亚 (yà) here works exactly like the English prefix 'sub-' (also 'Asia' in other contexts, but in scientific compounds it's always 'sub-'):
亚热带 'subtropical,'
亚音速 'subsonic,'
亚原子 'subatomic.'
原子 (yuánzǐ) = 'atom,' literally 'origin-particle,' a 19th-century calque of Greek atomos.
