J. I. Sakai

Current loop coalescence model of solar flares

A computer simulation and theoretical study of the physical characteristics of the explosive coalescence of current-carrying loops is presented. Characteristics of the explosive coalescence include a large impulsive increase of the kinetic energies of electrons and ions, the simultaneous heating and acceleration of electrons and ions in high and low energy ranges, and a break in the energy spectra of electrons and ions. A characteristic double subpeak structure is found in the quasi-periodic oscillations found in the time profiles of the solar flares of June 7, 1980 and November 26, 1982 which can be explained in terms of the coalescence instability of two current loops. 41 references.

A NUMERICAL MODEL OF A CORONAL MASS EJECTION: SHOCK DEVELOPMENT WITH IMPLICATIONS FOR THE ACCELERATION OF GeV PROTONS

The initiation and evolution of the coronal mass ejection, which occurred on 1998 May 2 in NOAA Active Region 8210, are modeled using a fully three-dimensional, global MHD code. The initial magnetic field for the model is based on magnetogram data from the Wilcox Solar Observatory, and the solar eruption is initiated by slowly evolving the boundary conditions until a critical point is reached where the configuration loses equilibrium. At this time, the field erupts, and a flux rope is ejected that achieves a maximum speed in excess of 1000 km s-1. The shock that forms in front of the rope reaches a fast-mode Mach number in excess of 4 and a compression ratio greater than 3 by the time it has traveled a distance of 5 R☉ from the surface. For such values, diffusive shock acceleration theory predicts a distribution of solar energetic protons with a cutoff energy of about 10 GeV. For this event, there appears to be no need to introduce an additional acceleration mechanism to account for solar energetic protons with energies below 10 GeV.

Generation of a Small-Scale Quasi-Static Magnetic Field and Fast Particles during the Collision of Electron-Positron Plasma Clouds

We present the results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counterstreaming instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and shows that, in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted into magnetic field energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collisionless change of structure, leading to large-scale, long-living structures and the production of high-energy particles. These processes may be important for understanding the production of high-energy particles in the region where two pulsar winds collide.

Loop coalescence in flares and coronal X-ray brightening

Characteristics of solar flares, such as their impulsive nature, time scale, heating, high-energy particle spectrum, and ..gamma..-ray oscillations, as well as recent X-ray photographs of coronal brightening, are explained by the nonlinear coalescence instability of current loops.

Waves in an Ultra-Relativistic Electron-Positron Plasma

Basic equations describing a macroscopic behaviour of ultra-relativistic plasma ( T ≫ m c 2 ) are formulated in a covariant form. Waves in an electron-positron plasma are investigated in a frame of two-fluid model equation. Dispersion relations for electrostatic wave, electromagnetic wave and Alfven wave propagating parallel to a constant magnetic field are shown. In the case that both electron and positron gases have same temperature, there does not exist the slow mode which corresponds to the ion acoustic wave. Right and left circular polarized waves propagate with the same dispersion relation which is different from one of the non-relativistic electron-ion plasma.

Particle acceleration by magnetic reconnection and shocks during current loop coalescence in solar flares

AbstractThis article reviews recent development of the theory of current loop coalescence and shock waves, giving particular attention to particle acceleration caused by these processes. First, explosive reconnection driven by the current loop coalescence and associated particle acceleration are studied by theoretical and magnetohydrodynamic simulation methods and the results are compared with observations of solar flares; this model gives a good explanation for the quasi-periodic structure of some solar flare bursts. Next follows a discussion of particle acceleration in association with fast magnetosonic shock waves. It is shown theoretically and by relativistic particle simulation that a quasi-perpendicular shock wave can accelerate trapped ions in the direction perpendicular to the ambient magnetic field up to speeds much greater than the Alfvén speed,

Particle-In-Cell simulations of circularly polarised Alfvén wave phase mixing : A new mechanism for electron acceleration in collisionless plasmas

\upsilon \sim \upsilon _A (m_i /m_e )^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}

Force-free equilibria and reconnection of the magnetic field lines in collisionless plasma configurations

. When the ambient magnetic field is rather strong (ωce ≳ ωpe), both ions and electrons can be accelerated to relativistic energies. For both the nonrelativistic and relativistic cases, the time needed for the acceleration is very short; it is