Halliday & Matthiessen (2014: 187):
These are so-called because they are closely associated with the meanings construed by the mood system: modality and temporality; and also intensity. This means that their neutral position in the clause is next to the Finite verbal operator, either just before it or just after it. But there are two other possible locations: before the Subject (ie in thematic position — those of temporality and modality have a strong tendency to function as Theme) and at the end of the clause as Afterthought.