6,295 research outputs found
Bi-Directional Energy Cascades and the Origin of Kinetic Alfv\'enic and Whistler Turbulence in the Solar Wind
The observed sub-proton scale turbulence spectrum in the solar wind raises
the question of how that turbulence originates. Observations of keV energetic
electrons during solar quite-time suggest them as possible source of free
energy to drive the turbulence. Using particle-in-cell simulations, we explore
how free energy in energetic electrons, released by an electron two-stream
instability drives Weibel-like electromagnetic waves that excite wave-wave
interactions. Consequently, both kinetic Alfv\'enic and whistler waves are
excited that evolve through inverse and forward magnetic energy cascades.Comment: 12 pages, 5 figures, Submitted to Physical Review Letter
Buneman instability in a magnetized current-carrying plasma with velocity shear
Buneman instability is often driven in magnetic reconnection. Understanding
how velocity shear in the beams driving the Buneman instability affects the
growth and saturation of waves is relevant to turbulence, heating, and
diffusion in magnetic reconnection. Using a Mathieu-equation analysis for weak
cosine velocity shear together with Vlasov simulations, the effects of shear on
the kinetic Buneman instability are studied in a plasma consisting of strongly
magnetized electrons and cold unmagnetized ions. In the linearly unstable
phase, shear enhances the coupling between oblique waves and the sheared
electron beam, resulting in a wider range of unstable eigenmodes with common
lower growth rates. The wave couplings generate new features of the electric
fields in space, which can persist into the nonlinear phase when electron holes
form. Lower hybrid instabilities simultaneously occur at
with a much lower growth
rate, and are not affected by the velocity shear.Comment: Accepted by Physics of Plasm
A General SU(2) Formulation for Quantum Searching with Certainty
A general quantum search algorithm with arbitrary unitary transformations and
an arbitrary initial state is considered in this work. To serach a marked state
with certainty, we have derived, using an SU(2) representation: (1) the
matching condition relating the phase rotations in the algorithm, (2) a concise
formula for evaluating the required number of iterations for the search, and
(3) the final state after the search, with a phase angle in its amplitude of
unity modulus. Moreover, the optimal choices and modifications of the phase
angles in the Grover kernel is also studied.Comment: 8 pages, 2 figure
A Novel Visualization Method for Detecting DDoS Network Attacks
With the rapid growth of networks in size and complexity, netwok administrators today are facing more and more challenges for protecting their networked computers and other devices from all kinds of attacks. Unlike the traditional methods of analyzing textual log data, a visual interactive system called DDoSViewer is proposed in this paper for detecting DDoS kind of network attacks. DDoSViewer is specifically designed for detecting DDoS attacks through the analysis of visual patterns. We will discuss the data sources, visual structures and interactive functions that are used in the proposed visualization system. We will also discuss the advantages and disadvantages of the existing visual solutions for DDoS detection. The extraction and analysis of network data, the calculation and display of graphic elementsÂż attributes and the pre-characteristics of DDoS attacks are all included in the new visualization technique. The experiments showed that the new system can detect DDoS attacks effectivel
A General Phase Matching Condition for Quantum Searching Algorithm
A general consideration on the phase rotations in quantum searching algorithm
is taken in this work. As four phase rotations on the initial state, the marked
states, and the states orthogonal to them are taken account, we deduce a phase
matching condition for a successful search. The optimal options for these phase
are obtained consequently.Comment: 3 pages, 3 figure
Evidence for s-wave pairing from measurement on lower critical field in
Magnetization measurements in the low field region have been carefully
performed on a well-shaped cylindrical and an ellipsoidal sample of
superconductor . Data from both samples show almost the same results.
The lower critical field and the London penetration depth
are thus derived. It is found that the result of normalized superfluid density
of can be well described by BCS
prediction with the expectation for an isotropic s-wave superconductivity.Comment: To appear in Phys. Rev.
Finite-temperature phase transitions in bilayer quantum Hall systems
In this paper, the influence of an in-plane magnetic field B_\parallel on the
finite-temperature phase transitions in nu=2 bilayer quantum Hall systems are
examined. It is found that there can exist two types of finite-temperature
phase transitions. The first is the Kosterlitz-Thouless (KT) transitions, which
can have an unusual non-monotonic dependence on B_\parallel; the second type
originates from the crossing of energy levels and always increases with
B_\parallel. Based on these results, we point out that the threshold
temperature observed in the inelastic light scattering experiments cannot be
the KT transition temperature, because the latter shows a totally different
B_\parallel-dependence as compared with the experimental observation. Instead,
it should be the level-crossing temperature, which we found agrees with the
B_\parallel-dependence observed. Moreover, combining the knowledge of these two
transition temperatures, a complete finite-temperature phase diagram is
presented.Comment: RevTeX, 5 pages with 3 EPS figures include
Partonic effects on anisotropic flows at RHIC
We report recent results from a multiphase transport (AMPT) model on the
azimuthal anisotropies of particle momentum distributions in heavy ion
collisions at the Relativistic Heavy Ion Collider. These include higher-order
anisotropic flows and their scaling, the rapidity dependence of anisotropic
flows, and the elliptic flow of charm quarks.Comment: 7 pages, 5 figures, talk given at "Hot Quarks 2004", July 18-24,
2004, Taos Valley, NM, US
Chemical Abundances of Planetary Nebulae in M33
Using spectroscopic data presented in Magrini et al. (2003), we have analyzed
with the photoionization code CLOUDY 94.00 (Ferland et al. 1998) 11 Planetary
Nebulae belonging to the spiral galaxy M 33. Central star temperatures and
nebular parameters have been determined. In particular the chemical abundances
of He/H, O/H, N/H, Ar/H, and S/H have been measured and compared with values
obtained via the Ionization Correction Factors (ICFs) method, when available.
Chemical abundance relationships have been investigated; in particular, a
correlation between N/H and N/O similar to the Galactic one (Henry 1989), and a
feeble anti-correlation between O/H and N/O have been found.
A gradient in O/H across the disc of M~33 is indicatively consistent with the
one found from HII regions in this galaxy (Vilchez et al 1988). Further studies
in the more external parts of M~33 are however needed to ascertain this point.
The present result shows that oxygen and helium abundances (with lower accuracy
also nitrogen, argon and sulphur) can be actually estimated from the brightest
PNe of a galaxy, even if the electron temperature cannot be measured. We also
found that the oxygen abundance is quite independent of the absolute magnitude
of the PN and consequently the brightest PNe are representative of the whole PN
population. This represents an important tool to measure the metallicity of
galaxies at the time of the formation of PNe progenitors.Comment: 17 pages, 6 figures, A&A accepte
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