  Houdebine et al (2017: H17) combined CaII data with projected rotational
velocities (v sin i) to construct rotation-activity correlations (RAC) in K-M
dwarfs. The RAC slopes were used to argue that a transition between dynamo
modes occurs at a spectral type between M2 and M3. H17 suggested that the
dynamo transition corresponds to a transition to complete convection (TTCC). An
independent study of GAIA data led Jao et al (2018) to suggest that the TTCC
sets in near M3.0V, close to the H17 result. However, the changes in a star
which cause TTCC signatures in GAIA data might not lead to changes in CaII
emission at an identical spectral type: the latter are also affected by
magnetic effects which depend on certain properties of convection in the core.
Here, we use CaII emission fluxes in a sample of ~600 M dwarfs, and attempt to
narrow down the transition from one dynamo mode to another: rather than relying
on RAC slopes, we quantify how the CaII emission flux varies with spectral type
to identify steps where the flux decreases significantly across a narrow range
of spectral types. We suggest that the dynamo mode transition may be narrowed
down to between M2.1 and M2.3. This is close to, but earlier than, the TTCC
location identified by Jao et al (2018). We suggest that the transition in
dynamo mode may be related to the existence of a small convective core which
occurs for a finite time interval in certain low mass stars.
