Intensity mapping of the 21cm signal of neutral hydrogen will yield exciting
insights into the Epoch of Reionisation and the nature of the first galaxies.
However, the large amount of data that will be generated by the next generation
of radio telescopes, such as the Square Kilometre Array (SKA), as well as the
numerous observational obstacles to overcome, require analysis techniques tuned
to extract the reionisation history and morphology. In this context, we
introduce a one-point statistic, to which we refer as the local variance,
$\sigma_\mathrm{loc}$, that describes the distribution of the mean differential
21cm brightness temperatures measured in two-dimensional maps along the
frequency direction of a light-cone. The local variance takes advantage of what
is usually considered an observational bias, the sample variance. We find the
redshift-evolution of the local variance to not only probe the reionisation
history of the observed patches of the sky, but also trace the ionisation
morphology. This estimator provides a promising tool to constrain the midpoint
of reionisation as well as gaining insight into the ionising properties of
early galaxies.