Some effects of geomagnetic activity and ionospheric height rises on mid-latitude spread-F occurrence

• Published in: Journal of Atmospheric and Terrestrial Physics
• Main Author: Bowman G.G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 1984
• Subjects: General Engineering; Atmospheric Science; General Earth and Planetary Sciences; Geophysics; General Environmental Science; 1902 Film; Television and Digital Media; 1908 Geophysics; Brisbane; Macquarie Island
• Online Access: https://espace.library.uq.edu.au/view/UQ:404865

The analysis investigates the base heights of the ionosphere (h′F) when spread-F is recorded at Brisbane, Australia, for 2 separate periods, namely July and August 1966 when spread-F occurs frequently and September and October 1966 when the activity is much lower. It is found that for July and August there is little tendency for spread-F to occur preferentially when the base height is above the average for the period. However, for the September-October period, spread-F occurs more often (by a factor of 2 or 3 depending on the geomagnetic activity) when the base height is above rather than below the average height. Also, this analysis shows that the overall spread-F occurrence (for both periods investigated) decreased to some extent following increased geomagnetic activity. This suppressed activity in the hours following geomagnetic activity is confirmed by superposed-epoch analyses using K indices (for Macquarie Island) as controls. It is suggested that the results of all the analyses might be explained by invoking a transition height in the ionosphere (controlled by the neutral-particle density of the upper atmosphere). Ionospheric off-vertical reflections from above this height would be recorded as spread-F traces in this model. This transition height would be low in July and August when the neutral-particle density is low and higher in September and October. It is further proposed that changes in the neutral-particle density could also be associated with reduced spread-F activity following increased geomagnetic activity, as well as influencing the diurnal, annual and sunspot-cycle variations of spread-F occurrence in mid-latitudes.