| At night,plasma irregularities are often generated in the ionosphere.In the low latitudinal and equatorial region,ionospheric irregularities are also known as equatorial spread F or equatorial plasma bubble.Equatorial spread F can cause ionospheric scintillation,which affects the quality of satellite communication and the accuracy of satellite navigation.Therefore,more and more attention has been paid to the study of equatorial spread F.The VHF radar,ionosonde and GPS scintillation/TEC receiver that were built in Hainan have accumulated a large amount of observational data,which makes it possible to systematically study the irregularities in the ionospheric F region in low latitudes of China.Based on the observations of VHF radar,ionosonde and GPS scintillation/TEC receiver at Sanya station,VHF radar and ionosonde at Fuke station,and C/NOFS satellite,the local generation rate of equatorial spread F,the simultaneous observation from multiple devices and thin layer irregularities in ionospheric F region in low latitudes of China were investigated in this paper.The research is divided into five parts:Understanding the local generation rate of equatorial spread-F(ESF)is important for forecasting ionospheric scintillation.In part one,using the GPS ionospheric scintillation/TEC and Sanya VHF radar data during March-April and September-October from 2010 to 2014,the occurrence of ionospheric scintillation,TEC fast fluctuation,and backscatter plume were studied.Through analyzing the simultaneous occurrence of ionospheric scintillation,TEC fast fluctuation and backscatter plume in 1 hour after local sunset,the local generation rate of ESF over Sanya was investigated.The results show that the monthly generation rate varies between 0% and 68%.A significant equinoctial asymmetry of local generation rate of ESF can be found in 2010,2013 and 2014.The local generation rate of ESF increases from 2010 to 2014 during March-April,while it does not have similar trend during September-October.The plasma vertical drift influenced by solar activity has a significant impact on the monthly generation rate of ESF.The equinoctial asymmetry of plasma vertical drift may contribute a lot to the equinoctial asymmetry of the generation rate of ESF.In part two,through concurrently measurements by Communication/Navigation Outage Forecasting System(C/NOFS)satellite,Sanya VHF radar and GPS ionospheric scintillation receiver on 12 March 2010,five plasma bubbles were found and three of them were observed by all those instruments.Two well-developed plumes with strong backscatter echoes were measured by Sanya VHF radar and their corresponding depletions were observed by C/NOFS satellite in Orbit 10317,10318 and 10319.Broad plasma depletions resulting from merging process were found in orbit of 10318.Theoccurrence time and geophysical positions of ionospheric scintillations correlate well with observations implemented by Sanya VHF radar and C/NOFS satellite.Observations from three types of instrument indicate that the spread F irregularities have distinct scale.There were longitudinal differences between Sanya VHF radar and C/NOFS as irregularities measured,and the eastward drift of developed bubbles are responsible for these differences.Generally,equatorial spread-F(ESF)irregularities initiated at the bottom of the ionospheric F region and manifested as plume structures with an altitude extent more than 50 km in the radar RTI(range-time-intensity)plot.In part three,by using the Fuke and Sanya VHF radars located at low latitude of China,evolution-type band-like structures of irregularities in F region were observed near local sunset.One interesting aspect is that the band-like structures initially appeared at high altitudes more than 400 km and then uplifted to higher altitude without zonal propagation.The lifetime of these band-like structures,with a similar pattern in the RTI plot for each beam of Fuke VHF radar,is less than 25 min.The band-like structures of F region irregularities were observed with big Doppler velocities and narrow spectral widths.During the presence of band-like structures,apparent amplitude scintillations(S4 > 0.2),total electron content(TEC)fast fluctuations(ROTI > 1),and range-type spread F were detected.It indicates that the evolution-type band-like structures of irregularities in the topside ionosphere are associated with the equatorial plasma bubbles(EPBs).The interchange instability process that occurs near the upper boundary of the equatorial plasma bubble may be the generation mechanism of the evolution-type band-like irregularities structures in the topside ionosphere.In part four,for the first time,a extremely rare F region top-type scattering layer(TSL)drifting downward were detected by Fuke VHF radar.The observational results indicate that the TSL generates at the altitude of ?450 km then grows vertically over time.The life time of this downdrafting TSL is about 50 min.The Doppler velocity of this TSL shows negative values indicating that the irregularity layer closes to the radar.The irregularity layer was detected with narrow spectral width ?20 m/s for the growth phase and ?5 m/s for the decay phase.The observations from Fuke ionosonde with frequencies from 7 to 15 MHz(except 12 MHz)indicated that the virtual height in the bottomside ionosphere descends with time corresponding to ambient westward electric field.The coexistence of background westward electric field and steepen plasma density gradient makes it is possible that the topside instability process results in the TSL.In part five,using the observational data obtained by beam 4 and 5 of Sanya VHF radar,two cases of post-midnight bottom-type scattering layers occurred on geomagnetic quiet days were investigated.The analytical results indicate that the post-midnight bottom-type scattering layers initiate at the altitude of 220-240 km.The thickness of the echo layers are relatively uniform,about 10 km,and the altitude rangedo not change with time.The altitude of the echo layers oscillate with time,and the Doppler spectrum widths are narrow.These characteristics are similar to those of the post-sunset bottom-type scattering layer.During the period of occurrence of post-midnight bottom-type scattering layer,spread echoes and satellite traces(ST)appeared in the ionograms recorded by Sanya ionosonde,indicating the presence of spread F irregularities and large-scale wave structures.The post-midnight bottom-type scattering layer may be caused by gradient drift instability driven by gravity wave disturbed wind.On another geomagnetic quiet day,both periodic plume echoes and interrupted post-midnight bottom-type scattering layer were observed by beam 5 of Sanya VHF radar,indicating that they were modulated by the large-scale wave structure.This further prove that the existence of large-scale wave structure after midnight can generate bottom-type irregularities. |
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