Hi Shelley,
The lesson on 'apparaître' is still being prepared but I will try to answer your question here-
In the sense of 'to appear' as in an event you can actually use both. You will hear "Le soleil est finalement apparu cet après-midi." or "Le soleil a finalement apparu... "The sun finally made an appearance this afternoon.)
The difference being emphasis on the action (using 'avoir') or the result (using 'être').
It is indeed a very subtle distinction and I would not worry too much about it ...
e.g
Les documents sont apparus/ont apparu ce matin.
The documents appeared/materialised this morning.
both are possible.
Hope this helps!