615 research outputs found
Quasi-separatrix layers and three-dimensional reconnection diagnostics for line-tied tearing modes
In three-dimensional magnetic configurations for a plasma in which no closed
field line or magnetic null exists, no magnetic reconnection can occur, by the
strictest definition of reconnection. A finitely long pinch with line-tied
boundary conditions, in which all the magnetic field lines start at one end of
the system and proceed to the opposite end, is an example of such a system.
Nevertheless, for a long system of this type, the physical behavior in
resistive magnetohydrodynamics (MHD) essentially involves reconnection. This
has been explained in terms comparing the geometric and tearing widths [1, 2].
The concept of a quasi-separatrix layer[3, 4] was developed for such systems.
In this paper we study a model for a line-tied system in which the
corresponding periodic system has an unstable tearing mode. We analyze this
system in terms of two magnetic field line diagnostics, the squashing
factor[3-5] and the electrostatic potential difference used in kinematic
reconnection studies[6, 7]. We discuss the physical and geometric significance
of these two diagnostics and compare them in the context of discerning
tearing-like behavior in line-tied modes. [1] G. L. Delzanno and J. M. Finn.
Physics of Plasmas, 15(3):032904, 2008. [2] Y.-M. Huang and E. G. Zweibel.
Physics of Plasmas, 16(4):042102, 2009. [3] E. R. Priest and P. D\'emoulin. J.
Geophys. Res., 100(A12):23443-23463, 1995. [4] P. D\'emoulin, J. C. Henoux, E.
R. Priest, and C. H. Mandrini. Astron. Astrophys., 308:643-655, Apr. 1996. [5]
V. S. Titov and G. Hornig. Advances in Space Research, 29(7):1087-1092, 2002.
[6] Y. Lau and J. M. Finn. The Astrophysical Journal, 350:672-691, Feb. 1990.
[7] Y. Lau and J. M. Finn. The Astrophysical Journal, 366:577-591, 1991.Comment: 13 pages, 9 figures, Submitted to Commun Nonlinear Sci Numer Simula
Activation and repression of mammalian gene expression by the c-myc protein.
One mechanism by which nuclear-localized oncogenes might transform cells is through an ability to regulate gene expression. We show that the c-myc protein stimulates the level of appropriately initiated expression from the human heat shock protein 70 (hsp70) promoter. Sequences required for full activation lie upstream of the transcription initiation site and are distinct from sequences necessary for basal expression. These sequences also appear distinct from promoter sequences necessary for heat induction, serum induction, and induction by the papovavirus T antigens. The c-myc protein inhibits appropriately initiated expression from the mouse metallothionein I (MT-I) promoter. A mutation that removes 138 amino acids of exon 2 produces a c-myc gene product that is capable of activating the hsp70 promoter but is no longer capable of inhibiting MT-I expression, suggesting that these two properties reside in different domains of the c-myc protein. Expression from the adenovirus EII promoter is slightly inhibited, while expression from the SV40 early promoter is minimally affected by the c-myc protein. Both the spectrum of promoters regulated by the c-myc protein and the sequence requirements for that regulation differ from those of previously characterized viral trans-activating proteins. The data suggest that the c-myc protein can both stimulate and inhibit transcription from mammalian promoters in a novel manner
Approximate Solution of the effective mass Klein-Gordon Equation for the Hulthen Potential with any Angular Momentum
The radial part of the effective mass Klein-Gordon equation for the Hulthen
potential is solved by making an approximation to the centrifugal potential.
The Nikiforov-Uvarov method is used in the calculations. Energy spectra and the
corresponding eigenfunctions are computed. Results are also given for the case
of constant mass.Comment: 12 page
Emergence of Anti-Cancer Drug Resistance: Exploring the Importance of the Microenvironmental Niche via a Spatial Model
Practically, all chemotherapeutic agents lead to drug resistance. Clinically,
it is a challenge to determine whether resistance arises prior to, or as a
result of, cancer therapy. Further, a number of different intracellular and
microenvironmental factors have been correlated with the emergence of drug
resistance. With the goal of better understanding drug resistance and its
connection with the tumor microenvironment, we have developed a hybrid
discrete-continuous mathematical model. In this model, cancer cells described
through a particle-spring approach respond to dynamically changing oxygen and
DNA damaging drug concentrations described through partial differential
equations. We thoroughly explored the behavior of our self-calibrated model
under the following common conditions: a fixed layout of the vasculature, an
identical initial configuration of cancer cells, the same mechanism of drug
action, and one mechanism of cellular response to the drug. We considered one
set of simulations in which drug resistance existed prior to the start of
treatment, and another set in which drug resistance is acquired in response to
treatment. This allows us to compare how both kinds of resistance influence the
spatial and temporal dynamics of the developing tumor, and its clonal
diversity. We show that both pre-existing and acquired resistance can give rise
to three biologically distinct parameter regimes: successful tumor eradication,
reduced effectiveness of drug during the course of treatment (resistance), and
complete treatment failure
Constraining the primordial spectrum of metric perturbations from gravitino and moduli production
We consider the production of gravitinos and moduli fields from quantum
vacuum fluctuations induced by the presence of scalar metric perturbations at
the end of inflation. We obtain the corresponding occupation numbers, up to
first order in perturbation theory, in terms of the power spectrum of the
metric perturbations. We compute the limits imposed by nucleosynthesis on the
spectral index for different models with constant . The results show
that, in certain cases, such limits can be as strong as , which is
more stringent than those coming from primordial black hole production.Comment: 16 pages, LaTeX, 5 figures. Corrected figures, new references
included. Final version to appear in Phys. Rev.
