5,234 research outputs found
Global MHD Simulations of Neptune's Magnetosphere
A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large-scale reconfiguration of magnetic topology and plasma distribution. During the “pole-on” magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole-on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning “off” and “on” when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations
The Elliptical Galaxy formerly known as the Local Group: Merging the Globular Cluster Systems
Prompted by a new catalogue of M31 globular clusters, we have collected
together individual metallicity values for globular clusters in the Local
Group. Although we briefly describe the globular cluster systems of the
individual Local Group galaxies, the main thrust of our paper is to examine the
collective properties. In this way we are simulating the dissipationless merger
of the Local Group, into presumably an elliptical galaxy. Such a merger is
dominated by the Milky Way and M31, which appear to be fairly typical examples
of globular cluster systems of spiral galaxies.
The Local Group `Elliptical' has about 700 +/- 125 globular clusters, with a
luminosity function resembling the `universal' one. The metallicity
distribution has peaks at [Fe/H] ~ -1.55 and -0.64 with a metal-poor to
metal-rich ratio of 2.5:1. The specific frequency of the Local Group Elliptical
is initially about 1 but rises to about 3, when the young stellar populations
fade and the galaxy resembles an old elliptical. The metallicity distribution
and stellar population corrected specific frequency are similar to that of some
known early type galaxies. Based on our results, we briefly speculate on the
origin of globular cluster systems in galaxies.Comment: 22 pages, Latex, 4 figures, 5 tables, submitted to A &
Suprathermal electrons at Saturn's bow shock
The leading explanation for the origin of galactic cosmic rays is particle
acceleration at the shocks surrounding young supernova remnants (SNRs),
although crucial aspects of the acceleration process are unclear. The similar
collisionless plasma shocks frequently encountered by spacecraft in the solar
wind are generally far weaker (lower Mach number) than these SNR shocks.
However, the Cassini spacecraft has shown that the shock standing in the solar
wind sunward of Saturn (Saturn's bow shock) can occasionally reach this
high-Mach number astrophysical regime. In this regime Cassini has provided the
first in situ evidence for electron acceleration under quasi-parallel upstream
magnetic conditions. Here we present the full picture of suprathermal electrons
at Saturn's bow shock revealed by Cassini. The downstream thermal electron
distribution is resolved in all data taken by the low-energy electron detector
(CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were
at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18
keV) measured a suprathermal electron signature at 31 of 508 crossings, where
typically only the lowest energy channels (<100 keV) were above background. We
show that these results are consistent with theory in which the "injection" of
thermal electrons into an acceleration process involves interaction with
whistler waves at the shock front, and becomes possible for all upstream
magnetic field orientations at high Mach numbers like those of the strong
shocks around young SNRs. A future dedicated study will analyze the rare
crossings with evidence for relativistic electrons (up to ~1 MeV).Comment: 22 pages, 5 figures. Accepted for publication in Ap
Gaseous-Hydrogen Pressurant Requirements for the Discharge of Liquid Hydrogen from a 3.96 Meter /13 ft/ Diameter Spherical Tank
Hydrogen gas pressure requirements during pressurization and transfer of liquid hydrogen from spherical tank
Reduced influence of perceptual context in mild traumatic brain injury is not an illusion
Perceptual grouping is impaired following mild traumatic brain injury (mTBI). This may affect visual size perception, a process influenced by perceptual grouping abilities. We conducted two experiments to evaluate visual size perception in people with self-reported history of mTBI, using two different size-contrast illusions: the Ebbinghaus Illusion (Experiment 1) and the MĂĽller-Lyer illusion (Experiment 2). In Experiment 1, individuals with mTBI and healthy controls were asked to compare the size of two target circles that were either the same size or different sizes. The target circles appeared by themselves (no-context condition), or were surrounded by smaller or larger circles (context condition). Similar levels of accuracy were evident between the groups in the no-context condition. However, size judgements by mTBI participants were more accurate in the context condition, suggesting that they processed the target circles separately from the surrounding circles. In Experiment 2, individuals with mTBI and healthy controls judged the length of parallel lines that appeared with arrowheads (context condition) or without arrowheads (no context condition). Consistent with Experiment 1, size judgements by mTBI participants were more accurate than size judgements by control participants in the context condition. These findings suggest that mTBI influences size perception by impairing perceptual grouping of visual stimuli in near proximity
Advanced engine study program
A design and analysis study was conducted to provide advanced engine descriptions and parametric data for space transfer vehicles. The study was based on an advanced oxygen/hydrogen engine in the 7,500 to 50,000 lbf thrust range. Emphasis was placed on defining requirements for high-performance engines capable of achieving reliable and versatile operation in a space environment. Four variations on the expander cycle were compared, and the advantages and disadvantages of each were assessed. Parametric weight, envelope, and performance data were generated over a range of 7,500 to 50,000 lb thrust and a wide range of chamber pressure and nozzle expansion ratio
- …