Study On Characteristics Of Ionospheric Structures And Irregularities In The Low Latitude Region

As the influence of factors such as solar and geomagnetic activities, there are many disturbences in the ionosphere, which will affect the propagation of radio waves, leading to the effect on the accuracy of navigation/ positioning, space flights, communication links, and so on. Especially in the low latitude region, the ionospheric valley structures vary a lot and the ionospheric irregularities frequently occur, often producing severe space weather events. The disturbences have complicated physical processes, and the mechanism is still an important issue in studies on space physics and space weather. As a consequence, the study on the low latitude ionospheric structures and irregularities is important to both the science and application. In this paper, with data from sounding rocket, ionosonde, GPS scintillation monitor and ROCSAT-1 satellite, we studied the characteristics of the ionospheric structures and irregularities in the low latitude region, including analysis and model study on in-situ observed E-F valley by sounding rocket, statistical study on spread F observed at low latitude stations as well as comparing with the results at high latitude, study on the correlation between spread F and scintillation, and their relationship with plasma bubbles and plasma blobs. The Main results are as follows:With density data from the sounding rocket launched from the Hainan ionospheric observatory in 2011, an ionospheric E-F valley was firstly in-situ observed in the low latitude region in the East Asian sector. The width of the observed valley was about 42.2 km, the depth was almost 47.0 %, and the altitude of the electron density minimum was 123.5 km. The valley was wide and deep. The valley parameters that obtained by a new 3-Chapman layer model agreed with the observation, impling that the observed E-F valley was formed by the quickly developing Chapman layers near sunrise, and the photochemical process at the height of 90 km had obvious effect.With data from ionosondes at six low latitude staions in 2004, we classified and did statistics on the spread F(SF). The results showed that the observed SF had four types: frequency spread F(FSF), mixed spread F(MSF), range spread F(RSF), and strong range spread F(SSF), whereof the branch spread F(BSF) never occurred. The occurrences of each type of SF had different monthly and seasonal variations with the total occurrence of SF, and the occurrences were also different at each station. In general, during nighttime, the RSF and SSF occurred before midnight, and the FSF and MSF occurred after midnight. At the three stations that closer to the Magnetic Equator, the total SF occurrences were higher and the epochs were earlier. However, at Jicamarca station(closest to the Magnetic Equator), the occurrence of SSF was not high. By contrast, in the results of analysis on data from ionosondes at three high latitude staions in 2006, the BSF occurred whereof the SSF never occurred. At high latitude the SF was observed during daytime as well as nighttime, and the majority was FSF and MSF. Besides, there were plasma total absorptions of radio waves(IPTAR) at high latitude, and the IPTAR had variations with diel, season and geomagnetic activity. However, there was no IPTAR at low latitude.With data of GPS amplitude scintillations and spread F from the low latitude station Vanimo in the Southern Hemisphere in 2003, we statistically analyzed the characteristics of scintillations and SF and their correlation. The results showed that the SF had four types: FSF, RSF, MSF and SSF, whereof the BSF never occurred. SSF and scintillations(S4≥0.3) usually occurred simultaneously and had similar periods and trends. Only the SSF had a high correlation(coefficient 0.7199 and p=0.0189) with the scintillation, while the other three types of SF were uncorrelated with the scintillation. The SSF occurrences were no more than 35% at Vanimo and Hatnan in each season in 2003, which was the reason for the view that the scintillation was independent of the SF without classification. With in-situ plasma density data from ROCSAT-1 satellite, we presented a case to confirm the view that both the scintillation and the SSF were caused by plasma bubbles(depletion) at low latitude.In general, ionospheric scintillations at low latitude are considered as signatures of equatorial plasma bubbles. In this paper, with GPS amplitude scintillation measured at Vanimo and the in situ measurement data from ROCSAT-1 satellite, we performed a case study on the concurrent observation of scintillations and a plasma blob observed in situ, and for the first time provides evidence of the view that scintillations can be associated with plasma blobs in the low-latitude ionosphere. During the case, SSF was also observed. The blob measured in situ had a scale size of about 800 km in F layer, and the ion density inside the blob was severely disturbed. The blob was mainly caused by O+ ion activity and was upgoing. We also analyzed the mechanism of scintillation produced by plasma blob, and suggested that the eastward polarization electric field should be associated with the plasma blob and its associated density disturbance which then causes the ionospheric amplitude scintillations.