| dc.description.abstract | The plasma density irregularities, which exist in ionosphere, can influence the propagation of radio waves. The ionosonde spread F and GPS phase fluctuations belong to this kind of phenomena. The observation of GPS phase fluctuations started from 1990s. However, the amount of research of this parameter is still few, and its occurrence pattern is not studied in detail. On the other hand, the ionosonde spread F has been investigated for over 50 years, and the behavior of its variations is well known. In this dissertation, we make use of the two different instruments, the ionosonde and the GPS, to observe the irregularities in different latitudes, concurrently. By comparing the two observations, we want to study about the variations of GPS phase fluctuations, and to understand the physical meaning behind the variations. The observation have been conducted at the geomagnetic equator, equatorial ionization anomaly region, and mid-latitudes during 1999 to 2000. We calculate the relative occurrences of spread F and GPS phase fluctuations, and then compare their monthly and daily variations.
The observations from Peru have been used for the study at the geomagnetic equatorial region. The results show that the spread F and the GPS phase fluctuations frequently occur during equinox and summer, however, the intense GPS phase fluctuations are more often in equinox. Moreover, the irregularities have larger altitudinal distribution range in the pre-midnight hours, and the GPS phase fluctuations are also more intense in this period. The variation patterns of spread F and GPS phase fluctuations can be explained based on ExB drift variation, and the comparison also confirm that the GPS phase fluctuations are related to the altitudinal range of the distribution of irregularities.
In Equatorial ionization anomaly region, the observations were carried out in Taiwan. In this region, the spread F mainly occurs in the pre-midnight period in equinox, and in the post-midnight period in summer. The GPS phase fluctuations have high occurrences around the midnight in equinox and summer. Moreover, there is almost no intense GPS phase fluctuations. Since the irregularities are magnetic field aligned, their altitudinal distribution range at the geomagnetic equator directly influence the intensity of GPS phase fluctuations at the equatorial ionization anomaly region.
The data from Wuhan is selected to study the variations at the mid-latitude region. The observations show that the spread F and the GPS phase fluctuations have high occurrences in summer. The spread F mainly occur in the post-midnight period, and the GPS phase fluctuations peak around midnight. The observations also indicate the absence of intense GPS phase fluctuations at Wuhan. The monthly variations of two instrument observations show that the pattern at Wuhan is very different from that at the equatorial ionization anomaly region. This result suggests that the physical mechanism of irregularities in the mid-latitude region is unrelated with the processes at the geomagnetic equatorial region. Moreover, it seems that the irregularities in mid-latitudes are related to the traveling ionospheric disturbances. | en_US |