Fill a large skillet with 1 1/2 inches water; bring to a boil. Add salmon; reduce heat, and simmer 8 minutes, or until fish flakes easily when tested with a fork. Remove from skillet; discard cooking liquid. Combine 1/4 cup each mayonnaise and buttermilk, 2 tablespoons dill, 1 tablespoon lime juice, and a pinch each of sugar, salt, and pepper in a small bowl. Arrange 1 cup arugula and 1 salmon fillet on each of 2 plates, and drizzle the dill mixture over the fish.