983 research outputs found
High-spectral resolution solar microwave observations
The application of high-spectral resolution microwave observations to the study of solar activity is discussed with particular emphasis on the frequency dependence of microwave emission from solar active regions. A shell model of gyroresonance emission from active regions is described which suggest that high-spectral resolution, spatially-resolved observations can provide quantitative information about the magnetic field distribution at the base of the corona. Corresponding observations of a single sunspot with the Owens Valley frequency-agile interferometer at 56 frequencies between 1.2 and 14 Ghs are presented. The overall form of the observed size and brightness temperature spectra was consistent with expectations based on the shell model, although there were differences of potential physical significance. The merits and weaknesses of microwave spectroscopy as a technique for measuring magnetic fields in the solar corona are briefly discussed
Planned improvements to the Owens Valley frequency-agile interferometer
Three small antennas will be added to the OVRO interferometer to form a five-element solar-dedicated array. This would provide up to 7 or 10 baselines (compared to the present 1 or 3). This would be sufficient to apply microwave diagnostics to most active region and burst sources. By using frequency-synthesis it would also provide an imaging capability comparable to that of an approximately 100 baseline interferometer. Expansion of the array is discussed
Solar flare hard X-ray spikes observed by RHESSI: a case study
In this paper, we analyze hard X-ray spikes observed by RHESSI to understand
their temporal, spectral, and spatial properties. A recently developed
demodulation code was applied to hard X-ray light curves in several energy
bands observed by RHESSI. Hard X-ray spikes were selected from the demodulated
flare light curves. We measured the spike duration, the energy-dependent time
delay, and count spectral index of these spikes. We also located the hard X-ray
source emitting these spikes from RHESSI mapping that was coordinated with
imaging observations in visible and UV wavelengths. We identify quickly varying
structures of <1 s during the rise of hard X-rays in five flares. These hard
X-ray spikes can be observed at photon energies over 100 keV. They exhibit
sharp rise and decay with a duration (FWHM) of less than 1 s. Energy-dependent
time lags are present in some spikes. It is seen that the spikes exhibit harder
spectra than underlying components, typically by 0.5 in the spectral index when
they are fitted to power-law distributions. RHESSI clean maps at 25-100 keV
with an integration of 2 s centered on the peak of the spikes suggest that hard
X-ray spikes are primarily emitted by double foot-point sources in magnetic
fields of opposite polarities. With the RHESSI mapping resolution of ~ 4 arsec,
the hard X-ray spike maps do not exhibit detectable difference in the spatial
structure from sources emitting underlying components. Coordinated
high-resolution imaging UV and infrared observations confirm that hard X-ray
spikes are produced in magnetic structures embedded in the same magnetic
environment of the underlying components. The coordinated high-cadence TRACE UV
observations of one event possibly reveal new structures on spatial scales <1-2
arsec at the time of the spike superposed on the underlying component. They are
probably sources of hard X-ray spikes.Comment: 20 pages, 11 figure
Investigating the potential for call combinations in a lifelong vocal learner
The ability for humans to create seemingly infinite meaning from a finite set of sounds has likely been a critical component in our success as a species, allowing the unbounded communication of information. Syntax, the combining of meaningful sounds into phrases, is one of the primary features of language that enables this extensive expressivity. The evolutionary history of syntax, however, remains largely debated, and it is only very recently that comparative data for syntax in animals have been revealed. Here, we provide further evidence for a structural basis of potential syntacticâlike call combinations in the vocal communication system of a groupâliving songbird. Acoustic analyses indicate that Western Australian magpies (Gymnorhina tibicen dorsalis) structurally combine generic alarm calls with acoustically distinct alert calls to produce an alarm alert sequence. These results are distinct from previous examples of call combinations as, to our knowledge, evidence for this capacity is yet to be demonstrated in the natural communication of a nonâhuman species that is capable of vocal learning throughout life. These findings offer prospects for experimental investigation into the presence and function of magpie call combinations, extending our understanding of animal vocal complexity
The Solar X-ray Limb
We describe a new technique to measure the height of the X-ray limb with
observations from occulted X-ray flare sources as observed by the RHESSI (the
Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has
model dependencies different from those present in traditional observations at
optical wavelengths, which depend upon detailed modeling involving radiative
transfer in a medium with complicated geometry and flows. It thus provides an
independent and more rigorous measurement of the "true" solar radius, meaning
that of the mass distribution. RHESSI's measurement makes use of the flare
X-ray source's spatial Fourier components (the visibilities), which are
sensitive to the presence of the sharp edge at the lower boundary of the
occulted source. We have found a suitable flare event for analysis,
SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique
here. Using a 4-minute integration over the 3-25 keV photon energy range, we
find arcsec, at 1 AU, where
