  Long-term stellar activity variations can affect the detectability of
long-period and Earth-analogue extrasolar planets. We have, for 54 stars,
analysed the long-term trend of five activity indicators:
log$R'_\mathrm{{HK}}$, the cross-correlation function (CCF) bisector span, CCF
full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to
the CCF; and studied their correlation with the RVs. The sign of the
correlations appears to vary as a function of stellar spectral type, and the
transition in sign signals a noteworthy change in the stellar activity
properties where earlier type stars appear more plage dominated. These
transitions become more clearly defined when considered as a function of the
convective zone depth. Therefore, it is the convective zone depth (which can be
altered by stellar metallicity) that appears to be the underlying fundamental
parameter driving the observed activity correlations. In addition, for most of
the stars, we find that the RVs become increasingly red-shifted as activity
levels increase, which can be explained by the increase in the suppression of
convective blue-shift. However, we also find a minority of stars where the RVs
become increasingly blue-shifted as activity levels increase. Finally, using
the correlation found between activity indicators and RVs, we removed RV
signals generated by long-term changes in stellar activity. We find that
performing simple cleaning of such long-term signals enables improved planet
detection at longer orbital periods.
