152 research outputs found
Turbulent Coronal Heating Mechanisms: Coupling of Dynamics and Thermodynamics
Context. Photospheric motions shuffle the footpoints of the strong axial
magnetic field that threads coronal loops giving rise to turbulent nonlinear
dynamics characterized by the continuous formation and dissipation of
field-aligned current sheets where energy is deposited at small-scales and the
heating occurs. Previous studies show that current sheets thickness is orders
of magnitude smaller than current state of the art observational resolution
(~700 km).
Aim. In order to understand coronal heating and interpret correctly
observations it is crucial to study the thermodynamics of such a system where
energy is deposited at unresolved small-scales.
Methods. Fully compressible three-dimensional magnetohydrodynamic simulations
are carried out to understand the thermodynamics of coronal heating in the
magnetically confined solar corona.
Results. We show that temperature is highly structured at scales below
observational resolution and nonhomogeneously distributed so that only a
fraction of the coronal mass and volume gets heated at each time.
Conclusions. This is a multi-thermal system where hotter and cooler plasma
strands are found one next to the other also at sub-resolution scales and
exhibit a temporal dynamics.Comment: A&A Letter, in pres
Turbulence, Energy Transfers and Reconnection in Compressible Coronal Heating Field-line Tangling Models
MHD turbulence has long been proposed as a mechanism for the heating of
coronal loops in the framework of the Parker scenario for coronal heating. So
far most of the studies have focused on its dynamical properties without
considering its thermodynamical and radiative features, because of the very
demanding computational requirements. In this paper we extend this previous
research to the compressible regime, including an energy equation, by using
HYPERION, a new parallelized, viscoresistive, three-dimensional compressible
MHD code. HYPERION employs a Fourier collocation -- finite difference spatial
discretization, and uses a third-order Runge-Kutta temporal discretization. We
show that the implementation of a thermal conduction parallel to the DC
magnetic field induces a radiative emission concentrated at the boundaries,
with properties similar to the chromosphere--transition region--corona system.Comment: 4 pages, 4 figures, Solar Wind 12 proceedings (in press
Magnetohydrodynamic Turbulent Cascade of Coronal Loop Magnetic Fields
The Parker model for coronal heating is investigated through a high
resolution simulation. An inertial range is resolved where fluctuating magnetic
energy E_M (k_perp) \propto k_\perp^{-2.7} exceeds kinetic energy E_K (k_\perp)
\propto k_\perp^{-0.6}. Increments scale as \delta b_\ell \simeq \ell^{-0.85}
and \delta u_\ell \simeq \ell^{+0.2} with velocity increasing at small scales,
indicating that magnetic reconnection plays a prime role in this turbulent
system. We show that spectral energy transport is akin to standard
magnetohydrodynamic (MHD) turbulence even for a system of reconnecting current
sheets sustained by the boundary. In this new MHD turbulent cascade, kinetic
energy flows are negligible while cross-field flows are enhanced, and through a
series of "reflections" between the two fields, cascade more than half of the
total spectral energy flow.Comment: 5 pages, 5 figures, to appear in Physical Review E - Rapid. Com
Nonlinear Dynamics of the Parker Scenario for Coronal Heating
The Parker or field line tangling model of coronal heating is studied
comprehensively via long-time high-resolution simulations of the dynamics of a
coronal loop in cartesian geometry within the framework of reduced
magnetohydrodynamics (RMHD). Slow photospheric motions induce a Poynting flux
which saturates by driving an anisotropic turbulent cascade dominated by
magnetic energy. In physical space this corresponds to a magnetic topology
where magnetic field lines are barely entangled, nevertheless current sheets
(corresponding to the original tangential discontinuities hypothesized by
Parker) are continuously formed and dissipated.
Current sheets are the result of the nonlinear cascade that transfers energy
from the scale of convective motions () down to the dissipative
scales, where it is finally converted to heat and/or particle acceleration.
Current sheets constitute the dissipative structure of the system, and the
associated magnetic reconnection gives rise to impulsive ``bursty'' heating
events at the small scales. This picture is consistent with the slender loops
observed by state-of-the-art (E)UV and X-ray imagers which, although apparently
quiescent, shine bright in these wavelengths with little evidence of entangled
features.