the uncertainties include statistical uncertainties from the method and a
systematic error. The standard VAL-C model predicts a value of 959.94 arcsec,
about 1 below our value.Comment: 12 pages, 5 figures, accepted for publication in Ap
On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy
We compare the photometric accuracy of spectra and images in flares observed
with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)}spacecraft. We
test the accuracy of the photometry by comparing the photon fluxes obtained in
different energy ranges from the spectral-fitting software SPEX with those
fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis,
Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the
fidelity of source geometries, and spatial spectra reconstructed with the five
image reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field-of-view, and time intervals on the quality of image
reconstruction. The detailed parameters and statistics are provided in an
accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean
have a robust convergence behavior and a photometric accuracy in the order of a
few percents, while MEM does not converge optimally for large degrees of
freedom (for large field-of-views and/or small pixel sizes), and MEM-Vis
suffers in the case of time-variable sources. This comparative study documents
the current status of the RHESSI spectral and imaging software, one year after
launch.Comment: 2 Figures, full version on
http://www.lmsal.com/~aschwand/eprints/2003_photo/index.htm
A new method of observing weak extended x-ray sources with RHESSI
We present a new method, fan-beam modulation, for observing weak extended
x-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI). This space-based solar x-ray and gamma-ray telescope has much greater
sensitivity than previous experiments in the 3-25 keV range, but is normally
not well suited to detecting extended sources since their signal is not
modulated by RHESSI's rotating grids. When the spacecraft is offpointed from
the target source, however, the fan-beam modulation time-modulates the
transmission by shadowing resulting from exploiting the finite thickness of the
grids. In this paper we detail how the technique is implemented and verify its
consistency with sources with clear known signals that have occurred during
RHESSI offpointing: microflares and the Crab Nebula. In both cases the results
are consistent with previous and complementary measurements. Preliminary work
indicates that this new technique allows RHESSI to observe the integrated hard
x-ray spectrum of weak extended sources on the quiet Sun.Comment: Publishe
Resolution of the age structure of the detrital zircon populations of two Lower Cretaceous sandstones from the Weald of England by fission track dating
Modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies, supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography. A computer program has been written and used to identify modes in the zircon fission track age distribution within two Lower Cretaceous sandstone samples from the Weald of southern England. Pronounced modes appear in one rock around 119 Ma, 160 Ma, 243 Ma and 309 Ma and in the other around 141 Ma, 175 Ma, 257 to 277 Ma and 394 to 453 Ma. The geological implications of these quite dissimilar zircon age spectra are discussed. It is concluded that they support the palaeogeographical models of Allen (1981) and indicate that the provenance of the first sample, from the Top Ashdown Sandstone member at Dallington in East Sussex, was almost entirely southerly, while that of the second, from the Netherside Sand member at Northchapel in West Sussex, was more varied, but predominantly westerly and northerl
Tidal Control of Jet Eruptions Observed by Cassini ISS
Observations by Cassini's Imaging Science Subsystem (ISS) of Enceladus' south polar region at high phase angles has revealed jets of material venting into space. Observations by Cassini's Composite Infrared Spectrometer (CIRS) have also shown that the south polar region is anomalously warm with hotspots associated with geological features called the Tiger Stripes. The Tiger Stripes are large rifts near the south pole of Enceladus, which are typically about 130 km in length, 2 km wide, with a trough 500 m deep, and are l1anked on each side by 100m tall ridges. Preliminary triangulation of jets as viewed at different times and with different viewing geometries in Cassini ISS images taken between 2005 and 2007 have constrained the locations of eight major eruptions of material and found all of them associated with the south polar fractures unofficially the 'Tiger Stripes', and found four of them coincident with the hotspots reported in 2006 by CIRS. While published ISS observations of jet activity suggest that individual eruption sites stay active on the timescale of years, any shorter temporal variability (on timescales of an orbital period, or 1.3 Earth days, for example) is more difficult to establish because of the spotty temporal coverage and the difficulty of visually isolating one jet from the forest of many seen in a typical image. Consequently, it is not known whether individual jets are continuously active, randomly active, or if they erupt on a predictable, periodic schedule. One mechanism that may control the timing of eruptions is diurnal tidal stress, which oscillates between compression/tension as well as right and left lateral shear at any given location throughout Enceladus' orbit and may allow the cracks to open and close regularly. We examine the stresses on the Tiger Stripe regions to see how well diurnal tidal stress caused by Enceladus' orbital eccentricity may possibly correlate with and thus control the observed eruptions. We then identify possible mechanisms by which tidal stress can provide access to the surface for volatile material and implications for observed jet activity
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