  Recent discoveries of water-rich, sub-Neptunian to Neptunian-massed
exoplanets with short-period orbits present a new parameter space for the study
of exoplanetary dynamos. We explore the geometry of the dynamo source region
within this parameter space using 1-D interior structure models. We model
planets with 4 chemically distinct layers that consist of (1) an iron core, (2)
a silicate layer, (3) an H2O layer, and (4) an H/He envelope. By varying the
total planetary mass in the range of 1 - 19 Earth masses, the mass fraction of
the H/He envelope between 0.1 - 5.1%, and the equilibrium temperature between
100 - 1000 K, a survey of the parameter space for potential dynamo source
region geometries is conducted. We find that due to the nature of the phase
diagram of water at pressure and temperature conditions of planetary interiors,
two different dynamo source region geometries are obtainable. Specifically, we
find that smaller planets and planets with thicker H/He envelopes are likely to
be in the regime of a thick-shelled dynamo. Massive planets and planets with
thin H/He envelopes are likely to be in the regime of a thin-shelled dynamo.
Also, small variations of these parameters can produce large interior structure
differences. This implies the potential to constrain these parameters based on
observations of a planet's magnetic field signature.
