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Coronal magnetic null points exist in abundance as demonstrated byextrapolations of the coronal field, and have been inferred to be important fora broad range of energetic events. These null points and their associatedseparatrix and spine field lines represent discontinuities of the field linemapping, making them preferential locations for reconnection. This field linemapping also exhibits strong gradients adjacent to the separatrix (fan) andspine field lines, that can be analysed using the `squashing factor', $Q$. Inthis paper we make a detailed analysis of the distribution of $Q$ in thepresence of magnetic nulls. While $Q$ is formally infinite on both the spineand fan of the null, the decay of $Q$ away from these structures is shown ingeneral to depend strongly on the null-point structure. For the generic case ofa non-radially-symmetric null, $Q$ decays most slowly away from the spine/fanin the direction in which $|{\bf B}|$ increases most slowly. In particular,this demonstrates that the extended, elliptical high-$Q$ halo around the spinefootpoints observed by Masson et al. (Astrophys. J., 700, 559, 2009) is ageneric feature. This extension of the $Q$ halos around the spine/fanfootpoints is important for diagnosing the regions of the photosphere that aremagnetically connected to any current layer that forms at the null. In light ofthis, we discuss how our results can be used to interpret the geometry ofobserved flare ribbons in `circular ribbon flares', in which typically acoronal null is implicated. We conclude that both the physics in the vicinityof the null and how this is related to the extension of $Q$ away from thespine/fan can be used in tandem to understand observational signatures ofreconnection at coronal null points.

Why are flare ribbons associated with the spines of magnetic null points generically elongated?