  Magnetic star-planet interactions (SPI) provide a detection method and
insight into exoplanet magnetic fields and, in turn, exoplanet interiors and
atmospheric environments. These signatures can be sporadic and difficult to
confirm for single-epoch observations of a system due to inhomogeneous stellar
magnetospheres and periodic variability in stellar magnetism. Thus an ideal SPI
search consists of multiple epochs containing observations on consecutive
nights spanning at least one complete planetary orbit. Such data sets are rare
but do exist for some of the most intensely studied hot Jupiter systems. One
such system is HD 189733 for which six suitable SPI data sets exist, the result
of spectroscopic monitoring to perform some of the first SPI searches and also
to study the star's magnetic field. Here we perform a uniform analysis of six
archival Ca II K data sets for HD 189733, spanning 2006 June through 2015 July,
in order to search for magnetic SPI signatures in the chromospheric line
variations. We find significant evidence for modulations of Ca II K with a
$2.29\pm0.04$ day period in the 2013 August data, which is consistent with the
planet's orbital period. The peak in the orbital variations occurs at
$\phi_\text{orb} \approx 0.9$, which corresponds to the SPI emission leading
the planet with a phase difference of $\Delta\phi \approx 40^\circ$ from the
sub-planetary point. This is consistent with the phase lead predictions of
non-linear force-free magnetic field SPI models. The stellar magnetic field
strength at the planet's orbit is greatest in 2013 August which, due to the
energy released in magnetic SPI scaling with $B_*$, lends strength to the SPI
interpretation.
