1,986 research outputs found
Instrumental oscillations in RHESSI count rates during solar flares
Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft
motion by studying persistent instrumental oscillations found in the
lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV
energy range during the decay phase of the flares of 2004 November 4 and 6.
Methods: The various motions of the RHESSI spacecraft which may contribute to
the manifestation of oscillations are studied. The response of each detector in
turn is also investigated. Results: We find that on 2004 November 6 the
observed oscillations correspond to the nutation period of the RHESSI
instrument. These oscillations are also of greatest amplitude for detector 5,
while in the lightcurves of many other detectors the oscillations are small or
undetectable. We also find that the variation in detector pointing is much
larger during this flare than the counterexample of 2004 November 4.
Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead
to clearly observable oscillations in count rates, posing a significant hazard
for data analysis. This issue is particularly problematic for detector 5 due to
its design characteristics. Dynamic correction of the RHESSI counts, accounting
for the livetime, data gaps, and the transmission of the bi-grid collimator of
each detector, is required to overcome this issue. These corrections should be
applied to all future oscillation studies.Comment: 8 pages, 10 figure
Automated identification of neurons and their locations
Individual locations of many neuronal cell bodies (>10^4) are needed to
enable statistically significant measurements of spatial organization within
the brain such as nearest-neighbor and microcolumnarity measurements. In this
paper, we introduce an Automated Neuron Recognition Algorithm (ANRA) which
obtains the (x,y) location of individual neurons within digitized images of
Nissl-stained, 30 micron thick, frozen sections of the cerebral cortex of the
Rhesus monkey. Identification of neurons within such Nissl-stained sections is
inherently difficult due to the variability in neuron staining, the overlap of
neurons, the presence of partial or damaged neurons at tissue surfaces, and the
presence of non-neuron objects, such as glial cells, blood vessels, and random
artifacts. To overcome these challenges and identify neurons, ANRA applies a
combination of image segmentation and machine learning. The steps involve
active contour segmentation to find outlines of potential neuron cell bodies
followed by artificial neural network training using the segmentation
properties (size, optical density, gyration, etc.) to distinguish between
neuron and non-neuron segmentations. ANRA positively identifies 86[5]% neurons
with 15[8]% error (mean[st.dev.]) on a wide range of Nissl-stained images,
whereas semi-automatic methods obtain 80[7]%/17[12]%. A further advantage of
ANRA is that it affords an unlimited increase in speed from semi-automatic
methods, and is computationally efficient, with the ability to recognize ~100
neurons per minute using a standard personal computer. ANRA is amenable to
analysis of huge photo-montages of Nissl-stained tissue, thereby opening the
door to fast, efficient and quantitative analysis of vast stores of archival
material that exist in laboratories and research collections around the world.Comment: 38 pages. Formatted for two-sided printing. Supplemental material and
software available at http://physics.bu.edu/~ainglis/ANRA
Isotopic evidence for iron mobility during subduction
Subduction zones are one of the most important sites of chemical interchange between the Earth's surface and interior. One means of explaining the high Fe3+/ΣFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SOX), carbonate (CO3 -), and/or iron (Fe3+) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (δ56Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R2 = 0.72) is observed between serpentinite bulk δ56Fe and Fe3+/ΣFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SOX or Fe(II)- Cl2 species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids. © 2016 Geological Society of America
Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops
We investigate the solar flare of 20 October 2002. The flare was accompanied
by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray
emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of
the HXR time profiles in different energy channels made with the Lomb
periodogram indicates two statistically significant time periods of about 16
and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR
emission in the impulsive phase of the flare. The 16-second QPP were more
pronounced in the thermal HXR emission and were observed both in the impulsive
and in the decay phases of the flare. Imaging analysis of the flare region, the
determined time periods of the QPP and the estimated physical parameters of
magnetic loops in the flare region allow us to interpret the observations as
follows. 1) In the impulsive phase energy was released and electrons were
accelerated by successive acts with the average time period of about 36 seconds
in different parts of two spatially separated, but interacting loop systems of
the flare region. 2) The 36-second periodicity of energy release could be
caused by the action of fast MHD oscillations in the loops connecting these
flaring sites. 3) During the first explosive acts of energy release the MHD
oscillations (most probably the sausage mode) with time period of 16 seconds
were excited in one system of the flare loops. 4) These oscillations were
maintained by the subsequent explosive acts of energy release in the impulsive
phase and were completely damped in the decay phase of the flare.Comment: 14 pages, 4 figure
Burkholderia pseudomallei traced to water treatment plant in Australia.
