The teams also exchanged draft picks. The Flyers got a third-round pick in next year's draft, and Tampa Bay received its 2009 fourth-round selection that it traded to Philadelphia in June.
The 6-foot, 205-pound Carle had a goal and an assist in 12 games for Tampa Bay this season. The former University of Denver standout won the Hobey Baker award as the nation's top college player in 2006.
Tampa Bay acquired the 24-year-old Carle in a four-player deal with the San Jose Sharks in July.
"Matt Carle is a good young defenseman and we are thrilled to add a player with his abilities to our roster," Flyers general manager Paul Holmgren said in a statement. "He will add skill and speed to our lineup and should fit in great with our team."
Eminger had two assists and eight penalty minutes in 12 games for Philadelphia this season. Downie played in six games for the Flyers this year, and was scoreless with 11 penalty minutes.
Downie received one of the longest suspensions in NHL history when he was banned for 20 games last year for leaving his feet to deliver a deliberate and dangerous hit to Ottawa forward Dean McAmmond's head.