The spatial and temporal structure of twin peaks and midday bite out in f_oF_2 (with associated height changes) in the Australian and South Pacific low midlatitude ionosphere

摘要

Morning and afternoon peaks in daytime critical frequency f_oF_2 defining a midday bite out were found to occur regularly across northern Australia during April 2008. This behavior was sufficiently repetitive to appear in the monthly median values of f_oF_2. The twin peak and bite out phenomenon was observable across the entire longitudinal range of ionosondes which were accessible for this study, from Niue in mid-Pacific to Learmonth on the western side of Australia. The high geographic density of ionosondes operating in Australia enabled the limited latitudinal range of occurrence to be established, the phenomenon ceasing to be present in the southern half of Australia or at equatorial latitudes. While strongest in 2008, the f_oF_2 bite out and associated variation in h_mF_2, the peak height of the ionosphere, continued to be seen in monthly April medians from sunspot minimum (2007–2008) to the current low sunspot maximum (2011–2013) while diminishing in magnitude. This phenomenon was also present around the September equinox, though not of such magnitude or consistency as during the April equinox. The morning and afternoon peaks in f_oF_2 occurred during periods of falling virtual and true height and were associated with the maximum compression of the ionosphere at this time as measured by the subpeak equivalent parabolic layer thickness ym. Meridional winds in the F2 layer are suggested as a driver of the twin peak enhancements in f_oF_2, which may possibly be related to a simultaneous occurrence of strong tidal influences as seen to be present in descending sporadic E.