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Tesi etd-03202019-133918


Thesis type
Tesi di laurea magistrale
Author
ARRO', GIUSEPPE
URN
etd-03202019-133918
Title
Riconnessione a scala elettronica in un plasma turbolento
Struttura
FISICA
Corso di studi
FISICA
Commissione
relatore Prof. Califano, Francesco
Parole chiave
  • plasma turbolento
  • scala elettronica
Data inizio appello
10/04/2019;
Consultabilità
completa
Riassunto analitico
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 of
magnetic 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 nonli-
nearly 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 turbu-
lence 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 hi-
ghly 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 bidi-
rectional 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 ob-
tained 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 intermit-
tent 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 tran-
sition 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.
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