628 research outputs found
RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003
We present data analysis and interpretation of a simple X-class flare
observed with RHESSI on November 3, 2003. In contrast to other X-class flares
observed previously, this flare shows a very simple morphology with well
defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward
motion of the LT source and increase in separation of the two FP sources are
consistent with magnetic reconnection models proposed for solar flares. In
addition, we find that the source motions are relatively slower during the more
active phases of hard X-ray emission; the emission centroid of the LT source
shifts toward higher altitudes with the increase of energy; the separation
between the LT emission centroids at two different photon energies is
anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic
fields could be a possible explanation of these features.Comment: To appear in the Astrophysical Journal Letters (12 pages, 4 figures
Exploring the geochemistry and shell ultrastructure of bivalves as archives of past ocean acidification events: recent Mytilidae in a natural acidic setting in Ischia (Italy, Mediterranean Sea)
Marine bivalve geochemistry and shell ultrastructure from modern low pH environments
Abstract. Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis (from the Mediterranean) and M. edulis (from the Wadden Sea) combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island of Ischia. The shells of transplanted mussels were compared with M. edulis collected at pH ~8.2 from Sylt (German Wadden Sea). Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.</jats:p
Positions and sizes of X-ray solar flare sources
<p><b>Aims:</b> The positions and source sizes of X-ray sources taking into account Compton backscattering (albedo) are investigated.</p>
<p><b>Methods:</b> Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source.</p>
<p><b>Results:</b> We show that the albedo effect can alter the true source positions and substantially increase the measured source sizes. The source positions are shifted by up to ~0.5” radially towards the disk centre and 5 arcsec source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. The X-ray sources therefore should have minimum observed sizes, and thus their FWHM source size (2.35 times second-moment) will be as large as ~7” in the 20-50 keV range for a disk-centered point source at a height of 1 Mm (~1.4”) above the photosphere. The source size and position change is greater for flatter primary X-ray spectra, a stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV.</p>
<p><b>Conclusions:</b> Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and we suggest that footpoint sources should be larger in X-rays than in either optical or EUV ranges.</p>
Fast Time Structure During Transient Microwave Brightenings: Evidence for Nonthermal Processes
Transient microwave brightenings (TMBs) are small-scale energy releases from
the periphery of sunspot umbrae, with a flux density two orders of magnitude
smaller than that from a typical flare. Gopalswamy et al (1994) first reported
the detection of the TMBs and it was pointed out that the radio emission
implied a region of very high magnetic field so that the emission mechanism has
to be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission.
It was not possible to decide between gyroresonance and gyrosynchrotron
processes because of the low time resolution (30 s) used in the data analysis.
We have since performed a detailed analysis of the Very Large Array data with
full time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can now
adequately address the question of the emission mechanism of the TMBs. We find
that nonthermal processes indeed take place during the TMBs. We present
evidence for nonthermal emission in the form of temporal and spatial structure
of the TMBs. The fast time structure cannot be explained by a thermodynamic
cooling time and therefore requires a nonthermal process. Using the physical
parameters obtained from X-ray and radio observations, we determine the
magnetic field parameters of the loop and estimate the energy released during
the TMBs. The impulsive components of TMBs imply an energy release rate of 1.3
x 10^22 erg/s so that the thermal energy content of the TMBs could be less than
10^24 erg.Comment: 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997
First principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2
We present an approach for the efficient calculation of vibrational Raman
intensities in periodic systems within density functional theory. The Raman
intensities are computed from the second order derivative of the electronic
density matrix with respect to a uniform electric field. In contrast to
previous approaches, the computational effort required by our method for the
evaluation of the intensities is negligible compared to that required for the
calculation of vibrational frequencies. As a first application, we study the
signature of 3- and 4-membered rings in the the Raman spectra of several
polymorphs of SiO2, including a zeolite having 102 atoms per unit cell.Comment: 4 pages, 2 figures, revtex4 Minor corrections; accepted in Phys. Rev.
Let
Dielectric Characterization of a Nonlinear Optical Material
Batisite was reported to be a nonlinear optical material showing second harmonic generation. Using dielectric spectroscopy and polarization measurements, we provide a thorough investigation of the dielectric and charge-transport properties of this material. Batisite shows the typical characteristics of a linear lossy dielectric. No evidence for ferro- or antiferroelectric polarization is found. As the second-harmonic generation observed in batisite points to a non-centrosymmetric structure, this material is piezoelectric, but most likely not ferroelectric. In addition, we found evidence for hopping charge transport of localized charge carriers and a relaxational process at low temperatures
Establishment and Initial Analysis of Deep Reef Stations (32-40 M) at the East Flower Garden Bank
Impact of anion polarizability on ion pairing in microhydrated salt clusters
Despite longstanding interest in the mechanism of salt dissolution in aqueous media, a molecular level understanding remains incomplete. Here, cryogenic ion trap vibrational action spectroscopy is combined with electronic structure calculations to track salt hydration in a gas phase model system one water molecule at a time. The infrared photodissociation spectra of microhydrated lithium dihalide anions [LiXX′(H2O)n]- (XX′ = I2, ClI and Cl2; n = 1–3) in the OH stretching region (3800–2800 cm-1) provide a detailed picture of how anion polarizability influences the competition among ion–ion, ion–water and water–water interactions. While exclusively contact ion pairs are observed for n = 1, the formation of solvent-shared ion pairs, identified by markedly red-shifted OH stretching bands (-1), originating from the bridging water molecules, is favored already for n = 2. For n = 3, Li+ reaches its maximum coordination number of four only in [LiI2(H2O)3]-, in accordance with the hard and soft Lewis acid and base principle. Water–water hydrogen bond formation leads to a different solvent-shared ion pair motif in [LiI2(H2O)3]- and network formation even restabilizes the contact ion pair motif in [LiCl2(H2O)3]-. Structural assignments are exclusively possible after the consideration of anharmonic effects. Molecular dynamics simulations confirm that the significance of large amplitude motion (of the water molecules) increases with increasing anion polarizability and that needs to be considered already at cryogenic temperatures
The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
The newly developed X-ray visibility forward fitting technique is applied to
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of a limb
flare to investigate the energy and height dependence on sizes, shapes, and
position of hard X-ray chromospheric footpoint sources. This provides
information about the electron transport and chromospheric density structure.
The spatial distribution of two footpoint X-ray sources is analyzed using
PIXON, Maximum Entropy Method, CLEAN and visibility forward fit algorithms at
nonthermal energies from to keV. We report, for the first
time, the vertical extents and widths of hard X-ray chromospheric sources
measured as a function of energy for a limb event. Our observations suggest
that both the vertical and horizontal sizes of footpoints are decreasing with
energy. Higher energy emission originates progressively deeper in the
chromosphere consistent with downward flare accelerated streaming electrons.
The ellipticity of the footpoints grows with energy from at keV to at keV. The positions of X-ray emission are in
agreement with an exponential density profile of scale height ~km.
The characteristic size of the hard X-ray footpoint source along the limb is
decreasing with energy suggesting a converging magnetic field in the footpoint.
The vertical sizes of X-ray sources are inconsistent with simple collisional
transport in a single density scale height but can be explained using a
multi-threaded density structure in the chromosphere.Comment: 7 pages, 7 figures, submitted to Ap
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