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We describe the data processing of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program). This first data release presents 33% (51.34 hr out of 155.7 hr, including overheads) of the total awarded time from 2020 January to 2021 December. Our observations were performed using the newly implemented on-the-fly mapping (OTFMAP) technique in the polarimetric mode. We present the pipeline steps to obtain homogeneously reduced high-level data products of polarimetric maps of galaxies for use in scientific analysis. Our approach has a general design and can be applied to sources smaller than the field of view of the HAWC+ array in any given band. We estimate that the OTFMAP polarimetric mode offers a reduction of observing overheads by a factor 2.34 and an improvement in sensitivity by a factor 1.80 when compared to the same on-source time polarimetric observations using the chopping and nodding mode. The OTFMAP is a significant optimization of the polarimetric mode of HAWC+, as it ultimately reduces the cost of operations of HAWC+/SOFIA by increasing the science collected per hour of observation up to an overall factor of 2.49. The OTFMAP polarimetric mode is the standard observing strategy of SALSA. The results and quantitative analysis of this first data release are presented in Papers IV and V of the series. [ABSTRACT FROM AUTHOR]

We present the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program) with a set of 14 nearby (<20 Mpc) galaxies with resolved imaging polarimetric observations using HAWC+ from 53 to 214 ÎĽ m at a resolution of 5″â€"18″ (90 pcâ€"1 kpc). We introduce the definitions of and background on extragalactic magnetism and present the scientific motivation and sample selection of the program. Here we focus on the general trends in the emissive polarization fraction. Far-infrared polarimetric observations trace the thermal polarized emission of magnetically aligned dust grains across the galaxy disks with polarization fractions of P = 0%â€"15% in the cold, T d = [19, 48] K, and dense, log 10 (N H I + H 2 [ cm âˆ' 2 ]) = [ 19.96 , 22.91 ] , interstellar medium. The spiral galaxies show a median 〈 P 154 ÎĽ m〉 = 3.3% ± 0.9% across the disks. We report the first polarized spectrum of starburst galaxies showing a minimum within 89â€"154 ÎĽ m. The falling 53â€"154 ÎĽ m polarized spectrum may be due to a decrease in the dust grain alignment efficiency produced by variations in dust temperatures along the line of sight in the galactic outflow. We find that the starburst galaxies and the star-forming regions within normal galaxies have the lowest polarization fractions. We find that 50% (seven out of 14) of the galaxies require a broken power law in the P âˆ' N H I + H 2 and P âˆ' T d relations with three different trends. Group 1 has a relative increase of anisotropic random B-fields produced by compression or shear of B-fields in the galactic outflows, starburst rings, and inner bars of galaxies, and groups 2 and 3 have a relative increase of isotropic random B-fields driven by star-forming regions in the spiral arms and/or an increase of dust grain alignment efficiency caused by shock-driven regions or evolutionary stages of a galaxy. [ABSTRACT FROM AUTHOR]