When is "here" an adverb or a noun?

In the sentence "I hope you are all paying attention, here is a sentence I made earlier", is here an adverb or a noun? I think it is a noun, but if I substitute a noun or a pronoun for here, the sentence loses its intended meaning.

John Lawler notes in a comment on a different answer here:

Alas, no. It's still an adverb. The construction Here/There is/are Noun Phrase allows the adverb to be fronted, with the subject Noun Phrase moved to the end, as the new information. It's said either referring to a physical place (pointing is appropriate), or metaphorically to refer to things that are being said in the conversation. Here is X generally means 'The next thing I say is X'.