Dynamics

  
Figure 1: Reconnection processes in triple helmet streamers. (see text)

To study the dynamical evolution of triple helmet streamers we solve the time dependent resistive MHD equations (1-8) with the help of a numerical MHD-code. As start configurations we use the stationary state triple helmet streamer configurations calculated in the last section. This equilibrium structure has a thin current sheet above the helmet streamer cusp. This cusp is a radial boundary for the last closed field line of the streamer configuration. The current distribution on the closed field lines in the helmet streamers is much smoother. We remark, that during a quasistatic evolution thin current sheets may form within each streamer, analogous to the formation of thin current sheets in the Earth's magnetotail [Wiegelmann and Schindler1995]. One difficulty is to choose the resistivity profile . The easiest choice is a constant resistivity in whole space. More realistic is to use a current dependent resitivity profile or to localize the resistivity at the equilibrium current sheets.

The results of our numerical experiments are presented schematically in figure 1. We find different regions where reconnection processes occur:

• (1) The helmet streamer cusp transforms to an X-point and very small plasmoids are ejected into space. This process is a possible mechanism for the acceleration of the slow solar wind.
• (2) Interaction of the outer streamers lead to the formation of a dome above the three streamers.
• (3-5) Within each streamer we find the formation of plasmoids. If we consider only one streamer, the acceleration of these plasmoids leads to a coronal mass ejection. But the two outer streamers and the dome have a stabilising effect and make it more difficult for a plasmoid to leave the configuration. One finds that triple streamers are more stable than a single streamer. The acceleration of the plasmoids in each streamer may cause an slow growth of the whole configuration and lead to a huge coronal mass ejection.