Observations of the solar corona [Schwenn et al.1997] with the coronagraph LASCO on the SOHO spacecraft showed, that the solar corona can be highly structured even during the solar activity minimum. The observation showed a triple structure in the streamer belt, which exists for several days. The observations further suggest, that the slow solar wind maybe caused by quasistationary small eruptions of the streamers. The observations further show, that these triple helmet streamers occasionally become unstable, which leads to extraordinarily huge coronal mass ejections. The aim of this paper is to make a first step to a better theoretical understanding of these dynamical phenomena.
The structure of helmet streamers and their stability has been studied both observationally and theoretically for a long time (e.g. pneuman:kopp71, pneuman:kopp71; cuperman:etal90, cuperman:etal90; cuperman:etal92, cuperman:etal92; koutchmy:livshits92, koutchmy:livshits92; wang:etal93, wang:etal93; cuperman:etal95, cuperman:etal95; wu:etal95, wu:etal95; hundhausen95, hundhausen95; bavassano:etal97, bavassano:etal97). A natural association seems to exist between helmet streamer stability and coronal mass ejections (CMEs) and sometimes CMEs are accompanied by erupting prominences or filaments. Coronal streamers are also thought to be the source regions of the slow solar wind and the activity processes discussed above may well contribute to this component of the solar wind.
The paper is outlined as follows. In section 2 we discuss the basic equations. Section 3 contains our model assumptions and their justification. In section 4 we present a method to calculate self-consistent, analytical stationary triple helmet streamer configurations. We use these stationary states as input for numerical experiments with a time-dependent MHD-code in section 5. In section 6 we summarize our results and give an outlook on future work.