The sun disc you are asking about is called the Aten(which basically means sun disc of Re)
It is often seen with different attachments to it depending on which god is worshiping the sun.If seen on stone carvings the rays of the sun beam down from it.
All the Aten discs I have seen appear to have the "arms" going upwards.
This Wikipedia page will tell you more,but I should warn you is quite long,as you are listening to it and not reading it.
http://en.wikipedia.org/wiki/Aten
The Pharaoh Akhenaten rebelled against the corrupt practices of the priestly class,and the hundreds of gods,headed by Amun.and changed the end of his kingly name to the Aten word,meaning solitary sun god.
When he died his son Smenkhare was murdered,and his second son Tutankamun was made king (to restore the king name to Amun not Aten) originally Tutankaten,but when the priests found that he was going to restore his Father's beliefs they murdered him too.