Research On The Hemispheric Asymmetry Of The Ionospheric Equatorial Ionization Anomaly And Scintillation Characteristics Over Southeast Asia

The ionosphere is an important part of the space environment Ionospheric equatorial anomaly (EIA) is a hot research direction in the low latitude regions of the ionosphere. The EIA is characterized by the F region electron density trough at the geomagnetic equator and two crests on either side of the equator. The development and variation of the EIA structure in electron density is also reflected in the ionospheric total electron content (TEC) because the F layer density heavily weights it. The EIA structure has been investigated extensively using both experimental and modeling techniques in the past, and its morphology is now largely known. But, some morphological changes of EIA, espically during the low solar activity periods, still need further research and analysis.In this paper, we mainly focus on the ionospheric morphological changes of EIA region over Southeast Asia using the data provided by the Russian Academy of Sciences (IZMIRAN) for more than a solar cycle term from 2000 to 2013. First, we analyze the morphological characteristics of the the ionospheric EIA over Southeast around 120°E. And then, we study the hemispheric asymmetry between the northern and southern crest regions of EIA during the geomagnetic quiet periods. Further more, we present a detailed analysis of the single peak phenomenon, when the zonal distribution of ionospheric TEC appears only one peak at the equatorial and low latitudes, during the low solar activity. Similarly, the worst source of ionospheric scintillation is at the equatorial anomaly region. In this paper, we also present the characteristics of the GPS and FY-2D geostationary meteorological satellite scintillation over northern crest region.The main contents and results of this paper can be summarized as follows:1. The morphology characteristics of EIA. The feature of the two-crested structure in TEC and NmF2 is obvious during the high solar activity years. Normally, the two-crested structure in NmF2 is more obvious than the structure in TEC of the corresponding period. The variation of the EIA crest in TEC and NmF2 shows an obvious seasonal and semiannual variation. In summer and winter month during the low solar activity years, there may only an obvious single crest in TEC at the equatorial and low latitudes. The winter anomaly phenomenon presented by the strength variation of TEC and NmF2 over the northern crest region is very clear, however, there is almost no winter anomaly over the southern crest region. The strength of EIA crests in TEC and NmF2 shows a very good correlation with the solar flux, but do not correlate well with the geomagnetic activity.2. The north-south asymmetry of EIA structure. In this part, we focus on the hemispheric asymmetry between the north and south crest regions of EIA during the geomagnetic quiet periods. The strength and latitudinal of the EIA crests are found to be asymmetric aboutthe magnetic equator. The conditions of solar activity have a significantly effect on EIA crest and the north-south asymmetry. The latitude distance of the two-crested structure in TEC and NmF2 has an obvious seasonal variation characteristic, increases in summer and decrease in winter. The minimum atitude distance presented in summer and autumn during the low solar activity period. And we also found the two-crested structure has an obvious southward position offset in winter during the low solar activity.In winter and summer, the variations of north-south asymmetry in TEC and NmF2 are very obvious for different solar activity conditions. For the same solar activity condition, the north-south asymmetry reversed between summer and winter. And in summer and winter, the north-south asymmetry also reversed between high and low solar activity periods.3. The single crest phenomenon in TEC at the equatorial anomaly region. The diurnal variations in TEC at EIA region are not always represented with two crests on both sides of the magnetic equator. Sometimes there is only an obvious single crest at the equatorial and low latitudes. In this part, we focus on analysis of the morphological features of the single crest phenomenon in TEC around the 120°E longitude during geomagnetic quiet periods. Single crest phenomena mainly occurred during the extremely low solar activity days. The north, south and magnetic equator single crest phenomenon are frequent in summer, winter and autumn months during the low solar activity days, respectively. During the single crest days, the north-south asymmetry of corresponding F2 layer NmF2 showed more obvious than the TEC structure, or even only a single peak.4. Photochemical process caused by solar EUV radiation also plays an important role in the anomaly region. Photo-ionization can produce more electrons under the subsolar point, and we think the original background electron density may effects the latitudinal distribution of ionospheric TEC and NmF2, which is called the geographic control effect in this paper. And the north-south asymmetry of EIA and single crest phenomenon can be explained by a combined theory of the transequatorial neutral wind, the subsolar point and geographic control effect.5. The ionospheric scintillation variations over the anomaly region are also studied using GPS measurements by Shenzhen station of Chinese Meridian Project, situated under the northern crest of the equatorial anomaly region. The results show that GPS scintillation events were essentially a nighttime phenomenon during period from December 2010 to April 2014. Scintillation events were mainly occurred at the inner edge of the northern crest of equatorial anomaly in China. And the scintillations are more probable to be observed in the west sector of the sky above Shenzhen. During the present period of study, a total number of 512 TEC depletions and 460 loss of lock events are observed. And both the events are likely to increase during the high solar activity period, especially the strong scintillations are often simultaneously accompanied by the TEC depletions and loss of lock of GPS receiver.6. Ionospheric scintillations observed by the FY-2D geostationary meteorological satellite over northern crest region. In this part, the ionospheric scintillations, described by the S4 index, observed by FY-2D are at first calibrated by and then compared with the simultaneous GPS observations around the FY-2D IPP. And then we compared analysis of the FY-2D and GPS scintillation characteristics.