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.
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.
In order to study chromospheric magnetosonic wave propagation including, for the first time, the effects of ion-neutral interactions in the partially ionized solar chromosphere, we have developed a ...new multi-fluid computational model, accounting for ionization and recombination reactions in gravitationally stratified magnetized collisional media. The two-fluid model used in our 2D numerical simulations treats neutrals as a separate fluid and considers charged species (electrons and ions) within the resistive MHD approach with Coulomb collisions and anisotropic heat flux determined by Braginskii's transport coefficients. The electromagnetic fields are evolved according to the full Maxwell equations and the solenoidality of the magnetic field is enforced with a hyperbolic divergence cleaning scheme. The initial density and temperature profiles are similar to VAL III chromospheric model in which dynamical, thermal and chemical equilibrium are considered to ensure comparison to existing MHD models and avoid artificial numerical heating. In this initial setup we include simple homogeneous flux tube magnetic field configuration and an external photospheric velocity driver to simulate the propagation of MHD waves in the partially ionized reactive chromosphere. In particular, we investigate the loss of chemical equilibrium and the plasma heating related to the steepening of fast magnetosonic wave fronts in the gravitationally stratified medium.
A&A 578, A85 (2015) We perform 2.5D hybrid simulations to investigate the origin and evolution of
relative drift speeds between protons and $\alpha$ particles in the
collisionless turbulent ...low-$\beta$ 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 $\alpha$ 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
$V_{\alpha p} = 0.5 V_\mathrm{A}$, 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.
A first approach to the shower reconstruction from the amplitudes of the Cerenkov photons is made using Artificial Neural Networks, (ANN). We show that ANN method gives good results for the ...photon-proton discrimination, the primary energy determination and encouraging perspectives for the determination of the location of the shower core.
The monitoring of the atmosphere is very relevant for Imaging Atmospheric Cherenkov Telescopes. Adverse weather conditions (strong wind, high humidity, etc.) may damage the telescopes and must ...therefore be monitored continuously to guarantee a safe operation, and the presence of clouds and aerosols affects the transmission of the Cherenkov light and consequently the performance of the telescopes. The ATmospheric CAlibration (ATCA) technical working group of the MAGIC collaboration aims to cover all aspects related to atmosphere monitoring and calibration. In this paper we give an overview of the ATCA goals and activities, which include the set-up and maintenance of appropriate instrumentation, proper analysis of its data, the realization of MC studies, and the correction of real data taken under non-optimal atmospheric conditions. The final goal is to reduce the systematic uncertainties in the determination of the \(\gamma\)-ray flux and energy, and to increase the duty cycle of the telescopes by establishing optimized data analysis methods specific for real atmospheric conditions.
We perform 2.5D hybrid simulations with massless fluid electrons and kinetic
particle-in-cell ions to study the temporal evolution of ion temperatures,
temperature anisotropies and velocity ...distribution functions in relation to the
dissipation and turbulent evolution of a broad-band spectrum of parallel and
obliquely propagating Alfv\'en-cyclotron waves. The purpose of this paper is to
study the relative role of parallel versus oblique Alfv\'en-cyclotron waves in
the observed heating and acceleration of minor ions in the fast solar wind. We
consider collisionless homogeneous multi-species plasma, consisting of
isothermal electrons, isotropic protons and a minor component of drifting
$\alpha$ particles in a finite-$\beta$ fast stream near the Earth. The kinetic
ions are modeled by initially isotropic Maxwellian velocity distribution
functions, which develop non-thermal features and temperature anisotropies when
a broad-band spectrum of low-frequency non-resonant, $\omega \leq 0.34
\Omega_p$, Alfv\'en-cyclotron waves is imposed at the beginning of the
simulations. The initial plasma parameter values, such as ion density,
temperatures and relative drift speeds, are supplied by fast solar wind
observations made by the \textit{Wind} spacecraft at 1AU. The imposed
broad-band wave spectra is left-hand polarized and resembles \textit{Wind}
measurements of Alfv\'enic turbulence in the solar wind. The imposed magnetic
field fluctuations for all cases are within the inertial range of the solar
wind turbulence and have a Kraichnan-type spectral slope $\alpha=-3/2$. We vary
the propagation angle from $\theta= 0^\circ$ to $\theta=30^\circ$ and
$\theta=60^\circ$, and find that the minor ion heating is most efficient for
the highly-oblique waves propagating at $60^\circ$, whereas the protons exhibit
perpendicular cooling at all propagation angles.
We perform 2.5D hybrid simulations to investigate the origin and evolution of relative drift speeds between protons and \(\alpha\) particles in the collisionless turbulent low-\(\beta\) 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 Alfvén-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 Alfvénic spectra with differentially streaming protons and \(\alpha\) 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 \(V_{\alpha p} = 0.5 V_\mathrm{A}\), 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.
The measurement of different reactions of
p
d annihilation at rest in a gaseous target has been performed using the OBELIX spectrometer at LEAR (CERN). A strong deviation from the OZI-rule prediction ...was found from the measurement of the ratio
R =
φπ
ωπ
in two regions of proton momenta,
P < 200 MeV/
c and
P > 400 MeV/
c:
R(
φπ
−
ωπ
−
) = (133 ± 26) × 10
−3
and (113 ± 30) × 10
−3, respectively. These values are about 30 times greater than the theoretical prediction. For the first time the excitation of the †-resonance was observed among the final-state products of
p
d annihilation. The existence of a broad enhancement in the 4π invariant mass at
m ≈ 1480 MeV, seen in previous experiments, was confirmed. A ≈ 100 MeV downward shift of the bump position, when the proton momentum increased up to
P > 400 MeV/
c, was also observed, while the positions of ω, ϱ and f
2(1270) did not change with the proton momentum. The following branching ratios were measured:
BR(
pd
→ π
−φ
p) = (6.62 ± 0.49) × 10
−4
, for
P < 200 MeV/
c;
BR(
pd
→ π
−φ
p) = (0.95 ± 0.22) × 10
−4
, for
P > 400 MeV/
c;
BR(
pd
→ π
−ω
p) = (49.7 ± 8.9) × 10
−4
, for
P < 200 MeV/
c;
BR(
pd
→ π
−ω
p) = (8.38 ± 1.09) × 10
−4
, for
P > 400 MeV/
c;
BR(
pd
→ 2π
−π
+
p) = (150 ± 6) × 10
−4
, for
P < 200 MeV/
c;
BR(
pd
→ 2π
−π
+
p) = (16.6 ± 0.9) × 10
−4
, for
P > 400 MeV/
c;
BR(
pd
→ 3π
−2π
+
p) = (326 ± 12) × 10
−4
, for
P < 200 MeV/
c;
BR(
pd
→ 3π
−2π
+
p) = (44 ± 7) × 10
−4
, for
P > 400 MeV/
c;
BR(
pd
→ Λ
K
+π
−) = (0.96 ± 0.19) × 10
−4
, for
P > 400 MeV/
c;
BR(
pd
→ Λ
K
+π
−π
0) = (3.5 ± 0.8) × 10
−4
, for
P > 400 MeV/
c;
BR(
pp
→ 2π
−2π
+) = (540 ± 20) × 10
−4
;
BR(
pp
→ 3π
−3π
+) = (251 ± 21) × 10
−4
.
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