the scope of this study but will be explored in forthcoming papers.

Observations and Data Sets
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AKARI observations and far-IR all-sky maps
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AKARI was designed as an all-sky survey mission in the mid- and far-IR wavelength regime [@Murakami07]. The satellite was equipped with a cryogenically cooled telescope of 68.5 cm aperture diameter and two scientific instruments, the Far-Infrared Surveyor (FIS; [@Kawada07]) and the Infrared Camera (IRC; [@Onaka07]). AKARI was launched in 2006 February and the all-sky survey was performed during the period 2006 April to 2007 August, which was the cold operation phase of the satellite, when it was cooled with liquid helium cryogen. Its 16-month cryogenic mission lifetime is thus by far the longest among the far-IR survey missions. The all-sky survey observations at far-IR wavelengths were performed by employing the FIS instrument [@Kawada07]. Four photometric bands were used: the N60 band (50–80 , centred at 65 ), the WIDE-S band (50–110 , centred at 90 ), the WIDE-L band (110–180 , centred at 140 ) and the N160 band (140–180 , centred at 160 ). The AKARI far-IR all-sky survey maps[^2], which have a better spatial resolution than the IRAS maps, were recently released publicly [@Doi15].

  