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The dust-HI correlation is used to characterize the emission properties ofdust in the diffuse interstellar medium. We cross-correlate sky maps fromPlanck, WMAP, and DIRBE, at 17 frequencies from 23 to 3000 GHz, with the Parkessurvey of the 21-cm line emission of neutral atomic hydrogen, over a contiguousarea of 7500 deg$^2$ centred on the southern Galactic pole. Our analysis yieldsfour specific results. (1) The dust temperature is observed to beanti-correlated with the dust emissivity and opacity. We interpret this resultas evidence for dust evolution within the diffuse ISM. The mean dust opacity ismeasured to be $(7.1 \pm 0.6) 10^{-27} cm^2/H \times (\nu/353\,GHz)^{1.53\pm0.03}$ for $100 < \nu <353$GHz. (2) We map the spectral index ofdust emission at millimetre wavelengths, which is remarkably constant at$\beta_{mm} = 1.51\pm 0.13$. We compare it with the far infrared spectral indexbeta_FIR derived from greybody fits at higher frequencies, and find asystematic difference, $\beta_{mm}-\beta_{FIR} = -0.15$, which suggests thatthe dust SED flattens at $\nu < 353\,$GHz. (3) We present spectral fits of themicrowave emission correlated with HI from 23 to 353 GHz, which separate dustand anomalous microwave emission. The flattening of the dust SED can beaccounted for with an additional component with a blackbody spectrum, whichaccounts for $(26 \pm 6)$% of the dust emission at 100 GHz and could representmagnetic dipole emission. Alternatively, it could account for an increasingcontribution of carbon dust, or a flattening of the emissivity of amorphoussilicates, at millimetre wavelengths. These interpretations make differentpredictions for the dust polarization SED. (4) We identify a Galacticcontribution to the residuals of the dust-HI correlation, which we model withvariations of the dust emissivity on angular scales smaller than that of ourcorrelation analysis.

Planck intermediate results. XVII. Emission of dust in the diffuse interstellar medium from the far-infrared to microwave frequencies