1,060 research outputs found
Up Front and Beyond the Centre Line: Australian Aborigines in Elite Australian Rules Football
Although there has been a substantial growth in the number of Aboriginal players in the Australian Football League over the past decade, issues of structural and institutional racism have not been explored. This investigation of the assignment of players by position revealed marked patterns of difference, which tend to reflect stereotypes about Aboriginal athletes. The results are similar to research conducted in the USA and the UK but suggest even stronger patterns of differentiation
A Panchromatic View of Brown Dwarf Aurorae
Stellar coronal activity has been shown to persist into the low-mass star
regime, down to late M-dwarf spectral types. However, there is now an
accumulation of evidence suggesting that at the end of the main sequence there
is a transition in the nature of the magnetic activity from chromospheric and
coronal to planet-like and auroral, from local impulsive heating via flares and
MHD wave dissipation to energy dissipation from strong large-scale
magnetospheric current systems. We examine this transition and the prevalence
of auroral activity in brown dwarfs through a compilation of multi-wavelength
surveys of magnetic activity, including radio, X-ray, and optical. We compile
the results of those surveys and place their conclusions in the context of
auroral emission as the consequence of large-scale magnetospheric current
systems that accelerate energetic electron beams and drive the particles to
impact the cool atmospheric gas. We explore the different manifestation of
auroral phenomena in brown dwarf atmospheres, like H, and define their
distinguishing characteristics. We conclude that large amplitude photometric
variability in the near infrared is most likely a consequence of clouds in
brown dwarf atmospheres, but that auroral activity may be responsible for
long-lived stable surface features. We report a connection between auroral
H emission and quiescent radio emission in ECMI pulsing brown dwarfs,
suggesting a potential underlying physical connection between the quiescent and
auroral emissions. We also discuss the electrodynamic engines powering brown
dwarf aurorae and the possible role of satellites around these systems to both
power the aurorae and seed the magnetosphere with plasma.Comment: 26 pages, 17 figures, and 2 tables; accepted to Ap
On a transition from solar-like coronae to rotation-dominated jovian-like magnetospheres in ultracool main-sequence stars
For main-sequence stars beyond spectral type M5 the characteristics of
magnetic activity common to warmer solar-like stars change into the brown-dwarf
domain: the surface magnetic field becomes more dipolar and the evolution of
the field patterns slows, the photospheric plasma is increasingly neutral and
decoupled from the magnetic field, chromospheric and coronal emissions weaken
markedly, and the efficiency of rotational braking rapidly decreases. Yet,
radio emission persists, and has been argued to be dominated by
electron-cyclotron maser emission instead of the gyrosynchrotron emission from
warmer stars. These properties may signal a transition in the stellar extended
atmosphere. Stars warmer than about M5 have a solar-like corona and
wind-sustained heliosphere in which the atmospheric activity is powered by
convective motions that move the magnetic field. Stars cooler than early-L, in
contrast, may have a jovian-like rotation-dominated magnetosphere powered by
the star's rotation in a scaled-up analog of the magnetospheres of Jupiter and
Saturn. A dimensional scaling relationship for rotation-dominated
magnetospheres by Fan et al. (1982) is consistent with this hypothesis
Optical observations of the AMPTE artificial comet and magnetotail barium releases
The first AMPTE artificial comet was observed with a low light level television camera operated aboard the NASA CV990 flying out of Moffett Field, California. The comet head, neutral cloud, and comet tail were all observed for four minutes with an unifiltered camera. Brief observations at T + 4 minutes through a 4554A Ba(+) filter confirmed the identification of the structures. The ion cloud expanded along with the neutral cloud at a rate of 2.3 km/sec (diameter) until it reached a final diameter of approx. 170 km at approx. T + 90 s. It also drifted with the neutral cloud until approx. 165 s. By T + 190 s it had reached a steady state velocity of 5.4 km/sec southward. A barium release in the magnetotail was observed from the CV990 in California, Eagle, Alaska, and Fairbanks, Alaska. Over a twenty-five minute period, the center of the barium streak drifted southward (approx. 500 m/sec), upward (24 km/sec) and eastward (approx 1 km/sec) in a nonrotating reference frame. An all-sky TV at Eagle showed a single auroral arc in the far North during this period
Space station WP-04 power system. Volume 2: Study results
Results of the phase B study contract for the definition of the space station Electric Power System (EPS) are presented in detail along with backup information and supporting data. Systems analysis and trades, preliminary design, advanced development, customer accommodations, operations planning, product assurance, and design and development phase planning are addressed. The station design is a hybrid approach which provides user power of 25 kWe from the photovoltaic subsystem and 50 kWe from the solar dynamic subsystem. The electric power is distributed to users as a utility service; single phase at a frequency of 20 kHz and voltage of 440VAC. The solar array NiH2 batteries of the photovoltaic subsystem are based on commonality to those used on the co-orbiting and solar platforms
Space station WP-04 power system. Volume 1: Executive summary
Major study activities and results of the phase B study contract for the preliminary design of the space station Electrical Power System (EPS) are summarized. The areas addressed include the general system design, man-tended option, automation and robotics, evolutionary growth, software development environment, advanced development, customer accommodations, operations planning, product assurance, and design and development phase planning. The EPS consists of a combination photovoltaic and solar dynamic power generation subsystem and a power management and distribution (PMAD) subsystem. System trade studies and costing activities are also summarized
All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities
We demonstrate a new technique for detecting radio transients based on
interferometric closure quantities. The technique uses the bispectrum, the
product of visibilities around a closed-loop of baselines of an interferometer.
