5,828 research outputs found
The radial dependence of the solar energetic particle flux
Researchers discuss the radial dependence of the peak flux and the fluence of solar flare produced energetic particles under the assumption that they propagate diffusively in the heliosphere
On the origin of the Trojan asteroids: Effects of Jupiter's mass accretion and radial migration
We present analytic and numerical results which illustrate the effects of
Jupiter's accretion of nebular gas and the planet's radial migration on its
Trojan companions. Initially, we approximate the system by the planar circular
restricted three-body problem and assume small Trojan libration amplitudes.
Employing an adiabatic invariant calculation, we show that Jupiter's
thirty-fold growth from a core to its present mass causes the
libration amplitudes of Trojan asteroids to shrink by a factor of about 2.5 to
of their original size. The calculation also shows that Jupiter's
radial migration has comparatively little effect on the Trojans; inward
migration from 6.2 to 5.2 AU causes an increase in Trojan libration amplitudes
of . In each case, the area enclosed by small tadpole orbits, if made
dimensionless by using Jupiter's semimajor axis, is approximately conserved.
Similar adiabatic invariant calculations for inclined and eccentric Trojans
show that Jupiter's mass growth leaves the asteroid's eccentricities and
inclinations essentially unchanged, while one AU of inward migration causes an
increase in both of these quantities by . Numerical integrations
confirm and extend these analytic results. We demonstrate that our predictions
remain valid for Trojans with small libration amplitudes even when the
asteroids have low, butComment: Submitted to Icarus - 13 Fig
Debris about asteroids: Where and how much?
We summarize several recent findings on the size and shape of the region within which material can stably orbit an asteroid. If the asteroid (with assumed density 2.38 g/cu cm) circles the Sun at 2.55 AU, co-planar prograde material will remain trapped whenever started on unperturbed circular orbits at less than about 220 R(sub A) (asteroid radii); co-planar retrograde particles are stable out twice as far. Our 3-D stability surface, which encloses several hundred numerically calculated orbits that start with various inclinations, is shaped like a sphere with its top and bottom sliced off; its dimensions scale like the Hill radius =(mu/3)(exp 1/3)R, where mu is the asteroid-to-solar mass ratio and R is the asteroid's orbital radius. If the asteroid moves along an elliptical orbit, a fairly reliable indicator of the dimensions of the hazard zone is the size of its Hill sphere at the orbit's pericenter. Grains with radii less than a few mm will be lost through the action of radiation forces which can induce escape or cause collisions with the asteroid on times scales of a few years; interplanetary micrometeoroids produce collisional break-up of these particles in approximately 10(exp 4) yrs. The effects of Jupiter and of asteroids that pass close to the target asteroid allow particles to diffuse from the system, again shrinking the hazard zone. None of the considered sources-primordial formation, debris spalled off the asteroid during micrometeoroid impact, captured interplanetary particles, feeder satellites, etc., seem capable of densely populating distant orbits from the asteroid. No certain detections of debris clouds or of binary asteroids have been made. Thus, it seems highly unlikely that a spacecraft fly-by targeted at 100 R(sub A) from the asteroid over its orbital pole would encounter any material
'A most active, enterprising officer': Captain John Perkins, the Royal Navy and the boundaries of slavery and liberty in the Caribbean
John Perkins was the most senior black officer in the Royal Navy during the American War of Independence and the French Revolutionary and Napoleonic Wars. He rose through the ranks from a carpenter's enslaved servant in 1759 to post captain in 1800, and went on to be one of the very first British officials to land in newly-independent Haiti in 1804. His career as a spy, gun-runner, naval officer and land owner was one of almost implausible adventure and speaks to the capacity of the maritime service to challenge and subvert race and slavery in the Caribbean. His very uniqueness, however, highlights the profound challenges for slaves and ex-slaves in trying to remake themselves as free people
The Social and Academic Standing of the Information Systems Discipline: General Theory Considered as Cultural Capital
Bourdieu’s concepts of social fields and social power provide a theoretical basis for arguing that the information systems (IS) field is engaged in an ongoing struggle with other disciplines for prestige and support. While IS has produced a considerable amount of high quality theory and research, it is by no means clear that this is understood by either the academy or the general public. It is argued that the discipline’s profile could be raised by the development and promulgation of a general theory of IS, similar in scope to the general theories found in other disciplines such as sociology. The political and cultural value of developing such a theory is discussed, as are a range of issues it is recommended that it should address
Three-Body Capture of Irregular Satellites: Application to Jupiter
We investigate a new theory of the origin of the irregular satellites of the
giant planets: capture of one member of a ~100-km binary asteroid after tidal
disruption. The energy loss from disruption is sufficient for capture, but it
cannot deliver the bodies directly to the observed orbits of the irregular
satellites. Instead, the long-lived capture orbits subsequently evolve inward
due to interactions with a tenuous circumplanetary gas disk.
We focus on the capture by Jupiter, which, due to its large mass, provides
the most stringent test of our model. We investigate the possible fates of
disrupted bodies, the differences between prograde and retrograde captures, and
the effects of Callisto on captured objects. We make an impulse approximation
and discuss how it allows us to generalize capture results from equal-mass
binaries to binaries with arbitrary mass ratios.
We find that at Jupiter, binaries offer an increase of a factor of ~10 in the
capture rate of 100-km objects as compared to single bodies, for objects
separated by tens of radii that approach the planet on relatively low-energy
trajectories. These bodies are at risk of collision with Callisto, but may be
preserved by gas drag if their pericenters are raised quickly enough. We
conclude that our mechanism is as capable of producing large irregular
satellites as previous suggestions, and it avoids several problems faced by
alternative models.Comment: 39 pages, 12 figures, 1 table, submitted to Icaru
Three-Body Encounters of Black Holes in Globular Clusters
Evidence has been mounting for the existence of black holes with masses from
10^2 to 10^4 M_Solar associated with stellar clusters. Such intermediate-mass
black holes (IMBHs) will encounter other black holes in the dense cores of
these clusters. The binaries produced in these interactions will be perturbed
by other objects as well thus changing the orbital characteristics of the
binaries. These binaries and their subsequent mergers due to gravitational
radiation are important sources of gravitational waves. We present the results
of numerical simulations of high mass ratio encounters, which help clarify the
interactions of intermediate-mass black holes in globular clusters and help
determine what types of detectable gravitational wave signatures are likely.Comment: 4 pages, 3 figures to appear in the proceedings of The Astrophysics
of Gravitational Wave Sources, College Park, MD, 24-26 April 200
Visualizing the Correlation between Human Brain Structure and Function
In order to help neuroscience researchers better investigate correlations between human brain structure and function, three visualization techniques were implemented in an existing neuroscience investigation software tool. These techniques aimed to produce effective visualizations of gray matter and white matter in the same virtual scene. The three techniques were: 1) wireframe rendering of the gray matter 2) multiple views of separate data sets with linked vantage point and 3) abstraction of white matter pathways drawn as arcs going outside the gray matter
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