8,394 research outputs found
A Constrained Tectonics Model for Coronal Heating
An analytical and numerical treatment is given of a constrained version of
the tectonics model developed by Priest, Heyvaerts, & Title [2002]. We begin
with an initial uniform magnetic field that is
line-tied at the surfaces and . This initial configuration is
twisted by photospheric footpoint motion that is assumed to depend on only one
coordinate () transverse to the initial magnetic field. The geometric
constraints imposed by our assumption precludes the occurrence of reconnection
and secondary instabilities, but enables us to follow for long times the
dissipation of energy due to the effects of resistivity and viscosity. In this
limit, we demonstrate that when the coherence time of random photospheric
footpoint motion is much smaller by several orders of magnitude compared with
the resistive diffusion time, the heating due to Ohmic and viscous dissipation
becomes independent of the resistivity of the plasma. Furthermore, we obtain
scaling relations that suggest that even if reconnection and/or secondary
instabilities were to limit the build-up of magnetic energy in such a model,
the overall heating rate will still be independent of the resistivity
B-L Cosmic strings and Baryogenesis
Cosmic strings arising from breaking of the gauge symmetry that
occurs in a wide variety of unified models can carry zero modes of heavy
Majorana neutrinos. Decaying and/or repeatedly self-interacting closed loops of
these ``'' cosmic strings can be a non-thermal source of heavy
right-handed Majorana neutrinos whose decay can contribute to the observed
baryon asymmetry of the Universe (BAU) via the leptogenesis route. The
cosmic strings are expected in GUT models such as SO(10), where they can be
formed at an intermediate stage of symmetry breaking well below the GUT scale
GeV; such light strings are not excluded by the CMB anisotropy
data and may well exist. We estimate the contribution of cosmic string
loops to the baryon-to-photon ratio of the Universe in the light of current
knowledge on neutrino masses and mixings implied by atmospheric and solar
neutrino measurements. We find that cosmic string loops can contribute
significantly to the BAU for symmetry breaking scale
\eta_{B-L}\gsim 1.7\times 10^{11}\gev. At the same time, in order for the
contribution of decaying cosmic string loops not to exceed the observed
baryon-to-photon ratio inferred from the recent WMAP results, the lightest
heavy right-handed Majorana neutrino mass must satisfy the constraint
M_1 \leq 2.4 \times 10^{12}(\eta_{B-L}/10^{13}\gev)^{1/2}\gev. This may have
interesting implications for the associated Yukawa couplings in the heavy
neutrino sector and consequently for the light neutrino masses generated
through see-saw mechanism.Comment: match with the published versio
TYPE II DNA: when the interfacial energy becomes negative
An important step in transcription of a DNA base sequence to a protein is the
initiation from the exact starting point, called promoter region. We propose a
physical mechanism for identification of the promoter region, which relies on a
new classification of DNAs into two types, Type-I and Type-II, like
superconductors, depending on the sign of the energy of the interface
separating the zipped and the unzipped phases. This is determined by the
energies of helical ordering and stretching over two independent length scales.
The negative interfacial energy in Type II DNA leads to domains of helically
ordered state separated by defect regions, or blobs, enclosed by the
interfaces. The defect blobs, pinned by non-coding promoter regions, would be
physically distinct from all other types of bubbles. We also show that the
order of the melting transition under a force is different for Type I and Type
II.Comment: 4 pages, 2 figures, Eq.(4) corrected in 4th versio
Dynamics of unbinding of polymers in a random medium
We have studied the aging effect on the dynamics of unbinding of a double
stranded directed polymer in a random medium. By using the Monte Carlo dynamics
of a lattice model in two dimensions, for which disorder is known to be
relevant, the unbinding dynamics is studied by allowing the bound polymer to
relax in the random medium for a waiting time and then allowing the two strands
to unbind. The subsequent dynamics is formulated in terms of the overlap of the
two strands and also the overlap of each polymer with the configuration at the
start of the unbinding process. The interrelations between the two and the
nature of the dependence on the waiting time are studied.Comment: 7 pages, latex, 3 figures, To appear in J. Chem. Phy
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