioni. In particolare si intende mettere in evidenza il ruolo degli strati di corrente e associata riconnessione magnetica nell’iniettare l’energia alle scale sub-ioniche, la formazione di anisotropie ioniche ed elettroniche, le “signatures” sulla funzione di distribuzione legate allo sviluppo dei processi menzionati sopra. Successivamente, con modello “full kinetic” (in cui vengono tagliate le onde di luce) si intende studiare la fisica degli elettroni negli strati di riconnessione, nello sviluppo della turbolenza verso le scale elettroniche, la possibilità di sviluppare un nuovo processo noto come “electron reconnection only” recentemente osservato dai satelliti.

6) The kinetics of collisionless shocks propagating through the inner heliosphere

D. Trotta, the UNICAL astrophysical plasmas group, D. Burgess

Collisionless shocks play a pivotal role in our understanding of the features of many astrophysical environments, ranging from solar flares to merging galaxy clusters. Shocks convert directed flow energy to thermal energy and, in the collisionless case, a small fraction of the energy is available to accelerate particles to high energies. Interplanetary shocks (generated in the heliosphere by fast coronal ejecta) and planetary bow shocks (formed due to interaction between the solar wind and planetary magnetospheres) are crucial ingredients to build up a comprehensive understanding of the interplanetary environment, and can be observed in-situ using the instrumentation on board of spacecrafts. Solar Orbiter will provide observations of interplanetary shocks in the inner heliosphere with unprecedented level of detail. Through the combination of state-of-the-art kinetic modelling and spacecraft data, we will be able to study the kinetic effects generated at the shock front and in the region immediately around it. These studies will elucidate poorly understood aspects of energy conversion in the heliosphere.

7) Turbulent fluctuations and features in particles velocity distribution

L. Sorriso-Valvo, C. Vasconez, et al.

Le fluttuazioni combinate di velocità, campo magnetico e corrente a piccola scala contribuiscono al flusso globale di energia turbolenta dalle grandi scale verso le scale cinetiche e dissipative. A seconda delle fluttuazioni da cui è dominato, tale flusso può attivare specifici processi di fisica del plasma quali risonanza, riconnessione ecc. La qualità dei dati di Solar Orbiter permettterà lo studio della relazione fra fluttuazioni turbolente e deformazioni delle VDF di protoni, alpha ed elettroni, allo scopo di stabilire il ruolo dei dettagli locali della turbolenza sui processi dissipativi nel vento solare, e la loro dipendenza dalla distanza eliocentrica.

8) Electrostatic turbulence and particle energization: comparing space and fusion plasmas

L. Sorriso-Valvo, N. Vianello, M. Zuin, M. Gobbin, F. Taccogna, C. Owen, D. Verscharen, R. Wicks, et al.

I regimi osservabili nel vento solare e nei plasmi di fusione sono estremamente differenti. Ciò nonostante, alcuni dei processi fisici sono condivisi dai due sistemi. Fra questi, la turbolenza elettrostatica, osservata ad alte frequenze dei plasmi spaziali e a frequenze più basse nelle macchine per la fusione, occupa un ruolo fondamentale soprattutto per l'energizzazione delle particelle. Le nuove misure di Solar Orbiter e misure nella macchina RFX (Padova) permetteranno uno studio comparato della turbolenza elettrostatica e del suo ruolo nell'energizzazione di particelle, rivelando similitudini e differenze fra i due sistemi.

9) Shear-driven kinetic heating and generation of non-gyrotropic particle distributions

D. Del Sarto, L. Franci, A. Ghizzo, P. Hellinger, S. Landi, L. Matteini, G. Nigro, E. Papini, F. Pegoraro

The low collision rate of the coronal plasma allows for the generation and maintenance in time of non-Maxwellian particle distributions, which are non-isotropic in the second order velocity moment (i.e., temperature or pressure). Both numerical simulations and in situ spacecraft measurements show that the local occurrence of non-gyrotropic ion velocity distributions is sometimes correlated to large values of the fluid vorticity. The latter is in turn related to the shear rate of the plasma. This suggests such a kind of pressure anisotropy to be caused by a kinetic heating that, for each particle species, taps from the energy stored in the shear flow of the related fluid velocity. This process is shown to be relevant when the local shear rate is not negligible with respect to the local gyration frequency. This kind of collisionless particle energisation is observed in kinetic turbulence simulations and corresponds to a reduction of the energy cascade rate. Solar Orbiter instruments are expected to provide measurements apt to test and analyse this mechanism for different species, in particular ions. In situ measurements will be compared with numerical simulations made with PIC-Hybrid (CAMELIA), full PIC (SMILEI), semi-Lagrangian Vlasov-Maxwell (VLEM) codes and with fluid codes based on extended 2-fluid models including the full pressure tensor dynamics.

