1,219 research outputs found
Identification of coronal heating events in 3D simulations
The solar coronal heating problem is an open question since 1939. One
proposed model for the transport and release of mechanical energy generated in
the sub-phorospheric layers and photosphere is the nanoflare model that
incorporates Ohmic heating which releases a part of the energy stored in the
magnetic field via magnetic reconnection. The problem with the verification of
this model is that we cannot resolve observationally small scale events.
Histograms of observable characteristics of flares, show powerlaw behavior, for
both energy release rate, size and total energy. Depending on the powerlaw
index of the energy release, nanoflares might be an important candidate for
coronal heating; we seek to find that index. In this paper, we employ a
numerical 3D-MHD simulation produced by the numerical code Bifrost, and a new
technique to identify the 3D heating events at a specific instant. The quantity
we explore is the Joule heating, which is explicitly correlated with the
magnetic reconnection because depends on the curl of the magnetic field. We are
able to identify 4136 events in a volume $24 \times 24 \times 9.5 \
\textrm{Mm}^3768 \times 786 \times 331$ grid cells) of a specific
snapshot. We find a powerlaw slope of the released energy per second, and two
powerlaw slopes of the identified volume. The identified energy events do not
represent all the released energy, but of the identified events, the total
energy of the largest events dominate the energy release. Most of the energy
release happens in the lower corona, while heating drops with height. We find
that with a specific identification method that large events can be resolved
into smaller ones, but at the expense of the total identified energy releases.
The energy release which cannot be identified as an event favours a low energy
release mechanism.Comment: 10 pages, 7 figure
Accelerated particle beams in a 3D simulation of the quiet Sun
Observational and theoretical evidence suggest that beams of accelerated
particles are produced in flaring events of all sizes in the solar atmosphere,
from X-class flares to nanoflares. Current models of these types of particles
in flaring loops assume an isolated 1D atmosphere. A more realistic environment
for modelling accelerated particles can be provided by 3D radiative
magnetohydrodynamics codes. Here, we present a simple model for particle
acceleration and propagation in the context of a 3D simulation of the quiet
solar atmosphere, spanning from the convection zone to the corona. We then
examine the additional transport of energy introduced by the particle beams.
The locations of particle acceleration associated with magnetic reconnection
were identified by detecting changes in magnetic topology. At each location,
the parameters of the accelerated particle distribution were estimated from
local conditions. The particle distributions were then propagated along the
magnetic field, and the energy deposition due to Coulomb collisions with the
ambient plasma was computed. We find that particle beams originate in extended
acceleration regions that are distributed across the corona. Upon reaching the
transition region, they converge and produce strands of intense heating that
penetrate the chromosphere. Within these strands, beam heating consistently
dominates conductive heating below the bottom of the transition region. This
indicates that particle beams qualitatively alter the energy transport even
outside of active regions.Comment: Accepted for publication in A&
Spectral analysis of 3D MHD models of coronal structures
We study extreme-ultraviolet emission line spectra derived from
three-dimensional magnetohydrodynamic models of structures in the corona. In
order to investigate the effects of increased magnetic activity at photospheric
levels in a numerical experiment, a much higher magnetic flux density is
applied at photospheric levels as compared to the Sun. Thus, we can expect our
results to highlight the differences between the Sun and more active, but still
solar-like stars. We discuss signatures seen in extreme-ultraviolet emission
lines synthesized from these models and compare them to signatures found in the
spatial distribution and temporal evolution of Doppler shifts in lines formed
in the transition region and corona. This is of major interest to test the
quality of the underlying magnetohydrodynamic model to heat the corona, i.e.
currents in the corona driven by photospheric motions (flux braiding).Comment: 10 pages, 3 figure
Photospheric Motions and Their Effects on the Corona: a Numerical Approach
We perform a number of numerical simulations of the solar corona with the aim
to understand how it responds to different conditions in the photosphere. By
changing parameters which govern the motion of the plasma at the photosphere we
study the behavior of the corona, in particular, the effects on the current
density generated. An MHD code is used to run simulations, using a 20x20x20
Mm^3 box with time spans ranging from one hundred to several hundreds of
minutes. All the experiments show a fast initial increase of the current
density, followed by a stabilization around an asymptotic value which depends
on the photospheric conditions. These asymptotic average current densities as
well as the turn-over points are discussed.Comment: 11 pages with 13 figures, accepted for publication in The
Astrophysical Journa
Electron Transport in Magnetic-Field-Induced Quasi-One-Dimensional Electron Systems in Semiconductor Nanowhiskers
Many-body effects on tunneling of electrons in semiconductor nanowhiskers are
investigated in a magnetic quantum limit. We consider the system with which
bulk and edge states coexist. We show that interaction parameters of edge
states are much smaller than those of bulk states and the tunneling conductance
of edge states hardly depends on temperature and the singular behavior of
tunneling conductance of bulk states can be observed.Comment: 4 pages, 4 figure
Ejection of cool plasma into the hot corona
We investigate the processes that lead to the formation, ejection and fall of
a confined plasma ejection that was observed in a numerical experiment of the
solar corona. By quantifying physical parameters such as mass, velocity, and
orientation of the plasma ejection relative to the magnetic field, we provide a
description of the nature of this particular phenomenon. The time-dependent
three-dimensional magnetohydrodynamic (3D MHD) equations are solved in a box
extending from the chromosphere to the lower corona. The plasma is heated by
currents that are induced through field line braiding as a consequence of
photospheric motions. Spectra of optically thin emission lines in the extreme
ultraviolet range are synthesized, and magnetic field lines are traced over
time. Following strong heating just above the chromosphere, the pressure
rapidly increases, leading to a hydrodynamic explosion above the upper
chromosphere in the low transition region. The explosion drives the plasma,
which needs to follow the magnetic field lines. The ejection is then moving
more or less ballistically along the loop-like field lines and eventually drops
down onto the surface of the Sun. The speed of the ejection is in the range of
the sound speed, well below the Alfven velocity. The plasma ejection is
basically a hydrodynamic phenomenon, whereas the rise of the heating rate is of
magnetic nature. The granular motions in the photosphere lead (by chance) to a
strong braiding of the magnetic field lines at the location of the explosion
that in turn is causing strong currents which are dissipated. Future studies
need to determine if this process is a ubiquitous phenomenon on the Sun on
small scales. Data from the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory (AIA/SDO) might provide the relevant information.Comment: 12 pages, 10 figure
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