2,604 research outputs found
An investigation of Fe XV emission lines in solar flare spectra
Previously, large discrepancies have been found between theory and
observation for Fe XV emission line ratios in solar flare spectra covering the
224-327 A wavelength range, obtained by the Naval Research Laboratory's S082A
instrument on board Skylab. These discrepancies have been attributed to either
errors in the adopted atomic data or the presence of additional atomic
processes not included in the modelling, such as fluorescence. However our
analysis of these plus other S082A flare observations (the latter containing Fe
XV transitions between 321-482 A), performed using the most recent Fe XV atomic
physics calculations in conjunction with a CHIANTI synthetic flare spectrum,
indicate that blending of the lines is primarily responsible for the
discrepancies. As a result, most Fe XV lines cannot be employed as electron
density diagnostics for solar flares, at least at the spectral resolution of
S082A and similar instruments (i.e. ~ 0.1 A). An exception is the intensity
ratio I(321.8 A)/I(327.0 A), which appears to provide good estimates of the
electron density at this spectral resolution.Comment: 6 pages, 3 figures, Astronomy & Astrophysics, in pres
RHESSI and SOHO/CDS Observations of Explosive Chromospheric Evaporation
Simultaneous observations of explosive chromospheric evaporation are
presented using data from the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) onboard SOHO. For
the first time, co-spatial imaging and spectroscopy have been used to observe
explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images
and spectra were used to determine the flux of non-thermal electrons
accelerated during the impulsive phase of an M2.2 flare. Assuming a
thick-target model, the injected electron spectrum was found to have a spectral
index of ~7.3, a low energy cut-off of ~20 keV, and a resulting flux of
>4x10^10 ergs cm^-2 s^-1. The dynamic response of the atmosphere was determined
using CDS spectra, finding a mean upflow velocity of 230+/-38 km s^-1 in Fe XIX
(592.23A), and associated downflows of 36+/-16 km s^-1 and 43+/-22 km s^-1 at
chromospheric and transition region temperatures, respectively, relative to an
averaged quiet-Sun spectra. The errors represent a 1 sigma dispersion. The
properties of the accelerated electron spectrum and the corresponding
evaporative velocities were found to be consistent with the predictions of
theory.Comment: 5 pages, 4 figures, ApJL (In Press
Convergence and segregation of the multiple rod pathways in mammalian retina.
Using a multidisciplinary approach, we demonstrate that three different pathways are responsible for the transmission of rod signals across the mouse retina. Each pathway serves a primarily nonoverlapping range of stimulus intensities, with ganglion cells receiving either segregated or convergent inputs. For both on-center (ON) and off-center (OFF) ganglion cells, the primary rod pathway carries signals with the lowest threshold, whereas the secondary rod pathway is less sensitive by approximately 1 log unit. In addition, OFF signaling uses a tertiary rod pathway that is approximately 1 log unit less sensitive than the secondary. Although some ganglion cells received rod inputs exclusively from one of the pathways, others showed convergent inputs. Using pharmacological and genetic approaches, we defined classes of ON and OFF ganglion cells for which the scotopic inputs derive only from the primary pathway or from both primary and secondary pathways. In addition, we observed a class of OFF ganglion cell receiving mixed input from primary and tertiary pathways. Interestingly, OFF ganglion cells receiving convergent inputs from all three rod pathways or from the secondary and tertiary pathways together were never observed. Overall, our data show a complex arrangement of convergence and segregation of rod inputs to ganglion cells in the mammalian retina
Magnetic structure of free cobalt clusters studied with Stern-Gerlach deflection experiments
We have studied the magnetic properties of free cobalt clusters in two
semi-independent Stern-Gerlach deflection experiments at temperatures between
60 and 307 K. We find that clusters consisting of 13 to 200 cobalt atoms
exhibit behavior that is entirely consistent with superparamagnetism, though
complicated by finite-system fluctuations in cluster temperature. By fitting
the data to the Langevin function, we report magnetic moments per atom for each
cobalt cluster size and compare the results of our two measurements and all
those performed previously. In addition to a gradual decrease in moment per
atom with increasing size, there are oscillations that appear to be caused by
geometrical shell structure. We discuss our observations in light of the two
competing models for Langevin-like magnetization behavior in free clusters,
superparamagnetism and adiabatic magnetization, and conclude that the evidence
strongly supports the superparamagnetic model
Inferring DNA sequences from mechanical unzipping data: the large-bandwidth case
The complementary strands of DNA molecules can be separated when stretched
apart by a force; the unzipping signal is correlated to the base content of the
sequence but is affected by thermal and instrumental noise. We consider here
the ideal case where opening events are known to a very good time resolution
(very large bandwidth), and study how the sequence can be reconstructed from
the unzipping data. Our approach relies on the use of statistical Bayesian
inference and of Viterbi decoding algorithm. Performances are studied
numerically on Monte Carlo generated data, and analytically. We show how
multiple unzippings of the same molecule may be exploited to improve the
quality of the prediction, and calculate analytically the number of required
unzippings as a function of the bandwidth, the sequence content, the elasticity
parameters of the unzipped strands
Trends in total column ozone measurements
