  We investigate the variability of exoplanetary radio emission using stellar
magnetic maps and 3D field extrapolation techniques. We use a sample of hot
Jupiter hosting stars, focusing on the HD 179949, HD 189733 and tau Boo
systems. Our results indicate two time-scales over which radio emission
variability may occur at magnetised hot Jupiters. The first is the synodic
period of the star-planet system. The origin of variability on this time-scale
is the relative motion between the planet and the interplanetary plasma that is
co-rotating with the host star. The second time-scale is the length of the
magnetic cycle. Variability on this time-scale is caused by evolution of the
stellar field. At these systems, the magnitude of planetary radio emission is
anticorrelated with the angular separation between the subplanetary point and
the nearest magnetic pole. For the special case of tau Boo b, whose orbital
period is tidally locked to the rotation period of its host star, variability
only occurs on the time-scale of the magnetic cycle. The lack of radio
variability on the synodic period at tau Boo b is not predicted by previous
radio emission models, which do not account for the co-rotation of the
interplanetary plasma at small distances from the star.
