Sally Rachel Jacobs has written:

'Magnesium reabsorption in the thick ascending limb of the loop of Henle'

The function of the ascending loop of henle is draw Sodium out of the filtrate. The ability of the ascending loop to actively transport sodium into the surrounding tissue is directly related to the amount of water that can be diffused out of the descending loop of henle. (main factor in water re-absorption)

Yes, the ascending limb of the loop of Henle.

absorption of water (from descending limb), absorption of Na+ (from ascending limb) 

the primary function of the ascending loop of henle in the kidney