580 research outputs found
Relative drifts and temperature anisotropies of protons and particles in the expanding solar wind -- 2.5D hybrid simulations
We perform 2.5D hybrid simulations to investigate the origin and evolution of
relative drift speeds between protons and particles in the
collisionless turbulent low- solar wind plasma. We study the generation
of differential streaming by wave-particle interactions and absorption of
turbulent wave spectra. Next we focus on the role of the relative drifts for
the turbulent heating and acceleration of ions in the collisionless fast solar
wind streams. The energy source is given by an initial broad-band spectrum of
parallel propagating Alfv\'en-cyclotron waves, which co-exists with the plasma
and is self-consistently coupled to the perpendicular ion bulk velocities. We
include the effect of a gradual solar wind expansion, which cools and
decelerates the minor ions. This paper for the first time considers the
combined effect of self-consistently initialized dispersive turbulent
Alfv\'enic spectra with differentially streaming protons and particles
in the expanding solar wind outflows within a 2.5D hybrid simulation study. In
the non-expanding wind, we find a threshold value of the differential streaming
, for which the relative drift speed remains
nearly steady. For ions, streaming below the threshold value, the waves act to
increase the magnitude of the relative drift speed. Ions, which stream faster
than the threshold value become subject to nonlinear streaming instability and
as the system evolves their bulk velocities decrease. We find that the solar
wind expansion strongly affects the relative drift speeds and significantly
slows down both ion species for all values of the relative drift speeds
considered in this study.Comment: 11 pages, 13 figures, submitted to A&
Nonlinear Terms of MHD Equations for Homogeneous Magnetized Shear Flow
We have derived the full set of MHD equations for incompressible shear flow
of a magnetized fluid and considered their solution in the wave-vector space.
The linearized equations give the famous amplification of slow magnetosonic
waves and describe the magnetorotational instability. The nonlinear terms in
our analysis are responsible for the creation of turbulence and self-sustained
spectral density of the MHD (Alfven and pseudo-Alfven) waves. Perspectives for
numerical simulations of weak turbulence and calculation of the effective
viscosity of accretion disks are shortly discussed in k-space.Comment: 13 pages, no figures; AIP Conference Proceedings 1356, Proceedings of
the School and Workshop on Space Plasma Physics (1--12 September 2010, Kiten,
Bulgaria), American Institute of Physics, Melville, NY, 201
Amplification of Slow Magnetosonic Waves by Shear Flow: Heating and Friction Mechanisms of Accretion Disks
Propagation of three dimensional magnetosonic waves is considered for a
homogeneous shear flow of an incompressible fluid. The analytical solutions for
all magnetohydrodynamic variables are presented by confluent Heun functions.
The problem is reduced to finding a solution of an effective Schroedinger
equation. The amplification of slow magnetosonic waves is analyzed in great
details. A simple formula for the amplification coefficient is derived. The
velocity shear primarily affects the incompressible limit of slow magnetosonic
waves. The amplification is very strong for slow magnetosonic waves in the
long-wavelength limit. It is demonstrated that the amplification of those waves
leads to amplification of turbulence. The phenomenology of Shakura-Sunyaev for
the friction in accretion disks is derived in the framework of the Kolmogorov
turbulence. The presented findings may be the key to explaining the anomalous
plasma heating responsible for the luminosity of quasars. It is suggested that
wave amplification is the keystone of the self-sustained turbulence in
accretion disks.Comment: 26pages, 11 figures, In Space Plasma Physics, Proceedings of the
School and Workshop on Space Plasma Physics, 31 August--7 September 2008,
Sozopol, Bulgaria, Editor: I. Zhelyazkov, American Institute of Physics, AIP
Conference Proceedings (2009). 5 new references are given in version 2.
Analytical expression for long wavelength amplificatio
Optimal purification of thermal graph states
In this paper, a purification protocol is presented and its performance is
proven to be optimal when applied to a particular subset of graph states that
are subject to local Z-noise. Such mixed states can be produced by bringing a
system into thermal equilibrium, when it is described by a Hamiltonian which
has a particular graph state as its unique ground state. From this protocol, we
derive the exact value of the critical temperature above which purification is
impossible, as well as the related optimal purification rates. A possible
simulation of graph Hamiltonians is proposed, which requires only bipartite
interactions and local magnetic fields, enabling the tuning of the system
temperature.Comment: 5 pages, 4 figures v2: published versio
On the origin of solar wind. Alfven waves induced jump of coronal temperature
Absorbtion of Alfven waves is considered to be the main mechanism of heating
in the solar corona. It is concluded that the sharp increase of the plasma
temperature by two orders of magnitude is related to a self-induced opacity
with respect to Alfven waves. The maximal frequency for propagation of Alfven
waves is determined by the strongly temperature dependent kinematic viscosity.
In such a way the temperature jump is due to absorption of high frequency
Alfven waves in a narrow layer above the solar surface. It is calculated that
the power per unit area dissipated in this layer due to damping of Alfven waves
blows up the plasma and gives birth to the solar wind. A model short
wave-length (WKB) evaluation takes into account the 1/f^2 frequency dependance
of the transversal magnetic field and velocity spectral densities. Such
spectral densities agree with old magnetometric data taken by Voyager 1 and
recent theoretical calculations in the framework of Langevin-Burgers MHD. The
present theory predicts existence of intensive high frequency MHD Alfven waves
in the cold layer beneath the corona. It is briefly discussed how this
statement can be checked experimentally. It is demonstrated that the magnitude
of the Alfven waves generating random noise and the solar wind velocity can be
expressed only in terms of satellite experimental data. It is advocated that
investigation of properties of the solar surface as a random driver by optical
methods is an important task for future solar physics. Jets of accretion disks
are speculated as a special case of the wind from magnetized turbulent plasma.Comment: 4 pages, no figures, minor corrections, final version for EPJ
A Simultaneous Quantum Secure Direct Communication Scheme between the Central Party and Other M Parties
We propose a simultaneous quantum secure direct communication scheme between
one party and other three parties via four-particle GHZ states and swapping
quantum entanglement. In the scheme, three spatially separated senders, Alice,
Bob and Charlie, transmit their secret messages to a remote receiver Diana by
performing a series local operations on their respective particles according to
the quadripartite stipulation. From Alice, Bob, Charlie and Diana's Bell
measurement results, Diana can infer the secret messages. If a perfect quantum
channel is used, the secret messages are faithfully transmitted from Alice, Bob
and Charlie to Diana via initially shared pairs of four-particle GHZ states
without revealing any information to a potential eavesdropper. As there is no
transmission of the qubits carrying the secret message in the public channel,
it is completely secure for the direct secret communication. This scheme can be
considered as a network of communication parties where each party wants to
communicate secretly with a central party or server.Comment: 4 pages, no figur
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