The paper shows the recurrence and cross recurrence plots of three time
series, concerning data of the solar activity. The data are the sunspot number
and the values of solar radio flux at 10.7 cm and of solar total irradiance,
which are known as highly correlated. To compare the series, the radio flux and
irradiance values are monthly averaged. Recurrence plots display the
oscillating behaviour with remarkable features. Moreover, cross recurrence
plots help in identifying time lags between the sunspot number maximum and the
maximum of radio or irradiance signals, in circumstances where the data values
are highly dispersed. Image processing is useful too, in enhancing the
monitoring. An interesting behaviour is displayed by cross recurrence plots of
irradiance, which are not symmetric with respect to the line of identity
