| The ionospheric plasma with uniform distribution will produce local disturbance and drift under the influence of tide,wind shear,gravity wave,solar activity and so on,which can result in ionospheric anomaly and irregularity.Ionospheric irregularities may cause rapid and intense random fluctuations in phase and amplitude of global navigation satellite system signals,which will lead to the interruption of satellite communication and errors in navigation.Therefore,observational study on ionospheric irregularities has great application significance.Based on the observation data of a very high frequency(VHF)coherent scatter radar,meteor radar,ionosonde at Fuke,Hainan island of Chinese Meridian Project and an ionospheric scintillation monitor receiver at Haikou station of China Research Institute of Radiowave Propagation,the characteristics of ionospheric E-region field-aligned irregularities(FAIs)in China low-latitude,a case of the daytime intense radar backscatter and strong scintillation related to the E-region irregularities and the statistical characteristics on the occurrence of E-region irregularities are investigated in this paper.Results are mainly as follows:1.Firstly,we classified the E-region FAIs echoes observed by Hainan COherent scatter Phased Array Radar(HCOPAR).According to morphological characteristics,the FAIs echoes can be classified into continuous echoes and quasi-periodic(QP)echoes.The continuous echoes can be divided into single-layer continuous echoes whose altitude decrease slowly with time,multi-layer continuous echoes appearing after sunset,and valley echoes.QP echoes can be classified into positive QP and negative QP.Secondly,observations of the HCOPAR have found some strange V-shaped echo pattern in the daytime range-time-intensity(RTI)plots between 100 and 180 km ranges.From 2013 to2015,we have found about 20 cases of daytime V-shaped echoes by manual search.Four cases with distinct echo traces are presented to display the common characteristics of the V-shaped echoes.They usually occur between 10:00-15:00 LT in the daytime with the enhanced top frequency and blanketing frequency of Es layer(f_tEs and f_bEs).Their left/right wings present negative/positive Doppler velocity.A case on 16 August 2015 was investigated by the joint observation of the HCOPAR,the Hainan Digisonde,and a scintillation receiver.The echo spectra show that the V-shaped echoes were backscattered from the type-2 irregularities.The formation of the V-shaped echo pattern was due to the leakage of the antenna sidelobe for the very intense scatter from Es layer.The irregularities were found traveling westward with the velocity of 41.12±2.08 m/s.While the occurrence of the intense irregularities,there were periodic peaks appearing on the f_tEs and f_bEs curves,and the maximum f_tEs-f_bEs reached 10.31 MHz.At the same time,the strong ionospheric scintillation event was recorded.The wind shear is considered to produce the sharp density gradient in the Es plasma and then induce the gradient drift instability(GDI).2.Based on the E-region echoes detected by HCOPAR from September 2015 to August 2016,we studied seasonal variation of E-region FAIs and compared the seasonal variation of Es,meteors and wind.The results show that the E-region FAIs over Fuke are found to occur mostly in the height ranges of 90-120 km and have maximum/minimum occurrence rate during summer/winter,which is consistent with the seasonal variation of diurnal tide.Meteoric input enhances the density of metal ions in E region.After 2 hours of sunrise,E-region are influenced by tidal wind to generate density gradient,which cause the development of GDI and come into being FAIs.From sunset,E-region FAIs expand slowly to 120 km,showing a multi-layer complex structure and recover rapidly to 100km after sunrise.In winter,the positive wind shear in the E-region may inhibit the development of GDI and reduce the occurrence of FAIs.In addition,the FAIs occurrence rate in daytime is higher than that in night in summer,which is different from the other three seasons.However,echoes SNR in daytime is lower than that in night.It may be that solar photo-ionization smoothes the plasma density gradients to weaken the radar backscatter.The f_tEs and f_bEs have no obvious seasonal variations.FAIs occurrence,however,is found to be related well with f_tEs-f_bEs.In summer,FAIs occurrence occur maximum and f_tEs-f_bEs shows large value,as compared to other seasons.During nighttime,f_tEs-f_bEs is 0.5-1 MHz higher than that in daytime while FAIs occurrence is maximum in nighttime.The results reveal that E-region FAIs show considerable correlation with the GDI induced by the density gradients in Es.During nighttime,the enhancement f_tEs-f_bEs indicates that Es has steep density gradients and may have several height regions where FAIs are excited.Hence,E-region echoes shows a multi-layer structure on the RTI plots.The statistical results show that seasonal variations of E region FAIs are well correlated with Es activity,zonal wind,wind shear,meteor count and tidal wind. |
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