10,951 research outputs found
Statistics of Solar Wind Electron Breakpoint Energies Using Machine Learning Techniques
Solar wind electron velocity distributions at 1 au consist of a thermal
"core" population and two suprathermal populations: "halo" and "strahl". The
core and halo are quasi-isotropic, whereas the strahl typically travels
radially outwards along the parallel and/or anti-parallel direction with
respect to the interplanetary magnetic field. With Cluster-PEACE data, we
analyse energy and pitch angle distributions and use machine learning
techniques to provide robust classifications of these solar wind populations.
Initially, we use unsupervised algorithms to classify halo and strahl
differential energy flux distributions to allow us to calculate relative number
densities, which are of the same order as previous results. Subsequently, we
apply unsupervised algorithms to phase space density distributions over ten
years to study the variation of halo and strahl breakpoint energies with solar
wind parameters. In our statistical study, we find both halo and strahl
suprathermal breakpoint energies display a significant increase with core
temperature, with the halo exhibiting a more positive correlation than the
strahl. We conclude low energy strahl electrons are scattering into the core at
perpendicular pitch angles. This increases the number of Coulomb collisions and
extends the perpendicular core population to higher energies, resulting in a
larger difference between halo and strahl breakpoint energies at higher core
temperatures. Statistically, the locations of both suprathermal breakpoint
energies decrease with increasing solar wind speed. In the case of halo
breakpoint energy, we observe two distinct profiles above and below 500 km/s.
We relate this to the difference in origin of fast and slow solar wind.Comment: Published in Astronomy & Astrophysics, 11 pages, 10 figure
Book Reviews
Book Reviews
The Oppenheimer Case: The Trial of a Security System
By Charles P. Curtis
New York: Simon and Schuster, 1955. Pp. xi, 281. 6.65
reviewer: J. Raymond Denney
=============================
Military Law under the Uniform Code of Military Justice
By William B. Aycock and Seymour W. Wurfel
Chapel Hill: University of North Carolina Press, 1955. Pp. xviii, 430.
reviewer: James B. Earle
=================================
Why Johnny Can\u27t Read
By Rudolf Flesch
New York: Harper & Brothers, 1955. Pp. ix, 222.
==================================
Plain Words: Their ABC
By Sir Ernest Gowers
New York: Alfred A. Knopf, 1955. Pp. viii, 298.
===================================
Effective Legal Writing
By Frank E. Cooper
Indianapolis, The Bobbs-Merrill Company, Inc., 1953. Pp. x, 313.
reviewer: J. Allen Smith
==================================
The Moral Decision: Right and Wrong in the Light of American law
By Edmond Cahn
Bloomington, Ind.: Indiana University Press, Pp. ix, 315. $5.00
reviewer: Samuel Enoch Stump
An Untriggered Search for Optical Bursts
We present an untriggered search for optical bursts with the ROTSE-I
telephoto array. Observations were taken which monitor an effective 256 square
degree field continuously over 125 hours to m_{ROTSE}=15.7. The uniquely large
field, moderate limiting magnitude and fast cadence of 10 minutes permits
transient searches in a new region of sensitivity. Our search reveals no
candidate events. To quantify this result, we simulate potential optical bursts
with peak magnitude, m_{p}, at t=10 s, which fade as f=(\frac{t}{t_{0}})
^{\alpha_{t}}, where \alpha_t < 0. Simple estimates based on observational
evidence indicate that a search of this sensitivity begins to probe the
possible region occupied by GRB orphan afterglows. Our observing protocol and
