He can achieve this form while running at high speeds through the Great Desert near Sand Blast City while also using Nate Morgan's Super Emerald to return to Knothole in regular time. He has only made this transformation once, and it seems probable that the environment influenced his change to this state as well as Polar Sonic and Eco Sonic.
Powers and abilities
As Solar Sonic, Sonic's already-amazing speed is enhanced to a new level, being capable of moving even faster than Ultra Sonic. He also has a degree of pyrokinesis, able to generate a trail of green flames from his body as he runs.