146 research outputs found
Disk Formation by AGB Winds in Dipole Magnetic Fields
We present a simple, robust mechanism by which an isolated star can produce
an equatorial disk. The mechanism requires that the star have a simple dipole
magnetic field on the surface and an isotropic wind acceleration mechanism. The
wind couples to the field, stretching it until the field lines become mostly
radial and oppositely directed above and below the magnetic equator, as occurs
in the solar wind. The interaction between the wind plasma and magnetic field
near the star produces a steady outflow in which magnetic forces direct plasma
toward the equator, constructing a disk. In the context of a slow (10 km/s)
outflow (10^{-5} M_sun/yr) from an AGB star, MHD simulations demonstrate that a
dense equatorial disk will be produced for dipole field strengths of only a few
Gauss on the surface of the star. A disk formed by this model can be
dynamically important for the shaping of Planetary Nebulae.Comment: 14 pages, 8 figures, 1 table, accepted by Ap
A turbulence-driven model for heating and acceleration of the fast wind in coronal holes
A model is presented for generation of fast solar wind in coronal holes,
relying on heating that is dominated by turbulent dissipation of MHD
fluctuations transported upwards in the solar atmosphere. Scale-separated
transport equations include large-scale fields, transverse Alfvenic
fluctuations, and a small compressive dissipation due to parallel shears near
the transition region. The model accounts for proton temperature, density, wind
speed, and fluctuation amplitude as observed in remote sensing and in situ
satellite data.Comment: accepted for publication in ApJ
Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona
We present a detailed analysis of oxygen ion velocity distributions in the
extended solar corona, based on observations made with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. Polar coronal holes at
solar minimum are known to exhibit broad line widths and unusual intensity
ratios of the O VI 1032, 1037 emission line doublet. The traditional
interpretation of these features has been that oxygen ions have a strong
temperature anisotropy, with the temperature perpendicular to the magnetic
field being much larger than the temperature parallel to the field. However,
recent work by Raouafi and Solanki suggested that it may be possible to model
the observations using an isotropic velocity distribution. In this paper we
analyze an expanded data set to show that the original interpretation of an
anisotropic distribution is the only one that is fully consistent with the
observations. It is necessary to search the full range of ion plasma parameters
to determine the values with the highest probability of agreement with the UVCS
data. The derived ion outflow speeds and perpendicular kinetic temperatures are
consistent with earlier results, and there continues to be strong evidence for
preferential ion heating and acceleration with respect to hydrogen. At
heliocentric heights above 2.1 solar radii, every UVCS data point is more
consistent with an anisotropic distribution than with an isotropic
distribution. At heights above 3 solar radii, the exact probability of isotropy
depends on the electron density chosen to simulate the line-of-sight
distribution of O VI emissivity. (abridged abstract)Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 679; May
20, 2008
Properties of solar polar coronal plumes constrained by Ultraviolet Coronagraph Spectrometer data
We investigate the plasma dynamics (outflow speed and turbulence) inside
polar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by the
UVCS instrument on SOHO at the minimum of solar cycle 22-23 with model
calculations. We consider Maxwellian velocity distributions with different
widths in plume and inter-plume regions. Electron densities are assumed to be
enhanced in plumes and to approach inter-plume values with increasing height.
Different combinations of the outflow and turbulence velocity in the plume
regions are considered. We compute line profiles and total intensities of the
\ion{H}{1} Ly and the \ion{O}{6} doublets. The observed profile shapes
and intensities are reproduced best by a small solar wind speed at low
altitudes in plumes that increases with height to reach ambient inter-plume
values above roughly 3-4 R_\sun combined with a similar variation of the
width of the velocity distribution of the scattering atoms/ions. We also find
that plumes very close to the pole give narrow profiles at heights above 2.5
R_\sun, which are not observed. This suggests a tendency for plumes to be
located away from the pole. We find that the inclusion of plumes in the model
computations provides an improved correspondence with the observations and
confirms previous results showing that published UVCS observations in polar
coronal holes can be roughly reproduced without the need for large temperature
anisotropy. The latitude distributions of plumes and magnetic flux
distributions are studied by analyzing data from different instruments on SOHO
and with SOLIS.Comment: 11 figure
Propagation of an Earth-directed coronal mass ejection in three dimensions
Solar coronal mass ejections (CMEs) are the most significant drivers of
adverse space weather at Earth, but the physics governing their propagation
through the heliosphere is not well understood. While stereoscopic imaging of
CMEs with the Solar Terrestrial Relations Observatory (STEREO) has provided
some insight into their three-dimensional (3D) propagation, the mechanisms
governing their evolution remain unclear due to difficulties in reconstructing
their true 3D structure. Here we use a new elliptical tie-pointing technique to
reconstruct a full CME front in 3D, enabling us to quantify its deflected
trajectory from high latitudes along the ecliptic, and measure its increasing
angular width and propagation from 2-46 solar radii (approximately 0.2 AU).
