Analysis of diurnal double maxima observed above Italy during 1975-1991

► This study tries to establish whether there is a sole cause/source of the DDM structures. ► Slant TEC data derived from Faraday rotation measurements from Italy have been used. ► DDM structures were verified by ionosonde measurements from Rome. ► Seasonal and geomagnetic trends of the DDMs were tested. ► Results indicated that neither geomagnetic nor substorm conditions are the sole cause of the DDM structures.