| In this dissertation,we select three representative stations in China’s sector,namely the mid-latitude coastal area Changchun station(125.3°E,43.8°N;magnetic latitude 33.6°N),the mid-latitude inland area Urumqi station(87.6°E,43.7°N;magnetic latitude 33.4°N)which is almost at the same geomagnetic latitude with Changchun,and low-latitude coastal area Hainan station(109.1°E,19.5°N;magnetic latitude 9.4°N),and obtain these Spread-F data sets from ionosondes to compare the low-and mid-latitude spread-F occurrence during high and low solar activity and during the full solar cycle,respectively.Then,using the nighttime atomic oxygen(OI)135.6 nm emission(the integral value of the square of the plasma density along the line of sight)observed by the Special Sensor Ultraviolet Spectrographic Imager(SSUSI)instrument on board the Defense Meteorological Satellite Program(DMSP),we investigate the seasonal variations of the wavelike longitudinal structures in EIA crests at a fixed local time(LT).Through the two works,we have learned the background plasma distribution of the global EIA region and the occurrence characteristics of Spread-F irregularity in our country.It lays a good foundation for our future work on study the ionospheric irregularity in the EIA region detected by SSUSI OI 135.6 nm emission.Our current main results are as follows:1.We analyze the ionosonde data sets from low-latitude station,Hainan and mid-latitude stations,Changchun and Urumqi.Comparisons of low-and mid-latitude spread-F occurrence rate during high and low solar activity are conducted.The results on spread-F occurrence rate mainly includes two aspects: the maximum spread-F occurrence rates over both low-and mid-latitude during low solar activity are larger than those during high solar activity;during both high and low solar activity,the maximum occurrence rate over low-latitude is greater than that over mid-latitude.In addition,there are also some detailed characteristics of spread-F occurrence.(1).Double-peak structure exists in spread-F occurrence during high and low solar activity over all three stations.During the equinox months of the high solar activity,the two occurrence peaks over low-latitude station occur in pre-midnight,while the two peaks during the solstices over mid-latitude stations of Changchun and Urumqi occur in the post-midnight period.In low solar activity,the two peaks appear in the solstices over both low-and mid-latitude stations.Over Hainan,the main peak during summer occurs around midnight and the sub-peak during winter occurs at post-midnight;OverChangchun and Urumqi,the two peaks occur at post-midnight.(2).Differences also exist in the detailed double-peak structure.First,the main peak over low-latitude occurs in vernal equinoctial months during high solar activity,but the maximum occurrence rates in February and April are relatively high and closer;the spread-F is easy to occur in summer during low solar activity over Hainan,but the maximum occurrence rates in May and July are also relatively high and closer.During high and low solar activity,the main peak values over low-latitude are very close,but they have notable differences over mid-latitude.A detailed analysis and comparison on characteristics of spread-F over low-latitude station,Hainan and mid-latitude stations,Changchun and Urumqi are given in this work.2.The characteristics of low-and mid-latitude spread-F will be investigated by analyzing ionosonde data sets over the full solar cycle from 2002 to 2012 over low-latitude Hainan as well as from 1992 to 2002 over mid-latitude Changchun and Urumqi.The results show that the variations of spread-F in different regions with season,local time,and F10.7 solar flux have some new features.(1).At low-latitude,the spread-F occurrence is largest in summer,and also has an increase in January especially at low solar activity;the month of peak occurrence changes with local time.(2).At mid-latitude,the spread-F occurrence has double-peak structure in summer and winter;the peak month also vary with local time,but only over Changchun station the peak month in winter always appears in January which not changes with local time.(3).The distributions of spread-F events with various duration are obviously different at these three stations.It is interesting that the mid-latitude anomalous occurrence of spread-F events during low solar activity(1996 and/or 1997)seems to be associated with short duration spread-F events.3.Nighttime atomic oxygen(OI)135.6 nm emission observed by the SSUSI instrument on board the DMSP F18 satellite is utilized to investigate the seasonal and longitudinal variations of the EIA crests at a fixed 20:00 LT from January 1,2012 to December 31,2015.The OI 135.6 nm emission intensity at the southern and northern EIA crests for each day is considered as a superposition of zonal mean and zonal wavenumber(WN)1-4 components.On the seasonal-longitudinal variations of the total OI 135.6 nm emission intensity at both EIA crests,the obvious double-peak structure appears in equinoxes for all 4 years.The longitudinal peaked structures are also prominent in equinoxes,and peak-3 structure dominates.While on the seasonal-longitudinal variations of the WN 1-4 component of the OI 135.6 nmemission intensity,the longitude of the peak emission intensity is usually stable in solstices and moves in equinoxes.The other main results are as follow:(1).the zonal mean emission intensity and amplitudes of WN 1-4 components at both EIA crests are also evident in equinoxes,and have a major semiannual and annual periods,except for amplitude of WN 4 at the southern EIA crest;the amplitude of WN 1 component is generally greater than WN 2-4;(2).the phases of WN 1-2 components at both EIA crests have a one-year period,and the seasonal variations of the phase of WN 1component at both EIA crests are in-phase but anti-phase in WN 2;(3).the phases of WN 1-2 components between the OI 135.6 nm emission intensity and the TEC at both EIA crests are almost consistent in April and solstices. |
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