The dependence of spectrum of transversally small-scale perturbations on the ionospheric conductivity is investigated. It is found that the perturbations under consideration have a periodic (Alfven, slow magnetosonic and compressible ballooning modes) and an aperiodic (flute and incompressible ballooning modes) components, which interact with each other through plasma pressure. The main attention is paid to the spectrum of periodic perturbations, because aperiodic one were considered before. It is shown that at a high conductivity the spectrum of periodic perturbations is identical to the corresponding spectrum at ideal conductivity. It is found that decrease of ionospheric conductivity leads to decrease of Alfven eigenmode frequencies, and at certain value of conductivity the first Alfven eigenmode vanishes. The frequencies of other Alfven eigenmodes change weakly at further decrease of conductivity. At the same time the type of interaction between Alfven and slow magnetosonic eigenmodes changes. When ionospheric conductivity decreases, the «reconnecting» curves of their spectra become closer, forming a simple intersection. It is shown that at usual conditions periodic perturbations are decaying and their decrement is proportional to ionospheric conductivity when the latter is low, and reverse proportional when it is high. In particular, the decrement is low in noon and night sectors of the magnetosphere, but becomes considerable in dawn and dusk sectors. On the basis of magnetometric measurements of low-latitude Intermagnet stations, it is shown that there is a qualitative correspondence between diurnal variations of frequency spectrum of magnetic field perturbations and the presented results of numerical simulation.