That's cool looking. I think that's die polishing lines. I had a bicentennial quarter similar to that - check my album for pictures. I had a thread about it.
@Shrews1994 It's actually a Worn Reverse Die strike. They are called striations on the field. Not Die polishing lines IMHO
To add to @paddyman98 post, Every time a coin is struck, metal moves and flows into the design. Even though the dies are very hard, over time and numerous strikes, the metal in the die will start to abrade. This can show up as the flow lines you see on the reverse of the OPs dime.[/user]