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The broad Mg II line in quasars has distinct variability properties comparedwith broad Balmer lines: it is less variable, and usually does not display a"breathing" mode, the increase in the average cloud distance when luminosityincreases. We demonstrate that these variability properties of Mg II can bereasonably well explained by simple Locally Optimally Emitting Cloud (LOC)photoionization models, confirming earlier photoionization results. In thefiducial LOC model, the Mg II-emitting gas is on average more distant from theionizing source than the H$\alpha$/H$\beta$ gas, and responds with a loweramplitude to continuum variations. If the broad-line region (BLR) is truncatedat a physical radius of $\sim 0.3$ pc (for a $10^{8.5}M_{\odot}$ BH accretingat Eddington ratio of 0.1), most of the Mg II flux will always be emitted nearthis outer boundary and hence will not display breathing. These resultsindicate that reverberation mapping results on broad Mg II, while generallymore difficult to obtain due to the lower line responsivity, can still be usedto infer the Mg II BLR size and hence black hole mass. But it is possible thatMg II does not have a well defined intrinsic BLR size-luminosity relation forindividual quasars, even though a global one for the general population maystill exist. The dramatic changes in broad H$\alpha$/H$\beta$ emission in theobservationally-rare changing-look quasars are fully consistent withphotoionization responses to extreme continuum variability, and the LOC modelprovides natural explanations for the persistence of broad Mg II inchanging-look quasars defined on H$\alpha$/H$\beta$, and the rare population ofbroad Mg II emitters in the spectra of massive inactive galaxies.

Understanding Broad Mg II Variability in Quasars with Photoionization: Implications for Reverberation Mapping and Changing-Look Quasars