72 research outputs found
Hybrid Simulation of Solar-Wind-Like Turbulence
We present 2.5D hybrid simulations of the spectral and thermodynamic evolution of an initial state of magnetic field and plasma variables that in many ways represents solar wind fluctuations. In accordance with Helios near-Sun high-speed stream observations, we start with Alfvnic fluctuations along a mean magnetic field in which the fluctuations in the magnitude of the magnetic field are minimized. Since fluctuations in the radial flow speed are the dominant free energy in the observed fluctuations, we include a field-aligned v(k) with an k(exp 1) spectrum of velocity fluctuations to drive the turbulent evolution. The flow rapidly distorts the Alfvnic fluctuations, yielding spectra (determined by spacecraft-like cuts) transverse to the field that become comparable to the k fluctuations, as in spacecraft observations. The initial near constancy of the magnetic field is lost during the evolution; we show this also takes place observationally. We find some evolution in the anisotropy of the thermal fluctuations, consistent with expectations based on Helios data. We present 2D spectra of the fluctuations, showing the evolution of the power spectrum and cross-helicity. Despite simplifying assumptions, many aspects of simulations and observations agree. The greatly faster evolution in the simulations is at least in part due to the small scales being simulated, but also to the non-equilibrium initial conditions and the relatively low overall Alfvnicity of the initial fluctuations
Three dimensional MHD Modeling of Vertical Kink Oscillations in an Active Region Plasma Curtain
Observations on 2011 August 9 of an X6.9-class flare in active region (AR)
11263 by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics
Observatory (SDO), were followed by a rare detection of vertical kink
oscillations in a large-scale coronal active region plasma curtain in EUV
coronal lines. The damped oscillations with periods in the range 8.8-14.9 min
were detected and analyzed recently. Our aim is to study the generation and
propagation of the MHD oscillations in the plasma curtain taking into account
realistic 3D magnetic and density structure of the curtain. We also aim at
testing and improving coronal seismology for more accurate determination of the
magnetic field than with standard method. We use the observed morphological and
dynamical conditions, as well as plasma properties of the coronal curtain based
on Differential Emission Measure (DEM) analysis to initialize a 3D MHD model of
its vertical and transverse oscillations by implementing the impulsively
excited velocity pulse mimicking the flare generated nonlinear fast
magnetosonic propagating disturbance interacting with the curtain obliquely.
The model is simplified by utilizing initial dipole magnetic field, isothermal
energy equation, and gravitationally stratified density guided by observational
parameters. Using the 3D MHD model, we are able to reproduce the details of the
vertical oscillations and study the process of their excitation by nonlinear
fast magnetosonic pulse, propagation, and damping, finding agreement with the
observations. We estimate the accuracy of simplified slab-based coronal
seismology by comparing the determined magnetic field strength to actual values
from the 3D MHD modeling results and demonstrate the importance of taking into
account more realistic magnetic geometry and density for improving coronal
seismology
Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfv\'en Waves
We present the numerical simulations of winds from evolved giant stars using
a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD)
code. This study extends our previous fully non-linear MHD wind simulations to
include a broadband frequency spectrum of Alfv\'en waves that drive winds from
red giant stars. We calculated four Alfv\'en wind models that cover the whole
range of Alfv\'en wave frequency spectrum to characterize the role of freely
propagated and reflected Alfv\'en waves in the gravitationally stratified
atmosphere of a late-type giant star. Our simulations demonstrate that, unlike
linear Alfv\'en wave-driven wind models, a stellar wind model based on plasma
acceleration due to broadband non-linear Alfv\'en waves, can consistently
reproduce the wide range of observed radial velocity profiles of the winds,
their terminal velocities and the observed mass loss rates. Comparison of the
calculated mass loss rates with the empirically determined mass loss rate for
alpha Tau suggests an anisotropic and time-dependent nature of stellar winds
from evolved giants.Comment: accepted by Ap
Slow magnetosonic waves and fast flows in active region loops
Recent EUV spectroscopic observations indicate that slow magnetosonic waves
are present in active region (AR) loops. Some of the spectral data were also
interpreted as evidence of fast (~100-300 km/s) quasi-periodic flows. We have
performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar
AR that contains impulsively generated waves and flows in coronal loops. The
model AR is initiated with a dipole magnetic field and gravitationally
stratified density, with an upflow driven steadily or periodically in localized
regions at the footpoints of magnetic loops. The resulting flows along the
magnetic field lines of the AR produce higher density loops compared to the
surrounding plasma by injection of material into the flux-tubes and the
establishment of siphon flow. We find that the impulsive onset of flows with
subsonic speeds result in the excitation of damped slow magnetosonic waves that
propagate along the loops and coupled nonlinearly driven fast mode waves. The
phase speed of the slow magnetosonic waves is close to the coronal sound speed.
When the amplitude of the driving pulses is increased we find that slow
shock-like wave trains are produced. When the upflows are driven periodically,
undamped oscillations are produced with periods determined by the periodicity
of the upflows. Based on the results of the 3D MHD model we suggest that the
observed slow magnetosonic waves and persistent upflows may be produced by the
same impulsive events at the bases of ARs.Comment: Accepted for publication in The Astrophysical Journa
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