1,242 research outputs found
The spectral evolution of impulsive solar X-ray flares. II.Comparison of observations with models
We study the evolution of the spectral index and the normalization (flux) of
the non-thermal component of the electron spectra observed by RHESSI during 24
solar hard X-ray flares. The quantitative evolution is confronted with the
predictions of simple electron acceleration models featuring the soft-hard-soft
behaviour. The comparison is general in scope and can be applied to different
acceleration models, provided that they make predictions for the behavior of
the spectral index as a function of the normalization. A simple stochastic
acceleration model yields plausible best-fit model parameters for about 77% of
the 141 events consisting of rise and decay phases of individual hard X-ray
peaks. However, it implies unphysically high electron acceleration rates and
total energies for the others. Other simple acceleration models such as
constant rate of accelerated electrons or constant input power have a similar
failure rate. The peaks inconsistent with the simple acceleration models have
smaller variations in the spectral index. The cases compatible with a simple
stochastic model require typically a few times 10^36 electrons accelerated per
second at a threshold energy of 18 keV in the rise phases and 24 keV in the
decay phases of the flare peaks.Comment: 9 pages, 4 figures, accepted for publication by A&
The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra
Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms
of purely collisional electron beam propagation, ignoring spatial evolution and
collective effects. In this paper we present self-consistent numerical
simulations of the spatial and temporal evolution of an electron beam subject
to collisional transport and beam-driven Langmuir wave turbulence. These
wave-particle interactions represent the background plasma's response to the
electron beam propagating from the corona to chromosphere and occur on a far
faster timescale than coulomb collisions. From these simulations we derive the
mean electron flux spectrum, comparable to such spectra recovered from high
resolution hard X-rays observations of solar flares with RHESSI. We find that a
negative spectral index (i.e. a spectrum that increases with energy), or local
minima when including the expected thermal spectral component at low energies,
occurs in the standard thick-target model, when coulomb collisions are only
considered. The inclusion of wave-particle interactions does not produce a
local minimum, maintaining a positive spectral index. These simulations are a
step towards a more complete treatment of electron transport in solar flares
and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal
energies maybe a better approximation instead of a sharp cut-off in the
injected electron spectrum.Comment: 6 pages, 5 figures, accepted by ApJ
The CFEPS Kuiper Belt Survey: Strategy and Pre-survey Results
We present the data acquisition strategy and characterization procedures for
the Canada-France Ecliptic Plane Survey (CFEPS), a sub-component of the
Canada-France-Hawaii Telescope Legacy Survey. The survey began in early 2003
and as of summer 2005 has covered 430 square degrees of sky within a few
degrees of the ecliptic. Moving objects beyond the orbit of Uranus are detected
to a magnitude limit of =23 -- 24 (depending on the image quality). To
track as large a sample as possible and avoid introducing followup bias, we
have developed a multi-epoch observing strategy that is spread over several
years. We present the evolution of the uncertainties in ephemeris position and
orbital elements as the objects progress through the epochs. We then present a
small 10-object sample that was tracked in this manner as part of a preliminary
survey starting a year before the main CFEPS project.
We describe the CFEPS survey simulator, to be released in 2006, which allows
theoretical models of the Kuiper Belt to be compared with the survey
discoveries since CFEPS has a well-documented pointing history with
characterized detection efficiencies as a function of magnitude and rate of
motion on the sky. Using the pre-survey objects we illustrate the usage of the
simulator in modeling the classical Kuiper Belt.Comment: to be submitted to Icaru
Bianchi I Quantum cosmology in the Bergmann-Wagoner theory
The Wheeler-DeWitt equation is considered in the context of generalized
scalar-tensor theories of gravitation for Bianchi type I cosmology. Exact
solutions are found for two selfinteracting potentials and arbitary coupling
function. The WKB wavefunctions are obtained and a family of solutions
satisfying the Hawking-Page regularity conditions of wormholes are found.Comment: 12 pages, Latex fil
N=1 Supergravity Chaotic Inflation in the Braneworld Scenario
We study a N=1 Supergravity chaotic inflationary model, in the context of the
braneworld scenario. It is shown that successful inflation and reheating
consistent with phenomenological constraints can be achieved via the new terms
in the Friedmann equation arising from brane physics. Interestingly, the model
satisfies observational bounds with sub-Planckian field values, implying that
chaotic inflation on the brane is free from the well known difficulties
associated with the presence of higher order non-renormalizable terms in the
superpotential. A bound on the mass scale of the fifth dimension, M_5 \gsim
1.3 \times 10^{-6} M_P, is obtained from the requirement that the reheating
temperature be higher than the temperature of the electroweak phase transition.Comment: 5 pages, 1 Table, Revtex
Observations of Low Frequency Solar Radio Bursts from the Rosse Solar-Terrestrial Observatory
The Rosse Solar-Terrestrial Observatory (RSTO; www.rosseobservatory.ie) was
