Model assumptions

  Throughout the rest of the paper we make the following assumptions:

• The streamer structure is extended in azimuth and varies only very slowly along this direction [Schwenn et al.1997]. Thus we can use a two-dimensional model with rotational invariance.
• The streamers are elongated in the radial direction. Thus we use the method of asymptotic expansion to calculate stationary states.
• The observations [Habbal et al.1997] show that a stationary slow solar wind might not exist, but that the slow solar wind is produced by many small eruptions. Thus stationary states of helmet streamers should be static equilibria.
• The aim of this work is to investigate the basic physical mechanisms of coronal mass ejections and small eruptions. Therefore we use as a first step a Cartesian geometry and neglect gravity. We are aware of the fact that both gravity and the basic geometry should have influence on the quantitative dynamic evolution of helmet streamers, but we use the correct magnetic field topology and the question whether an instability occurs or not should not be influenced by this model assumption.