Observational Study On The Low-latitude Ionospheric Field-aligned Irregularities Over Hainan

Plasma irregularities in the ionosphere may cause rapid variations in phase and amplitude of radio signals and lead to detrimental effect on navigation and communication system.Therefore,it is important to understand the underlying generation mechanism and the occurrence characteristics of these ionospheric irregularities.Based on the observations from two ground-based instruments of Chinese Meridian Project,i.e.,a very high frequency?VHF?coherent scatter radar installed at Fuke,Hainan Island of China and a collocated Digisonde,we conducted a comprehensive study of the characteristics and generation mechanism of the low-latitude ionospheric E-and F-region field-aligned irregularities?FAIs?.Our work includes the studies on the characteristics of ionospheric E-region quasi-periodic?QP?echoes,the strong correlation between QP echoes and plasma drift in the E-region,different structures of ionospheric F-region echoes,the occurrence characteristics of nighttime F-region echoes,case studies of typical daytime F-region FAIs echoes,as well as the formation and evolution of F-region FAIs during geomagnetic disturbances.The main work and conclusions are as follows:In the first part,we used the data of Hainan COherent scatter Phased Array Radar?HCOPAR?in 2013 to observe the ionospheric E-region QP echoes.According to the striation tilts of QP echoes in altitude-time-SNR?signal to noise ratio?plots,QP echoes can be classified into two types,i.e.,positive QP?PQP?and negative QP?NQP?.Combined with the simultaneous observations from Hainan Digisonde in E-region,we analysed the correlation between the QP echoes and background Es layer and found that the intensity of QP echoes is positively correlated with the density gradients in Es layer,as indicated by the difference between E-region critical frequency and blanket frequency?i.e.,1?=f_oEs-f_bEs).It is proved that the generation of the QP echoes is related with the gradient drift instabilities induced by the steep density gradients in Es layer.In addition,the relationship between the morphology of the QP echoes and the background plasma drift was investigated and a surprising consistency between the striation tilts of the QP echoes and the meridian plasma drift was found.A negative echo striation of the QP FAIs was recorded when the measured plasma drift direction was southward,whereas a positive echo striation was observed during the northward drift.Furthermore,a QP echo with continuous trace but with three consecutive changes in its striation direction was recorded,and the background meridian plasma drift changed its direction three times simultaneously.Thus,it can be concluded that the morphology of the QP echoes striation may be controlled by the background meridian netural wind in the E region,i.e.,the northward/southward drifting striated FAIs in the observation region of the coherent scatter radar might induce the positive/negative QP echo striation.In the second part,we used the data of HCOPAR in 2014-2017 to make long-term observation of the ionospheric F-region FAIs to investigate the morphology and occurrence characteristics of F-region FAIs.Firstly,we introduced the typical observation results of the F-region FAIs echoes and classified the F-region FAIs echoes according to four different different criteria:?a?based on its morphological characteristics,the FAIs echoes could be classified into bottom-type and topside plume structure,?b?based on the generation location,the FAIs echoes could be classified into Type-1 and Type-2,explained in detail,Type-1 echoes are those freshly-generated in the field of view?FoV?of the radar,whereas Type-2 echoes are those generated elsewhere and drifting into the radar FoV,?c?based on the development phase,the FAIs echoes could be divided into Evolution-type?E-type?and Decay-type?D-type?,which cooresponding to FAIs which are in growth and decay phase,?d?based on the local time when they were observed,the FAIs echoes could be separated into post-sunset FAIs,post-midnight FAIs and daytime FAIs.Secondly,we studied the dependence of F-region FAIs occurrence over Fuke on solar activity,season and local time during the period of 2014-2017.The results reveal that the FAIs occurrence rate decreases with decreasing solar activity.Under high solar flux conditions,the FAIs over Fuke had maximum/minimum occurrence rate during equinox/solstice months.Besides,equinoctial asymmetry,i.e.,higher occurrence rate in the March equinox than that in the September equinox,was found only obvious during solar maximum,while solstitial asymmetry,i.e.,higher occurrence in the June solstice that that in the December solstice,was found during both solar maximum and minimum.FAIs prefer to commence after 19:30 LT with the highest occurrence rate between 20:00 LT to midnight.The intensity of the FAIs echoes is typically stronger in the pre-midnight sector than that in the post-midnight sector.Meanwhile,it is shown that the FAIs echoes can grow much higher altitudes in the pre-midnight sector than in the post-midnight sector.Last but not least,we also analysed two typical cases of daytime F-region FAIs.The irregularities emerged in the geomagnetical quiet condition and were irrelevant to the storm-time perturbation electric field as other daytime cased observed by other instruments.These two FAIs appeared in the topside F₂ layer.They presented small Doppler velocities?near zero?and narrow spectral widths?less than 40 m/s?and the fan sector maps show that the FAIs moved northward with almost no zonal speed which are different from the normal post-sunset F region FAIs observed over Fuke.More than 2h after the emergency of the daytime irregularities over Fuke,the Shaoyang Digisonde situated⁸70 km north to the HCOPAR recorded the spread-F in ionospheric F₁layer.According to the echo altitudes,the spread F may connect the daytime FAIs via magnetic field line.The strong photoioniztion after sunrise made it difficult to generate the plasma bubbles in the sunlit ionosphere.Consequently,the two midday FAIs over Fuke were already in decay phase and may drift along the magnetic field lines from higher altitudes in the south and are most likely the remnant of previous night's plasma irregularities.In the third part,we investigated the influence of geomagnetic disturbance on the formation and evolution of the F-region FAIs via two case studies.The initiation or suppression of equatorial/low-latitude FAIs during geomagnetic disturbances largely hinges on the competitive effects of storm-time perturbation electric fields.Based on the statiscal results of the HCOPAR observations in period of 2014-2017,most of the F-region FAIs over Fuke emerged under geomagnetic quiet condition,and only a few cases of FAIs might be related with geomagnetic disturbances.In the first case,we presented a study of the F-region FAIs formation and evolution during the 7-8 September 2017 geomagnetic storm.The under-shielding prompt penetration electric field?PPEF?associated with sudden southward turning of the interplanetary magnetic field?IMF?Bz resulted in large ascent of the F layer,making conducive conditions at the bottomside of the layer for the growth of Rayleigh-Taylor instability and the development of the plasma irregularities in the post-sunset hours.The irregularities persisted into the postmidnight sector when the southward IMF Bz gradually decreased to the quiet time values.In addition,the base height of F layer at Fuke also showed a large elevation after midnight during two consecutive substorm onsets,suggesting that the substorm-induced over-shielding penetration electric field may take over and modify the ambient zonal electric field in low-latitude ionosphere and induce the irregularities in the postmidnight sector.Moreover,different from the quiet time eastward movement of the irregularities observed over Fuke,the storm time irregularities displayed no zonal drift at the initial period and subsequently began drifting westward.The reversal of background plasma zonal drift velocity observed by Hainan Digisonde characterized the storm time zonal drift pattern of the irregularities.In the second case,we presented a study of the F-region FAIs which emerged very late in post-midnight sector?04:37-05:21 LT?under the geomagnetically disturbed condition during 13-14 November 2015.The base height of F layer displayed large elevation in post-midnight sector before the FAIs were recorded by the HCOPAR.Over-shielding penetration electric field which may result from both substorm onsets and rapid northward turning of IMF Bz was considered to be responsible for the large uplift of F layer in post-midnight sector and the subsequent formation of F-region FAIs.