Helmet streamer influence on the evolution of magnetic flux ropes

Abstract

Context. Solar eruptions play a major role in space weather. Understanding the physical mechanisms that influence their evolution is essential for improving future predictions about the geo-effectiveness of an event. Helmet streamers (HSs) are present in the solar corona during both periods of minimum and maximum solar activity. This magnetic structure features a current sheet of low magnetic energy, towards which coronal mass ejection events tend to deflect. However, there is also a closed magnetic field region underlying this current sheet where eruptions are often confined. This makes it an interesting structure to study, as the inherent complexity of the structure hinders the predictability of the eruption.Aims. The aim of this study is to investigate the influence of the HS on the evolution and potential confinement of a magnetic flux rope (MFR). We explore magnetic configurations involving the MFR and HS that are more likely to allow the MFR to rise through the overlying magnetic field, with the ultimate goal of establishing simple parameters that can help predict the conditions under which an MFR can ascend or remain confined.Methods. Through 2.5D magnetohydrodynamic numerical simulations, we emulated the dynamics of MFRs in the presence of an HS. We analysed the dynamics of different magnetic field configurations, paying special attention to the mechanisms that facilitate the ascent or confinement of the MFR.Results. We find that the null point reconnection mechanism plays a fundamental role in the dynamics of the MFR. Depending on the initial configuration, null point reconnection can either confine the ascent by disrupting the MFR or facilitate its rise by reducing the strapping flux above it. We also identify a critical value in the relationship between the magnetic flux that the MFR must traverse during its ascent and its own magnetic flux. We find that if the strapping flux above the MFR is less than two-thirds of its own poloidal flux, the MFR is able to ascend.Conclusions. In our simulations, null point reconnection plays a major role in facilitating the ascent of the MFR. A key factor in predicting whether the MFR will rise is the initial ratio between its poloidal flux and the strapping magnetic flux above it.