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Discussion of TSI: \cite{Shapiro_2011}, \cite{Shapiro_2013}, \cite{Shapiro_2014}  Some of the now CoRoT and Kepler paper to include would be:   Stellar cycles in HD 49933 by \cite{Garcia_2010}, see also \cite{Salabert_2011}  Oscillation amplitudes and activity \cite{Chaplin_2011}  Photospheric activity index: \cite{Mathur_2013}, \cite{Mathur_2014}  The sweat spot: \cite{Bonanno_2014}  The sounding stellar cycles with Kepler project: \cite{Karoff_2009}, \cite{Karoff_2013}  \section{Magnetic fields}  It is not possible to measure stellar magnetic fields directly with e.g. a Hall sensor. Instead the effect of magnetic fields can be observed through mainly: 

A number of studies have been published analyzing photometric variability in {\it Kepler} light curves \citep{2010ApJ...713L.155B,2011AJ....141...20B,2013Natur.500..427B,2014ApJ...788L...9B,2015arXiv151203454B,2011AJ....141...50W,2013MNRAS.436.1883W}, without attributing this variability to stellar cycles. We will come back to these studies in the next sections.  \section{Rotation}  The study of stellar rotation was pioneered Robert P. Kraft in a series of papers \cite{Kraft_1965a,Kraft_1965b,Anderson_1966,Kraft_1967a,Kraft_1967b}. These observations, together with the early observation from Odie Wilson led to the famous Skumanich spin-down law \cite{Skumanich_1972}. \cite{Skumanich_1972}, which connects to rotation of a star to its evolutionary stage. The simple idea is that a star, as it goes through life, loses angular momentum to a stellar wind and thus spin down. The general idea behind the Skumanich spin-down law is still accepted today, but the theory no contains a lot more details and if fact also, a lot more unknowns.   Stars like the Sun are formed from rotating molecular cloud. As these cloud contract, they will stars to spin-up, in order to conserve angular momentum. This means that Sun-like stars, as they arrive on the main-sequence, will be rotating relatively fast. Naively, we would assume that Sun-like stars would have close to homogenous rotation when they arrive on the main sequence, but even this assumption is like not corrects (ref). We would then expect that the star spin-down through out the convection zone and thus build up radial differential rotation in the convection zone. This is how ever also not correct. This was proven from the first helioseismic observations of progrande and retrograde sectroal oscillations modes by \cite{1984Natur.310...19D} and \cite{1985Natur.317..591B}. These observations revealed that the solar convection zone rotated at more or less the same rotation profile as the solar surface leaving all the radial shear to the thin border between the radiation and the convection zone. A region that have later been names the {\it tachocline} \cite{1992A&A...265..106S}. \cite{1998ApJ...505..390S} used observations from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft to calculate was should properly be names the standard to rotation profile. This profile shows that the decrease in angular velocity with increasing latitude seen on the surface continues all the way down to the tachocline, where a strong shear layer change the latitudinal rotation profile of the convection zone to a solid body rotation rotation in the radiation zone.  Recently,