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In the present paper, we use Si IV 1393.755 \AA\ spectral line observed bythe Interface Region Imaging Spectrograph (IRIS) in the quiet-Sun to determinephysical nature of the solar transition region (TR) oscillations. We analyzethe properties of these oscillations using wavelet tools (e.g., power,cross-power, coherence, and phase difference) along with the stringent noisemodel (i.e., power-law + constant). We estimate the period of the intensity andDoppler velocity oscillations at each chosen location in the quiet-Sun (QS) andquantify the distribution of the statistically significant power and associatedperiods in one bright and two dark regions. In the bright TR region, the meanperiods in intensity and velocity are 7 min, and 8 min respectively. In thedark region, the mean periods in intensity and velocity are 7 min, and 5.4 minrespectively. We also estimate the phase difference between the intensity andDoppler velocity oscillations at each location. The statistical distribution ofphase difference is estimated, which peaks at -119\degree $\pm$ 13\degree,33\degree $\pm$ 10\degree, 102\degree $\pm$ 10\degree\ in the bright region,while at -153\degree $\pm$ 13\degree, 6\degree $\pm$ 20\degree, 151\degree$\pm$ 10\degree\ in the dark region. The statistical distribution reveals thatthe oscillations are caused by propagating slow magnetoacoustic wavesencountered with the TR. Some of these locations may also be associated withthe standing slow waves. Even, in the given time domain, several locationsexhibit presence of both propagating and standing oscillations at differentfrequencies.

Spectroscopic study of solar transition region oscillations in the quiet-Sun observed by IRIS using Si IV spectral line