Burkholderia pseudomallei was isolated from environmental specimens 1 year after an outbreak of acute melioidosis in a remote coastal community in northwestern Australia. B. pseudomallei was isolated from a water storage tank and from spray formed in a pH-raising aerator unit. Pulsed-field gel electrophoresis confirmed the aerator and storage tank isolates were identical to the outbreak strain, WKo97
DMTPC: A dark matter detector with directional sensitivity
By correlating nuclear recoil directions with the Earth's direction of motion
through the Galaxy, a directional dark matter detector can unambiguously detect
Weakly Interacting Massive Particles (WIMPs), even in the presence of
backgrounds. Here, we describe the Dark Matter Time-Projection Chamber (DMTPC)
detector, a TPC filled with CF4 gas at low pressure (0.1 atm). Using this
detector, we have measured the vector direction (head-tail) of nuclear recoils
down to energies of 100 keV with an angular resolution of <15 degrees. To study
our detector backgrounds, we have operated in a basement laboratory on the MIT
campus for several months. We are currently building a new, high-radiopurity
detector for deployment underground at the Waste Isolation Pilot Plant facility
in New Mexico.Comment: 4 pages, 2 figures, proceedings for the CIPANP 2009 conference, May
26-31, 200
Energy Release from Impacting Prominence Material Following the 2011 June 7 Eruption
Solar filaments exhibit a range of eruptive-like dynamic activity, ranging from the full or partial eruption of the filament mass and surrounding magnetic structure as a coronal mass ejection to a fully confined or failed eruption. On 2011 June 7, a dramatic partial eruption of a filament was observed by multiple instruments on board the Solar Dynamics Observatory (SDO) and Solar-Terrestrial Relations Observatory. One of the interesting aspects of this event is the response of the solar atmosphere as non-escaping material falls inward under the influence of gravity. The impact sites show clear evidence of brightening in the observed extreme ultraviolet wavelengths due to energy release. Two plausible physical mechanisms for explaining the brightening are considered: heating of the plasma due to the kinetic energy of impacting material compressing the plasma, or reconnection between the magnetic field of low-lying loops and the field carried by the impacting material. By analyzing the emission of the brightenings in several SDO/Atmospheric Imaging Assembly wavelengths, and comparing the kinetic energy of the impacting material (7.6 10(exp 26) - 5.8 10(exp 27) erg) to the radiative energy (approx. 1.9 10(exp 25) - 2.5 10(exp 26) erg), we find the dominant mechanism of energy release involved in the observed brightening is plasma compression
The Variability of Seyfert 1.8 and 1.9 Galaxies at 1.6 microns
We present a study of Seyfert 1.5-2.0 galaxies observed at two epochs with
the Hubble Space Telescope (HST) at 1.6 microns. We find that unresolved
nuclear emission from 9 of 14 nuclei varies at the level of 10-40% on
timescales of 0.7-14 months, depending upon the galaxy. A control sample of
Seyfert galaxies lacking unresolved sources and galaxies lacking Seyfert nuclei
show less than 3% instrumental variation in equivalent aperture measurements.
This proves that the unresolved sources are non-stellar and associated with the
central pc of active galactic nuclei. Unresolved sources in Seyfert 1.8 and 1.9
galaxies are not usually detected in HST optical surveys, however high angular
resolution infrared observations will provide a way to measure time delays in
these galaxies.Comment: accepted by ApJLetters (emulateapj latex
Moments of Inertia of Nuclei in the Rare Earth Region: A Relativistic versus Non-Relativistic Investigation
A parameter free investigation of the moments of inertia of ground state
rotational bands in well deformed rare-earth nuclei is carried out using
Cranked Relativistic Hartree-Bogoliubov (CRHB) and non-relativistic Cranked
Hartree-Fock-Bogoliubov (CHFB) theories. In CRHB theory, the relativistic
fields are determined by the non-linear Lagrangian with the NL1 force and the
pairing interaction by the central part of finite range Gogny D1S force. In
CHFB theory, the properties in particle-hole and particle-particle channels are
defined solely by Gogny D1S forces. Using an approximate particle number
projection before variation by means of the Lipkin Nogami method improves the
agreement with the experimental data, especially in CRHB theory. The effect of
the particle number projection on the moments of inertia and pairing energies
is larger in relativistic than in non-relativistic theory.Comment: 18 pages + 2 PostScript figure
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