  Interest in stealth coronal mass ejections (CMEs) is increasing due to their
relatively high occurrence rate and space weather impact. However, typical CME
signatures such as extreme-ultraviolet dimmings and post-eruptive arcades are
hard to identify and require extensive image processing techniques. These weak
observational signatures mean that little is currently understood about the
physics of these events. We present an extensive study of the magnetic field
configuration in which the stealth CME of 3 March 2011 occurred. Three distinct
episodes of flare ribbon formation are observed in the stealth CME source
active region (AR). Two occurred prior to the eruption and suggest the
occurrence of magnetic reconnection that builds the structure which will become
eruptive. The third occurs in a time close to the eruption of a cavity that is
observed in STEREO-B 171A data; this subsequently becomes part of the
propagating CME observed in coronagraph data. We use both local (Cartesian) and
global (spherical) models of the coronal magnetic field, which are complemented
and verified by the observational analysis. We find evidence of a coronal null
point, with field lines computed from its neighbourhood connecting the stealth
CME source region to two ARs in the northern hemisphere. We conclude that
reconnection at the null point aids the eruption of the stealth CME by removing
field that acted to stabilise the pre-eruptive structure. This stealth CME,
despite its weak signatures, has the main characteristics of other CMEs, and
its eruption is driven by similar mechanisms.
