  We report the time variability of the late-time radio emission in a Type-I
superluminous supernova (SLSN), PTF10hgi, at z = 0.0987. The Karl G. Jansky
Very Large Array 3 GHz observations at 8.6 and 10 years after the explosion
both detected radio emission with a ~40% decrease in flux density in the second
epoch. This is the first report of a significant variability of the late-time
radio light curve in a SLSN. Through combination with previous measurements in
two other epochs, we constrained both the rise and decay phases of the radio
light curve over three years, peaking at approximately 8-9 years after the
explosion with a peak luminosity of L(3GHz) = 2 x 10^21 W/Hz. Possible
scenarios for the origin of the variability are an active galactic nucleus
(AGN) in the host galaxy, an afterglow caused by the interaction between an
off-axis jet and circumstellar medium, and a wind nebula powered by a
newly-born magnetar. Comparisons with models show that the radio light curve
can be reproduced by both the afterglow model and magnetar wind nebula model.
Considering the flat radio spectrum at 1-15 GHz and an upper limit at 0.6 GHz
obtained in previous studies, plausible scenarios are a low-luminosity
flat-spectrum AGN or a magnetar wind nebula with a shallow injection spectral
index.
