Statistical analysis of compact brightenings in IRIS Mg II h and k lines

Context. Compact brightenings (CBs) are frequently observed by the Interface Region Imaging Spectrograph (IRIS) in ultraviolet radiation. They appear as small and intense short-time phenomena located in solar active regions. Aims. Our main goal is to characterize and classify different CBs based on the Mg II h & k lines profiles, determine their visibility in the far-ultraviolet range, and relate them to well-defined UV-bursts and Ellerman bombs. This information is used to diagnose their formation height in the context of established 1D non-local thermodynamic equilibrium (NLTE) models with hot spots. Methods. We present a statistical analysis based on the IRIS Mg II spectra in large dense rasters, which we divided between three locations: emerging flux (EMF) areas, plages, and around sunspots. We developed an algorithm to search for CBs using proxies based on the enhancement of contrasts at different wavelengths around the Mg II k line center and their visibility in Si IV, C II, and Mg II triplet lines. Three types of Mg II profiles are differentiated using the R parameter (ratio between the intensity at 2800Å and the integrated intensity of Mg II k line), and are described as follows: the enhancement of intensity in the peaks or line center (Type 1), in the close wings (Type 2), and in the far wings (Type 3) of Mg II lines. Results. Of all the 2053 identified CBs, most of them (53%) are classified as Type 2, 27% as Type 1, and 20% as Type 3. It seems that each CB type, except Type 2, prefers a different location, suggesting various formation scenarios and magnetic field configurations. Type 3 CBs are mainly found around sunspots and in plages and Type 1 mostly in EMF regions. We also found a correlation between Mg II k, Si IV, C II, and Mg II UV triplet lines. Signatures of emission in Si IV, C II, and Mg II UV triplet lines are present for, respectively, 55%, 73%, and 37% of all CBs. The strongest emission in those lines appears for Type 1 CBs. Conclusions. For the CB classification we defined a new R parameter that reflects their formation height and allows us to divide CBs into three different types according to the grid of 1D models: Type 1 form in the chromosphere (> 650 km), Type 2 between 650 and 480 km at the temperature minimum region (TMR) and Type 3 below 480 km. We parametrized all the 2053 CBs and determined their mutual dependencies. In particular, we investigated the occurrence of possible Ellerman bombs, UV bursts, and IRIS bombs among all CBs, which constitute, respectively, 13%, 6%, and 2.4% all CBs. We found that contrast parameters related to cool and hot lines are correlated, and this correlation is more significant for CBs located above the TMR. This correlation may suggest a common formation region. The use of modern machine learning tools will help us better understand the nature of small-scale chromospheric activity.