3,243 research outputs found
Conditions for electron-cyclotron maser emission in the solar corona
Context. The Sun is an active source of radio emission ranging from long
duration radio bursts associated with solar flares and coronal mass ejections
to more complex, short duration radio bursts such as solar S bursts, radio
spikes and fibre bursts. While plasma emission is thought to be the dominant
emission mechanism for most radio bursts, the electron-cyclotron maser (ECM)
mechanism may be responsible for more complex, short-duration bursts as well as
fine structures associated with long-duration bursts. Aims. We investigate the
conditions for ECM in the solar corona by considering the ratio of the electron
plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The
ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods.
Two-dimensional electron density, magnetic field, plasma frequency, and
electron cyclotron frequency maps of the off- limb corona were created using
observations from SDO/AIA and SOHO/LASCO, together with potential field
extrapolations of the magnetic field. These maps were then used to calculate
{\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results.
We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is
possible at heights < 1.07 R_sun in an active region near the limb; that is,
where magnetic field strengths are > 40 G and electron densities are greater
than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities
(> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region.
Conclusions. This demonstrates that the condition for ECM emission is satisfied
within areas of the corona containing large magnetic fields, such as the core
of a large active region. Therefore, ECM could be a possible emission mechanism
for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure
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
Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS
New calculations of radiative rates and electron impact excitation cross
sections for Fe XI are used to derive emission line intensity ratios involving
3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These
ratios are subsequently compared with observations of a solar active region,
obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph
(SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating
that enhanced the instrumental sensitivity for features in the 170 - 225 A
wavelength range, observed in second-order between 340 and 450 A. This
enhancement led to the detection of many emission lines not seen on previous
SERTS flights, which were measured with the highest spectral resolution (0.03
A) ever achieved for spatially resolved active region spectra in this
wavelength range. However, even at this high spectral resolution, several of
the Fe XI lines are found to be blended, although the sources of the blends are
identified in the majority of cases. The most useful Fe XI electron density
diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves
lines close in wavelength and free from blends, and which varies by a factor of
11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature
sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be
due to Fe XI, the first time (to our knowledge) this feature has been
identified in the solar spectrum. Similarly, there are new identifications of
the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are
blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, 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
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