1,371 research outputs found
A comparative study of the nonuniqueness problem of the potential equation
The nonuniqueness problem occurring at transonic speeds with the conservative potential equation is investigated numerically. The study indicates that the problem is not an inviscid phenomenon, but results from approximate treatment of shock waves inherent in the conservative potential model. A new bound on the limit of validity of the conservative potential model is proposed
Oscillation of solar radio emission at coronal acoustic cut-off frequency
Recent SECCHI COR2 observations on board STEREO-A spacecraft have detected
density structures at a distance of 2.5--15~R propagating with periodicity of
about 90~minutes. The observations show that the density structures probably
formed in the lower corona. We used the large Ukrainian radio telescope URAN-2
to observe type IV radio bursts in the frequency range of 8--32~MHz during the
time interval of 08:15--11:00~UT on August 1, 2011. Radio emission in this
frequency range originated at the distance of 1.5--2.5 R according to the
Baumbach-Allen density model of the solar corona. Morlet wavelet analysis
showed the periodicity of 80~min in radio emission intensity at all
frequencies, which demonstrates that there are quasi-periodic variations of
coronal density at all heights. The observed periodicity corresponds to the
acoustic cut-off frequency of stratified corona at a temperature of 1~MK. We
suggest that continuous perturbations of the coronal base in the form of
jets/explosive events generate acoustic pulses, which propagate upwards and
leave the wake behind oscillating at the coronal cut-off frequency. This wake
may transform into recurrent shocks due to the density decrease with height,
which leads to the observed periodicity in the radio emission. The recurrent
shocks may trigger quasi-periodic magnetic reconnection in helmet streamers,
where the opposite field lines merge and consequently may generate periodic
density structures observed in the solar wind.Comment: 10 pages, 6 figures, accepted in A&
Coarse Graining RNA Nanostructures for Molecular Dynamics Simulations
A series of coarse-grained models have been developed for the study of the
molecular dynamics of RNA nanostructures. The models in the series have one to
three beads per nucleotide and include different amounts of detailed structural
information. Such a treatment allows us to reach, for the systems of thousands
of nucleotides, a time scale of microseconds (i.e. by three orders of magnitude
longer than in the full atomistic modelling) and thus to enable simulations of
large RNA polymers in the context of bionanotechnology. We find that the
3-beads-per-nucleotide models, described by a set of just a few universal
parameters, are able to describe different RNA conformations and are comparable
in structural precision to the models where detailed values of the backbone
P-C4' dihedrals taken from a reference structure are included. These findings
are discussed in the context of the RNA conformation classes
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