[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (N e ) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2°E, 18.3°N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles defined as the value of Àdh/d(ln(N e )), and the effective scale height (H m ), which is defined as the scale height in the Chapman-a function to approximate the N e profiles. As a measure of the slope of the height profiles of the topside electron density, the derived VSH and H m show marked diurnal and seasonal variations and solar activity dependences. Their features are discussed in terms of thermal structures in the lower topside ionosphere. We also investigate the quantitative relationships between H m , VSH, and plasma scale height (H p ) over Arecibo. The similarities and differences in these scale heights are discussed. Results suggest that both the contributions from topside temperature structure and diffusion processes can also greatly control VSH and H m through changing the profile shape.

[1] We analyzed 11 years ' (1998-2008) worth of the total electron content (TEC) data derived at the Jet Propulsion Laboratory (JPL) from Global Positioning System (GPS) observations to investigate the overall climatological features of the ionosphere in a new way. The global ionospheric maps (GIM) of JPL TEC are averaged globally and over low-, middle-, and high-latitude ranges in the southern (northern) hemisphere and both hemispheres to identify their capability of capturing the overall features of the ionosphere. These mean TEC data show strong annual/semiannual, solar cycle, and 27-day variations. The mean TEC presents stronger solar activity sensitivity at lower-latitude bands. Moreover, the saturation effect exists in these mean TEC versus solar index F 10.7 , more pronounced at low latitudes, while the mean TEC increases faster with higher solar EUV fluxes, being evident at high latitudes. The annual asymmetry (differences in June and December solstices) can be detected in the mean TEC averaged globally and at low latitudes under all solar epochs as well as at middle and high latitudes under most solar activities. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination. Both the hemispheric differences and annual asymmetry are more marked with increasing solar activity. The annual components of the mean TEC are stronger in the southern hemisphere, and the semiannual components are of similar phases and comparable amplitudes in conjugate hemispheres, which suggest close couplings of the ionosphere in both hemispheres. Further, the mean TEC averaged in one hemisphere can reliably be used as ionospheric indices to monitor the solar activity variabilities and to capture the overall climatological features of the ionosphere over specified regions, while it should be cautioned that the mitigation of the dominant annual components with opposite phases in conjugate hemispheres leaves a significant semiannual component in the mean TEC averaged in both hemispheres, especially under low solar activity.

[1] We collected the ionospheric electron density (N e ) profiles from the FORMOSAT-3/ COSMIC (F3/C) radio occultation measurements to investigate the seasonal behaviors of daytime N e in the altitude range of 200-560 km. Harmonic analysis of the N e at different altitudes provides unprecedented detail of the seasonal behaviors of N e at low solar activity (LSA). Global maps of seasonal harmonic components indicate that there are strong annual and semiannual variations in daytime N e , which have distinct latitudinal and altitudinal dependency. The semiannual component predominates over the annual variation in the equatorial regions and at high latitudes in the East Asian and South Atlantic sectors at low altitudes, and at higher altitudes the semiannual component predominates in the equatorial region, but recedes in other regions. The semiannual variation peaks in equinoctial months in most regions, while it has maxima in solstice months, first in the South Pacific region (around 30°S, 120°W) at 250 km altitude and expanding over the South Pacific and South Atlantic oceans at higher altitudes. Moreover, there is a region around 45°S, 30°W with a dominant semiannual component, moving toward east-north with increasing altitude in the range of 200-270 km. These two interesting features are novel but are not reported yet. The relative amplitude of the annual component of N e has hemispheric asymmetry, which is prominent at high altitudes in the Southern Hemisphere. The winter/seasonal anomaly widely exists in the Northern Hemisphere and southern low latitudes and in Indian Ocean region at low altitudes but gradually disappears at higher altitudes. Further, in equatorial regions, a new finding is the obvious wave-like pattern in the longitudinal structure of the amplitudes of seasonal harmonic components in equatorial regions, which supports possible couplings of sources with lower atmospheric origins in the longitudinal variations of N e .