• plasma turbolento
• scala elettronica

A plasma is a collection of a really high number of charged particles whose dynamics is essentially determined by collective, long range electromagnetic interaction between its costituents rather than by collisions.
On sufficiently large temporal and spatial scales, it is possible to describe the plasma dynamics by using the so called ideal magnetohydrodynamic model (ideal MHD) which treats all charged species (electrons and ions) as a single fluid. Ideal MHD allows to describe a large number of plasma properties and phenomena among which the connection between magnetic field and plasma which implies that magnetic field lines cannot break during plasma evolution. This property of an ideal plasma has crucial consequences on the system dynamics by preventing the transition to different energetic states unconnected by the magnetic topology.
If the system locally develops small scale structures, the description provided by ideal MHD ceases to be valid and connection between plasma and magnetic field lines no longer holds. As a consequence the global magnetic topology changes as field lines can now break and reconnect, eventually leading to new magnetic configurations which aren’t topologically equivalent to the former magnetic state.
Magnetic reconnection processes play a fundamental role both in laboratory pla- smas and in problems of astrophysical interest as a change in the global topology ofmagnetic field lines has strong consequences on particles trajectories and so on trans- port phenomana. Furthermore, the new magnetic configuration corresponds to a lower energy state with respect to the initial one and this energy defect is converted into plasma kinetic and internal energy, so as a consequence of reconnection, particles are accelerated and the system is heated.
Standard models of collisionless magnetic reconnection predict that electrons and ions decouple from the magnetic field on different scales. This implies that field lines breaking arises in a tiny diffusion region where both ions and electrons are demagnetized and this region is embedded in a wider current sheet where only ions are demagnetized while electrons are still connected to the magnetic field. Magnetic reconnection is accompained by the generation of bidirectional opposite jets of both ions and electrons coming out from the reconnection site. This kind of behaviour has been observed experimentally in many situations.
In turbulent plasma dynamics energy injected in the system at large scales is nonlinearly transferred to smaller scales and this causes the formation of localized structures, such as current sheets, inside which the connection between plasma and magnetic field lines may be broken. So magnetic reconnection plays a crucial role in plasma turbulence too as reconnection sites are spontaneously created by turbulent dynamics and contribute to the cascading energy transfer across the scales.
The solar wind and the Earth’s magnetosphere represent a natural laboratory for the study of plasma turbulence and collisionless magnetic reconnection thanks to highly accurate in situ satellite measurements. The MMS space mission has recently seen evidence in the turbulent Earth’s magnetosheath of unusual reconnection events characterized by the presence of an electron scale current sheet in which divergent bidirectional electron jets weren’t accompained by any ion outflows. These new phenomena were dubbed "electron only reconnection events".
From an intuitive point of view it is clear that the formation of electron scale current sheets requires a significative accumulation of magnetic energy at scales between ions typical scales and electrons typical scales so it is difficult to imagine these electron only reconnection events happening in a turbulent plasma in which energy, injected at very large scales, is transferred continuosly from large scales down to ion and then to electron scales. Hence reconnection sites that develope in these systems are always made up of an electron scale diffusion region surrounded by an ion scale current sheet. To obtain the electron only kind of reconnection it is necessary to have a mechanism (such as an instability) which injects energy close to ion scales because in this situation ions are nearly demagnetized and magnetic fluctuations can give rise to a purely electronic dynamics.
In this thesis the generation and the role of electron only reconnection events in a turbulent system have been investigated by means of an eulerian hybrid Vlasov-Maxwell (HVM) simulation of a freely decaying turbulent plasma. The HVM model which treats ions as kinetic and electrons as a fluid, has been integrated in a 2D-3V phase space. By injecting magnetic energy close to ion characteristic scales, electron only reconnection events have been observed to develop and reproduce all the features revealed by the MMS mission in the magnetosheath such as the lack of ion outflows from the reconnection site. The mechanism leading to the formation of electron scale current sheets has been identified and reconnection events have been analized in detail.
The role of electron only reconnection events in the developement of the turbulent energy cascade has been studied as well. The analisys of the statistical properties of magnetic fluctuations shows an intermittent turbulent behaviour at large scales while a non-intermittent turbulent regime is revealed at sub-ion scales. It has been verified that the transition from the intermittent behaviour to the non-intermittent one happens at a scale close to the typical width of the reconnection current sheets that are generated by the turbulent dynamics.
The results obtained from this simulation have been compared with the results obtained from another simulation of a freely decaying turbulent plasma in which energy was injected at scales larger than ion scales. In this simulation reconnection events occurr as well but now all the reconnection sites show the typical structure expected for standard reconnection events in which the diffusion region is surrounded by an ion scale current sheet and electron jets are accompained by ion flows. By analizing the statistical properties of magnetic fluctuations, the transition from large scale intermittent behaviour to small scale non-intermittent beahviour has been detected again with the only difference that the transition between these two different regimes occours at a larger scale with respect to the first simulation. Again, the scale at which the transition occours is really close to the mean width of the current sheets and this result suggests that the break in the turbulent behaviour of the system is linked to the energy conversion that takes place inside the reconnection region which causes the transfer of energy from the magnetic field to particles.