Study On Low Latitudinal Ionospheric Irregularity By Ground-Based Airglow Imager And Satellite Observations

In the ionosphere,there are many plasma irregularrities,which are important space weather phenomena.These plasma irregularrities usually have a great impact on the trans-ionospheric radio wave signals.They significantly affect high frequency communication,satellite positioning navigation,Satellite-to-Ground communication.Thus,it is very important to study the morphological features and evolutions of ionospheric irregularrities.In our study,we used optical instruments from the Chinese Meridian Project and all-sky imager,Communication/Navigation Outage Forecasting System(C/NOFS)satellite,Global Navigation Satellite System(GNSS)network,a digisonde(located at Fuke)and the data of Modern Era Retrospective Analysis for Research and Applications(MERRA)to study some cases of equatorial plasma bubbles(EPBs)and edge plasma enhancements of equatorial plasma depletions(EPEEPDs)over the China.In the study,we found that atmospheric gravity waves were caused by typhoon,which could play an important role in seeding the planar wave-like structures of EPBs.The result indicates that formation of some special ionospheric irregularities may be related to activities of the lower atmosphere.Besides,in the study of EPEEPDs,we discussed a possible mechanism of the EPEEPDs.The mechanism indicates that the plasma is more likely to be redistributed and exhibits as depletion and enhancement.The result deepens our understanding of evolutions of ionospheric irregularities.The results are given as follows:1.We studied evolutions of EPBs on 04-05 October 2013(case 1)and 29-30 September 2013(case 2),which observed by two all-sky imagers located in the Fuke(109.1°E,19.5°N)and Guiping(110.7°E,23.8°N),China.The case 1 shows that: all EPBs present planar wave-like structures.They were parallel to each other.Based on observations of MERRA and typhoon,we found that atmospheric gravity waves were caused by typhoon,which could play an important role in seeding the planar wave-like structures of EPBs.Besides,before midnight,these EPBs have an angle with the geomagnetic meridian and the angle is about 30°.Near midnight,the higher latitude part of one EPB suddenly began to rotate with an eastward zonal drift,resulting a “C” shaped EPB which was observed for the first time by the all-sky imager.After midnight,merging evolution of fossil EPBs was observed for the first time by the all-sky imager.The case 2 shows that: there are “I” shaped,“S” shaped,and “Y” shaped EPBs occurring in the case.The angle of “I” shaped EPBs and different shapes of EPBs should be related to zonal wind velocities and conductivity at different geographical latitudes.During the development of “Y” shaped EPBs,the zonal drift velocity of the east branch was greater than that of the west branch.2.We used an all-sky imager(located at Qujing)(103.8°E,25.5°N)and the C/NOFS satellite to study occurrence characteristics of EPEEPDs.We found EPEEPDs accompanied by EPBs should occur.We suggested that the polarized electric field of evolutionary process of EPDs was the mechanism of the EPEEPDs generation.The generation mechanism indicated that some plasma redistributed during the EPDs development.Equatorial plasma depletions(EPDs)appearing at nighttime low in latitudes were widely studied before.However,structures of EPEEPDs have not been well addressed.In this part,we used four years observations(2012-2015)of an all-sky imager and the C/NOFS to study EPEEPDs in detail.The results show those EPEEPDs only appeared at the east and west edges of EPDs,and there are no EPEEPDs in poleward edges of EPDs.Four years observation results show EPEEPDs is a high incidence phenomenon in the edges of EPDs;and the average incidence reaches about 82%.The occurrence of them varying with geographic longitude and local time are similar to that of EPDs,and there is an occurrence peaked in between 20:00-22:00 LT.3.We used an all-sky imager(located at Qujing),a digisonde(located at Fuke),and GNSS network to study a special EPB event on 08 November 2015.Over the past years,many studies have shown that the EPBs occur mainly after sunset,and they usually drift eastward.In our study,we caught a special EPBs event in the southern China.Those EPBs emerged shortly before sunrise,and kept developing and dissipated at about one hour after sunrise.Evolutions of EPBs around sunrise were observed for the first time by an all-sky imager and GNSS network.Meanwhile,results from all-sky imager and GNSS network both showed the EPBs drifted westward rather than the eastward drift of normally post-sunset EPBs.Besides,the observations of all-sky imager show break and recombination processes of EPBs.Meanwhile,those EPBs occurred in the recovery phase of a geomagnetic storm.The geomagnetic storm may be the main reason of triggering those EPBs.