Approximate solution of the Duffin-Kemmer-Petiau equation for a vector Yukawa potential with arbitrary total angular momenta
The usual approximation scheme is used to study the solution of the
Duffin-Kemmer-Petiau (DKP) equation for a vector Yukawa potential in the
framework of the parametric Nikiforov-Uvarov (NU) method. The approximate
energy eigenvalue equation and the corresponding wave function spinor
components are calculated for arbitrary total angular momentum in closed form.
Further, the approximate energy equation and wave function spinor components
are also given for case. A set of parameter values is used to obtain the
numerical values for the energy states with various values of quantum levelsComment: 17 pages; Communications in Theoretical Physics (2012). arXiv admin
note: substantial text overlap with arXiv:1205.0938, and with
arXiv:quant-ph/0410159 by other author
de Sitter Vacua in String Theory
We outline the construction of metastable de Sitter vacua of type IIB string
theory. Our starting point is highly warped IIB compactifications with
nontrivial NS and RR three-form fluxes. By incorporating known corrections to
the superpotential from Euclidean D-brane instantons or gaugino condensation,
one can make models with all moduli fixed, yielding a supersymmetric AdS
vacuum. Inclusion of a small number of anti-D3 branes in the resulting warped
geometry allows one to uplift the AdS minimum and make it a metastable de
Sitter ground state. The lifetime of our metastable de Sitter vacua is much
greater than the cosmological timescale of 10^10 years. We also prove, under
certain conditions, that the lifetime of dS space in string theory will always
be shorter than the recurrence time.Comment: 12 pages, 2 figs, added comments on the thin wall approximation to
tunnelin
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Flow characteristics of Newtonian and non-Newtonian fluids in a vessel stirred by a 60° pitched blade impeller
Mean and rms velocity characteristics of two Newtonian flows at Reynolds numbers of 12,800 (glycerin solution) and 48,000 (water) and of a non-Newtonian flow (0.2% CMC solution, at a power number similar to the Newtonian glycerin flow) in a mixing vessel stirred by a 60° pitched blade impeller have been measured by laser Doppler velocimetry (LDV). The velocity measurements, resolved over 360° and 1.08° of impeller rotation, showed that the mean flow of the two power number matched glycerin and CMC flows were similar to within 3% of the impeller tip velocity and the turbulence intensities generally lower in the CMC flow by up to 5% of the tip velocity. The calculated mean flow quantities showed similar discharge coefficient and pumping efficiency in all three flows and similar strain rate between the two power number matched glycerin and CMC flows; the strain rate of the higher Reynolds number Newtonian flow was found to be slightly higher. The energy balance around the impeller indicated that the CMC flow dissipated up to 9% more of the total input power and converted 7% less into the turbulence compared to the glycerin flow with the same power input which could lead to less effective mixing processes where the micro-mixing is important
Inflation and Preheating in NO models
We study inflationary models in which the effective potential of the inflaton
field does not have a minimum, but rather gradually decreases at large .
In such models the inflaton field does not oscillate after inflation, and its
effective mass becomes vanishingly small, so the standard theory of reheating
based on the decay of the oscillating inflaton field does not apply. For a long
time the only mechanism of reheating in such non-oscillatory (NO) models was
based on gravitational particle production in an expanding universe. This
mechanism is very inefficient. We will show that it may lead to cosmological
problems associated with large isocurvature fluctuations and overproduction of
dangerous relics such as gravitinos and moduli fields. We also note that the
setting of initial conditions for the stage of reheating in these models should
be reconsidered. All of these problems can be resolved in the context of the
recently proposed scenario of instant preheating if there exists an interaction
of the inflaton field with another scalar field
. We show that the mechanism of instant preheating in NO models is much
more efficient than the usual mechanism of gravitational particle production
even if the coupling constant is extremely small, .Comment: 10 pages, revte
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