  Observational evidence of star-planet magnetic interactions (SPMI) in compact
exo-systems have been looked for in the past decades. Indeed planets in
close-in orbit can be magnetically connected to their host star, and channel
Alfv\'en waves carrying large amounts of energy towards the central star. The
strength and temporal modulation of SPMIs are primarily set by the magnetic
topology of the host star and the orbital characteristics of the planet. As a
result, SPMI signals can be modulated over the rotational period of the star,
the orbital period of the planet, or a complex combination of the two. The
detection of SPMI thus have to rely on multiple-epochs and multiple-wavelengths
observational campaigns. We present a new method to characterize SPMIs and
apply it to Kepler-78, a late G star with a super-Earth on an 8.5 hours orbit.
We model the corona of Kepler-78 using the large-scale magnetic topology of the
star observed with Zeeman-Doppler-Imaging. We show that the closeness of
Kepler-78b allows the interaction to channel energy flux densities up to a few
kW m$^{-2}$ towards the central star. We show that this flux is large enough to
be detectable in classical activity tracers such as H$\alpha$. It is
nonetheless too weak to explain the modulation observed by \citet{Moutou2016}.
We furthermore demonstrate how to predict the temporal modulation of SPMI
signals in observed systems such as Kepler-78. The methodology presented here
thus paves the road towards denser, specific observational campaigns that would
allow a proper identification of SPMIs in compact star-planet systems.
