PRONE
The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.
Standing up is a move-equivalent action that provokes an attack of opportunity.
The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 cover bonus to Armor Class against ranged attacks at greater than 5ft, but takes a –4 penalty to AC against melee attacks. Ranged attacks from attackers with a reach able to normally attack the prone character give no cover AC bonus against the ranged attack and attacker does not receive a bonus to hit.
Standing up is a move-equivalent action that provokes an attack of opportunity.
Examples:
* A Zen Archer with point blank master may shoot an adjacent prone character at his standard attack bonus.
* Fighter 10ft away with a reach weapon attacks a prone character at -4 to his AC.
* A large humanoid with a bow 10ft away from prone character attacks at his standard attack bonus.
* A wizard with scorching ray who successfully cast defensively at 5ft away from prone character uses standard attack bonus.
I'm not sure if casting and using scorching ray provokes two AoO. One for casting and one for ranged attack. As I understand it, the range attack is a free action and is a part of spell. So, if cast defensively then there would be no AoO.
Defining the prone AC bonus vs range as a cover bonus will allow things like soft cover to not stack. Improved precise shot would also eliminate the AC bonus.
The philosophy of being prone vs range and it giving you a bonus to AC is because you are reducing your profile to the attack and using the ground as hard cover. But if the attacker is directly over you, then you don't get that cover. Attackers with Reach weapons or ones that could normally just touch you with their normal reach but are using a ranged weapon also negate the cover bonus since they are able to guide the attack to a 0 range of prone target.
I also considered if the prone target is larger and how that would affect a range modifier, but decided there was no easy answer that I liked. Perhaps, huge and larger creature who falls prone do not get AC bonus vs range attacks. They're just too big to gain cover from the ground and reduce their profile, but I'm not sure I agree with that.
I do have a tweak with reach weapons. I think I may have listed the modifier wrong in my examples. The reach weapon would get no penalty for cover, but defender would still get the -4 AC with melee attack. The range rules for a reach weapon only applies to cover, so it's still considered a melee weapon.
Unlike 3.5, if you have an ally in front of you with your reach weapon, the target will get a +4 AC for soft cover. Soft cover and prone cover don't stack. Ranged attackers would only get the -4 to attack rather than -8 if shooting a prone target with an ally in the way.