Research On Ionospheric Irregularities Observed In Low Latitude (Hainan) Region

In the last twenty years, great improvements have been made in the study field of ionospheric irregularities (such as the various type of spread F and sporadic E) at Low latitude, especially near equatorial region. But still some basic features of these phenomena at low and middle latitudes (such as the variations with longitude, season, and from day to day), are not properly understood. Since the DPS-4 digisonde was established at Hainan station (with 19.4°N, 109.0°E, Dip lat. 9°N, D. 1°W) in March 2002, ionograms were recorded at 15-min intervals from March 2002 to February 2006 comprising a database of about 130,000 observations. In this thesis, we statistically studied the temporal distribution of the occurrence for the spread-F, Es and scintillation as a function of season, solar activity and magnetic activity at Hainan station. The following are main results from our study.Using the Spread-F (SF) data observed by the DPS-4 digisonde at Hainan ionospheric observatory station from March 2002 to February 2006, the properties of four types of SF, i.e., frequency, range, mixed, and strong SF, are statistically studied. The results show that during the high solar activity year (2002) the Frequency SF (FSF) seldom took place. The Range SF (RSF) mainly took place in July, August and September. The Mixed (MSF) mainly took place in March. The Strong SF (SSF) mainly took place in March, April, and September. During the middle solar activity years (2003 and 2004), the FSF mainly took place in May and June. The RSF seldom took place in every month in 2003 and mainly took place in September 2004. The MSF mainly took place in December 2003 and January, June, and July 2004. The SSF mainly took place in October 2003 and March, April, August, and October 2004. During the low solar activity year (2005), the FSF and MSF mainly took place in June and July. RSF and SSF mainly took place in the equinoxes. The main difference between Hainan and Paulista (Brazil) lies in seasonal variation and the main difference between Hainan and Ahmedabad (India) lies in the seasonal occurrence rate.During equinoxes and summer, the occurrence frequency of all types of spread-F shows a little increase when Kp varies from 0 to 1 and then a decrease when Kp varies from 1 to 8. During winter, the occurrence frequency in days of each type of spread-F is small, however, it also shows an increase with smaller values of Kp and then shows a decrease with higher values of Kp.The response of equatorial ionosphere during geomagnetic storms is one of the important issues related to space weather studies and how they affect the occurrence characteristics of equatorial spread-F is one of the outstanding research fields. In this study, we use the data from DPS-4 digisonde and GPS L-band scintillation monitor during the nine strong storms period in 2004 in Hainan China to investigate the effects of geomagnetic storms on different type spread-F. The results show that: 1) During non-Equatorial Spread-F (ESF) active winter and summer seasons, Strong range Spread F mainly occurred in the main and earlier phase of storm, and occurred before midnight. Mixed Spread-F and Frequency Spread-F mainly occurred in the initial and recovery phase of storm, and occurred after midnight. It was obvious that spread-F were affected by the storm, in particular, the occurrence of SSF during storm depended on the local time of storm phase. When the minimum of Dst index of storm belonged to the intervals between 2200 and 0600 LT, between 0700 and 1600 LT, and between 1700 and 2100 LT, storm was apt to intrigue, suppress, and non-affect SSF, respectively. 2) During the ESF active equinox seasons, none of spread-F appeared during storm, it seemed storm suppressed the spread-F in these seasons. 3) When SSF emerged in the main phase of storm, h'F rised greatly. We also compared and discussed our results with other regional observations during the storm.Using the observation data (Jul. 2003-Jun. 2005) from GPS scintillation monitor at CSSAR Hainan station, a morphological analysis for L-band ionospheric scintillations over Hainan region in the fall phase of solar cycle has been performed. The main results are the following. Scintillations mainly occurred from postsunset to near midnight. The onset time of scintillations occurred when the peaks of storms occurred on near 22LT in equinox months, and had some time delay in winter months, and had some time delay which mainly took place near midnight in summer months. The occurrence rate and intensity of scintillations were evidently enhanced in equinox months, and were evidently reduced in winter and summer months. The scintillations mainly occurred on the magnetic quiet days, which mainly took place near the two equinoxes. The scintillations may also occurred in magnetic disturbance/ storm days, which mostly appearred in winter and summer months. Compared with the geomagnetic activity, the solar activity did not evidently affect the scintillation occurrence.