established at Birr Castle, Co. Offaly, Ireland (53 05'38.9", 7 55'12.7") in
2010 to study solar radio bursts and the response of the Earth's ionosphere and
geomagnetic field. To date, three Compound Astronomical Low-cost Low-frequency
Instrument for Spectroscopy and Transportable Observatory (CALLISTO)
spectrometers have been installed, with the capability of observing in the
frequency range 10-870 MHz. The receivers are fed simultaneously by biconical
and log-periodic antennas. Nominally, frequency spectra in the range 10-400 MHz
are obtained with 4 sweeps per second over 600 channels. Here, we describe the
RSTO solar radio spectrometer set-up, and present dynamic spectra of a sample
of Type II, III and IV radio bursts. In particular, we describe fine-scale
structure observed in Type II bursts, including band splitting and rapidly
varying herringbone features
The CORALIE survey for southern extra-solar planets XV. Discovery of two eccentric planets orbiting HD4113 and HD156846
We report the detection of two very eccentric planets orbiting HD4113 and
HD156846 with the CORALIE Echelle spectrograph mounted on the 1.2-m Euler Swiss
telescope at La Silla. The first planet, HD4113b, has minimum mass of
, a period of days and an
eccentricity of . It orbits a metal rich G5V star at
AU which displays an additional radial velocity drift of 28 m s/yr
observed during 8 years. The combination of the radial-velocity data and the
non-detection of any main sequence stellar companion in our high contrast
images taken at the VLT with NACO/SDI, characterizes the companion as a
probable brown dwarf or as a faint white dwarf. The second planet, \object{HD
156846 b}, has minimum mass of M, a period
of days, an eccentricity of and is located
at AU from its parent star. HD156846 is a metal rich G0 dwarf and is
also the primary of a wide binary system ( AU, years). Its
stellar companion, \object{IDS 17147-1914 B}, is a M4 dwarf. The very high
eccentricities of both planets can be explained by Kozai oscillations induced
by the presence of a third object.Comment: 4 pages, 5 figures, A&A Letter accepte
Non‐stationary control of the NAO on European rainfall and its implications for water resource management
Water resource forecasting generally centres on understanding hydrological variability over coming months or years, so that water managers can prepare for extremes such as droughts or floods (Chang & Guo, 2020; Hao et al., 2018). Some forecasting systems seek to project further into the future to allow long‐term planning of infrastructure and resilience to extremes and climate change (Svensson et al., 2015). These systems can rely directly or indirectly on outputs from Global Climate Models (GCMs; such as gridded reanalysis datasets) to forecast hydrological conditions (Bhatt & Mall, 2015; Ionita & Nagavciuc, 2020). In the North Atlantic region, in particular Western Europe, the North Atlantic Oscillation (NAO) is used as an indicator for hydrometeorological conditions given its leading control on winter rainfall totals (Hurrell & Deser, 2010; Scaife et al., 2008, 2014). A dipole of pressure anomalies over the North Atlantic, the NAO's positive phase (greater than average pressure gradient; NAO+) results in wetter conditions in northwest Europe with dryer conditions in southwest Europe (Rust et al., 2018; Trigo et al., 2004). Its negative phase (weaker than average pressure gradient; NAO−) results in the inverse effect on rainfall (Folland et al., 2015; and as shown by the correlation coefficients in Figure 1). Given this relationship, and, considering the role of winter rainfall variability in groundwater drought development (e.g., reduced winter recharge) and generation of late winter/early spring floods, the NAO offers a potential explanatory variable when understanding the behaviour of some hydrological extremes
Gravitational Microlensing Evidence for a Planet Orbiting a Binary Star System
The study of extra-solar planetary systems has emerged as a new discipline of
observational astronomy in the past few years with the discovery of a number of
extra-solar planets. The properties of most of these extra-solar planets were
not anticipated by theoretical work on the formation of planetary systems. Here
we report observations and light curve modeling of gravitational microlensing
event MACHO-97-BLG-41, which indicates that the lens system consists of a
planet orbiting a binary star system. According to this model, the mass ratio
of the binary star system is 3.8:1 and the stars are most likely to be a late K
dwarf and an M dwarf with a separation of about 1.8 AU. A planet of about 3
Jupiter masses orbits this system at a distance of about 7 AU. If our
interpretation of this light curve is correct, it represents the first
discovery of a planet orbiting a binary star system and the first detection of
a Jovian planet via the gravitational microlensing technique. It suggests that
giant planets may be common in short period binary star systems.Comment: 11 pages, with 1 color and 2 b/w Figures included (published version
The role of chaotic resonances in the solar system
Our understanding of the Solar System has been revolutionized over the past
decade by the finding that the orbits of the planets are inherently chaotic. In
extreme cases, chaotic motions can change the relative positions of the planets
around stars, and even eject a planet from a system. Moreover, the spin axis of
a planet-Earth's spin axis regulates our seasons-may evolve chaotically, with
adverse effects on the climates of otherwise biologically interesting planets.
Some of the recently discovered extrasolar planetary systems contain multiple
planets, and it is likely that some of these are chaotic as well.Comment: 28 pages, 9 figure
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