Three-dimensional Electromagnetic Particle Code Research Articles

Wave generation and particle acceleration in electron-positron plasmas

The dynamics of an electron-positron plasma cloud moving perpendicular to an ambient magnetic field with background plasmas is investigated by using a three-dimensional electromagnetic particle code. It is shown that the Alfvén waves with large amplitude and linear polarization are excited dominantly, while the electromagnetic waves are weakly excited because of the relativistic effect of the cloud particles. When the cloud density becomes large, Langmuir waves leading to particle acceleration are also generated to compensate the charges escaping from the cloud along the magnetic field. Using a two-dimensional electromagnetic and relativistic particle code, it is shown that high-frequency Langmuir waves can be generated by the plasma maser mechanism from low-frequency Alfvén waves excited by a temperature anisotropy. It is confirmed by the simulation that the plasma maser mechanism does not require an electron (or positron) beam component for the growth of Langmuir waves.

Electromagnetic waves emitted from an electron-positron plasma cloud moving across a magnetic field.

We have investigated the dynamics of an electron-positron plasma cloud moving perpendicular to an ambient magnetic field in a vacuum and also with background plasmas using a three-dimensional electromagnetic particle code. Simulation results show that charge sheaths are formed at both sides of the cloud and a polarized electric field is created inside the cloud by the coherent motion of cloud particles. This polarized electric field leads to the E\ifmmode\times\else\texttimes\fi{}B drift motion of the cloud, and some of the cloud particles expand along the magnetic field from the charge sheaths. Consequently, linearly polarized electromagnetic waves are excited from the cloud. It should be noted that the kinetic energy of clouds is transformed very efficiently into the emitted wave energy during about one cyclotron period. This mechanism may be responsible to the emission of strong electromagnetic waves observed in electron-positron plasmas. \textcopyright{} 1996 The American Physical Society.

Particle Simulation of a Collision between a Plasma Cloud and a Current Loop

We present a model of a collision between a plasma cloud and a current loop by performing simulations with a three-dimensional electromagnetic particle code. The loop heating and particle acceleration in such collision processes are crucially important in understanding the triggering mechanisms of solar flares. Theories and observations previously studied have revealed that plasmoids produced in the coalescence process of current loops escape from the interaction region. Furthermore, they may move to another magnetic loop, colliding with it and triggering a flare. Based on this idea, we investigate this simulation study as a flare model. A cloud is pushed across an ambient magnetic field to a current loop along the field. Simulation results show that a great deal of the released energy from the cloud is transferred into loop kinetic energy, resulting in heating of the loop. If the initial kinetic energy of the cloud is large enough to compress and bend the ambient magnetic field distinctly, nonthermal electrons are produced in the loop, which have a broken power law spectrum. Those heated electrons are responsible for soft X-ray and hard X-ray emissions from the loop. In addition, electrons in the cloud are also heated. The high-energy electrons in the loop and the cloud are almost identical, with the same maximum energies, of the order of 30 times the thermal energy. The high-energy electrons in the cloud correspond to a hard X-ray source above the loop apex.

Coalescence of two current loops with a kink instability simulated by a three-dimensional electromagnetic particle code

We have studied the dynamics of a coalescence of current loops using three-dimensional electromagnetic (EM) particle simulation code. Our focus is the investigation of such kinetic processes as energy trasnfer, heating particles, and electromagnetic emissions associated with a current loop coalescence which cannot be studied by MHD simulations. First, the two loops undergo a pinching oscillation due to a pressure imbalance between the inside and outside of the current loop. During the pinching oscillation, a kinetic kink instability is excited and electrons in the loops are heated perpendicularly to an ambient magnetic field. Next, the two current loops collide and coalesce, while at the same time a helical structure grows further. Subsequently, the perturbed current, which is due to these helically bunched electrons, can drive a whistler instability. It should be noted in this case that the whistler wave is excited by the kinetic kink instability and not a beam instability. After the coalescence of two helical loops, tilting motions can be observed in the direction of left-hand rotation, and the helical structure will relax resulting in strong plasma heating mostly in the direction perpendicular to the ambient magnetic field. It is also shown that high-frequency electromagnetic waves can be emitted from the region where the two loops coalesce and propagate strongly in the direction of the electron drift velocity. These processes may be important in understanding heating mechansims for coronal loops as well as radio wave emission mechanisms from active regions of solar plasmas.

Study of nonlinear Alfvén waves in an electron–positron plasma with a three-dimensional electromagnetic particle code

Results from three-dimensional (3-D) electromagnetic particle simulations of Alfvén waves generated by an electron beam in a nonrelativistic electron–positron plasma are presented. The results show that electrostatic modes are excited due to the beam instability. The bunches of the particles (electrons and positrons) caused by electrostatic waves are directly involved in the generation of Alfvén waves. The Alfvén waves propagate along the beam as damped solitons accelerating the background particles. The simulation results are in good agreement with theoretical analysis.

Solar wind-magnetosphere interaction as simulated by a 3-D EM particle code

Results are presented for simulating the solar-wind-magnetosphere interaction with a new three-dimensional electromagnetic particle code. Previously, such global simulations were done with MHD codes while lower-dimensional particle or hybrid codes served to account for microscopic processes and such transport parameters as have to be introduced ad hoc in MHD. The kinetic model presented attempts to combine the macroscopic and microscopic tasks. It relies only on the Maxwell curl equations and the Lorentz equation for particles, which are ideally suited for computers. The preliminary results shown are for an unmagnetized solar-wind plasma streaming past a dipolar magnetic field. The results show the formation of a bow shock and a magnetotail, the penetration of energetic particles into cusp and radiation belt regions, and dawn-dusk asymmetries. >