Large-scale travelling atmospheric disturbances in the night ionosphere during the solar–terrestrial event of 23 May 2002

This paper examines the night of 23 May 2002 as observed by a large number of Australian ionosondes (19) as well as others situated in New Guinea, Indonesia and China. The arrival of a solar Coronal Mass Ejection (CME) and subsequent negative Bz turnings in the solar wind resulted in a magnetic storm with two bursts of energy inputs into the auroral zones. The energy depositions produced two successive rise and falls in ionospheric height over a 300 km height range within the period 12.30–21.00 UT. The two events were seen in the night-side hemisphere by all ionosondes at Southeast Asian longitudes in the southern hemisphere, as well as in the northern hemisphere. In this paper, the simultaneity and spatial variability of these events is investigated. The first event, after an initial expansion towards the equator, ended with a retreat in the area of height rise back towards the auroral zone. The second event was of greater complexity and did not show such a steady variation in rise and fall times with latitude. Such events are often described as large-scale travelling atmospheric/ionospheric disturbances (LTADs or LTIDs). In the southern hemisphere, the front of the initial height rise was found to move at a speed up to 1300 m/s as was also measured by Tsugawa et al. [2006. Geomagnetic conjugate observations of large-scale travelling ionospheric disturbances using GPS networks in Japan and Australia. Journal of Geophysical Research 111, A02302] from small changes in GPS TEC. The front was uniform across the widest longitudinal range of observation (52° or 5360 km).The relationship between the subsequent fall in ionospheric height and an associated temporary increase in foF2 was found to be consistent with previous observations. Ionospheric drivers that move ionization up and down magnetic field lines are suggested as the common cause of the relationship between foF2 and height.