Abstract
Dispersive Alfven waves (DAWs) offer, an alternative to magnetic reconnection, opportunity to accelerate solar flare particles in order to alleviate the problem of delivering flare energy to denser parts of the solar atmosphere to match xray observations. Here, we focus on the effect of DAW polarisation, left, right, circular and elliptical, in the different regimes inertial and kinetic, aiming to study these effects on the efficiency of particle acceleration. We use 2.5D particleincell simulations to study how the particles are accelerated when DAW, triggered by a solar flare, propagates in the transversely inhomogeneous plasma that mimics solar coronal loop. (1) In the inertial regime, fraction of accelerated electrons (along the magnetic field) in the density gradient regions is ≈20% by the time when DAW develops three wavelengths and is increasing to ≈30% by the time when DAW develops thirteen wavelengths. In all considered cases, ions are heated in the transverse to the magnetic field direction and fraction of heated particles is ≈35%. (2) The case of right circular, left and right elliptical polarisation DAWs, with the electric field in the nonignorable transverse direction exceeding several times that of in the ignorable direction, produce more pronounced parallel electron beams (with larger maximal electron velocities) and transverse ion beams in the ignorable direction. In the inertial regime, such polarisations yield the fraction of accelerated electrons 20%. In the kinetic regime, this increases to 35%. (3) The parallel electric field that is generated in the density inhomogeneity regions is independent of the electronion mass ratio and stays of the order 0.03 ω_{pe}cm_{e} /e which for solar flaring plasma parameters exceeds Dreicer electric field by eight orders of magnitude. (4) Electron beam velocity has the phase velocity of the DAW. Thus, electron acceleration is via Landau damping of DAWs. For the Alfven speeds of V_{A} = 0.3c, the considered mechanism can accelerate electrons to energies circa 20 keV. (5) The increase of mass ratio from m_{i} /m_{e} = 16 to 73.44 increases fraction of accelerated electrons from 20% to 30–35% (depending on DAW polarisation). For the mass ratio m_{i} /m_{e} = 1836, the fraction of accelerated electrons would be >35%. (6) DAWs generate significant density and temperature perturbations that are located in the density gradient regions. DAWs propagating in the transversely inhomogeneous plasma can effectively accelerate electrons along the magnetic field and heat ions across it.
The author would like to thank EPSRCfunded Collaborative Computational Plasma Physics (CCPP) project lead by Professor T. D. Arber (Warwick) for providing EPOCH ParticleinCell code and Dr. K. Bennett (Warwick) for CCPP related programming support. Computational facilities used are that of Astronomy Unit, Queen Mary University of London and STFCfunded UKMHD consortium at St Andrews University. The author is financially supported by HEFCEfunded South East Physics Network (SEPNET).
I. INTRODUCTION
II. THE MODEL
III. RESULTS
IV. CONCLUSIONS
Key Topics
 Electric fields
 51.0
 Magnetic fields
 38.0
 Electrostatic waves
 35.0
 Electron beams
 22.0
 Alfven waves
 21.0
Figures
(Color online) Contour (intensity) plots of the following physical quantities at (the time when DAW develops three wavelengths): (a) B_{x} (x, y, t = t_{end} ) – B _{0}, (b) B_{y} (x, y, t = t_{end} ), (c) B_{z} (x, y, t = t_{end} ), (d) E_{x} (x, y, t = t_{end} ), (e) E_{y} (x, y, t = t_{end} ), (f) E_{z} (x, y, t = t_{end} ), (g) J_{x} (x, y, t = t_{end} ), (h) J_{y} (x, y, t = t_{end} ), and (i) J_{z} (x, y, t = t_{end} ). See text for the normalisation used. This figure pertains to the numerical run L16 (see Table II for the run parameters).
(Color online) Contour (intensity) plots of the following physical quantities at (the time when DAW develops three wavelengths): (a) B_{x} (x, y, t = t_{end} ) – B _{0}, (b) B_{y} (x, y, t = t_{end} ), (c) B_{z} (x, y, t = t_{end} ), (d) E_{x} (x, y, t = t_{end} ), (e) E_{y} (x, y, t = t_{end} ), (f) E_{z} (x, y, t = t_{end} ), (g) J_{x} (x, y, t = t_{end} ), (h) J_{y} (x, y, t = t_{end} ), and (i) J_{z} (x, y, t = t_{end} ). See text for the normalisation used. This figure pertains to the numerical run L16 (see Table II for the run parameters).
(Color online) Contour (intensity) plots of the following physical quantities at (the time when DAW develops three wavelengths): (a) n_{e} (x, y, t = t_{end} ), (b) n_{i} (x, y, t = t_{end} ), (c) T_{e} (x, y, t = t_{end} ), and (d) T_{i} (x, y, t = t_{end} ). Here, physical quantities are in SI units. This figure pertains to the numerical run L16.
