251 research outputs found
Macroscopic electromagnetic stress tensor for ionized media
Following the arguments presented by Mansuripur [Opt. Express 16, 14821-14835
(2008)], we suggest a form for the macroscopic electromagnetic stress tensor
appropriate for ionized media. The generalized Lorentz force includes the
effects of polarization forces as well as those on the free charge and current
densities. The resulting tensor is written in terms of the fields D, B, E, and
H. Its expression for a fully ionized medium subject to an external
electromagnetic field is discussed, as are the plasma conservation equations.
An apparatus is suggested for its experimental discrimination.Comment: 16 pages, 2 figures, fixed some nonsense with the tubular source, to
appear in JP
Comment on "Plasma ionization by annularly bounded helicon waves" [Phys . Plasmas 13, 063501 (2006)]
The neoclassical calculation of the helicon wave theory contains a
fundamental flaw. Use is made of a proportional relationship between the
magnetic field and its curl to derive the Helmholtz equation describing helicon
wave propagation; however, by the fundamental theorem of Stokes, the curl of
the magnetic field must be perpendicular to that portion of the field
contributing to the local curl. Reexamination of the equations of motion
indicates that only electromagnetic waves propagate through a stationary region
of constant pressure in a fully ionized, neutral medium.Comment: 7 pages, 1 figure, to be published in Phys. Plasmas,
http://link.aip.org/link/?PHPAEN/16/054701/
Potential formulation of the dispersion relation for a uniform, magnetized plasma with stationary ions in terms of a vector phasor
The derivation of the helicon dispersion relation for a uniform plasma with
stationary ions subject to a constant background magnetic field is reexamined
in terms of the potential formulation of electrodynamics. Under the same
conditions considered by the standard derivation, the nonlinear self-coupling
between the perturbed electron flow and the potential it generates is
addressed. The plane wave solution for general propagation vector is determined
for all frequencies and expressed in terms of a vector phasor. The behavior of
the solution as described in vacuum units depends upon the ratio of
conductivity to the magnitude of the background field. Only at low conductivity
and below the cyclotron frequency can significant propagation occur as
determined by the ratio of skin depth to wavelength.Comment: 10 pages, 6 figures, major revision, final version, to appear in Po
Molecular Decay of the Tooth Gene Enamelin (ENAM) Mirrors the Loss of Enamel in the Fossil Record of Placental Mammals
Vestigial structures occur at both the anatomical and molecular levels, but studies documenting the co-occurrence of morphological degeneration in the fossil record and molecular decay in the genome are rare. Here, we use morphology, the fossil record, and phylogenetics to predict the occurrence of “molecular fossils” of the enamelin (ENAM) gene in four different orders of placental mammals (Tubulidentata, Pholidota, Cetacea, Xenarthra) with toothless and/or enamelless taxa. Our results support the “molecular fossil” hypothesis and demonstrate the occurrence of frameshift mutations and/or stop codons in all toothless and enamelless taxa. We then use a novel method based on selection intensity estimates for codons (ω) to calculate the timing of iterated enamel loss in the fossil record of aardvarks and pangolins, and further show that the molecular evolutionary history of ENAM predicts the occurrence of enamel in basal representatives of Xenarthra (sloths, anteaters, armadillos) even though frameshift mutations are ubiquitous in ENAM sequences of living xenarthrans. The molecular decay of ENAM parallels the morphological degeneration of enamel in the fossil record of placental mammals and provides manifest evidence for the predictive power of Darwin's theory
Local Differences in Human Immunodeficiency Virus Prevalence: A Comparison of Social Venue Patrons, Antenatal Patients, and Sexually Transmitted Infection Patients in Eastern Kinshasa
This study compares the sexual behavior and HIV prevalence of men and women at social venues where people meet new sexual partners in Eastern Kinshasa with the HIV prevalence and behavior of STI treatment and antenatal clinic patients in the same area
Critical evaluation of the neoclassical model for the equilibrium electrostatic field in a tokamak
The neoclassical prescription to use an equation of motion to determine the
electrostatic field within a tokamak plasma is fraught with difficulties.