Beyond 7 solar radii, we show that its motion is determined by an aerodynamic
drag in the solar wind and, using our reconstruction as input for a 3D
magnetohydrodynamic simulation, we determine an accurate arrival time at the
Lagrangian L1 point near Earth.Comment: 5 figures, 2 supplementary movie
First results on Martian carbon monoxide from Herschel/HIFI observations
We report on the initial analysis of Herschel/HIFI carbon monoxide (CO)
observations of the Martian atmosphere performed between 11 and 16 April 2010.
We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz
and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of
temperature and the mean volume mixing ratio of carbon monoxide. The derived
temperature profile agrees within less than 5 K with general circulation model
(GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower
values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in
agreement with the 900 ppm derived from Herschel/SPIRE observations in November
2009.Comment: Accepted for publication in Astronomy and Astrophysics (special issue
on HIFI first results); minor changes to match published versio
A Dust Cloud of Ganymede Maintained by Hypervelocity Impacts of Interplanetary Micrometeoroids
A dust cloud of Ganymede has been detected by in-situ measurements with the
dust detector onboard the Galileo spacecraft. The dust grains have been sensed
at altitudes below five Ganymede radii (Ganymede radius = ). Our
analysis identifies the particles in the dust cloud surrounding Ganymede by
their impact direction, impact velocity, and mass distribution and implies that
they have been kicked up by hypervelocity impacts of micrometeoroids onto the
satellite's surface. We calculate the radial density profile of the particles
ejected from the satellite by interplanetary dust grains. We assume the yields,
mass and velocity distributions of the ejecta obtained from laboratory impact
experiments onto icy targets and consider the dynamics of the ejected grains in
ballistic and escaping trajectories near Ganymede. The spatial dust density
profile calculated with interplanetary particles as impactors is consistent
with the profile derived from the Galileo measurements. The contribution of
interstellar grains as projectiles is negligible. Dust measurements in the
vicinities of satellites by spacecraft detectors are suggested as a beneficial
tool to obtain more knowledge about the satellite surfaces, as well as dusty
planetary rings maintained by satellites through the impact ejecta mechanism.Comment: 31 pages, 10 figures, accepted for Planetary and Space Scienc
Influence of fast interstellar gas flow on dynamics of dust grains
The orbital evolution of a dust particle under the action of a fast
interstellar gas flow is investigated. The secular time derivatives of
Keplerian orbital elements and the radial, transversal, and normal components
of the gas flow velocity vector at the pericentre of the particle's orbit are
derived. The secular time derivatives of the semi-major axis, eccentricity, and
of the radial, transversal, and normal components of the gas flow velocity
vector at the pericentre of the particle's orbit constitute a system of
equations that determines the evolution of the particle's orbit in space with
respect to the gas flow velocity vector. This system of differential equations
can be easily solved analytically. From the solution of the system we found the
evolution of the Keplerian orbital elements in the special case when the
orbital elements are determined with respect to a plane perpendicular to the
gas flow velocity vector. Transformation of the Keplerian orbital elements
determined for this special case into orbital elements determined with respect
to an arbitrary oriented plane is presented. The orbital elements of the dust
particle change periodically with a constant oscillation period or remain
constant. Planar, perpendicular and stationary solutions are discussed.
The applicability of this solution in the Solar system is also investigated.
We consider icy particles with radii from 1 to 10 micrometers. The presented
solution is valid for these particles in orbits with semi-major axes from 200
to 3000 AU and eccentricities smaller than 0.8, approximately. The oscillation
periods for these orbits range from 10^5 to 2 x 10^6 years, approximately.Comment: 22 pages, 3 figures; Accepted for publication in Celestial Mechanics
and Dynamical Astronom
Clostridium difficile infection in Polish pediatric outpatients with inflammatory bowel disease
The prevalence of Clostridium difficile infection (CDI) in pediatric patients with inflammatory bowel disease (IBD) is still not sufficiently recognized. We assessed the prevalence of CDI and recurrences in outpatients with IBD. In addition, the influence of IBD therapy on CDI and antimicrobial susceptibility of the potentially causative C. difficile strains was assessed. This was a prospective, single-center, observational study. All specimens were obtained between January 2005 and January 2007 from the IBD outpatient service and screened for C. difficile and its toxins. C. difficile isolates were genotyped by PCR ribotyping. Diagnosis of Crohn’s disease (CD) and ulcerative colitis (UC) was based on Porto criteria. Severity of disease was assessed using the Hyams scale (for Crohn’s disease) and the Truelove–Witts scale (for ulcerative colitis). One hundred and forty-three fecal samples from 58 pediatric IBD patients (21 with Crohn’s disease and 37 with ulcerative colitis) were screened. The risk of C. difficile infection was 60% and was independent of disease type (CD or UC) (χ2 = 2.5821, df = 3, p = 0.4606). About 17% of pediatric IBD patients experienced a recurrence of CDI. All C. difficile strains were susceptible to metronidazole, vancomycin and rifampin. A high prevalence of C. difficile infection and recurrences in pediatric outpatients with IBD was observed, independent of disease type. There was no significant correlation between C. difficile infection and IBD therapy. PCR ribotyping revealed C. difficile re-infection and relapses during episodes of IBD in pediatric outpatients
Germline MSH2 and MLH1 mutational spectrum in HNPCC families from Poland and the Baltic States.
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