(Color online) Contour (intensity) plots of the following physical quantities at (the time when DAW develops three wavelengths): (a) n_{e} (x, y, t = t_{end} ), (b) n_{i} (x, y, t = t_{end} ), (c) T_{e} (x, y, t = t_{end} ), and (d) T_{i} (x, y, t = t_{end} ). Here, physical quantities are in SI units. This figure pertains to the numerical run L16.
Time evolution (at t = 0 and ) of electron and ion velocity distribution functions (on a loglinear plot): (a) f_{e} ( _{x} ,t = 0) dashed (inner) curve and f_{e} ( _{x} , t = t_{end} ) solid (outer) curve, (b) f_{e} ( _{y} , t = 0) dashed (inner) curve and f_{e} ( _{y} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (c) f_{e} ( _{z} , t = 0) dashed (inner) curve and f_{e} ( _{z} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (d) f_{i} ( _{x} , t = 0) dashed (inner) curve and f_{i} ( _{x} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (e) f_{i} ( _{y} , t = 0) dashed (inner) curve and f_{i} ( _{y} , t = t_{end} ) solid (outer) curve, and (f) f_{i} ( _{z} , t = 0) dashed (inner) curve and f_{i} ( _{z} , t = t_{end} ) solid (outer) curve. Time evolution (at 20 time intervals between t = 0 and t = t_{end} ) of the following: (g) maxE_{x} (x, y, t), triangles connected with a solid curve and (h) index, diamonds connected with dashed curve, according to Eq. (8), index, triangles connected with a solid curve, according to Eq. (9). This figure pertains to the numerical run L16.
Time evolution (at t = 0 and ) of electron and ion velocity distribution functions (on a loglinear plot): (a) f_{e} ( _{x} ,t = 0) dashed (inner) curve and f_{e} ( _{x} , t = t_{end} ) solid (outer) curve, (b) f_{e} ( _{y} , t = 0) dashed (inner) curve and f_{e} ( _{y} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (c) f_{e} ( _{z} , t = 0) dashed (inner) curve and f_{e} ( _{z} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (d) f_{i} ( _{x} , t = 0) dashed (inner) curve and f_{i} ( _{x} , t = t_{end} ) solid (outer) curve (note that the both curves overlap), (e) f_{i} ( _{y} , t = 0) dashed (inner) curve and f_{i} ( _{y} , t = t_{end} ) solid (outer) curve, and (f) f_{i} ( _{z} , t = 0) dashed (inner) curve and f_{i} ( _{z} , t = t_{end} ) solid (outer) curve. Time evolution (at 20 time intervals between t = 0 and t = t_{end} ) of the following: (g) maxE_{x} (x, y, t), triangles connected with a solid curve and (h) index, diamonds connected with dashed curve, according to Eq. (8), index, triangles connected with a solid curve, according to Eq. (9). This figure pertains to the numerical run L16.
As in Fig. 3 but for the numerical run R16.
As in Fig. 3 but for the numerical run R16.
As in Fig. 3 but for the numerical run EL16.
As in Fig. 3 but for the numerical run EL16.
As in Fig. 3 but for the numerical run ER16.
As in Fig. 3 but for the numerical run ER16.
As in Fig. 3 but for the numerical run EL16_{1}.
As in Fig. 3 but for the numerical run EL16_{1}.
As in Fig. 3 but for the numerical run ER16_{1}.
As in Fig. 3 but for the numerical run ER16_{1}.
As in Fig. 3 but for the numerical run L73.
As in Fig. 3 but for the numerical run L73.
(Color online) As in Fig. 1 but for the numerical run R73.
(Color online) As in Fig. 1 but for the numerical run R73.
(Color online) As in Fig. 2 but for the numerical run R73.
(Color online) As in Fig. 2 but for the numerical run R73.
As in Fig. 3 but for the numerical run R73.
As in Fig. 3 but for the numerical run R73.
As in Fig. 3 but for the numerical run EL73.
As in Fig. 3 but for the numerical run EL73.
As in Fig. 3 but for the numerical run ER73.
As in Fig. 3 but for the numerical run ER73.
As in Fig. 3 but for the numerical run EL73_{1}.
As in Fig. 3 but for the numerical run EL73_{1}.
As in Fig. 3 but for the numerical run ER73_{1}.
As in Fig. 3 but for the numerical run ER73_{1}.
(Color online) As in Fig. 1 but for the numerical run L16Long.
(Color online) As in Fig. 1 but for the numerical run L16Long.
As in Fig. 3 but for the numerical run L16Long.
As in Fig. 3 but for the numerical run L16Long.
Tables
Numerical simulation parameters.
Numerical simulation parameters.
Numerical simulation run identification and physical parameters. I stands for inertial and K for kinetic. Reg. stands for regime.
Numerical simulation run identification and physical parameters. I stands for inertial and K for kinetic. Reg. stands for regime.
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