Stellar magnetic ﬁelds are generated by dynamo mechanisms deep inside the stars. However, the type of dynamo cannot be the same for every star. It depends on the internal structure of the star which, in turn, depends on the stellar mass. In the Sun, for example, it is thought that ﬁeld generation primarily occurs in a thin layer between the radiative core and the outer convective zone. This type of dynamo cannot operate in very low mass stars that are fully convective and therefore lack such an interface. The differences in the underlying dynamo manifest themselves in the properties of the magnetic ﬁelds at the stellar surface. Using Zeeman-Doppler imaging (ZDI), the strength and geometry of stellar magnetic ﬁelds can be mapped. In this poster, we use ~100 Zeeman Doppler maps to probe the underlying dynamos of ~50 stars. Introduction Saturated and unsaturated regimes The poloidal and toroidal components both follow the well known activity-rotation relation from X-ray studies. Both display the unsaturated and saturated regimes with the transition occurring at a Rossby number of ~0.1. The similarity between the two plots indicates that the poloidal and toroidal ﬁelds are generated from each other. The magnetic energy at the stellar surface can be broken down into various components that reﬂect the geometry of the magnetic ﬁeld. The most commonly used components are the poloidal/toroidal components as well as the axisymmetric component. Field geometry When plotting the the toroidal magnetic energy against the poloidal magnetic energy, we ﬁnd that the sample splits into two subsamples. Stars more massive than 0.5 M⊙ (circles) are able to generate toroidal ﬁeld more efﬁciently and follow a different power law to stars less massive than 0.5 M⊙ (pentagons). This change in power laws is likely due to a change in internal structure as stars cross the fully convective boundary. See poster by Lisa Lehmann for further discussion. When plotting the percentage of magnetic energy in axisymmetric modes against the percentage of magnetic energy that is toroidal, we ﬁnd that the most toroidal stars also have the most axisymmetric ﬁelds. This is a strong indication that toroidal ﬁelds are generated via differential rotation - an axisymmetric ﬁeld generation mechanism. Conclusions In this poster, we have analysed ~100 ZDI maps of ~50 stars. This is the largest sample of stars gathered to date to study the behaviour of toroidal ﬁelds as a function of various parameters. We also ﬁnd that the internal structure of a star affects the relative strengths of the poloidal and toroidal components. Fully convective stars appear to be remain dominantly poloidal whereas stars with radiative cores are able to generate dominantly toroidal ﬁelds. Lastly, we ﬁnd that the most toroidal stars also have the most axisymmetric ﬁelds. On the other hand, the orientation of the magnetic axes of dominantly poloidal stars appear to be unconstrained relative to their rotation axis. We ﬁnd that that the magnetic ﬁeld strengths follow the activity-rotation relation. This provides another link between X-ray emission and the magnetic ﬁelds that generate them. Victor See1, M. Jardine1, A. A. Vidotto2, J.-F. Donati3, C. P. Folsom4 et al. The magnetic ﬁeld geometry of cool stars 1University of St Andrews, 2Trinity College Dublin, 3University of Toulouse, 4University of Grenoble arXiv: 1508.01403 See et al. (2015) wcvs@st-andrews.ac.uk