Individual #D-layer__D_region  the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves
  part of:  ionosphere
  type:  region.location__part  the extended spatial location of something; "the farming regions of France"; "religions in all parts of the world"; "regions of outer space"

No statement uses or specializes D-layer; click here to add one.