You could indeed call it "supine" as distinct from "prone".  But I just found this in p 271's rules for PUSH attacks, where we have "knocked down":

 A knocked-down combatant is considered
    hindered and suffers a -2 AC penalty.

Page 264 tells us hindered means -2 AC but says nothing about it causing any penalty to attacks.  

rhialto wrote:

Never encountered in all my days, but my first instinct is "a prone attacker cannot attack".[...] I'd likely levy a -2 or -4 attack roll modifier [..]

I totally have encountered this in several game systems over the years.  In systems where getting up from prone takes your whole action, every player wants to attack from prone, even at a penalty, rather than spend that round not getting to roll something.

So that means it's telling that p. 264's "Table 139: Attack Roll Modifiers" does not include any entry for Attacker is Prone.
   Surely if attacking from prone were possible it would be at a penalty.  Sure, a movie-hero could hamstring their opponent from prone, but doing so should require them to heroically overcome more than just the -1 penalty for 'defender is in a higher position'.  They have options, but fewer than a standing combatant's, and they are therefore easier to predict and to defend against, amounting to a penalty worse than that -1...

And since there's nothing listed for it, I suspect the intention was that prone characters are not supposed to be able to attack at all, but are assumed to get back to their feet before renewing the fight.  (If they weren't treated as prone only temporarily because of the attacker having had the drop on them; per p. 266's "A surprized party may be considered prone if caught completely unawares (e.g., sleeping, eating, reading).").

But if a prone creature could not attack, that would mean that Pyromancers' Fire Web spells, which keep the victim prone for multiple rounds, would be an instant, no-challenge, encounter-winner.  And we don't really want that, do we?