It is important to ensure the best available data are used in any determination of possible trends in total ozone in order to have the most accurate estimates of any trends and the associated uncertainties. Accordingly, the existing total ozone records were examined in considerable detail. Once the best data set has been produced, the statistical analysis must examine the data for any effects that might indicate changes in the behavior of global total ozone. The changes at any individual measuring station could be local in nature, and herein, particular attention was paid to the seasonal and latitudinal variations of total ozone, because two dimensional photochemical models indicate that any changes in total ozone would be most pronounced at high latitudes during the winter months. The conclusions derived from this detailed examination of available total ozone can be split into two categories, one concerning the quality and the other the statistical analysis of the total ozone record
The Influence of Magnetic Field on Oscillations in the Solar Chromosphere
Two sequences of solar images obtained by the Transition Region and Coronal
Explorer in three UV passbands are studied using wavelet and Fourier analysis
and compared to the photospheric magnetic flux measured by the Michelson
Doppler Interferometer on the Solar Heliospheric Observatory to study wave
behaviour in differing magnetic environments. Wavelet periods show deviations
from the theoretical cutoff value and are interpreted in terms of inclined
fields. The variation of wave speeds indicates that a transition from dominant
fast-magnetoacoustic waves to slow modes is observed when moving from network
into plage and umbrae. This implies preferential transmission of slow modes
into the upper atmosphere, where they may lead to heating or be detected in
coronal loops and plumes.Comment: 8 pages, 6 figures (4 colour online only), accepted for publication
in The Astrophysical Journa
Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella
Recent calculations of atomic data for Fe XV have been used to generate
theoretical line ratios involving n = 3-4 transitions in the soft X-ray
spectral region (52-83 A), for a wide range of electron temperatures and
densities applicable to solar and stellar coronal plasmas. A comparison of
these with solar flare observations from a rocket-borne spectrograph (XSST)
reveals generally good agreement between theory and experiment. In particular,
the 82.76 A emission line in the XSST spectrum is identified, for the first
time to our knowledge in an astrophysical source. Most of the Fe XV transitions
which are blended have had the species responsible clearly identified, although
there remain a few instances where this has not been possible. The line ratio
calculations are also compared with a co-added spectrum of Capella obtained
with the Chandra satellite, which is probably the highest signal-to-noise
observation achieved for a stellar source in the 25-175 A soft X-ray region.
Good agreement is found between theory and experiment, indicating that the Fe
XV lines are reliably detected in Chandra spectra, and hence may be employed as
diagnostics to determine the temperature and/or density of the emitting plasma.
However the line blending in the Chandra data is such that individual emission
lines are difficult to measure accurately, and fluxes may only be reliably
determined via detailed profile fitting of the observations. The co-added
Capella spectrum is made available to hopefully encourage further exploration
of the soft X-ray region in astronomical sources.Comment: 27 pages, 10 figures, Astrophysical Journal, in pres
Plasma Diagnostics of Active Region Evolution and Implications for Coronal Heating
A detailed study is presented of the decaying solar active region NOAA 10103
observed with the Coronal Diagnostic Spectrometer (CDS), the Michelson Doppler
Imager (MDI) and the Extreme-ultraviolet Imaging Telescope (EIT) onboard the
Solar and Heliospheric Observatory (SOHO). Electron density maps formed using
Si X (356.03A/347.41A) show that the density varies from ~10^10 cm^-3 in the
active region core, to ~7x10^8 cm^-3 at the region boundaries. Over the five
days of observations, the average electron density fell by ~30%. Temperature
maps formed using Fe XVI(335.41A)/Fe XIV(334.18A) show electron temperatures of
\~2.34x10^6 K in the active region core, and ~2.10x10^6 K at the region
boundaries. Similarly to the electron density, there was a small decrease in
the average electron temperature over the five day period. The radiative,
conductive, and mass flow losses were calculated and used to determine the
resultant heating rate (P_H). Radiative losses were found to dominate the
active region cooling process. As the region decayed, the heating rate
decreased by almost a factor of five between the first and last day of
observations. The heating rate was then compared to the total unsigned magnetic
flux (Phi_tot), yielding a power-law of the form P_H ~ Phi_tot^(0.81 +/- 0.32).
This result suggests that waves rather than nanoflares may be the dominant
heating mechanism in this active region.Comment: 9 pages, 11 figures. MNRAS, In Pres
Dielectric relaxation of DNA aqueous solutions
We report on a detailed characterization of complex dielectric response of
Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40
Hz - 110 MHz). Results reveal two broad relaxation modes of strength
20<\Delta\epsilon_LF<100 and 5<\Delta\epsilon_HF<20, centered at 0.5
kHz<\nu_LF<70 kHz and 0.1 MHz<\nu_HF<15 MHz. The characteristic length scale of
the LF process, 50<L_LF<750nm, scales with DNA concentration as
c_DNA^{-0.29\pm0.04} and is independent of the ionic strength in the low added
salt regime. Conversely, the measured length scale of the LF process does not
vary with DNA concentration but depends on the ionic strength of the added salt
as I_s^{-1} in the high added salt regime. On the other hand, the
characteristic length scale of the HF process, 3<L_HF<50 nm, varyes with DNA
concentration as c_DNA^{-0.5} for intermediate and large DNA concentrations. At
low DNA concentrations and in the low added salt limit the characteristic
length scale of the HF process scales as c_DNA^{-0.33}. We put these results in
perspective regarding the integrity of the double stranded form of DNA at low
salt conditions as well as regarding the role of different types of counterions
in different regimes of dielectric dispersion. We argue that the free DNA
counterions are primarily active in the HF relaxation, while the condensed
counterions play a role only in the LF relaxation. We also suggest theoretical
interpretations for all these length scales in the whole regime of DNA and salt
concentrations and discuss their ramifications and limitations.Comment: 15 pages, 9 figure
- …