Magnetic field changes preceding filament eruptions and coronal mass ejections

Solar filaments (or prominences) can be represented by twisted flux ropes in a bipolar magnetic environment. In such models, the dipped field lines of the flux rope carry the filament material and parasitic polarities in the filament channel are responsible for the existence of the lateral feet of filaments. Most filaments eventually erupt, in many cases as part of a coronal mass ejection (CME). Such eruptions are often preceded by detectable changes in the photospheric magnetic field in the vicinity of the filament. We first review recent observations of such changes due to large-scale flows or variations of the background magnetic field, and we discuss their role in eruptions. We then focus on emerging flux in the vicinity of filament channels. It has been suggested that magnetic reconnection between the emerging flux and the pre-existing coronal field can trigger filament eruptions and CMEs. For a particular event, observed with Hinode/XRT, we observe signatures of such reconnection, but no eruption of the filament. We present a numerical simulation of this event and we briefly argue why no eruption took place in this case.