35,991 research outputs found
Silicon abundance from RESIK solar flare observations
The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and
active region X-ray spectra in four channels covering the wavelength range 3.8
-- 6.1 \AA in its operational period between 2001 and 2003. Several highly
ionized silicon lines were observed within the range of the long-wavelength
channel (5.00 -- 6.05 \AA). The fluxes of the \sixiv Ly- line (5.217
\AA) and the \sixiii line (5.688 \AA) during 21 flares with
optimized pulse-height analyzer settings on RESIK have been analyzed to obtain
the silicon abundance relative to hydrogen in flare plasmas. As in previous
work, the emitting plasma for each spectrum is assumed to be characterized by a
single temperature and emission measure given by the ratio of emission in the
two channels of GOES. The silicon abundance is determined to be (\sixiv) and (\sixiii) on a logarithmic scale with
H = 12. These values, which vary by only very small amounts from flare to flare
and times within flares, are and times the
photospheric abundance, and are about a factor of three higher than RESIK
measurements during a period of very low activity. There is a suggestion that
the Si/S abundance ratio increases from active regions to flares.Comment: To be published, Solar Physic
Protein search for multiple targets on DNA
Protein-DNA interactions are crucial for all biological processes. One of the
most important fundamental aspects of these interactions is the process of
protein searching and recognizing specific binding sites on DNA. A large number
of experimental and theoretical investigations have been devoted to uncovering
the molecular description of these phenomena, but many aspects of the
mechanisms of protein search for the targets on DNA remain not well understood.
One of the most intriguing problems is the role of multiple targets in protein
search dynamics. Using a recently developed theoretical framework we analyze
this question in detail. Our method is based on a discrete-state stochastic
approach that takes into account most relevant physical-chemical processes and
leads to fully analytical description of all dynamic properties. Specifically,
systems with two and three targets have been explicitly investigated. It is
found that multiple targets in most cases accelerate the search in comparison
with a single target situation. However, the acceleration is not always
proportional to the number of targets. Surprisingly, there are even situations
when it takes longer to find one of the multiple targets in comparison with the
single target. It depends on the spatial position of the targets, distances
between them, average scanning lengths of protein molecules on DNA, and the
total DNA lengths. Physical-chemical explanations of observed results are
presented. Our predictions are compared with experimental observations as well
as with results from a continuum theory for the protein search. Extensive Monte
Carlo computer simulations fully support our theoretical calculations
RESIK observations of He-like Ar X-ray line emission in solar flares
The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s,
p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal
spectrometer aboard CORONAS-F between 2001 and 2003. The three line features
include the Ar XVII w (resonance line), a blend of x and y (intercombination
lines), and z (forbidden line), all of which are blended with Ar XVI
dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies
with electron temperature Te mostly because of unresolved dielectronic
satellites. With temperatures estimated from GOES X-ray emission, the observed
G ratios agree fairly well with those calculated from CHIANTI and other data.
With a two-component emission measure, better agreement is achieved. Some S XV
and S XVI lines blend with the Ar lines, the effect of which occurs at
temperatures greater than 8MK, allowing the S/Ar abundance ratio to be
determined. This is found to agree with coronal values. A nonthermal
contribution is indicated for some spectra in the repeating-pulse flare of 2003
February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted,
June 2008
A solar spectroscopic absolute abundance of argon from RESIK
Observations of He-like and H-like Ar (Ar XVII and Ar XVIII) lines at 3.949
Angstroms and 3.733 Angstroms respectively with the RESIK X-ray spectrometer on
the CORONAS-F spacecraft, together with temperatures and emission measures from
the two channels of GOES, have been analyzed to obtain the abundance of Ar in
flare plasmas in the solar corona. The line fluxes per unit emission measure
show a temperature dependence like that predicted from theory, and lead to
spectroscopically determined values for the absolute Ar abundance, A(Ar) = 6.44
pm 0.07 (Ar XVII) and 6.49 pm 0.16 (Ar XVIII) which are in agreement to within
uncertainties. The weighted mean is 6.45 pm 0.06, which is between two recent
compilations of the solar Ar abundance and suggest that the photospheric and
coronal abundances of Ar are very similar.Comment: 4 figure
The Solar Flare Iron Abundance
The abundance of iron is measured from emission line complexes at 6.65 keV
(Fe line) and 8 keV (Fe/Ni line) in {\em RHESSI} X-ray spectra during solar
flares. Spectra during long-duration flares with steady declines were selected,
with an isothermal assumption and improved data analysis methods over previous
work. Two spectral fitting models give comparable results, viz. an iron
abundance that is lower than previous coronal values but higher than
photospheric values. In the preferred method, the estimated Fe abundance is
(on a logarithmic scale, with ),
or times the photospheric Fe abundance. Our estimate is based on
a detailed analysis of 1,898 spectra taken during 20 flares. No variation from
flare to flare is indicated. This argues for a fractionation mechanism similar
to quiet-Sun plasma. The new value of has important implications
for radiation loss curves, which are estimated.Comment: Accepted by Astrophysical Journa
Implementation of the 64-meter-diameter Antennas at the Deep Space Stations in Australia and Spain
The management and construction aspects of the Overseas 64-m Antenna Project in which two 64-m antennas were constructed at the Tidbinbilla Deep Space Communications Complex in Australia, and at the Madrid Deep Space Communications Complex in Spain are described. With the completion of these antennas the Deep Space Network is equipped with three 64-m antennas spaced around the world to maintain continuous coverage of spacecraft operations. These antennas provide approximately a 7-db gain over the capabilities of the existing 26-m antenna nets. The report outlines the project organization and management, resource utilization, fabrication, quality assurance, and construction methods by which the project was successfully completed. Major problems and their solutions are described as well as recommendations for future projects
Sequence Heterogeneity Accelerates Protein Search for Targets on DNA
The process of protein search for specific binding sites on DNA is
fundamentally important since it marks the beginning of all major biological
processes. We present a theoretical investigation that probes the role of DNA
sequence symmetry, heterogeneity and chemical composition in the protein search
dynamics. Using a discrete-state stochastic approach with a first-passage
events analysis, which takes into account the most relevant physical-chemical
processes, a full analytical description of the search dynamics is obtained. It
is found that, contrary to existing views, the protein search is generally
faster on DNA with more heterogeneous sequences. In addition, the search
dynamics might be affected by the chemical composition near the target site.
The physical origins of these phenomena are discussed. Our results suggest that
biological processes might be effectively regulated by modifying chemical
composition, symmetry and heterogeneity of a genome.Comment: 10 pages, 5 figure
Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium
The abundance of potassium is derived from X-ray lines observed during flares
by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57
A. The lines include those emitted by He-like K and Li-like K dielectronic
satellites, which have been synthesized using the CHIANTI atomic code and newly
calculated atomic data. There is good agreement of observed and synthesized
spectra, and the theoretical behavior of the spectra with varying temperature
estimated from the ratio of the two GOES channels is correctly predicted. The
observed fluxes of the He-like K resonance line per unit emission measure gives
log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9.
This is higher than photospheric abundance estimates by a factor 5.5, a
slightly greater enhancement than for other elements with first ionization
potential (FIP) less than about 10 eV. There is, then, the possibility that
enrichment of low-FIP elements in coronal plasmas depends weakly on the value
of the FIP which for K is extremely low (4.34 eV). Our work also suggests that
fractionation of elements to form the FIP effect occurs in the low chromosphere
rather than higher up, as in some models.Comment: 14 pages, 3 figure
- …