3,566 research outputs found
Solar Wind Electric Fields in the Ion Cyclotron Frequency Range
Measurements of fluctuations of electric fields in the frequency range from a
fraction of one Hz to 12.5 Hz are presented, and corrected for the Lorentz
transformation of magnetic fluctuations to give the electric fields in the
plasma frame. The electric fields are large enough to provide the dominant
force on the ions of the solar wind in the region near the ion cyclotron
frequency of protons, larger than the force due to magnetic fluctuations. They
provide sufficient velocity space diffusion or heating to counteract
conservation of magnetic moment in the expanding solar wind to maintain nearly
isotropic velocity distributions
Intelligent search for distributed information sources using heterogeneous neural networks
As the number and diversity of distributed information sources on the Internet exponentially increase, various search services are developed to help the users to locate relevant information. But they still exist some drawbacks such as the difficulty of mathematically modeling retrieval process, the lack of adaptivity and the indiscrimination of search. This paper shows how heteroge-neous neural networks can be used in the design of an intelligent distributed in-formation retrieval (DIR) system. In particular, three typical neural network models - Kohoren's SOFM Network, Hopfield Network, and Feed Forward Network with Back Propagation algorithm are introduced to overcome the above drawbacks in current research of DIR by using their unique properties. This preliminary investigation suggests that Neural Networks are useful tools for intelligent search for distributed information sources
Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence
Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show
the spectral behavior of classical Kolmogorov fluid turbulence over an inertial
subrange and departures from this at short wavelengths, where energy should be
dissipated. Here we present the first measurements of the electric field
fluctuation spectrum over the inertial and dissipative wavenumber ranges in a
plasma. The inertial subrange is observed and
agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed
in this regime is shown to be consistent with the Alfv\'en speed. At smaller
wavelengths the electric spectrum is softer and is consistent
with the expected dispersion relation of short-wavelength kinetic Alfv\'en
waves. Kinetic Alfv\'en waves damp on the solar wind ions and electrons and may
act to isotropize them. This effect may explain the fluid-like nature of the
solar wind.Comment: submitted; 4 pages + 3 figure
Large parallel and perpendicular electric fields on electron spatial scales in the terrestrial bow shock
Large parallel ( 100 mV/m) and perpendicular ( 600 mV/m) electric
fields were measured in the Earth's bow shock by the vector electric field
experiment on the Polar satellite. These are the first reported direct
measurements of parallel electric fields in a collisionless shock. These fields
exist on spatial scales comparable to or less than the electron skin depth (a
few kilometers) and correspond to magnetic field-aligned potentials of tens of
volts and perpendicular potentials up to a kilovolt. The perpendicular fields
are amongst the largest ever measured in space, with energy densities of
of order 10%. The measured parallel electric field
implies that the electrons can be demagnetized, which may result in stochastic
(rather than coherent) electron heating
Phonons in random alloys: the itinerant coherent-potential approximation
We present the itinerant coherent-potential approximation(ICPA), an analytic,
translationally invariant and tractable form of augmented-space-based,
multiple-scattering theory in a single-site approximation for harmonic phonons
in realistic random binary alloys with mass and force-constant disorder.
We provide expressions for quantities needed for comparison with experimental
structure factors such as partial and average spectral functions and derive the
sum rules associated with them. Numerical results are presented for Ni_{55}
Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for
weak force-constant disorder and the latter for strong. We present results on
dispersion curves and disorder-induced widths. Direct comparisons with the
single-site coherent potential approximation(CPA) and experiment are made which
provide insight into the physics of force-constant changes in random alloys.
The CPA accounts well for the weak force-constant disorder case but fails for
strong force-constant disorder where the ICPA succeeds.Comment: 19 pages, 12 eps figures, uses RevTex
Whistler Wave Observations by \textit{Parker Solar Probe} During Encounter : Counter-Propagating Whistlers Collocated with Magnetic Field Inhomogeneities and their Application to Electric Field Measurement Calibration
Observations of the young solar wind by the Parker Solar Probe (PSP) mission
reveal the existence of intense plasma wave bursts with frequencies between
-- (tens of Hz up to Hz) in the
spacecraft frame. The wave bursts are often collocated with inhomogeneities in
the solar wind magnetic field, such as local dips in magnitude or sudden
directional changes. The observed waves are identified as electromagnetic
whistler waves that propagate either sunward, anti-sunward, or in
counter-propagating configurations during different burst events. Being
generated in the solar wind flow the waves experience significant Doppler
down-shift and up-shift {of wave frequency} in the spacecraft frame for sunward
and anti-sunward waves, respectively. Their peak amplitudes can be larger than
~nT, where such values represent up to of the background magnetic
field during the interval of study. The amplitude is maximum for propagation
parallel to the background magnetic field. We (i) evaluate the properties of
these waves by reconstructing their parameters in the plasma frame, (ii)
estimate the effective length of the PSP electric field antennas at whistler
frequencies, and (iii) discuss the generation mechanism of these waves
Prelude to THEMIS tail conjunction study
A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 <I>R<sub>E</sub></I> downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 <I>R<sub>E</sub></I> downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction
Whistler waves generated inside magnetic dips in the young solar wind: observations of the Search-Coil Magnetometer on board Parker Solar Probe
Context. Whistler waves are electromagnetic waves produced by electron-driven
instabilities, that in turn can reshape the electron distributions via
wave-particle interactions. In the solar wind, they are one of the main
candidates for explaining the scattering of the strahl electron population into
the halo at increasing radial distances from the Sun and for subsequently
regulating the solar wind heat flux. However, it is unclear what type of
instability dominates to drive whistlers in the solar wind. Aims. Our goal is
to study whistler wave parameters in the young solar wind sampled by Parker
Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter
to discriminate between the generation mechanisms of these waves. Methods. We
analyze the cross-spectral matrices of magnetic fieldfluctuations measured by
the Search-Coil Magnetometer (SCM) and processed by the Digital Fields Board
(DFB) from the FIELDS suite during PSP's first perihelion. Results. Among the
2701 wave packets detected in the cross spectra, namely individual bins in time
and frequency, most were quasi-parallel to the background magnetic field but a
significant part (3%) of observed waves had oblique (> 45{\deg}) WNA. The
validation analysis conducted with the time-series waveforms reveal that this
percentage is a lower limit. Moreover, we find that about 64% of the whistler
waves detected in the spectra are associated with at least one magnetic dip.
Conclusions. We conclude that magnetic dips provides favorable conditions for
the generation of whistler waves. We hypothesize that the whistlers detected in
magnetic dips are locally generated by the thermal anisotropy as quasi-parallel
and can gain obliqueness during their propagation. We finally discuss the
implication of our results for the scattering of the strahl in the solar wind.Comment: 15 pages, 14 figures, recommended for publication in A&
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Letter processing and font information during reading: beyond distinctiveness, where vision meets design
Letter identification is a critical front end of the
reading process. In general, conceptualizations of the identification process have emphasized arbitrary sets of distinctive features. However, a richer view of letter processing incorporates principles from the field of type design, including an emphasis on uniformities across letters within a font. The importance of uniformities is supported by a small body of research indicating that consistency of font increases letter identification efficiency. We review design concepts and the relevant literature, with the goal of stimulating further thinking about letter processing during reading
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