10) Modelling Solar Orbiter VDFs to identify pristine solar wind kinetic properties close to the Sun

L. Matteini, P. Hellinger, S. Landi, L. Franci, A. Verdini and E. Papini

Particle distribution functions (VDFs) in the solar wind contain fundamental information about the physical mechanisms responsible for plasma heating and acceleration close to the Sun. However, pristine VDF kinetic signatures of coronal processes are not preserved during expansion and can have been significantly reprocessed when observed in situ. Firstly, because the weakly collisional solar wind expansion acts directly on particle distributions, inducing slow secular changes in their initial properties. Secondly, the evolution of the solar wind plasma is constantly modified by the interaction with the underlying developed turbulence, including kinetic-scale waves and structures that populate the cascade. Lastly, deviations from Maxwellian generated in the corona can be further enhanced during expansion leading to the onset of kinetic instabilities that can substantially reshape VDFs as a function of radial distance. To correctly link Solar Orbiter in situ measurements with remote sensing observations is then crucial to identify and disentangle all these contributions. Partially, this is mitigated by the fact that Orbiter will explore inner heliospheric regions, sampling then more pristine plasma conditions. On the other hand, to capture well VDF signatures that origin from the corona and acceleration region, some ad hoc modelling is needed. This can be achieved by the use of the Hybrid-Expanding-Box model (HEB), which self-consistently couples a kinetic description of the solar wind ions with large scale expansion. Results from this modelling can be directly compared with SWA measurements of different ion populations taken between 0.3 and 1AU, in order to constrain the origin and evolution of non-thermal properties, like temperature anisotropy and relative ion-drifts.

References:

Hellinger et al., “Hybrid simulations of the expanding solar wind: Temperatures and drift velocities”, GRL 2003

Matteini et al., “Ion Kinetics in the Solar Wind: Coupling Global Expansion to Local Microphysics”, Space Sci. Rev. 2011

Matteini et al., “Signatures of kinetic instabilities in the solar wind”, JGR 2013

Franci et al., “Solar Wind Turbulent Cascade from MHD to Sub-ion Scales: Large-size 3D Hybrid Particle-in-cell Simulations”, ApJ 2018

Hellinger et al., “Turbulence versus Fire-hose Instabilities: 3D Hybrid Expanding Box Simulations”, ApJ 2019

11) Effective dissipation, irreversibility and fluctuations in solar wind turbulence.

D. Del Sarto, A. Ghizzo

The solar wind is a low-collision plasma and, as such, is often described within the hamiltonian Vlasov model paradigm. One of the fundamental open questions, in these purely collisionless models, is to understand not only the mechanisms of transfer of (kinetic) energy among scales but also that of “information”, i.e., entropy, and the way the two compete in the dynamics and in self-organization: entropy variation is formally forbidden in a continuum Vlasov model as long as fluctuations of the distribution function are small, while collisions and dissipation can induce entropy growth in numerical Vlasov models and in a real low-collision plasma, as well. However, even in a continuum Vlasov model, large amplitude fluctuations of the distribution function appear as source terms of the Vlasov equation and can be compatible with large amlitude reversible fluctuations (both increase and decrease) of the entropy, which can be related to self-organization processes. The two kind of “source” terms in Vlasov-type models challenge our understanding of the way that turbulent fluctuations and collisions can intervene and/or compete in a real low-collision plasma such as the solar wind. Solar Orbiter in situ measurements can help to shed light between effective dissipation mechanisms related to irreversible information loss (e.g., by collisions) and related to reversible redistribution of the information among scales (because of fluctuations of the distribution function). In situ measurements will be compared with “noise-less” Vlasov simulations with the VLEM code and “noisy”simulations with the PIC code SMILEI.

References:

[1] A. Ghizzo and D. Del Sarto, “Low- and High- frequency nature of oblique filamentation modes. II. Vlasov-Maxwell simulations of collisionless heating process “, Accepted: to appear on Phys. Plasmas (2020).

[2] A. Ghizzo, D. Del Sarto, “Effects of fluctuations and momentum transfer in relativistic counter-propagating electron beams“, submitted (2020).

[3] M. Sarrat, A. Ghizzo, D. Del Sarto, L. Serrat, “Parallel implementation of a relativistic semi-Lagrangian Vlasov-Maxwell solver“, EpJ D, 71, 271 (2017).

[4] J.Derouillat, A.Beck, F.Pérez, T.Vinci, M. Chiaramello, A.Grassi, M. Flé, G.Bouchard, I. Plotnikov, N.Aunai, J. Dargent, C. Riconda, M.Grech, “SMILEI: a collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation”, Comp. Phys. Comm. 222, 351 (2018).