Modeling And Statistic Studies Of Mid-Latitude Ionosphere

The Earth's ionosphere is strongly coupled with thermosphere and magnetosphere and can have significant effects on both civilian and military systems, including HF communications, surveillance, and navigation systems. Therefore, the study of ionosphere has become more and more important. This paper is based on the incoherent scatter radar (ISR) measurements over Millstone Hill (42.6oN, 288.5oE) and combines numerical simulation with data assimilation technique. Variations of mid-latitude ionosphere and the ionosphere/ thermosphere coupling process are systematically analyzed. Main results of this dissertation are outlined as follows: First, the ISR measurements for more than two solar cycles during the period 1976-2002 over Millstone Hill has provided a good opportunity to examine variations of the middle latitude ionosphere, such as the bottomside electron density profile parameters B0 and B1, and the ionospheric characteristics (F2-layer peak parameters NmF2, hmF2 and scale height H, etc.). To improve the statistics, we also include the ionosonde observations over Millstone Hill for the period 1989-1990 and 1998-2004 to get the F2 layer peak parameters NmF2 and hmF2. On the other hand, the statistical results are also compared with the newly updated International Reference Ionosphere model (IRI-2001) in order to validate its prediction. The statistical study revealed that: (1) The F2 peak density 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. Our study on the dependence of NmF2 and hmF2 on solar activity indicates that they increase with daily F107 index and saturate (or increase with a much lower rate) for very high F107, however, they show almost linear dependence with the solar proxy index F107p=(F107+F107A)/2, where F107A is the 81-day running mean of daily F107. This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F107p. (2) We found that it appropriate to use the modified Chapman α-layer function with the varying scale height for representing the electron density height variation. The determined scale height H also undergoes appreciable changes with local time, season, and solar activity. It increases with increasing solar activity, and shows almost linear dependence with the solar proxy index F107p. The temporal variations of the effective topside scale height H0 can be explained in terms of those in the slab thickness, as suggested previously. (3) Diurnal, seasonal, and solar activity variations of the bottomside electron density profile parameters B0 and B1, representing the F2 layer thickness and shape, are studied. These results are compared with the latest IRI model. Our statistical study is characterized by morning and afternoon falls in the diurnal variation of B0 for seasons other than summer and a ~15% change in B1 over a solar cycle, features not fully well represented by the standard IRI model. The standard IRI B1, however, is very close to observations in terms of the diurnal variation. Next, in additional to observational data, we think it is very important and beneficial to have theoretical and numerical models for physics-based investigations. This paper has established such a time-dependent theoretical model that can be used to study the behaviors of electron and ion variations for the mid-latitude ionosphere over the altitude range of 100-600 km. The model includes major dynamic and photo-chemical processes for 4 ion species with 21 chemical reactions. This model uses external models of neutral composition, and winds and the solar EUV model as inputs. With the help of the model, it is very convenient to carry out both the ionospheric morphology and the mechanism study, in particular, for the IS observations at Millstone Hill. Theoretical calculations of the ionospheric lower transition height (LTH), a level of equal O+ and molecular ion densities, were performed and compared with empirical models. We give a substantial extension to the prior work by including the AE-C data of ion composition analysis and by detailed quantitative studies of the LTH simulation, and creating a new LTH empirical model based on our simulations. Finally, our theoretical ionospheric model is also employed to investigate the ionospheric behavior as observed by the incoherent-scatter radar at Millstone Hill during the September 21-27, 1998 storm. The observed NmF2 presented a significant negative phase on September 25, and a G condition (hmF2 < 200 km) was also observed. The model results based on the standard input parameters (climatological model values) are in good agreement with the observed electron densities under quiet conditions, but there are large discrepancies during disturbed periods. The exospherictemperature Tex, neutral winds, atomic oxygen density [O] and molecular nitrogen density [N2], and solar flux are inferred from the ISR ion temperature profiles and from the electron density profiles, based on the data assimilation idea. Our calculated results show that the maximum Tex is higher than 1700 K, and an averaged decrease in [O] is a factor of 2.2 and an increase in [N2] at 300 km is about 1.8 times for the disturbed day, September 25, relative to the quiet day level. Therefore, the large change of [N2]/[O] ratio gives a good explanation for the negative phase at Millstone Hill during this storm. Furthermore, at the disturbed nighttime the observations show a strong NmF2 decrease, accompanied by a significant hmF2 increase after the sudden storm commencement (SSC). Simulations are carried out based on the inferred Tex. It is found that the uplift of F2 layer during the period from sunset to post-midnight is mainly associated with the large equatorward winds, and a second rise in hmF2 after midnight results from the depleted Ne in the bottom-side of F2 layer due to the increased recombination, while the 'midnight collapse'of hmF2 is attributed to the large-scale traveling atmospheric disturbances. In conclusion, the results presented in this work are very important not only for ionospheric (space) weather and prediction but also for understanding various physical processes involved at middle or subauroral latitudes. The data assimilation technique discussed here makes it feasible to extract the thermospheric information from various ionospheric data and apply it back to future forecasting. Such a technique may provide a solution to some space weather problems and further our understanding on the ionosphere-thermosphere coupling system.