The Seyfert 1 galaxy Ark 120. Spectral variability in 1992-2005

We have studied the variability of the H beta line and the adjacent continuum in the spectrum of the Seyfert galaxy Ark 120, based on spectral observations of the galaxy's nucleus obtained in the Crimea in 1992-2005, supplemented by published data for 1988-1996. Irregular variability on various timescales (years to days) can be accompanied by periodic brightness variations in both the continuum and the H beta line, with a period of P similar to 430 days and an amplitude of Delta m similar to 0.2(m) in the continuum, which were traced for more than 13 cycles. In total, in 1988-2005, the flux variations in the line lag those in the continuum by 55 +/- 9 days if calculated from the peak of the cross correlation function, or by 72 +/- 7 days, if calculated from the centroid of the CCF. The delay is correlated with the continuum brightness, increasing when the continuum flux increases. The H beta line profiles indicate both a high degree of diversity and the presence of features that recur after various extended time intervals. Analysis of the evolution of the differences between each individual normalized line pro. le and the mean normalized pro. le indicates systematic motion of excesses relative to the average pro. le from negative to positive radial velocities. In contrast, parts of the H beta line with low radiation relative to the mean normalized pro. le evolve in the opposite direction (from the red to the blue H beta wing). This pattern is also typical for the rotating broad-line region, if this region has the form of a disk. The rotation period exceeds 9000-10000 days, or 25-27 years. The size of the broad-line region calculated form this period corresponds to a reverberation time of no fewer than 30 days, consistent with the results of cross-correlation analysis.