Herein we examine two popular expressions for the equilibrium electrostatic
field so determined and show that one fails to withstand a formal scrutiny
thereof while the other fails to respect the vector nature of the diamagnetic
current. Reconsideration of the justification for the presence of the
equilibrium electrostatic field indicates that no field is needed for a neutral
plasma when considering the net bound current defined as the curl of the
magnetization. With any shift in the toroidal magnetic flux distribution, a
dynamic electric field is generated with both radial and poloidal components,
providing an alternate explanation for any measurements thereof.Comment: 13 pages, 2 figures, moderate revision, final version, edited for
length, to appear in MR
Sweat Rates During Continuous and Interval Aerobic Exercise: Implications for NASA Multipurpose Crew Vehicle (MPCV) Missions
Aerobic deconditioning is one of the effects spaceflight. Impaired crewmember performance due to loss of aerobic conditioning is one of the risks identified for mitigation by the NASA Human Research Program. Missions longer than 8 days will involve exercise countermeasures including those aimed at preventing the loss of aerobic capacity. The NASA Multipurpose Crew Vehicle (MPCV) will be NASA's centerpiece architecture for human space exploration beyond low Earth orbit. Aerobic exercise within the small habitable volume of the MPCV is expected to challenge the ability of the environmental control systems, especially in terms of moisture control. Exercising humans contribute moisture to the environment by increased respiratory rate (exhaling air at 100% humidity) and sweat. Current acceptable values are based on theoretical models that rely on an "average" crew member working continuously at 75% of their aerobic capacity (Human Systems Integration Requirements Document). Evidence suggests that high intensity interval exercise for much shorter durations are equally effective or better in building and maintaining aerobic capacity. This investigation will examine sweat and respiratory rates for operationally relevant continuous and interval aerobic exercise protocols using a variety of different individuals. The results will directly inform what types of aerobic exercise countermeasures will be feasible to prescribe for crewmembers aboard the MPCV
Stratus Ocean Reference Station (20˚S, 85˚W), mooring recovery and deployment cruise R/V Revelle cruise dana 03, November 10 - November 26, 2003
The Ocean Reference Station at 20°S, 85°W under the stratus clouds west of northern
Chile and Peru is being maintained to provide ongoing, climate-quality records of surface
meteorology, of air-sea fluxes of heat, freshwater, and momentum, and of upper ocean
temperature, salinity, and velocity variability. The Stratus Ocean Reference Station,
hereafter ORS Stratus, is supported by the National Oceanic and Atmospheric
Administrations (NOAA) Climate Observation Program. It is recovered and redeployed
annually, with cruises that have come in October or November.
During the November 2003 cruise of Scripps Institution of Oceanography's R/V Roger
Revelle to the ORS Stratus site, the primary activities where the recovery of the WHOI
surface mooring that had been deployed in October 2002, the deployment of a new WHOI
surface mooring at that site, the in-situ calibration of the buoy meteorological sensors by
comparison with instrumentation put on board by Chris Fairall of the NOAA
Environmental Technology Laboratory (ETL), and observations of the stratus clouds and
lower atmosphere by NOAA ETL and Jason Tomlinson from Texas A&M.
The ORS Stratus buoys are equipped with two Improved Meteorological systems, which
provide surface wind speed and direction, air temperature, relative humidity, barometric
pressure, incoming shortwave radiation, incoming longwave radiation, precipitation rate,
and sea surface temperature. The IMET data are made available in near real time using
satellite telemetry. The mooring line carries instruments to measure ocean salinity,
temperature, and currents. On some deployments, additional instrumentation is attached to
the mooring to measure rainfall and bio-optical variability. The ETL instrumentation used
during the 2003 cruise included a cloud radar, radiosonde balloons, and sensors for mean
and turbulent surface meteorology.
In addition to this work, buoy work was done in support of the Ecuadorian Navy Institute
of Oceanography (INOCAR) and of the Chilean Navy Hydrographic and Oceanographic
Service (SHOA). The surface buoy, oceanographic instrumentation, and upper 500 m of
an INOCAR surface mooring at 2°S, 84°W that had been vandalized were recovered and
transferred to the Ecuadorian Navy vessel B. A. E. Calicuchima. A tsunami warning
mooring was installed at 75°W, 20°S for SHOA. SHOA personnel onboard were trained
during the cruise by staff from the NOAA Pacific Marine Environmental Laboratory
(PMEL) and National Data Buoy Center (NDBC). The cruise hosted two teachers
participating in NOAA's Teacher at Sea Program, Deb Brice from San Marcos, California
and Viviana Zamorano from Arica, Chile.Funding was provided by the National Oceanic and Atmospheric Administration uncer Contract Number NA17RJ1223
An Early and Comprehensive Millimetre and Centimetre Wave and X-ray Study of SN 2011dh: a Non-Equipartition Blast Wave Expanding into a Massive Stellar Wind
Only a handful of supernovae (SNe) have been studied in multiwavelengths from the radio to X-rays, starting a few days after the explosion. The early detection and classification of the nearby Type IIb SN 2011dh/PTF 11eon in M51 provides a unique opportunity to conduct such observations. We present detailed data obtained at one of the youngest phase ever of a core-collapse SN (days 3–12 after the explosion) in the radio, millimetre and X-rays; when combined with optical data, this allows us to explore the early evolution of the SN blast wave and its surroundings. Our analysis shows that the expanding SN shock wave does not exhibit equipartition (ϵe/ϵB ∼ 1000), and is expanding into circumstellar material that is consistent with a density profile falling like R−2. Within modelling uncertainties we find an average velocity of the fast parts of the ejecta of 15 000 ± 1800 km s−1, contrary to previous analysis. This velocity places SN 2011dh in an intermediate blast wave regime between the previously defined compact and extended SN Type IIb subtypes. Our results highlight the importance of early (∼1 d) high-frequency observations of future events. Moreover, we show the importance of combined radio/X-ray observations for determining the microphysics ratio ϵe/ϵB
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