1,217 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
Alpha Clustering and the stellar nucleosynthesis of carbon
The astrophysical S--factor and reaction rates for the triple--alpha process
are calculated in the direct--capture model. It is shown that the stellar
carbon production is extremely sensitive to small variations in the N--N
interaction.Comment: 2 pages LaTe
Galileo dust data from the jovian system: 2000 to 2003
The Galileo spacecraft was orbiting Jupiter between Dec 1995 and Sep 2003.
The Galileo dust detector monitored the jovian dust environment between about 2
and 370 R_J (jovian radius R_J = 71492 km). We present data from the Galileo
dust instrument for the period January 2000 to September 2003. We report on the
data of 5389 particles measured between 2000 and the end of the mission in
2003. The majority of the 21250 particles for which the full set of measured
impact parameters (impact time, impact direction, charge rise times, charge
amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in
radius), most of them originating from Jupiter's innermost Galilean moon Io.
Their impact rates frequently exceeded 10 min^-1. Surprisingly large impact
rates up to 100 min^-1 occurred in Aug/Sep 2000 when Galileo was at about 280
R_J from Jupiter. This peak in dust emission appears to coincide with strong
changes in the release of neutral gas from the Io torus. Strong variability in
the Io dust flux was measured on timescales of days to weeks, indicating large
variations in the dust release from Io or the Io torus or both on such short
timescales. Galileo has detected a large number of bigger micron-sized
particles mostly in the region between the Galilean moons. A surprisingly large
number of such bigger grains was measured in March 2003 within a 4-day interval
when Galileo was outside Jupiter's magnetosphere at approximately 350 R_J
jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003
provided the first actual comparison of in-situ dust data from a planetary ring
with the results inferred from inverting optical images.Comment: 59 pages, 13 figures, 6 tables, submitted to Planetary and Space
Scienc
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
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
figure
Jovian Dust Streams: A monitor of Io's volcanic plume activity
Streams of high speed dust particles originate from Jupiter's innermost
Galilean moon Io. After release from Io, the particles collect electric charges
in the Io plasma torus, gain energy from the co-rotating electric field of
Jupiter's magnetosphere, and leave the Jovian system into interplanetary space
with escape speeds over . Galileo, which was the first
orbiter spacecraft of Jupiter, has continuously monitored the dust streams
during 34 revolutions about the planet between 1996 and 2002. The observed dust
fluxes exhibit large orbit-to-orbit variability due to systematic and
stochastic changes. After removal of the systematic variations, the total dust
emission rate of Io has been calculated. It varies between and
, and is typically in the range of 0.1 to . We compare the dust emission rate with other markers of volcanic
activity on Io like large-area surface changes caused by volcanic deposits and
sightings of volcanic plumes.Comment: Geophysical Research Letters, accepted, 1 color figure, 1 b/w figur
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
Binaural Interaction in the Nucleus Laminaris of the Barn Owl : A Quantitative Model
A quantitative, neuronal model is proposed for the computation of interaural time difference (ITD) in the auditory system of the barn owl. The model uses a general, probabilistic approach, and is composed of two stages, the characteristics of which are based on anatomical and physiological evidence. Excitatory inputs from both ears, phase-locked to the waveform of tonal stimuli, together with phase-independent inhibitory inputs are summated linearly. The result is transformed into a probability of spike generation by a sigmoid nonlinearity, constituting a stochastic, ’soft’ threshold with saturation. The model incorporates inhibition as a control parameter on the nonlinearity, and includes the usual crosscorrelation-type models as a special case. It has a minimum number of parameters, the values of which can be estimated from physiological data in a straightforward manner. This simple, general model accounts for the binaural response properties of physiologically recorded neurons. In particular, it explains the experimentally observed ITD-tuning and the increase of phase-locking from input to output neurons. The model predicts that a decrease in inhibition causes a non-monotonic change in sensitivity to ITD
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