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Did you mean a circumflex or tilde? If so,then it is probably the περισπομένη, which (in Attic Greek) symbolised the rising then falling of pitch in a long syllable. It is only used on a long vowel.
If you meant a macron, then it symbolises a long alpha, iota or upsilon.
I'm afraid I don't quite follow you. Does this ~ equal a circumflex, and does it therefore function as an accent? Or is this ~ called a macron? And, if a macron makes an alpha, iota or upsilon long, how would that compare with the circumflex, as it does the same thing? I hope not to burden you with these tedious questions.
Is that so? That might very well be the case. If the ~ would indeed be a separate, relevant, Greek character, I should expect to be able to find it in Smyth's grammar, but I cannot. This sheds a favourable light on your point of view, benissimus.