Variations of electron density based on long-term incoherent scatter radar and ionosonde measurements over Millstone Hill

[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 F2 layer peak density NmF2, peak height hmF2, 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 NmF2, hmF2, and H indicates that the peak parameters (NmF2 and hmF2) of the F2 layer increase with daily F10.7 index and saturate (or increase with a much lower rate) for very high F10.7; however, they show almost linear dependence with the solar proxy index F10.7p = (F10.7 + F10.7A)/2, where F10.7A is the 81-day running mean of daily F10.7. This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F10.7p. The rate of change in the ionospheric characteristics with solar activity exhibits a seasonal and local time variation. Over Millstone Hill, NmF2 in summer is characterized by the evening peak in its diurnal variation, and NmF2 exhibits winter anomaly under low and high solar activity levels. The temporal variations of the topside effective scale height H0 can be explained in terms of those in the slab thickness. The IRI model overestimates the Ne 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.