In Parallel Mode, SPIRE and PACS are operating in photometry mode simultaneously, carrying out large-area mapping observations. SPIRE is observing in its three photometric bands (250, 350 and 500 µm), while PACS is taking data in its red band (130-210 µm) and in one of the blue bands (60-85 µm or 85-130 µm).

The SPIRE PACS Parallel Mode is treated as a 4^th Herschel instrument, i.e. it has its own Astronomical Observing Template (AOT), its own user interface in HSpot and the two instruments are operated in a way that the spacecraft infrastructure is optimally used.

The advantage of Parallel Mode is ensured by the compatibility of the two instruments and their operating modes. In practice, simultaneous observations in five bands are made possible without significant degradation in instrument performance. However, considering the fact that the PACS and SPIRE integration times are identical, the relative instrument sensitivities have to be carefully inspected to judge the real scientific benefit of using Parallel Mode.

In Parallel Mode, the detectors' sampling frequency of the SPIRE instrument is reduced to 10 Hz, in respect to the 18.2 Hz of the SPIRE-only operations. Observations performed during the PV phase demonstrated that this does not affect the quality of SPIRE data, especially when scanning at 20"/s (slow mode). However, scanning at 60"/s (fast mode) has a small impact, mostly on glitches detection, removal and timeline reconstruction, and, as a second order effect, on beam smearing.

The PACS instrument applies an additional data compression with respect to PACS-prime operations, in order to make sure that the data rate in the five bands (3 for SPIRE and 2 for PACS) stays within the allowed limits. In the blue channel, data frames are averaged on-board 8 by 8, but the compression is left unchanged; in the red channel, a 4 by 4 frames averaging is applied. As a consequence, in Parallel Mode the PSF is more elongated in the blue channel than in PACS-prime scan mapping.

Parallel Mode will be used during dedicated Observation Days (ODs) of ~21 hours, although no single observation could last longer than 18 hours. The reason for this is to allow uninterrupted observations with the PACS and SPIRE photometers to reduce operational overheads caused by cooler recycling or switching between sub-instruments. Photometers cooler recycling takes about 2.5 hours each and it is run almost in parallel for the two instruments (30 minutes apart). It is carried out during the 3 hours of the Daily Telecommunication Period (DTCP), prior to a dedicated Parallel Mode OD. The cooler hold time is ~48 hours for SPIRE and up to ~60 hours for PACS: the preferred and most effective use of Parallel Mode is to carry out observations during windows of 2 days.