Fig. 4From: Occurrence features of intermediate descending layer and Sporadic E observed over the higher mid-latitude ionospheric station of MoscowExamples of daily HTI plots showing a 4-h periodicity of IDL occurrence over Moscow station during: a 18 December 2011, b 31 January 2011, c 26 October 2011, d 5 December 2013, e 25 January 2013 and f 20 November 2015. Dark solid lines are drawn to help the reader identify the IDLs altitude descentBack to article page