3,455 research outputs found
Gas-dynamic shock heating of post-flare loops due to retraction following localized, impulsive reconnection
We present a novel model in which shortening of a magnetic flux tube
following localized, three-dimensional reconnection generates strong
gas-dynamic shocks around its apex. The shortening releases magnetic energy by
progressing away from the reconnection site at the Alfven speed. This launches
inward flows along the field lines whose collision creates a pair of
gas-dynamic shocks. The shocks raise both the mass density and temperature
inside the newly shortened flux tube. Reconnecting field lines whose initial
directions differ by more that 100 degrees can produce a concentrated knot of
plasma hotter that 20 MK, consistent with observations. In spite of these high
temperatures, the shocks convert less than 10% of the liberated magnetic energy
into heat - the rest remains as kinetic energy of bulk motion. These
gas-dynamic shocks arise only when the reconnection is impulsive and localized
in all three dimensions; they are distinct from the slow magnetosonic shocks of
the Petschek steady-state reconnection model
On Flux Rope Stability and Atmospheric Stratification in Models of Coronal Mass Ejections Triggered by Flux Emergence
Flux emergence is widely recognized to play an important role in the
initiation of coronal mass ejections. The Chen-Shibata (2000) model, which
addresses the connection between emerging flux and flux rope eruptions, can be
implemented numerically to study how emerging flux through the photosphere can
impact the eruption of a pre-existing coronal flux rope. The model's
sensitivity to the initial conditions and reconnection micro-physics is
investigated with a parameter study. In particular, we aim to understand the
stability of the coronal flux rope in the context of X-point collapse and the
effects of boundary driving in both unstratified and stratified atmospheres. In
the absence of driving, we assess the behavior of waves in the vicinity of the
X-point. With boundary driving applied, we study the effects of reconnection
micro-physics and atmospheric stratification on the eruption. We find that the
Chen-Shibata equilibrium can be unstable to an X-point collapse even in the
absence of driving due to wave accumulation at the X-point. However, the
equilibrium can be stabilized by reducing the compressibility of the plasma,
which allows small-amplitude waves to pass through the X-point without
accumulation. Simulations with the photospheric boundary driving evaluate the
impact of reconnection micro-physics and atmospheric stratification on the
resulting dynamics: we show the evolution of the system to be determined
primarily by the structure of the global magnetic fields with little
sensitivity to the micro-physics of magnetic reconnection; and in a stratified
atmosphere, we identify a novel mechanism for producing quasi-periodic behavior
at the reconnection site behind a rising flux rope as a possible explanation of
similar phenomena observed in solar and stellar flares.Comment: Submitted Feb 28, 2014 to, accepted Aug 14, 2014 by Astronomy &
Astrophysics. 13 pages, 10 figures, 2 table
Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical Modeling
Numerical simulations of the helical () kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation enforces
reconnection between the legs of the rope if modes with two helical turns are
dominant as a result of high initial twist in the range . Such
reconnection is complex, involving also the ambient field. In addition to
breaking up the original rope, it can form a new, low-lying, less twisted flux
rope. The new flux rope is pushed downward by the reconnection outflow, which
typically forces it to break as well by reconnecting with the ambient field.
The top part of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights, producing
a "failed eruption." The helical current sheet associated with the instability
is squeezed between the approaching legs, temporarily forming a double current
sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently
strong electric fields that it would be able to accelerate particles. It may
also form plasmoids, or plasmoid-like structures, which trap energetic
particles and propagate out of the reconnection region up to the top of the
erupting flux rope along the helical current sheet. The kinking of a highly
twisted flux rope involving leg-leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which ejected a
relatively compact hard X-ray and microwave source and was associated with a
fast coronal mass ejection.Comment: Solar Physics, in pres
Tri-trophic transfer of zinc to newly emerged seven-spotted ladybirds (Coccinella septempunctata) from sewage sludge amended soil
Multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma
We present results from the first self-consistent multi-fluid simulations of
chromospheric magnetic reconnection in a weakly ionized reacting plasma. We
simulate two dimensional magnetic reconnection in a Harris current sheet with a
numerical model which includes ion-neutral scattering collisions, ionization,
recombination, optically thin radiative loss, collisional heating, and thermal
conduction. In the resulting tearing mode reconnection the neutral and ion
fluids become decoupled upstream from the reconnection site, creating an excess
of ions in the reconnection region and therefore an ionization imbalance. Ion
recombination in the reconnection region, combined with Alfv\'{e}nic outflows,
quickly removes ions from the reconnection site, leading to a fast reconnection
rate independent of Lundquist number. In addition to allowing fast
reconnection, we find that these non-equilibria partial ionization effects lead
to the onset of the nonlinear secondary tearing instability at lower values of
the Lundquist number than has been found in fully ionized plasmas.These
simulations provide evidence that magnetic reconnection in the chromosphere
could be responsible for jet-like transient phenomena such as spicules and
chromospheric jets.Comment: 8 Figures, 32 pages tota
Investigation of the wear resistance of high chromium white irons
High chromium white irons are commonly used throughout the mineral processing industry to handle erosive and erosivecorrosive slurries. These alloys are used in critical wear areas in the form of castings, or deposited as weld overlays onto steel substrates. The wear resistance of these alloys is due to their microstructure, which comprises hard rod like carbides dispersed in a matrix of austenite or martensite. The microstructure and carbide morphologies of these alloys can vary significantly depending on the chemical composition and the production process. This investigation uses a simple slurry pot test apparatus with slurries containing a mixed proportion of quartz particles to produce a mixed mode of impact and low angle erosion. A variety of different commercially produced castings and weld overlays ranging from low carbide volume fraction hypoeutectic alloys to hypereutectic alloys having a high carbide volume fraction were tested. The wear mechanism and hence the wear resistance of the alloys tested has been related to the matrix and the carbide morphologies.G.D. Nelson, G.L.F. Powell and V.M. Linto
A SLAM II simulation model for analyzing space station mission processing requirements
Space station mission processing is modeled via the SLAM 2 simulation language on an IBM 4381 mainframe and an IBM PC microcomputer with 620K RAM, two double-sided disk drives and an 8087 coprocessor chip. Using a time phased mission (payload) schedule and parameters associated with the mission, orbiter (space shuttle) and ground facility databases, estimates for ground facility utilization are computed. Simulation output associated with the science and applications database is used to assess alternative mission schedules
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