38 research outputs found
Aspects of the Mass Distribution of Interstellar Dust Grains in the Solar System from In-Situ Measurements
The in-situ detection of interstellar dust grains in the Solar System by the
dust instruments on-board the Ulysses and Galileo spacecraft as well as the
recent measurements of hyperbolic radar meteors give information on the
properties of the interstellar solid particle population in the solar vicinity.
Especially the distribution of grain masses is indicative of growth and
destruction mechanisms that govern the grain evolution in the interstellar
medium. The mass of an impacting dust grain is derived from its impact velocity
and the amount of plasma generated by the impact. Because the initial velocity
and the dynamics of interstellar particles in the Solar System are well known,
we use an approximated theoretical instead of the measured impact velocity to
derive the mass of interstellar grains from the Ulysses and Galileo in-situ
data. The revised mass distributions are steeper and thus contain less large
grains than the ones that use measured impact velocities, but large grains
still contribute significantly to the overall mass of the detected grains. The
flux of interstellar grains with masses is determined to
be . The comparison of radar data
with the extrapolation of the Ulysses and Galileo mass distribution indicates
that the very large () hyperbolic meteoroids detected by
the radar are not kinematically related to the interstellar dust population
detected by the spacecraft.Comment: 14 pages, 11 figures, to appear in JG
X-ray Halos and Large Grains in the Diffuse Interstellar Medium
Recent observations with dust detectors on board the interplanetary
spacecraft Ulysses and Galileo have recorded a substantial flux of large
interstellar grains with radii between 0.25 and 2.0 mu entering the solar
system from the local interstellar cloud. The most commonly used interstellar
grain size distribution is characterized by a a^-3.5 power law in grain radii
a, and extends to a maximum grain radius of 0.25 mu. The extension of the
interstellar grain size distribution to such large radii will have a major
effect on the median grain size, and on the amount of mass needed to be tied up
in dust for a given visual optical depth. It is therefore important to
investigate whether this population of larger dust particles prevails in the
general interstellar medium, or if it is merely a local phenomenon. The
presence of large interstellar grains can be mainly inferred from their effect
on the intensity and radial profiles of scattering halos around X-ray sources.
In this paper we examine the grain size distribution that gives rise to the
X-ray halo around Nova Cygni 1992. The results of our study confirm the need to
extend the interstellar grain size distribution in the direction of this source
to and possibly beyond 2.0 mu. The model that gives the best fit to the halo
data is characterized by: (1) a grain size distribution that follows an a^-3.5
power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0
mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044
and 0.0022, respectively, consistent with solar abundances constraints.
Additional observations of X-ray halos probing other spatial directions are
badly needed to test the general validity of this result.Comment: 17 pages, incl. 1 figure, accepted for publ. by ApJ Letter
In Situ Measurements of Interstellar Dust
We present the mass distribution of interstellar grains measured in situ by
the Galileo and Ulysses spaceprobes as cumulative flux. The derived in situ
mass distribution per logarithmic size interval is compared to the distribution
determined by fitting extinction measurements. Large grains measured in situ
contribute significantly to the overall mass of dust in the local interstellar
cloud. The problem of a dust-to-gas mass ratio that contradicts cosmic
abundances is discussed.Comment: 4 pages and two figure
Four years of Ulysses dust data: 1996 to 1999
The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (, perihelion distance 1.3 AU, aphelion distance 5.4 AU). Between
January 1996 and December 1999 the spacecraft was beyond 3 AU from the Sun and
crossed the ecliptic plane at aphelion in May 1998. In this four-year period
218 dust impacts were recorded with the dust detector on board. We publish and
analyse the complete data set of both raw and reduced data for particles with
masses to g. Together with 1477 dust impacts
recorded between launch of Ulysses and the end of 1995 published earlier
\cite{gruen1995c,krueger1999b}, a data set of 1695 dust impacts detected with
the Ulysses sensor between October 1990 and December 1999 is now available. The
impact rate measured between 1996 and 1999 was relatively constant with about
0.2 impacts per day. The impact direction of the majority of the impacts is
compatible with particles of interstellar origin, the rest are most likely
interplanetary particles. The observed impact rate is compared with a model for
the flux of interstellar dust particles. The flux of particles several
micrometers in size is compared with the measurements of the dust instruments
on board Pioneer 10 and Pioneer 11 beyond 3 AU (Humes 1980, JGR, 85,
5841--5852, 1980). Between 3 and 5 AU, Pioneer results predict that Ulysses
should have seen five times more ( sized) particles than
actually detected.Comment: accepted by Planetary and Space Science, 22 pages, 8 figures (1
colour figure
Interstellar Dust Inside and Outside the Heliosphere
In the early 1990s, after its Jupiter flyby, the Ulysses spacecraft
identified interstellar dust in the solar system. Since then the in-situ dust
detector on board Ulysses continuously monitored interstellar grains with
masses up to 10e-13 kg, penetrating deep into the solar system. While Ulysses
measured the interstellar dust stream at high ecliptic latitudes between 3 and
5 AU, interstellar impactors were also measured with the in-situ dust detectors
on board Cassini, Galileo and Helios, covering a heliocentric distance range
between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the
inner solar system is altered by the solar radiation pressure force,
gravitational focussing and interaction of charged grains with the time varying
interplanetary magnetic field. The grains act as tracers of the physical
conditions in the local interstellar cloud (LIC). Our in-situ measurements
imply the existence of a population of 'big' interstellar grains (up to 10e-13
kg) and a gas-to-dust-mass ratio in the LIC which is a factor of > 2 larger
than the one derived from astronomical observations, indicating a concentration
of interstellar dust in the very local interstellar medium. Until 2004, the
interstellar dust flow direction measured by Ulysses was close to the mean apex
of the Sun's motion through the LIC, while in 2005, the data showed a 30 deg
shift, the reason of which is presently unknown. We review the results from
spacecraft-based in-situ interstellar dust measurements in the solar system and
their implications for the physical and chemical state of the LIC.Comment: 10 pages, 2 b/w figures, 1 colour figure; submitted to Space Science
Review
OPTIMIST: A new conflict resolution algorithm for ACT-R.
