Variability of D-region electron densities at Tromso

The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromso has been used for a study of D-region electron densities by the differential-absorption method on a number of days during 1978-79. Received signals are generally stratified in several layers, typically over 60-80 km. Strong stable echoes are seen down to 55 km during periods of enhanced riometer absorption. Inferred electron densities vary between about 100-1000 per cu cm at about 60-80 km and show well-defined features which persist for about 10-20 min. During periods of high absorption, enhanced electron densities about 600 cm per cu cm are observed below 65 km. During a polar cap absorption event, the inferred electron densities at 60-70 km show a very stable profile. Possible sources of D-region ionization at high latitudes are briefly discussed.