  Using a potential field source surface (PFSS) model, we recently analyzed the
global topology of the background coronal magnetic field for a sequence of
coronal mass ejections (CMEs) that occurred on 2010 August 1-2. Here we repeat
this analysis for the background field reproduced by a magnetohydrodynamic
(MHD) model that incorporates plasma thermodynamics. As for the PFSS model, we
find that all three CME source regions contain a coronal hole that is separated
from neighboring coronal holes by topologically very similar pseudo-streamer
structures. However, the two models yield very different results for the size,
shape, and flux of the coronal holes. We find that the helmet-streamer cusp
line, which corresponds to a source-surface null line in the PFSS model, is
structurally unstable and does not form in the MHD model. Our analysis
indicates that generally, in MHD configurations, this line rather consists of a
multiple-null separator passing along the edge of disconnected flux regions.
Some of these regions are transient and may be the origin of so-called streamer
blobs. We show that the core topological structure of such blobs is a
three-dimensional "plasmoid", consisting of two conjoined flux ropes of
opposite handedness, which connect at a spiral null point of the magnetic
field. Our analysis reveals that such plasmoids appear also in pseudo-streamers
on much smaller scales. These new insights into the coronal magnetic topology
provide some intriguing implications for solar energetic particle events and
for the properties of the slow solar wind.
