Diurnal variation of VLF signals
  This paper presents an introduction to Chapman's theory of a production layer in the ionosphere relevant to the monitoring of VLF signals and Sudden Ionospheric Distubances (SIDs). It shows how the diurnal pattern of VLF signal strength can be derived, and how this pattern may be fitted to VLF signal strength measurements. It also shows through an example some of the possible problems for fitting this diurnal pattern (the fact that D-layer height cannot always be derived directly from measurements) and some of the advantages when a `fit' has been obtained (increased sensitivity of the instrument without any changes to hardware). 
Richard Kaye
astro-ph.IM physics.space-ph