19,604 research outputs found
Lightning induced currents in aircraft wiring using low level injection techniques
Various techniques were studied to predict the transient current induced into aircraft wiring bundles as a result of an aircraft lightning strike. A series of aircraft measurements were carried out together with a theoretical analysis using computer modeling. These tests were applied to various aircraft and also to specially constructed cylinders installed within coaxial return conductor systems. Low level swept frequency CW (carrier waves), low level transient and high level transient injection tests were applied to the aircraft and cylinders. Measurements were made to determine the transfer function between the aircraft drive current and the resulting skin currents and currents induced on the internal wiring. The full threat lightning induced transient currents were extrapolated from the low level data using Fourier transform techniques. The aircraft and cylinders used were constructed from both metallic and CFC (carbon fiber composite) materials. The results show the pulse stretching phenomenon which occurs for CFC materials due to the diffusion of the lightning current through carbon fiber materials. Transmission Line Matrix modeling techniques were used to compare theoretical and measured currents
Tributes to Professor Robert Berkley Harper
In 1977, I began teaching at The University of Pittsburgh Law School and in short order one of my closest friends during my tenure there was Professor Robert “Bob” Harper. I wondered when I was hired whether I was selected because I looked strikingly similar to Bob, and perhaps the faculty thought my favoring Professor Harper would make my assimilation into the law school faculty that much easier. Students constantly called me Professor Harper and, indeed, many on the faculty called me Bob for several years; I never bothered to correct them. I thought if they paid that little attention to detail in law school, I would just let them go through life missing some of the finer points their education, and life for that matter, has to offer
QUASAT: An orbiting very long baseline interferometer program using large space antenna systems
QUASAT, which stands for QUASAR SATELLITE, is the name given to a new mission being studied by NASA. The QUASAT mission concept involves a free flying Earth orbiting large radio telescope, which will observe astronomical radio sources simultaneously with ground radio telescopes. The primary goal of QUASAT is to provide a system capable of collecting radio frequency data which will lead to a better understanding of extremely high energy events taking place in a variety of celestial objects including quasars, galactic nuclei, interstellar masers, radio stars and pulsars. QUASAT's unique scientific contribution will be the increased resolution in the emission brightness profile maps of the celestial objects
Migration as Adaptation? Exploring The Scope for Co-ordinating Environmental and Migration Policies in the European Union
Electrostatics of ions inside the nanopores and trans-membrane channels
A model of a finite cylindrical ion channel through a phospholipid membrane
of width separating two electrolyte reservoirs is studied. Analytical
solution of the Poisson equation is obtained for an arbitrary distribution of
ions inside the trans-membrane pore. The solution is asymptotically exact in
the limit of large ionic strength of electrolyte on the two sides of membrane.
However, even for physiological concentrations of electrolyte, the
electrostatic barrier sizes found using the theory are in excellent agreement
with the numerical solution of the Poisson equation. The analytical solution is
used to calculate the electrostatic potential energy profiles for pores
containing charged protein residues. Availability of a semi-exact interionic
potential should greatly facilitate the study of ionic transport through
nanopores and ion channels
On the incidence of weak magnetic fields in DA white dwarfs
Context: About 10% of white dwarfs have magnetic fields with strength in the
range between about 10^5 and 3x10^8 G. It is not known whether the remaining
white dwarfs are not magnetic, or if they have a magnetic field too weak to be
detected with the techniques adopted in the large surveys. Aims. We describe
the results of the first survey specifically devised to clarify the detection
frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using
the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular
spectropolarimetric measurements of a small sample of cool (DA6 - DA8) white
dwarfs. Using FORS and UVES archive data, we have also revised numerous white
dwarf field measurements previously published in the literature. Results: We
have discovered an apparently constant longitudinal magnetic field of \sim9.5
kG in the DA6 white dwarf WD2105-820. This star is the first weak-field white
dwarf that has been observed sufficiently to roughly determine the
characteristics of its field. The available data are consistent with a simple
dipolar morphology with magnetic axis nearly parallel to the rotation axis, and
a polar strength of \simeq 56 kG. Our re-evaluation of the FORS archive data
for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG
had previously been correctly identified in at least three white dwarfs.
Conclusions: We find that the probability of detecting a field of kG strength
in a DA white dwarf is of the order of 10% for each of the cool and hot DA
stars. If there is a lower cutoff to field strength in white dwarfs, or a field
below which all white dwarfs are magnetic, the current precision of
measurements is not yet sufficient to reveal it.Comment: Accepted for publication in Astronomy & Astrophysic
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with
magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000
K and 50000 K. Through a least-squares minimization scheme with an evolutionary
algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95
previously published plus 14 newly discovered within SDSS, and five discovered
by SEGUE). Since we were limited to a single spectrum for each object we used
only centered magnetic dipoles or dipoles which were shifted along the magnetic
dipole axis. We also statistically investigated the distribution of
magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
A periodically active pulsar giving insight into magnetospheric physics
PSR B1931+24 (J1933+2421) behaves as an ordinary isolated radio pulsar during
active phases that are 5-10 days long. However, the radio emission switches off
in less than 10 seconds and remains undetectable for the next 25-35 days, then
it switches on again. This pattern repeats quasi-periodically. The origin of
this behaviour is unclear. Even more remarkably, the pulsar rotation slows down
50% faster when it is on than when it is off. This indicates a massive increase
in magnetospheric currents when the pulsar switches on, proving that pulsar
wind plays a substantial role in pulsar spin-down. This allows us, for the
first time, to estimate the currents in a pulsar magnetospheric during the
occurrence of radio emission.Comment: 12 pages, 2 figure
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