1. Generation of Alfven Waves by Magnetic Reconnection
Hiromitsu Kigure, Kazunari Shibata (Kyoto U.), Takaaki Yokoyama (Tokyo U.), Satoshi Nozawa (Ibaraki U.), Kunio Takahashi (NAOJ)
Abstract
Results for 2.5-dimensional magnetohydrodynamical (MHD) simulations are reported for the magnetic reconnection of non-perfectly antiparallel magnetic fields. The magnetic field has a component perpendicular to the computational plane, that is, guide field. The angle between magnetic field lines in two half regions is a key parameter in our simulations. Alfven waves are generated at the reconnection point and propagate along the reconnected field line. The energy fluxes of the Alfven waves and slow-mode MHD waves generated by the magnetic reconnection are measured. Each flux shows the similar time evolution independent of The percentage of the energies (time integral of energy fluxes) carried by the Alfven waves and slow-mode waves to the released magnetic energy are calculated. The Alfven waves carry 32.9%, 36.1%, and 34.4% of the released magnetic energy at the maximum in the case of =0.2, 1.0, and 10.0 respectively, where is the plasma (the ratio of gas pressure to magnetic pressure). The slow-mode waves carry 20.0%, 26.8%, and 62.9% of the energy at the maximum. Implications of these results for solar coronal heating and acceleration of high-speed solar wind are discussed.
Time Evolution
Left figures show the time evolution of the logarithmic pressure, and right figures (except the bottom figure) show the time evolution of the magnetic field component perpendicular to the initial field in the case of and The bottom figure of the right panel shows the Bz distribution at t=30.0.
MHD Equations, Numerical Method, Resistivity, Initial Condition, etc.
These equations are numerically solved by the Lax-Wendroff module in CANS (Coordinated Astronomical Numerical Softwares), which is an integrated code to simulate astrophysical magnetohydrodynamic phenomena.
These are schematic pictures of the initial magnetic field. To decide the initial magnetic field distribution, there are two parameters. One is the plasma beta, and another is the angle between the magnetic field lines in two half regions.
The gas pressure distribution is designed to satisfy the total pressure equilibrium, and the density distribution is decided under the condition that initial sound velocity is unity in the all computational domain. The periodic boundary condition is adopted at the all boundaries.
Energy Fluxes of Alfven Waves and Slow-mode Waves
The energy fluxes carried by the Alfven waves and slow-mode waves generated by the magnetic reconnection are measured on a line where The definitions are as follows:
Percentage of the Energies Carried by Waves to the Released Magnetic Energy
, where means the difference from the initial value. The plus sign is on the the y=10.0 line and the minus sign on the y=－10.0 line.
We also measure the time integral of each flux:
The origin of the energies transported by the Alfven waves and slow-mode waves is the magnetic energy released by the reconnection so that the magnetic energy in the simulation domain is also measured.
These figures show the percentage of the energies carried by the Alfven waves and slow-mode waves to the released magnetic energy. The horizontal axis shows the parameter , and the vertical axis shows the percentage. As decreases the percentage of the energy carried by the Alfven waves increases up to 32.9%, 36.1%, and 34.4% in the case of =0.2, 1.0, and 10.0 respectively, and then decreases. The percentage is maximum when = 80° or 90°.　The percentage of the energy carried by the slow-mode　waves is almost constant when is relatively large in the cases of =0.2 and 1.0 while it gradually decreases in the case of =10.0. The maximum
values are 20.0%, 26.8%, and 62.9% in the case of =0.2, 1.0, and 10.0 respectively.
The total non-radiative energy input to the solar coronal hole was estimated at ergs cm-2 s-1 by Withbroe (1988). For the acceleration of high-speed solar wind, some ergs cm-2 s-1 is required to be deposited at distances of several solar radii (see, e.g., Parker 1991, and references therein). If the solar wind is accelerated by the energy flux of Alfven waves, this means that 20% of the energy released by reconnection events in the solar corona is transferred as a form of Alfven wave. Our results show that the energy larger than the required can be carried by the Alfven wave independent of around the parametric region of These energies are converted to thermal energy through the dissipation of Alfven waves.
Time evolution of the energy flux, F, of (a)Alfven waves and (b)slow-mode waves in each case. Time evolution of the time integral of energy flux carried by (c)Alfven waves and (d)slow-mode waves in each case. The plasma beta is 0.2.