Several studies have suggested recently that a more dynamic conflict resolution mechanism in the ACT-R cognitive architecture (Anderson & Lebiere, 1998) could improve the decision-making behaviour of cognitive models. This part of ACT-R theory is revisited and a new solution is proposed. The new algorithm (OPTIMIST) has been implemented as an overlay to the ACT-R architecture, and can be used as an alternative mechanism. The operation of the new algorithm is tested in a model of the classical Yerkes and Dodson experiement of animals' learning. When OPTIMIST is used, the resulting model fits the data better than the previous model (e.g. R2 (R squared) increases from .85 to .93 in one example)
Hand antisepsis without decreasing efficacy by shortening the rub-in time of alcohol-based handrubs to 15 seconds
BackgroundA previous study among neonatal intensive care unit (NICU) nurses showed that the antibacterial efficacy of alcohol-based handrubs (ABHR) can be achieved in 15 s instead of 30 s with a significant increase in the frequency of hand antisepsis. This study aimed to examine 15-s vs 30-s antisepsis performance by measuring microbial load on fingertips and compliance among nurses in a low-risk gynaecological ward.MethodsAn independent trained observer monitored the frequency and compliance with hand antisepsis during shifts in a crossover design. Fingertips including thumbs were rinsed in soy broth before hand rubbing at the beginning of a shift and then hourly to determine the bacterial load. Performance activity was assigned to the contamination class of the Fulkerson scale. Immediately before the lunch break, volunteers cleaned their hands for a randomly determined application time of 15 or 30 s.ResultsExamination of bacterial load on fingertips revealed no difference between 15 vs 30 s application time. Controlled hand antisepsis before the lunch break also showed no difference in efficacy for either test series. Participants rubbing for 15 s were more likely to perform hand antisepsis compared with those rubbing for 30 s ( P=0.2). The compliance increased from 54.7% to 69.5% in the 15-s trial.DiscussionShortening the duration for hand antisepsis did not decrease efficacy. Shortening the application time to 15 s should be considered within the critical components of a successful multimodal intervention strategy to improve hand-hygiene compliance in clinical practice
The Local Bubble, Local Fluff, and Heliosphere
The properties of the Local Bubble, Local Fluff complex of nearby
interstellar clouds, and the heliosphere are mutually constrained by data and
theory. Observations and models of the diffuse radiation field, interstellar
ionization, pick-up ion and anomalous cosmic-ray populations, and interstellar
dust link the physics of these regions. The differences between the
one-asymmetric-superbubble and two-superbubble views of the Local Bubble are
discussed.Comment: 10 pages, 2 figure
One year of Galileo dust data from the Jovian system: 1996
The dust detector system onboard Galileo records dust impacts in circumjovian
space since the spacecraft has been injected into a bound orbit about Jupiter
in December 1995. This is the sixth in a series of papers dedicated to
presenting Galileo and Ulysses dust data. We present data from the Galileo dust
instrument for the period January to December 1996 when the spacecraft
completed four orbits about Jupiter (G1, G2, C3 and E4). Data were obtained as
high resolution realtime science data or recorded data during a time period of
100 days, or via memory read-outs during the remaining times. Because the data
transmission rate of the spacecraft is very low, the complete data set (i. e.
all parameters measured by the instrument during impact of a dust particle) for
only 2% (5353) of all particles detected could be transmitted to Earth; the
other particles were only counted. Together with the data for 2883 particles
detected during Galileo's interplanetary cruise and published earlier, complete
data of 8236 particles detected by the Galileo dust instrument from 1989 to
1996 are now available. The majority of particles detected are tiny grains
(about 10 nm in radius) originating from Jupiter's innermost Galilean moon Io.
These grains have been detected throughout the Jovian system and the highest
impact rates exceeded . A small number of grains has been
detected in the close vicinity of the Galilean moons Europa, Ganymede and
Callisto which belong to impact-generated dust clouds formed by (mostly
submicrometer sized) ejecta from the surfaces of the moons (Kr\"uger et al.,
Nature, 399, 558, 1999). Impacts of submicrometer to micrometer sized grains
have been detected thoughout the Jovian system and especially in the region
between the Galilean moons.Comment: accepted for Planetary and Space Science, 33 pages, 6 tables, 10
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