The different regimes of weak and strong MHD turbulence that develop, and
their influence on coronal heating scalings, are shown to depend on the loop
parameters, and this dependence is quantitatively characterized: weak
turbulence regimes and steeper spectra occur in {\it stronger loop fields} and
lead to {\it larger heating rates} than in weak field regions.Comment: 22 pages, 18 figures, uses emulateapj, for mpeg file associated to
Figure 17e see (temporarily) http://www.df.unipi.it/~rappazzo/arxiv/jfl.mpg,
ApJ, in pres
CASE STUDY ON EFFECTS OF THE MANDATORY VALIDATION ON BUS COMMERCIAL SPEED
The paper aims to define the new operational requirements and procedures to allow the GTT (Torino public transport company) to implement mandatory validation without negative impacts on both the company and the users. To this end, a four-step methodology has been put forward: a) choice of the reference route and trip sampling; b) data acquisition; c) boarding time analysis and d) future scenario definition.
Attained results show that the most unfavourable situation for the company is banning people from boarding the bus/tram through any door (the case today) because it requires, in order to maintain the same time interval at bus stops, an increase of trips in the morning peak hour. Thus, the present system limits the outcomes negatively for the users in terms of waiting time. However, a change could lead to such positive consequences as fuller passenger cooperation to validate tickets/passes and a more ordered boarding, thus reducing fraud and improving the image of the company
Magnetic moment non-conservation in magnetohydrodynamic turbulence models
The fundamental assumptions of the adiabatic theory do not apply in presence
of sharp field gradients as well as in presence of well developed
magnetohydrodynamic turbulence. For this reason in such conditions the magnetic
moment is no longer expected to be constant. This can influence particle
acceleration and have considerable implications in many astrophysical problems.
Starting with the resonant interaction between ions and a single parallel
propagating electromagnetic wave, we derive expressions for the magnetic moment
trapping width (defined as the half peak-to-peak difference in the
particle magnetic moment) and the bounce frequency . We perform
test-particle simulations to investigate magnetic moment behavior when
resonances overlapping occurs and during the interaction of a ring-beam
particle distribution with a broad-band slab spectrum.
We find that magnetic moment dynamics is strictly related to pitch angle
for a low level of magnetic fluctuation, , where is the constant and uniform background magnetic field.
Stochasticity arises for intermediate fluctuation values and its effect on
pitch angle is the isotropization of the distribution function .
This is a transient regime during which magnetic moment distribution
exhibits a characteristic one-sided long tail and starts to be influenced by
the onset of spatial parallel diffusion, i.e., the variance
grows linearly in time as in normal diffusion. With strong fluctuations
isotropizes completely, spatial diffusion sets in and
behavior is closely related to the sampling of the varying magnetic field
associated with that spatial diffusion.Comment: 13 pages, 10 figures, submitted to PR
Coronal Heating, Weak MHD Turbulence and Scaling Laws
Long-time high-resolution simulations of the dynamics of a coronal loop in
cartesian geometry are carried out, within the framework of reduced
magnetohydrodynamics (RMHD), to understand coronal heating driven by motion of
field lines anchored in the photosphere. We unambiguously identify MHD
anisotropic turbulence as the physical mechanism responsible for the transport
of energy from the large scales, where energy is injected by photospheric
motions, to the small scales, where it is dissipated. As the loop parameters
vary different regimes of turbulence develop: strong turbulence is found for
weak axial magnetic fields and long loops, leading to Kolmogorov-like spectra
in the perpendicular direction, while weaker and weaker regimes (steeper
spectral slopes of total energy) are found for strong axial magnetic fields and
short loops. As a consequence we predict that the scaling of the heating rate
with axial magnetic field intensity , which depends on the spectral index
of total energy for given loop parameters, must vary from for weak
fields to for strong fields at a given aspect ratio. The predicted
heating rate is within the lower range of observed active region and quiet Sun
coronal energy losses.Comment: 4 pages, 5 figures, uses emulateapj, complies with published versio
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