Top: quasi separator layer map indicating the locations of rapid changes in the field line mapping across the core region of the domain using the invariant measure defined by Titov (2007). The data represent ±45 Mm in the x and y around the centre of the domain. The data are logarithmically scaled over values from 2 to 1000. White areas have low values. Bottom: QSL map and its relation to the null points close to the active region.