Acceleration of electrons and ions by strong lower-hybrid turbulence in solar flares (original) (raw)
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Electron trapping and acceleration by kinetic Alfvén waves in solar flares
Astronomy & Astrophysics, 2016
Context. Theoretical models and spacecraft observations of solar flares highlight the role of wave-particle interaction for non-local electron acceleration. In one scenario, the acceleration of a large electron population up to high energies is due to the transport of electromagnetic energy from the loop-top region down to the footpoints, which is then followed by the energy being released in dense plasma in the lower atmosphere. Aims. We consider one particular mechanism of non-linear electron acceleration by kinetic Alfvén waves. Here, waves are generated by plasma flows in the energy release region near the loop top. We estimate the efficiency of this mechanism and the energies of accelerated electrons. Methods. We use analytical estimates and test-particle modelling to investigate the effects of electron trapping and acceleration by kinetic Alfvén waves in the inhomogeneous plasma of the solar corona. Results. We demonstrate that, for realistic wave amplitudes, electrons can be accelerated up to 10−1000 keV during their propagation along magnetic field lines. Here the electric field that is parallel to the direction of the background magnetic field is about 10 to 10 3 times the amplitude of the Dreicer electric field. The acceleration mechanism strongly depends on electron scattering which is due to collisions that only take place near the loop footpoints. Conclusions. The non-linear wave-particle interaction can play an important role in the generation of relativistic electrons within flare loops. Electron trapping and coherent acceleration by kinetic Alfvén waves represent the energy cascade from large-scale plasma flows that originate at the loop-top region down to the electron scale. The non-diffusive character of the non-linear electron acceleration may be responsible for the fast generation of high-energy particles.
New Ion-Wave Path in the Energy Cascade
Physical Review Letters, 2011
We present the results of kinetic numerical simulations that demonstrate the existence of a novel branch of electrostatic nonlinear waves driven by particle trapping processes. These waves have an acoustic-type dispersion with phase speed comparable to the ion thermal speed and would thus be heavily Landau damped in the linear regime. At variance with the ion-acoustic waves, this novel electrostatic branch can exist at a small but finite amplitude even for low values of the electron to ion temperature ratio. Our results provide a new interpretation of observations in space plasmas, where a significant level of electrostatic activity is observed in the high frequency region of the solar-wind turbulent spectra.
Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble
Journal of Geophysical Research: Space Physics, 2013
Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasmadepleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X = À18 R E. The bubble is bounded by both sharp leading (@b z /@x < 0) and trailing (@b z /@x > 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvénic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E Â B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B z , or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed.
2007
The Sun is an active star that manifests its activity not only in the appearance of the wellknown sunspots but also in flares. During a flare, a huge amount of the stored magnetic energy is suddenly released and transfered into: a local heating of the corona, mass motions (e.g., jets and coronal mass ejections), an enhanced emission of electromagnetic radiation (from the radio up to the γ-ray range), and a generation of energetic particles (e.g., electrons, protons, and heavy ions). A substantial part of the flare released energy is carried by the energetic electrons, which are the source of the nonthermal radio and X-ray radiation. They can be detected by different means: with radio (e.g., from the ground-based radio observatories of the Astrophysical Institute Potsdam, Germany and the Nançay radioheliograph, France, respectively), with X-ray (e.g., RHESSI satellite), and with in-situ (e.g., WIND spacecraft) measurements. One of the most important and still open questions in solar ...
On cyclotron wave heating and acceleration of solar wind ions in the outer corona
Journal of Geophysical Research, 2001
Observatory (SOHO) [Kohlet al., 1998] in the solar coronal holes have been interpreted and modeled by invoking wave-particle cyclotron resonance . However, in the model of Cranmer et al. [1999a, 1999b] and in other subsequent models the assumption of a rigid slope of the wave spectrum was made in calculating the wave energy absoftion by the different ion species. In the present paper it is shown that a self-consistent treatment of the wave damping and absorption is necessary and leads to substantially different results. On the basis of quasi-linear theory, the interaction between the ions and the ion-cyclotron waves ] is studied. The total energy conservation equation, including the kinetic energy of the resonant particles and the wave energy, is derived and discussed in detail. The spectral evolution equation for cyclotron waves, when being controlled by the wave growth/damping rate and WKB effects, is solved self-consistently together with the full set of anisotropic multifluid equations for the ions including the cyclotron-resonance wave heating and acceleration rates. From the numerical results we reach the following conclusions: (1) It is physically questionable to use a spectrum with a fixed spectral slope near the cyclotron resonance when one calculates the partition of wave energy among the different ionic species and the kinetic degrees of freedom parallel and perpendicular to the magnetic field. This assumption neglects the important effects of wave absorption and the concurrent reshaping of the wave spectrum, and thus leads in the dissipation domain to extremely low amplitudes of the waves and to difficulties in supplying enough energy to balance the wave absorption at the cyclotron resonances. (2) If the spectrum is allowed to evolve self-consistently and concurrently with the particles' heating and acceleration through wave absorption, such a high perpendiculax temperature and corresponding large temperature anisotropy as observed by UVCS do not occur or cannot be maintained. We conclude that the UVCS oxygen ion observations have not yet been explained satisfactorily by the cyclotron-resonance theory.
The electron acceleration at shock waves in the solar corona
Astronomy & Astrophysics, 2007
Context. In the solar corona, shock waves are generated by flares and/or coronal mass ejections. They are able to accelerate electrons up to high energies and can thus be observed as type II bursts in the nonthermal solar radio radiation. In-situ measurements of shock waves in interplanetary space have shown that shock waves attached by whistler waves are preferably accompanied by the production of energetic electrons. Aims. Motivated by these observations, we study the interaction of electrons with such whistlers, which are excited by the protons accelerated by the shock. Methods. We start with a resonant whistler wave-proton interaction that accounts for the initial whistler wave generation. Then, we consider resonant whistler wave-electron interaction, treated with a relativistic approach that is responsible for the electron energization in the whistler wave field. Results. As a result, we show that electrons can be accelerated by a resonant wave particle (i.e., whistler-electron) interaction. This mechanism acts in the case of quasi-perpendicular shock waves. After acceleration, the energetic electrons are reflected by the associated shock wave back into the upstream region. The theoretical results are compared with observations, e.g., solar type II radio bursts.
A laboratory study of collisional electrostatic ion cyclotron waves
Journal of Geophysical Research, 1986
The effects of neutral-particle collisions on current-driven electrostatic ion cyclotron (EIC) waves are studied in a Q machine with a cesium (Cs +) plasma. We find that even when vin --0.312ci, EIC waves of substantial amplitude (/Xn/n of several percent) can be excited.