1,551 research outputs found
Data-driven model of the solar corona above an active region
We aim to reproduce the structure of the corona above a solar active region
as seen in the extreme ultraviolet (EUV) using a three-dimensional
magnetohydrodynamic (3D MHD) model. The 3D MHD data-driven model solves the
induction equation and the mass, momentum, and energy balance. To drive the
system, we feed the observed evolution of the magnetic field in the photosphere
of the active region AR 12139 into the bottom boundary. This creates a hot
corona above the cool photosphere in a self-consistent way. We synthesize the
coronal EUV emission from the densities and temperatures in the model and
compare this to the actual coronal observations. We are able to reproduce the
overall appearance and key features of the corona in this active region on a
qualitative level. The model shows long loops, fan loops, compact loops, and
diffuse emission forming at the same locations and at similar times as in the
observation. Furthermore, the low-intensity contrast of the model loops in EUV
matches the observations. In our model the energy input into the corona is
similar as in the scenarios of fieldline-braiding or flux-tube tectonics, that
is, energy is transported to the corona through the driving of the vertical
magnetic field by horizontal photospheric motions. The success of our model
shows the central role that this process plays for the structure, dynamics, and
heating of the corona.Comment: 5 pages, 3 Figures, published in A&A letter
Thermodynamic laws in isolated systems
The recent experimental realization of exotic matter states in isolated
quantum systems and the ensuing controversy about the existence of negative
absolute temperatures demand a careful analysis of the conceptual foundations
underlying microcanonical thermostatistics. Here, we provide a detailed
comparison of the most commonly considered microcanonical entropy definitions,
focussing specifically on whether they satisfy or violate the zeroth, first and
second law of thermodynamics. Our analysis shows that, for a broad class of
systems that includes all standard classical Hamiltonian systems, only the
Gibbs volume entropy fulfills all three laws simultaneously. To avoid
ambiguities, the discussion is restricted to exact results and analytically
tractable examples.Comment: footnotes 19, 26 and outlook section adde
Time parameters and Lorentz transformations of relativistic stochastic processes
Rules for the transformation of time parameters in relativistic Langevin
equations are derived and discussed. In particular, it is shown that, if a
coordinate-time parameterized process approaches the relativistic
Juttner-Maxwell distribution, the associated proper-time parameterized process
converges to a modified momentum distribution, differing by a factor
proportional to the inverse energy.Comment: 5 pages, 1 figur
Meaning of temperature in different thermostatistical ensembles
Depending on the exact experimental conditions, the thermodynamic properties
of physical systems can be related to one or more thermostatistical ensembles.
Here, we survey the notion of thermodynamic temperature in different
statistical ensembles, focusing in particular on subtleties that arise when
ensembles become non-equivalent. The 'mother' of all ensembles, the
microcanonical ensemble, uses entropy and internal energy (the most
fundamental, dynamically conserved quantity) to derive temperature as a
secondary thermodynamic variable. Over the past century, some confusion has
been caused by the fact that several competing microcanonical entropy
definitions are used in the literature, most commonly the volume and surface
entropies introduced by Gibbs. It can be proved, however, that only the volume
entropy satisfies exactly the traditional form of the laws of thermodynamics
for a broad class of physical systems, including all standard classical
Hamiltonian systems, regardless of their size. This mathematically rigorous
fact implies that negative 'absolute' temperatures and Carnot efficiencies
are not achievable within a standard thermodynamical framework. As an important
offspring of microcanonical thermostatistics, we shall briefly consider the
canonical ensemble and comment on the validity of the Boltzmann weight factor.
We conclude by addressing open mathematical problems that arise for systems
with discrete energy spectrum.Comment: 11 pages, 1 figur
Current systems of coronal loops in 3D MHD simulations
We study the magnetic field and current structure associated with a coronal
loop. Through this we investigate to what extent the assumptions of a
force-free magnetic field break down and where they might be justified. We
analyse a 3D MHD model of the solar corona in an emerging active region with
the focus on the structure of the forming coronal loops. The lower boundary of
this simulation is taken from a model of an emerging active region. As a
consequence of the emerging magnetic flux and the horizontal motions at the
surface a coronal loop forms self-consistently. We investigate the current
density along magnetic field inside (and outside) this loop and study the
magnetic and plasma properties in and around it. We find that the total current
along the loop changes its sign from being antiparallel to parallel to the
magnetic field. This is caused by the inclination of the loop together with the
footpoint motion. Around the loop the currents form a complex non-force-free
helical structure. This is directly related to a bipolar current structure at
the loop footpoints at the base of the corona and a local reduction of the
background magnetic field (i.e. outside the loop) caused by the plasma flow
into and along the loop. The locally reduced magnetic pressure in the loop
allows the loop to sustain a higher density, which is crucial for the emission
in extreme UV. The acting of the flow on the magnetic field hosting the loop
turns out to be also responsible for the observed squashing of the loop. The
complex magnetic field and current system surrounding it can be modeled only in
3D MHD models where the magnetic field has to balance the plasma pressure. A 1D
coronal loop model or a force-free extrapolation can not capture the current
system and the complex interaction of the plasma and the magnetic field in the
coronal loop, despite the fact that the loop is under low- conditions.Comment: 10 pages, 11 figures, published in A&
Earthquake recurrence as a record breaking process
Extending the central concept of recurrence times for a point process to
recurrent events in space-time allows us to characterize seismicity as a record
breaking process using only spatiotemporal relations among events. Linking
record breaking events with edges between nodes in a graph generates a complex
dynamical network isolated from any length, time or magnitude scales set by the
observer. For Southern California, the network of recurrences reveals new
statistical features of seismicity with robust scaling laws. The rupture length
and its scaling with magnitude emerges as a generic measure for distance
between recurrent events. Further, the relative separations for subsequent
records in space (or time) form a hierarchy with unexpected scaling properties
RLFC: Random Access Light Field Compression using Key Views and Bounded Integer Encoding
We present a new hierarchical compression scheme for encoding light field
images (LFI) that is suitable for interactive rendering. Our method (RLFC)
exploits redundancies in the light field images by constructing a tree
structure. The top level (root) of the tree captures the common high-level
details across the LFI, and other levels (children) of the tree capture
specific low-level details of the LFI. Our decompressing algorithm corresponds
to tree traversal operations and gathers the values stored at different levels
of the tree. Furthermore, we use bounded integer sequence encoding which
provides random access and fast hardware decoding for compressing the blocks of
children of the tree. We have evaluated our method for 4D two-plane
parameterized light fields. The compression rates vary from 0.08 - 2.5 bits per
pixel (bpp), resulting in compression ratios of around 200:1 to 20:1 for a PSNR
quality of 40 to 50 dB. The decompression times for decoding the blocks of LFI
are 1 - 3 microseconds per channel on an NVIDIA GTX-960 and we can render new
views with a resolution of 512X512 at 200 fps. Our overall scheme is simple to
implement and involves only bit manipulations and integer arithmetic
operations.Comment: Accepted for publication at Symposium on Interactive 3D Graphics and
Games (I3D '19
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