237 research outputs found
The Dynamic Formation of Prominence Condensations
We present simulations of a model for the formation of a prominence
condensation in a coronal loop. The key idea behind the model is that the
spatial localization of loop heating near the chromosphere leads to a
catastrophic cooling in the corona (Antiochos & Klimchuk 1991). Using a new
adaptive grid code, we simulate the complete growth of a condensation, and find
that after approx. 5,000 s it reaches a quasi-steady state. We show that the
size and the growth time of the condensation are in good agreement with data,
and discuss the implications of the model for coronal heating and SOHO/TRACE
observations.Comment: Astrophysical Journal latex file, 20 pages, 7 b-w figures (gif files
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
Within-population variability in a moth sex pheromone blend, part 2: selection towards fixation
Criteria for Flux Rope Eruption: Non Equilibrium versus Torus Instability
The coronal magnetic configuration of an active region typically evolves
quietly during few days before becoming suddenly eruptive and launching a
coronal mass ejection (CME). The precise origin of the eruption is still
debated. Among several mechanisms, it has been proposed that a loss of
equilibrium, or an ideal magneto-hydrodynamic (MHD) instability such as the
torus instability, could be responsible for the sudden eruptivity. Distinct
approaches have also been formulated for limit cases having circular or
translation symmetry. We revisit the previous theoretical approaches, setting
them in the same analytical framework. The coronal field results from the
contribution of a non-neutralized current channel added to a background
magnetic field, which in our model is the potential field generated by two
photospheric flux concentrations. The evolution on short Alfvenic time scale is
governed by ideal MHD. We show analytically first that the loss of equilibrium
and the stability analysis are two different views of the same physical
mechanism. Second, we identify that the same physics is involved in the
instability of circular and straight current channels. Indeed, they are just
two particular limiting case of more general current paths. A global
instability of the magnetic configuration is present when the current channel
is located at a coronal height, h, large enough so that the decay index of the
potential field, (d ln |Bp|) / (d ln h) is larger than a critical value. At the
limit of very thin current channels, previous analysis found a critical decay
index of 1.5 and 1 for circular and straight current channels, respectively.
However, with current channels being deformable and as thick as expected in the
corona, we show that this critical index has similar values for circular and
straight current channels, typically in the range [1.1,1.3].Comment: 12 pages, 4 figure
Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating
It has been established that cold plasma condensations can form in a magnetic
loop subject to localized heating of the footpoints. In this paper, we use
grid-adaptive numerical simulations of the radiative hydrodynamic equations to
parametrically investigate the filament formation process in a pre-shaped loop
with both steady and finite-time chromospheric heating. Compared to previous
works, we consider low-lying loops with shallow dips, and use a more realistic
description for the radiative losses. We demonstrate for the first time that
the onset of thermal instability satisfies the linear instability criterion.
The onset time of the condensation is roughly \sim 2 hr or more after the
localized heating at the footpoint is effective, and the growth rate of the
thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the
amplitude and the decay length scale characterizing this localized
chromospheric heating. We show how single or multiple condensation segments may
form in the coronal portion. In the asymmetric heating case, when two segments
form, they approach and coalesce, and the coalesced condensation later drains
down into the chromosphere. With a steady heating, this process repeats with a
periodicity of several hours. While our parametric survey confirms and augments
earlier findings, we also point out that steady heating is not necessary to
sustain the condensation. Once the condensation is formed, it can keep growing
also when the localized heating ceases. Finally, we show that the condensation
can survive continuous buffeting by perturbations resulting from the
photospheric p-mode waves.Comment: 43 pages, 18 figure
On the structure and evolution of a polar crown prominence/filament system
Polar crown prominences are made of chromospheric plasma partially circling
the Suns poles between 60 and 70 degree latitude. We aim to diagnose the 3D
dynamics of a polar crown prominence using high cadence EUV images from the
Solar Dynamics Observatory (SDO)/AIA at 304 and 171A and the Ahead spacecraft
of the Solar Terrestrial Relations Observatory (STEREO-A)/EUVI at 195A. Using
time series across specific structures we compare flows across the disk in 195A
with the prominence dynamics seen on the limb. The densest prominence material
forms vertical columns which are separated by many tens of Mm and connected by
dynamic bridges of plasma that are clearly visible in 304/171A two-color
images. We also observe intermittent but repetitious flows with velocity 15
km/s in the prominence that appear to be associated with EUV bright points on
the solar disk. The boundary between the prominence and the overlying cavity
appears as a sharp edge. We discuss the structure of the coronal cavity seen
both above and around the prominence. SDO/HMI and GONG magnetograms are used to
infer the underlying magnetic topology. The evolution and structure of the
prominence with respect to the magnetic field seems to agree with the filament
linkage model.Comment: 24 pages, 14 figures, Accepted for publication in Solar Physics
Journal, Movies can be found at http://www2.mps.mpg.de/data/outgoing/panesar
Changes in globus pallidus with (pre)term kernicterus
OBJECTIVE: We report serial magnetic resonance (MR) and sonographic
behavior of globus pallidus in 5 preterm and 3 term infants with
kernicterus and describe the clinical context in very low birth weight
preterm infants. On the basis of this information, we suggest means of
diagnosis and prevention. METHODS: Charts and MR and ultrasound images of
5 preterm infants and 3 term infants with suspected bilirubin-associated
brain damage were reviewed. Included were preterm infants with severe
hearing loss, quadriplegic hypertonia, and abnormal hypersignal of globus
pallidus on T2-weighted MR imaging (MRI). In 1 infant who died on day 150,
the diagnosis was confirmed during the neonatal period. The others were
picked up as outpatients and scanned at 12 or 22 months' corrected age.
Three instances of term kernicterus were included for comparison of serial
MRI in the neonatal period and early infancy: they were caused by
glucose-6-phosphate dehydrogenase deficiency, urosepsis, and dehydration
plus fructose 1-6 biphosphatase deficiency. RESULTS: Five preterm infants
of 25 to 29 weeks' gestational age presented with total serum bilirubin
(TSB) levels below exchange transfusion thresholds commonly advised. Mixed
acidosis was present in 3 infants around the TSB peak. The
bilirubin/albumin molar ratio was >0.5 in all, in the absence of
displacing drugs. All failed to pass bedside hearing screen tests and had
severe hearing loss on auditory brain response testing. Symmetrical
homogeneous hyperechogenicity of globus pallidus was the alerting feature
in 1 infant. Globus pallidus was hyperintense on T1-weighted MR images in
this child. The other infants presented with severe developmental delay as
a result of dyskinetic quadriplegia and hearing loss. Globus pallidus was
normal on T1- but hyperintense on T2-weighted MR images at 12
Electric current circuits in astrophysics
Cosmic magnetic structures have in common that they are anchored
in a dynamo, that an external driver converts kinetic energy into internal
magnetic energy, that this magnetic energy is transported as Poynting fl ux across the magnetically dominated structure, and that the magnetic energy
is released in the form of particle acceleration, heating, bulk motion,
MHD waves, and radiation. The investigation of the electric current system is
particularly illuminating as to the course of events and the physics involved.
We demonstrate this for the radio pulsar wind, the solar flare, and terrestrial
magnetic storms
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
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