1,440 research outputs found
The Creation and Propagation of Radiation: Fields Inside and Outside of Sources
We present a new algorithm for computing the electromagnetic fields of
currents inside and outside of finite current sources, for arbitrary time
variations in the currents. Unexpectedly, we find that our solutions for these
fields are free of the concepts of differential calculus, in that our solutions
only involve the currents and their time integrals, and do not involve the time
derivatives of the currents. As examples, we give the solutions for two
configurations of current: a planar solenoid and a rotating spherical shell
carrying a uniform charge density. For slow time variations in the currents, we
show that our general solutions reduce to the standard expressions for the
fields in classic magnetic dipole radiation. In the limit of extremely fast
turn-on of the currents, we show that for our general solutions the amount of
energy radiated is exactly equal to the magnetic energy stored in the static
fields a long time after current creation. We give three associated problem
statements which can be used in courses at the undergraduate level, and one
problem statement suitable for courses at the graduate level. These problems
are of physical interest because: (1) they show that current systems of finite
extent can radiate even during time intervals when the currents are constant;
(2) they explicitly display transit time delays across a source associated with
its finite dimensions; and (3) they allow students to see directly the origin
of the reaction forces for time-varying systemsComment: 25 pages, 5 figure
Sunward-propagating Alfv\'enic fluctuations observed in the heliosphere
The mixture/interaction of anti-sunward-propagating Alfv\'enic fluctuations
(AFs) and sunward-propagating Alfv\'enic fluctuations (SAFs) is believed to
result in the decrease of the Alfv\'enicity of solar wind fluctuations with
increasing heliocentric distance. However, SAFs are rarely observed at 1 au and
solar wind AFs are found to be generally outward. Using the measurements from
Voyager 2 and Wind, we perform a statistical survey of SAFs in the heliosphere
inside 6 au. We first report two SAF events observed by Voyager 2. One is in
the anti-sunward magnetic sector with a strong positive correlation between the
fluctuations of magnetic field and solar wind velocity. The other one is in the
sunward magnetic sector with a strong negative magnetic field-velocity
correlation. Statistically, the percentage of SAFs increases gradually with
heliocentric distance, from about 2.7% at 1.0 au to about 8.7% at 5.5 au. These
results provide new clues for understanding the generation mechanism of SAFs
An investigation of the anabolic actions of biosynthetic human growth hormone after injury by burning
Previous clinical trials in normal subjects and post-operative patients have shown that biosynthetic growth hormone preparations increase nitrogen retention. It has been suggested that their administration to injured patients may be beneficial. A clinical trial is presented of twelve adult burned patients of whom six were allocated to receive biosynthetic human growth hormone (somatropin) and six to form a control group. Injury by burning is followed by increases in resting energy expenditure and urinary nitrogen excretion, accompanied by insulin resistance and glucose intolerance. There is a generalised fall in plasma protein concentrations, including the somatomedin, insulin-like growth factor-I. Somatropin administration causes no change in the rate of protein oxidation, the positivity of nitrogen balance or, either serum somatomedin or plasma protein concentrations. It causes an increase in the insulin-resistance already present in burned patients. Two further studies are presented in which somatropin is compared with a placebo in both unburned and burned rats. Injury by burning causes weight-loss, an increase in urinary nitrogen excretion, a fall in the serum albumin and somatomedin concentrations, and a reduction in the strength of healing laparotomy wounds. Somatropin administration to unburned rats causes a small rise in the serum somatomedin concentration and a transient increase in wound-strength. It causes no increase in the positivity of nitrogen balance or weight-gain. Its administration to the burned rat causes no effect on the serum somatomedin concentration, nitrogen balance, weight-gain or wound-healing. These three studies show that somatropin has no anabolic effect soon after injury by burning. I postulate that this failure reflects the changes observed after injury in somatomedin concentrations and the responsiveness of somatomedins to somatropin. I conclude that somatropin and related compounds are not suitable for use as anabolic agents soon after injury by burning
Laboratory observation of a nonlinear interaction between shear Alfv\'{e}n waves
An experimental investigation of nonlinear interactions between shear
Alfv\'{e}n waves in a laboratory plasma is presented. Two Alfv\'{e}n waves,
generated by a resonant cavity, are observed to beat together, driving a low
frequency nonlinear psuedo-mode at the beat frequency. The psuedo-mode then
scatters the Alfv\'{e}n waves, generating a series of sidebands. The observed
interaction is very strong, with the normalized amplitude of the driven
psuedo-mode comparable to the normalized magnetic field amplitude () of the interacting Alfv\'{e}n waves.Comment: 10 pages, 4 figures, submitted to Phys. Rev. Let
Accuracy and consistency of grass pollen identification by human analysts using electron micrographs of surface ornamentation
• Premise of the study: Humans frequently identify pollen grains at a taxonomic rank above species. Grass pollen is a classic case of this situation, which has led to the development of computational methods for identifying grass pollen species. This paper aims to provide context for these computational methods by quantifying the accuracy and consistency of human identification.
