both the parallel wire carries current in same direction , then why the magnetic field is upward in c2d2 and downward in c1d1 ?
Here is the answer to your query.
In a straight current carrying conductor the magnetic field is circular (using right hand thumb rule). From one side the magnetic field line seems emerging out and from the other side it seems going inwards.
So to reduce this ambiguity, if we consider the outside direction of magnetic field of one current carrying conductor so we need to consider the outside direction of the magnetic field on the other conductor also.
You can understand it from the following diagram:
Hope, you understand the concept.