Falling Prone: If the mount falls prone, the rider is dismounted and falls prone in an unoccupied space of the rider’s choice adjacent to the now-prone mount. However, if the mount is flying when it is knocked prone, it instead falls. The rider isn’t dismounted unless the mount lands and falls prone itself. A rider who is knocked prone can immediately make a saving throw. On a roll of 9 or lower, the rider is dismounted and falls prone in an unoccupied space of the rider’s choice adjacent to the mount. On a roll of 10 or higher, the rider is neither dismounted nor knocked prone. A rider who voluntarily drops prone falls prone in an unoccupied space of the rider’s choice adjacent to the mount.
The power's description doesn't explicitly mention the Melee keyword for the sphere's attack, therefore that attack is not a Melee attack.
The only restriction is that the attack's target has to be adjacent to the sphere, but a close blast 1 or a [non-provoking] ranged 1 attack has the same effect. This means that the range restriction cannot be used to guess any implicit keywords of the attack in that way.
Compare the wording of these Conjuration powers, some of which explicitly mention the proxy's attack's type:
[...] You conjure a Medium ball of lightning that occupies 1 square within range, and the ball makes the following attack, which is a close burst 1. [...]
When a creature falls past an enemy, the creature does not trigger opportunity actions, such as opportunity attacks, from that enemy that are triggered by movement."
Best Answer
The rider falls prone in an adjacent square
From the DMG and the Rules Compendium[ddi]