• Methods: We measured the ability of nine human analysts to identify 12 species of grass pollen using scanning electron microscopy images. These are the same images that were used in computational identifications. We have measured the coverage, accuracy, and consistency of each analyst, and investigated their ability to recognize duplicate images.
• Results: Coverage ranged from 87.5% to 100%. Mean identification accuracy ranged from 46.67% to 87.5%. The identification consistency of each analyst ranged from 32.5% to 87.5%, and each of the nine analysts produced considerably different identification schemes. The proportion of duplicate image pairs that were missed ranged from 6.25% to 58.33%.
• Discussion: The identification errors made by each analyst, which result in a decline in accuracy and consistency, are likely related to psychological factors such as the limited capacity of human memory, fatigue and boredom, recency effects, and positivity bias
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
Intermittency and the passive nature of the magnitude of the magnetic field
It is shown that the statistical properties of the magnitude of the magnetic
field in turbulent electrically conducting media resemble, in the inertial
range, those of passive scalars in fully developed three-dimensional fluid
turbulence. This conclusion, suggested by the data from Advanced Composition
Explorer, is supported by a brief analysis of the appropriate
magnetohydrodynamic equations
Comparison of electromagnetic and gravitational radiation: What we can learn about each from the other
We compare the nature of electromagnetic fields and gravitational fields in linearized general relativity. We carry out this comparison both mathematically and visually. In particular, the “lines of force” visualizations of electromagnetism are contrasted with the recently introduced tendex/vortex eigenline technique for visualizing gravitational fields. Specific solutions, visualizations, and comparisons are given for an oscillating point quadrupole source. Among the similarities illustrated are the quasistatic nature of the near fields, the transverse 1/r nature of the far fields, and the interesting intermediate field structures connecting these two limiting forms. Among the differences illustrated are the meaning of field line motion and of the flow of energy
The clinical assessment study of the foot (CASF): study protocol for a prospective observational study of foot pain and foot osteoarthritis in the general population.
BACKGROUND: Symptomatic osteoarthritis (OA) affects approximately 10% of adults aged over 60 years. The foot joint complex is commonly affected by OA, yet there is relatively little research into OA of the foot, compared with other frequently affected sites such as the knee and hand. Existing epidemiological studies of foot OA have focussed predominantly on the first metatarsophalangeal joint at the expense of other joints. This three-year prospective population-based observational cohort study will describe the prevalence of symptomatic radiographic foot OA, relate its occurrence to symptoms, examination findings and life-style-factors, describe the natural history of foot OA, and examine how it presents to, and is diagnosed and managed in primary care. METHODS: All adults aged 50 years and over registered with four general practices in North Staffordshire, UK, will be invited to participate in a postal Health Survey questionnaire. Respondents to the questionnaire who indicate that they have experienced foot pain in the preceding twelve months will be invited to attend a research clinic for a detailed clinical assessment. This assessment will consist of: clinical interview; physical examination; digital photography of both feet and ankles; plain x-rays of both feet, ankles and hands; ultrasound examination of the plantar fascia; anthropometric measurement; and a further self-complete questionnaire. Follow-up will be undertaken in consenting participants by postal questionnaire at 18 months (clinic attenders only) and three years (clinic attenders and survey participants), and also by review of medical records. DISCUSSION: This three-year prospective epidemiological study will combine survey data, comprehensive clinical, x-ray and ultrasound assessment, and review of primary care records to identify radiographic phenotypes of foot OA in a population of community-dwelling older adults, and describe their impact on symptoms, function and clinical examination findings, and their presentation, diagnosis and management in primary care
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