Peera Pongkitiwanichakul | Kasetsart University (original) (raw)
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Papers by Peera Pongkitiwanichakul
Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important ro... more Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important role in the heating and acceleration of the fast solar wind. Although such interactions have been studied extensively for "parallel" waves, whose wave vectors k are aligned with the background magnetic field B 0 , much less is known about interactions between ions and oblique A/IC waves, for which the angle θ between k and B 0 is nonzero. In this paper, we present new numerical results on resonant cyclotron interactions between protons and oblique A/IC waves in collisionless low-beta plasmas such as the solar corona. We find that if some mechanism generates oblique high-frequency A/IC waves, then these waves initially modify the proton distribution function in such a way that it becomes unstable to parallel waves. Parallel waves are then amplified to the point that they dominate the wave energy at the large parallel wave numbers at which the waves resonate with the particles. Pitch-angle scattering by these waves then causes the plasma to evolve towards a state in which the proton distribution is constant along a particular set of nested "scattering surfaces" in velocity space, whose shapes have been calculated previously. As the distribution function approaches this state, the imaginary part of the frequency of parallel A/IC waves drops continuously towards zero, but oblique waves continue to undergo cyclotron damping while simultaneously causing protons to diffuse across these kinetic shells to higher energies. We conclude that oblique A/IC waves can be more effective at heating protons than parallel A/IC waves, because for oblique waves the plasma does not relax towards a state in which proton damping of oblique A/IC waves ceases.
We develop a model for stochastic acceleration of electrons in solar flares. As in several previo... more We develop a model for stochastic acceleration of electrons in solar flares. As in several previous models, the electrons are accelerated by turbulent fast magnetosonic waves ("fast waves") via transit-time-damping (TTD) interactions. (In TTD interactions, fast waves act like moving magnetic mirrors that push the electrons parallel or anti-parallel to the magnetic field). We also include the effects of Coulomb collisions and the waves' parallel electric fields. Unlike previous models, our model is two-dimensional in both momentum space and wavenumber space and takes into account the anisotropy of the wave power spectrum FkF_kFk and electron distribution function frmef_{\rm e}frme. We use weak turbulence theory and quasilinear theory to obtain a set of equations that describes the coupled evolution of FkF_kFk and frmef_{\rm e}frme. We solve these equations numerically and find that the electron distribution function develops a power-law-like non-thermal tail within a restricted range of energies Ein(Ermnt,Ermmax)E\in (E_{\rm nt}, E_{\rm max})Ein(Ermnt,Ermmax). We obtain approximate analytic expressions for ErmntE_{\rm nt}Ermnt and ErmmaxE_{\rm max}Ermmax, which describe how these minimum and maximum energies depend upon parameters such as the electron number density and the rate at which fast-wave energy is injected into the acceleration region at large scales. We contrast our results with previous studies that assume that FkF_kFk and frmef_{\rm e}frme are isotropic, and we compare one of our numerical calculations with the time-dependent hard-x-ray spectrum observed during the June 27, 1980 flare. In our numerical calculations, the electron energy spectra are softer (steeper) than in models with isotropic FkF_kFk and frmef_{\rm e}frme and closer to the values inferred from observations of solar flares.
Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important ro... more Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important role in the heating and acceleration of the fast solar wind. Although such interactions have been studied extensively for "parallel" waves, whose wave vectors k are aligned with the background magnetic field B 0 , much less is known about interactions between ions and oblique A/IC waves, for which the angle θ between k and B 0 is nonzero. In this paper, we present new numerical results on resonant cyclotron interactions between protons and oblique A/IC waves in collisionless low-beta plasmas such as the solar corona. We find that if some mechanism generates oblique high-frequency A/IC waves, then these waves initially modify the proton distribution function in such a way that it becomes unstable to parallel waves. Parallel waves are then amplified to the point that they dominate the wave energy at the large parallel wave numbers at which the waves resonate with the particles. Pitch-angle scattering by these waves then causes the plasma to evolve towards a state in which the proton distribution is constant along a particular set of nested "scattering surfaces" in velocity space, whose shapes have been calculated previously. As the distribution function approaches this state, the imaginary part of the frequency of parallel A/IC waves drops continuously towards zero, but oblique waves continue to undergo cyclotron damping while simultaneously causing protons to diffuse across these kinetic shells to higher energies. We conclude that oblique A/IC waves can be more effective at heating protons than parallel A/IC waves, because for oblique waves the plasma does not relax towards a state in which proton damping of oblique A/IC waves ceases.
We develop a model for stochastic acceleration of electrons in solar flares. As in several previo... more We develop a model for stochastic acceleration of electrons in solar flares. As in several previous models, the electrons are accelerated by turbulent fast magnetosonic waves ("fast waves") via transit-time-damping (TTD) interactions. (In TTD interactions, fast waves act like moving magnetic mirrors that push the electrons parallel or anti-parallel to the magnetic field). We also include the effects of Coulomb collisions and the waves' parallel electric fields. Unlike previous models, our model is two-dimensional in both momentum space and wavenumber space and takes into account the anisotropy of the wave power spectrum FkF_kFk and electron distribution function frmef_{\rm e}frme. We use weak turbulence theory and quasilinear theory to obtain a set of equations that describes the coupled evolution of FkF_kFk and frmef_{\rm e}frme. We solve these equations numerically and find that the electron distribution function develops a power-law-like non-thermal tail within a restricted range of energies Ein(Ermnt,Ermmax)E\in (E_{\rm nt}, E_{\rm max})Ein(Ermnt,Ermmax). We obtain approximate analytic expressions for ErmntE_{\rm nt}Ermnt and ErmmaxE_{\rm max}Ermmax, which describe how these minimum and maximum energies depend upon parameters such as the electron number density and the rate at which fast-wave energy is injected into the acceleration region at large scales. We contrast our results with previous studies that assume that FkF_kFk and frmef_{\rm e}frme are isotropic, and we compare one of our numerical calculations with the time-dependent hard-x-ray spectrum observed during the June 27, 1980 flare. In our numerical calculations, the electron energy spectra are softer (steeper) than in models with isotropic FkF_kFk and frmef_{\rm e}frme and closer to the values inferred from observations of solar flares.