The bispectrum is calibration independent, resistant to interference, and
computationally efficient, so it can be built into correlators for real-time
transient detection. Our technique could find celestial transients anywhere in
the field of view and localize them to arcsecond precision. At the Karl G.
Jansky Very Large Array (VLA), such a system would have a high survey speed and
a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth.
The ability to localize dispersed millisecond pulses to arcsecond precision in
large volumes of interferometer data has several unique science applications.
Localizing individual pulses from Galactic pulsars will help find X-ray
counterparts that define their physical properties, while finding host galaxies
of extragalactic transients will measure the electron density of the
intergalactic medium with a single dispersed pulse. Exoplanets and active stars
have distinct millisecond variability that can be used to identify them and
probe their magnetospheres. We use millisecond time scale visibilities from the
Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect
dispersed pulses and reject local interference. The computational and data
efficiency of the bispectrum will help find transients on a range of time
scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include
discussion of non-Gaussian statistics of techniqu
A Deep Radio Limit for the TRAPPIST-1 System
The first nearby very-low mass star planet-host discovered, TRAPPIST-1,
presents not only a unique opportunity for studying a system of multiple
terrestrial planets, but a means to probe magnetospheric interactions between a
star at the end of the main sequence and its close-in satellites. This
encompasses both the possibility of persistent coronal solar-like activity,
despite cool atmospheric temperatures, and the presence of large-scale
magnetospheric currents, similar to what is seen in the Jovian system.
Significantly, the current systems include a crucial role for close-in
planetary satellites analogous to the role played by the Galilean satellites
around Jupiter. We present the first radio observations of the seven-planet
TRAPPIST-1 system using the Karl G. Jansky Very Large Array, looking for both
highly circularly polarized radio emission and/or persistent quiescent
emissions. We measure a broadband upper flux density limit of <8.1 Jy
across 4-8 GHz, and place these observations both in the context of
expectations for stellar radio emission, and the possible electrodynamic
engines driving strong radio emissions in very-low mass stars and brown dwarfs,
with implications for future radio surveys of TRAPPIST-1 like planet-hosts. We
conclude that magnetic activity of TRAPPIST-1 is predominantly coronal and does
not behave like the strong radio emitters at the stellar/sub-stellar boundary.
We further discuss the potential importance of magnetic field topology and
rotation rates, demonstrating that a TRAPPIST-1 like planetary system around a
rapidly rotating very-low mass star can generate emission consistent with the
observed radio luminosities of very-low mass stars and brown dwarfs.Comment: 9 pages, accepted by AAS Journal
Electron beam induced radio emission from ultracool dwarfs
We present the numerical simulations for an electron-beam-driven and
loss-cone-driven electron-cyclotron maser (ECM) with different plasma
parameters and different magnetic field strengths for a relatively small region
and short time-scale in an attempt to interpret the recent discovered intense
radio emission from ultracool dwarfs. We find that a large amount of
electromagnetic field energy can be effectively released from the beam-driven
ECM, which rapidly heats the surrounding plasma. A rapidly developed
high-energy tail of electrons in velocity space (resulting from the heating
process of the ECM) may produce the radio continuum depending on the initial
strength of the external magnetic field and the electron beam current. Both
significant linear polarization and circular polarization of electromagnetic
waves can be obtained from the simulations. The spectral energy distributions
of the simulated radio waves show that harmonics may appear from 10 to
70 ( is the electron plasma frequency) in the
non-relativistic case and from 10 to 600 in the relativistic
case, which makes it difficult to find the fundamental cyclotron frequency in
the observed radio frequencies. A wide frequency band should therefore be
covered by future radio observations.Comment: 10 pages, 19 figures, accepted for publication in the Astrophysical
Journa
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