image sensitivity result in a broad region of high detection efficiency for
light curves to the bright and slowly varying side of a boundary running from
[\alpha_{t},m_{p}]=[-2.0,6.0] to [-0.3,13.2]. Within this region, the
integrated rate of brief optical bursts is less than 1.1\times 10^{-8} {\rm
s}^{-1} {\rm deg}^{-2}. At 22 times the observed GRB rate from BATSE,
this suggests a limit on \frac{\theta_{opt}}{\theta_{\gamma}}\lesssim 5 where
\theta_{opt} and \theta_{\gamma} are the optical and gamma-ray collimation
angles, respectively. Several effects might explain the absence of optical
bursts, and a search of the kind described here but more sensitive by about 4
magnitudes should offer a more definitive probe.Comment: 8 pages, 6 figures, 1 tabl
A Search for Early Optical Emission from Short and Long Duration Gamma-ray Bursts
Gamma-ray bursts of short duration may harbor vital clues to the range of
phenomena producing bursts. However, recent progress from the observation of
optical counterparts has not benefitted the study of short bursts. We have
searched for early optical emission from six gamma-ray bursts using the ROTSE-I
telephoto array. Three of these events were of short duration, including GRB
980527 which is among the brightest short bursts yet observed. The data consist
of unfiltered CCD optical images taken in response to BATSE triggers delivered
via the GCN. For the first time, we have analyzed the entire 16 degree by 16
degree field covered for five of these bursts. In addition, we discuss a search
for the optical counterpart to GRB 000201, a well-localized long burst. Single
image sensitivities range from 13th to 14th magnitude around 10 s after the
initial burst detection, and 14 - 15.8 one hour later. No new optical
counterparts were discovered in this analysis suggesting short burst optical
and gamma-ray fluxes are uncorrelated.Comment: 8 pages, 2 figures, subm. to ApJ Let
Observations of the Optical Counterpart to XTE J1118+480 During Outburst by the ROTSE-I Telescope
The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of
2000. As detected by the Rossi X-ray Timing Explorer (RXTE), the first outburst
began in early January and the second began in early March. Routine imaging of
the northern sky by the Robotic Optical Transient Search Experiment (ROTSE)
shows the optical counterpart to XTE J1118+480 during both outbursts. These
data include over 60 epochs from January to June 2000. A search of the ROTSE
data archives reveal no previous optical outbursts of this source in selected
data between April 1998 and January 2000. While the X-ray to optical flux ratio
of XTE J1118+480 was low during both outbursts, we suggest that they were full
X-ray novae and not mini-outbursts based on comparison with similar sources.
The ROTSE measurements taken during the March 2000 outburst also indicate a
rapid rise in the optical flux that preceded the X-ray emission measured by the
RXTE by approximately 10 days. Using these results, we estimate a pre-outburst
accretion disk inner truncation radius of 1.2 x 10^4 Schwarzschild radii.Comment: 9 pages, 1 table, 2 figure
IMAPS Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas
We use the absorption features from neutral argon at 1048 and 1066 A to
determine interstellar abundances or their lower limits toward nine early-type
stars. These features were observed with the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) along sight lines with low reddening and low
fractional abundances of molecular hydrogen. We find that the interstellar Ar I
is below its solar and B-star abundance with respect to hydrogen toward zeta
Pup, gamma2 Vel and beta Cen A with (logarithmic) reduction factors
-0.37+/-0.09, -0.18+/-0.10, and -0.61+/-0.12 dex, respectively.
