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Context. DG Tau is a low-mass pre-main sequence star, whose stronglyaccreting protoplanetary disk exhibits a so-far enigmatic behavior: itsmid-infrared thermal emission is strongly time-variable, even turning the 10$\mu$m silicate feature from emission to absorption temporarily. Aims. We lookfor the reason for the spectral variability at high spatial resolution and atmultiple epochs. Methods. We study the temporal variability of the mid-infraredinterferometric signal, observed with the VLTI/MIDI instrument at six epochsbetween 2011 and 2014. We fit a geometric disk model to the observedinterferometric signal to obtain spatial information about the disk. We alsomodel the mid-infrared spectra by template fitting to characterize the profileand time dependence of the silicate emission. We use physically motivatedradiative transfer modeling to interpret the mid-infrared interferometricspectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 $\mu$mabsorption feature related to amorphous silicate grains. The outer disk (r>1-3au) spectra show a crystalline silicate feature in emission, similar to thespectra of comet Hale-Bopp. The striking difference between the inner and outerdisk spectral feature is highly unusual among T Tauri stars. The mid-infraredvariability is dominated by the outer disk. The strength of the silicatefeature changed by more than a factor of two. Between 2011 and 2014 thehalf-light radius of the mid-infrared-emitting region decreased from 1.15 to0.7 au. Conclusions. For the origin of the absorption we discuss four possibleexplanations: a cold obscuring envelope, an accretion heated inner disk, atemperature inversion on the disk surface and a misaligned inner geometry. Thesilicate emission in the outer disk can be explained by dusty material highabove the disk plane, whose mass can change with time, possibly due toturbulence in the disk.

Mid-infrared interferometric variability of DG Tau: implications for the inner-disk structure