[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

[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] Measurements from the incoherent scatter radar (ISR) and ionosonde over Millstone Hill (42.6°N, 288.5°E) are analyzed to explore ionospheric temporal variations. The F 2 layer peak density N m F 2 , peak height h m F 2 , and scale height H are derived from a Chapman a layer fitting to observed ISR electron density profiles. Diurnal, seasonal, and solar activity variations of the ionospheric characteristics are presented. Our study on the solar activity dependence of N m F 2 , h m F 2 , and H indicates that the peak parameters (N m F 2 and h m F 2 ) of the F 2 layer increase with daily F 10.7 index and saturate (or increase with a much lower rate) for very high F 10.7 ; however, they show almost linear dependence with the solar proxy index F 10.7p = (F 10.7 + F 10.7A )/2, where F 10.7A is the 81-day running mean of daily F 10.7 . This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F 10.7p . The rate of change in the ionospheric characteristics with solar activity exhibits a seasonal and local time variation. Over Millstone Hill, N m F 2 in summer is characterized by the evening peak in its diurnal variation, and N m F 2 exhibits winter anomaly under low and high solar activity levels. The temporal variations of the topside effective scale height H 0 can be explained in terms of those in the slab thickness. The IRI model overestimates the N e effective topside scale height over Millstone Hill; therefore our analysis for the effective topside scale height from the Millstone Hill measurements might help to improve the IRI topside profiles at middle latitudes.Citation: Lei, J., L. Liu, W. Wan, and S.-R. Zhang (2005), Variations of electron density based on long-term incoherent scatter radar and ionosonde measurements over Millstone Hill, Radio Sci., 40, RS2008,