While Ar can condense onto the surfaces of dust grains in the interiors of
dense clouds, it is unlikely that argon atoms are depleted by this process in
the low-density lines of sight considered in this study. Instead, we propose
that the relatively large photoionization cross section of Ar makes it much
easier to hide in its ionized form than H. In regions that are about half
ionized, this effect can lower Ar I/H I by -0.11 to -0.96 dex, depending on the
energy of the photoionizing radiation and its intensity divided by the local
electron density. We apply this interpretation for the condition of the gas in
front of beta Cen A, which shows the largest deficiency of Ar. Also, we
determine the expected magnitudes of the differential ionizations for He, N, O,
Ne and Ar in the partly ionized, warm gas in the local cloud around our solar
system. For the local cloud and others that can be probed by UV studies, the
observed Ar I to H I ratio may be a good discriminant between two possible
alternatives, collisional ionization or photoionization, for explaining the
existence of partly ionized regions.Comment: 36 pages, 3 figure
The Social and Political Dimensions of the Ebola Response: Global Inequality, Climate Change, and Infectious Disease
The 2014 Ebola crisis has highlighted public-health vulnerabilities in Liberia, Sierra
Leone, and Guinea â countries ravaged by extreme poverty, deforestation and
mining-related disruption of livelihoods and ecosystems, and bloody civil wars in
the cases of Liberia and Sierra Leone. Ebolaâs emergence and impact are grounded
in the legacy of colonialism and its creation of enduring inequalities within African
nations and globally, via neoliberalism and the Washington Consensus. Recent
experiences with new and emerging diseases such as SARS and various strains of
HN influenzas have demonstrated the effectiveness of a coordinated local and
global public health and education-oriented response to contain epidemics. To what
extent is international assistance to fight Ebola strengthening local public health and
medical capacity in a sustainable way, so that other emerging disease threats, which
are accelerating with climate change, may be met successfully? This chapter
considers the wide-ranging socio-political, medical, legal and environmental factors
that have contributed to the rapid spread of Ebola, with particular emphasis on the
politics of the global and public health response and the role of gender, social
inequality, colonialism and racism as they relate to the mobilization and
establishment of the public health infrastructure required to combat Ebola and other
emerging diseases in times of climate change
A quantum point contact for neutral atoms
We show that the conductance of neutral atoms through a tightly confining
waveguide constriction is quantized in units of lambda_dB^2/pi, where lambda_dB
is the de Broglie wavelength of the incident atoms. Such a constriction forms
the atom analogue of an electron quantum point contact and is an example of
quantum transport of neutral atoms in an aperiodic system. We present a
practical constriction geometry that can be realized using a microfabricated
magnetic waveguide, and discuss how a pair of such constrictions can be used to
study the quantum statistics of weakly interacting gases in small traps.Comment: 5 pages with 3 figures. To appear in Phys. Rev. Let
All-Optical Broadband Excitation of the Motional State of Trapped Ions
We have developed a novel all-optical broadband scheme for exciting,
amplifying and measuring the secular motion of ions in a radio frequency trap.
Oscillation induced by optical excitation has been coherently amplified to
precisely control and measure the ion's secular motion. Requiring only laser
line-of-sight, we have shown that the ion's oscillation amplitude can be
precisely controlled. Our excitation scheme can generate coherent motion which
is robust against variations in the secular frequency. Therefore, our scheme is
ideal to excite the desired level of oscillatory motion under conditions where
the secular frequency is evolving in time. Measuring the oscillation amplitude
through Doppler velocimetry, we have characterized the experimental parameters
and compared them with a molecular dynamics simulation which provides a
complete description of the system.Comment: 8 pages, 10 figure
Aharonov-Bohm Oscillations in a One-Dimensional Wigner Crystal-Ring
We calculate the magnetic moment (`persistent current') in a strongly
correlated electron system --- a Wigner crystal --- in a one-dimensional
ballistic ring. The flux and temperature dependence of the persistent current
in a perfect ring is shown to be essentially the same as for a system of
non-interacting electrons. In contrast, by incorporating into the ring geometry
a tunnel barrier that pins the Wigner crystal, the current is suppressed and
its temperature dependence is drastically changed. The competition between two
temperature effects --- the reduced barrier height for macroscopic tunneling
and loss of quantum coherence --- may result in a sharp peak in the temperature
dependence. The character of the macroscopic quantum tunneling of a Wigner
crystal ring is dictated by the strength of pinning. At strong pinning the
tunneling of a rigid Wigner crystal chain is highly inhomogeneous, and the
persistent current has a well-defined peak at
independent of the barrier height ( is the sound velocity of the Wigner
crystal, is the length of the ring). In the weak pinning regime, the Wigner
crystal tunnels through the barrier as a whole and if the effect of
the barrier is to suppress the current amplitude and to shift the crossover
temperature from to . ( is the
amplitude of the pinning potential, is the drift velocity of a Wigner crystal ring with lattice spacing
). For very weak pinning, , the influence of the barrier on the
persistent current of a Wigner crystal ring is negligibly small.Comment: 30 pages, RevTeX, 2 figures available on reques
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