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We present an initial study of one of the first ALMA Band 3 observations ofthe Sun with the aim to characterise the diagnostic potential of brightnesstemperatures measured with ALMA on the Sun. The observation covers 48min at acadence of 2s targeting a Quiet Sun region at disk-centre. Corresponding timeseries of brightness temperature maps are constructed with the first version ofthe Solar ALMA Pipeline (SoAP) and compared to simultaneous SDO observations.The angular resolution of the observations is set by the synthesized beam(1.4x2.1as). The ALMA maps exhibit network patches, internetwork regions andalso elongated thin features that are connected to large-scale magnetic loopsas confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate bestwith the SDO/AIA 171, 131 and 304 channels in that they exhibit networkfeatures and, although very weak in the ALMA maps, imprints of large-scaleloops. A group of compact magnetic loops is very clearly visible in ALMA Band3. The brightness temperatures in the loop tops reach values of about8000-9000K and in extreme moments up to 10 000K. ALMA Band 3 interferometricobservations from early observing cycles already reveal temperature differencesin the solar chromosphere. The weak imprint of magnetic loops and thecorrelation with the 171, 131, and 304 SDO channels suggests though that theradiation mapped in ALMA Band 3 might have contributions from a larger range ofatmospheric heights than previously assumed but the exact formation height ofBand 3 needs to be investigated in more detail. The absolute brightnesstemperature scale as set by Total Power measurements remains less certain andmust be improved in the future. Despite these complications and the limitedangular resolution, ALMA Band 3 observations have large potential forquantitative studies of the small-scale structure and dynamics of the solarchromosphere.

The Sun at millimeter wavelengths I. Introduction to ALMA Band 3 observations