34,537 research outputs found
Global energetic neutral atom (ENA) measurements and their association with the Dst index
We present a new global magnetospheric index that measures the intensity of the Earth\u27s ring current through energetic neutral atoms (ENAs). We have named it the Global Energetic Neutral Index (GENI), and it is derived from ENA measurements obtained by the Imaging Proton Spectrometer (IPS), part of the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment on the POLAR satellite. GENI provides a simple orbit-independent global sum of ENAs measured with IPS. Actual ENA measurements for the same magnetospheric state look different when seen from different points in the POLAR orbit. In addition, the instrument is sensitive to weak ion populations in the polar cap, as well as cosmic rays. We have devised a method for removing the effects of cosmic rays and weak ion fluxes, in order to produce an image of “pure” ENA counts. We then devised a method of normalizing the ENA measurements to remove the orbital bias effect. The normalized data were then used to produce the GENI. We show, both experimentally and theoretically the approximate proportionality between the GENI and the Dst index. In addition we discuss possible implications of this relation. Owing to the high sensitivity of IPS to ENAs, we can use these data to explore the ENA/Dst relationship not only during all phases of moderate geomagnetic storms, but also during quiescent ring current periods
Identifying features predictive of faculty integrating computation into physics courses
Computation is a central aspect of 21st century physics practice; it is used
to model complicated systems, to simulate impossible experiments, and to
analyze mountains of data. Physics departments and their faculty are
increasingly recognizing the importance of teaching computation to their
students. We recently completed a national survey of faculty in physics
departments to understand the state of computational instruction and the
factors that underlie that instruction. The data collected from the faculty
responding to the survey included a variety of scales, binary questions, and
numerical responses. We then used Random Forest, a supervised learning
technique, to explore the factors that are most predictive of whether a faculty
member decides to include computation in their physics courses. We find that
experience using computation with students in their research, or lack thereof
and various personal beliefs to be most predictive of a faculty member having
experience teaching computation. Interestingly, we find demographic and
departmental factors to be less useful factors in our model. The results of
this study inform future efforts to promote greater integration of computation
into the physics curriculum as well as comment on the current state of
computational instruction across the United States
An extended model of the quantum free-electron laser
Previous models of the quantum regime of operation of the Free Electron Laser
(QFEL) have performed an averaging and the application of periodic boundary
conditions to the coupled Maxwell - Schrodinger equations over short, resonant
wavelength intervals of the interaction. Here, an extended, one-dimensional
model of the QFEL interaction is presented in the absence of any such averaging
or application of periodic boundary conditions, the absence of the latter
allowing electron diffusion processes to be modeled throughout the pulse. The
model is used to investigate how both the steady-state (CW) and pulsed regimes
of QFEL operation are affected. In the steady-state regime it is found that the
electrons are confined to evolve as a 2-level system, similar to the previous
QFEL models. In the pulsed regime Coherent Spontaneous Emission (CSE) due to
the shape of the electron pulse current distribution is shown to be present in
the QFEL regime for the first time. However, unlike the classical case, CSE in
the QFEL is damped by the effects of quantum diffusion of the electron
wavefunction. Electron recoil from the QFEL interaction can also cause a
diffusive drift between the recoiled and non-recoiled parts of the electron
pulse wavefunction, effectively removing the recoiled part from the primary
electron-radiation interaction.Comment: Submitted to Optics Expres
Experimental Study of the Role of Atomic Interactions on Quantum Transport
We report an experimental study of quantum transport for atoms confined in a
periodic potential and compare between thermal and BEC initial conditions. We
observe ballistic transport for all values of well depth and initial
conditions, and the measured expansion velocity for thermal atoms is in
excellent agreement with a single-particle model. For weak wells, the expansion
of the BEC is also in excellent agreement with single-particle theory, using an
effective temperature. We observe a crossover to a new regime for the BEC case
as the well depth is increased, indicating the importance of interactions on
quantum transport.Comment: 4 pages, 3 figure
A Preliminary Study of the Visual Field in Athletics
If a basketball player sees an area half as large as another player, he has less chance of sizing up the entire situation on the floor at any moment. Likewise, if one player secs less at a given time than he did previously or will in the future, he is probably less effective at that given time. This study attempts first to discover whether or not there is a reason to suspect a measurable relationship between the size of the visual field and successful basketball playing. If there is, obviously a test of visual fields would be advantageous to a coach deciding which inexperienced candidates should be trained. Secondly, this study investigates the effects of fatigue upon the size of the visual field, i.e., does a tired man actually see less? If he does see less, what are the characteristics of the recovery of normal vision? Rather than prepare and conduct an extensive survey immediately, this study was designed to precede a more complete research, to suggest tentative conclusions, and to reveal the difficulties and uncontrollable variations in such work
Vertex Expansion for the Bianchi I model
A perturbative expansion of Loop Quantum Cosmological transitions amplitudes
of Bianchi I models is performed. Following the procedure outlined in [1,2] for
isotropic models, it is shown that the resulting expansion can be written in
the form of a series of amplitudes each with a fixed number of transitions
mimicking a spin foam expansion. This analogy is more complete than in the
isotropic case, since there are now the additional anisotropic degrees of
freedom which play the role of `colouring' of the spin foams. Furthermore, the
isotropic expansion is recovered by integrating out the anisotropies.Comment: 15 pages, 2 figure
Family Size and Educational Attainment in England and Wales
In this paper, we use linked census data from England and Wales to investigate whether having a large number of siblings leads to lower educational attainment. There is a large literature suggesting that with large sibship size, parental resources will be diluted and this, in turn, will lead to lower educational attainment. Using twin births and the sex composition of the sibling group as instrumental variables, we find that the evidence of a family size effect on educational attainment is rather uncertain. Similar results are obtained when we use occupational attainment as the dependent variable. We also demonstrate the confounding of birth order and family size effects, and show that an adjusted birth order index proposed by Booth and Kee provides an effective solution to this estimation problem
Local spinfoam expansion in loop quantum cosmology
The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and
Bianchi I models defined by loop quantum cosmology have recently been
translated into a spinfoam-like formalism. The construction is facilitated by
the presence of a massless scalar field which is used as an internal clock. The
implicit integration over the matter variable leads to a nonlocal spinfoam
amplitude. In this paper we consider a vacuum Bianchi I universe and show that
by choosing an appropriate regulator a spinfoam expansion can be obtained
without selecting a clock variable and that the resulting spinfoam amplitude is
local.Comment: 12 page
High-resolution imaging of ultracold fermions in microscopically tailored optical potentials
We report on the local probing and preparation of an ultracold Fermi gas on
the length scale of one micrometer, i.e. of the order of the Fermi wavelength.
The essential tool of our experimental setup is a pair of identical,
high-resolution microscope objectives. One of the microscope objectives allows
local imaging of the trapped Fermi gas of 6Li atoms with a maximum resolution
of 660 nm, while the other enables the generation of arbitrary optical dipole
potentials on the same length scale. Employing a 2D acousto-optical deflector,
we demonstrate the formation of several trapping geometries including a tightly
focussed single optical dipole trap, a 4x4-site two-dimensional optical lattice
and a 8-site ring lattice configuration. Furthermore, we show the ability to
load and detect a small number of atoms in these trapping potentials. A site
separation of down to one micrometer in combination with the low mass of 6Li
results in tunneling rates which are sufficiently large for the implementation
of Hubbard-models with the designed geometries.Comment: 15 pages, 6 figure
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