Previous models of Mercury's core magnetic field based on high altitude
data from first MESSENGER flybys revealed an axisymmetric. Here we use
low altitude MESSENGER data covering the entire mission period to
construct spherical harmonic models based on various spatial norms.
Although we find a dominantly axisymmetric field, our models
nevertheless include detectable deviations from axisymmetry. These
non-axisymmetric features appear at high latitudes, resembling intense
geomagnetic flux patches at Earth's core-mantle boundary. Based on this
core field morphology, we then attempt to infer Mercury's internal
structure. More specifically, assuming that Mercury's high-latitude
non-axisymmetric features are concentrated by downwellings at the edge
of the planet's inner core tangent cylinder, and accounting for the
presence of a stably stratified layer at the top of Mercury's core, we
establish a relation between the inner core size and the thickness of
the stratified layer. Considering plausible ranges, we propose that
Mercury's inner core size is about 500-660 km, which corresponds to a
stratified layer thickness of 880-500 km, respectively.