9 research outputs found
Particle-in-cell simulations of circularly polarised Alfvén wave phase mixing: A new mechanism for electron acceleration in collisionless plasmas
In this work we used Particle-In-Cell simulations to study the interaction of circularly polarised Alhén waves with one dimensional plasma density inhomogeneities transverse to the uniform magnetic field (phase mixing) in collisionless plasmas. In our preliminary work we reported discovery of a new electron acceleration mechanism, in which progressive distortion of the Alfvén wave front, due to the differences in local Alfvén speed, generates an oblique (nearly parallel to the magnetic field) electrostatic field. The latter accelerates electrons through the Landau resonance. Here we report a detailed study of this novel mechanism, including: (i) analysis of broadening of the ion distribution function due to the presence of Alfvén waves; and (ii) the generation of compressive perturbations due to both weak non-linearity and plasma density inhomogeneity. The amplitude decay law in the inhomogeneous regions, in the kinetic regime, is demonstrated to be the same as in the MHD approximation described by Heyvaerts & Priest (1983, A&A, 117, 220)
A weakly nonlinear Alfvénic pulse in a transversely inhomogeneous medium
The interaction of a weakly nonlinear Alfvénic pulse with an Alfvén speed inhomogeneity in the direction perpendicular to the magnetic field is investigated. Identical to the phase mixing experienced by a harmonic Alfvén wave, sharp transverse gradients are generated in the pulse by the inhomogeneity. In the initial stage of the evolution of an initially plane Alfvénic pulse, the transverse gradients efficiently generate transversely propagating fast magnetoacoustic waves. However, high resolution full MHD numerical simulations of the developed stage of the pulse evolution show that the generation saturates due to destructive wave interference. It is shown that the weakly non-linear description of the generated fast magnetoacoustic wave is well described by the driven wave equation proposed in Nakariakov et al. (1997), and a simple numerical code (2D MacCromack), which solves it with minimal CPU resources, produces identical results to those obtained from the full MHD code (Lare2d, Arber et al. 2001). A parametric study of the phenomenon is undertaken, showing that, contrary to one's expectations, steeper inhomogeneities of the Alfvén speed do not produce higher saturation levels of the fast wave generation. There is a certain optimal gradient of the inhomogeneity that ensures the maximal efficiency of the fast wave generation
Coronal Loop Oscillations Observed with AIA - Kink-Mode with Cross-Sectional and Density Oscillations
A detailed analysis of a coronal loop oscillation event is presented, using
data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) for the first time. The loop oscillation event occurred on
2010 Oct 16, 19:05-19:35 UT, was triggered by an M2.9 GOES-class flare, located
inside a highly inclined cone of a narrow-angle CME. This oscillation event had
a number of unusual features: (i) Excitation of kink-mode oscillations in
vertical polarization (in the loop plane); (ii) Coupled cross-sectional and
density oscillations with identical periods; (iii) no detectable kink amplitude
damping over the observed duration of four kink-mode periods ( min);
(iv) multi-loop oscillations with slightly () different periods;
and (v) a relatively cool loop temperature of MK. We employ a
novel method of deriving the electron density ratio external and internal to
the oscillating loop from the ratio of Alfv\'enic speeds deduced from the flare
trigger delay and the kink-mode period, i.e.,
. The coupling of the kink mode and
cross-sectional oscillations can be explained as a consequence of the loop
length variation in the vertical polarization mode. We determine the exact
footpoint locations and loop length with stereoscopic triangulation using
STEREO/EUVI-A data. We model the magnetic field in the oscillating loop using
HMI/SDO magnetogram data and a potential field model and find agreement with
the seismological value of the magnetic field, G, within a
factor of two.Comment: ApJ (in press, accepted May 10, 2011
Fundamental Physical Processes in Coronae: Waves, Turbulence, Reconnection, and Particle Acceleration
Our understanding of fundamental processes in the solar corona has been
greatly progressed based on the space observations of SMM, Yohkoh, Compton GRO,
SOHO, TRACE, RHESSI, and STEREO. We observe now acoustic waves, MHD
oscillations, turbulence-related line broadening, magnetic configurations
related to reconnection processes, and radiation from high-energy particles on
a routine basis. We review a number of key observations in EUV, soft X-rays,
and hard X-rays that innovated our physical understanding of the solar corona,
in terms of hydrodynamics, MHD, plasma heating, and particle acceleration
processes.Comment: Proc. IAU Symp. 247, Waves and Oscillations in the Solar Atmosphere:
Heating and Magneto-Seismology, (ed. R. Erdelyi
A mechanism for parallel electric field generation in the MHD limit: possible implications for the coronal heating problem in the two stage mechanism
We solve numerically ideal, 2.5D, MHD equations in Cartesian coordinates,
with a plasma beta of 0.0001 starting from the equilibrium that mimics a
footpoint of a large curvature radius solar coronal loop or a polar region
plume. On top of such an equilibrium, a purely Alfv\'enic, linearly polarised,
plane wave is launched. In the context of the coronal heating problem a new two
stage mechanism of plasma heating is presented by putting emphasis, first, on
the generation of parallel electric fields within an ideal MHD description
directly, rather than focusing on the enhanced dissipation mechanisms of the
Alfv\'en waves and, second, dissipation of these parallel electric fields via
{\it kinetic} effects. It is shown that a single Alfv\'en wave harmonic with
frequency Hz and longitudinal wavelength Mm, for a
putative Alfv\'en speed of 4328 km s, the generated parallel electric
field could account for 10% of the necessary coronal heating requirement. We
conjecture that wide spectrum (10 Hz) Alfv\'en waves, based on the
observationally constrained spectrum, could provide the necessary coronal
heating requirement. The exact amount of energy that could be deposited by such
waves through our mechanism of parallel electric field generation can only be
calculated once a more complete parametric study is done. Thus, the
"theoretical spectrum" of the energy stored in parallel electric fields versus
frequency needs to be obtained.Comment: Astron. Astrophys. (accepted, in press) (2006) - FULL pape