| dc.description.abstract | After extensive research efforts in both observations and theoretical simulation, it
is generally believed that the F-region irregularities in the ionosphere above the
equator are generated by the combined effects from the gravitational Rayleigh-Taylor
(GRT) instability and the ExB gradient drift instability. Such generated irregularities
initiate near the bottom of ionosphere due to small disturbances. They are amplified
by GRT and ExB and gradually move upward, eventually penetrate the F-region peak
of the ionosphere. However, observations from Satellites AE-C and AE-E suggest the
existence of F-region bottomside sinusoidal (BSS) irregularities. Due to the lack of
numerical modeling on such features, the mechanism responsible for their occurrence
is not well understood, which is one of the purposes of this study.
Another goal of this study is to understand the relationship between the
occurrence of ionospheric irregularities and controlling factors such as the longitude,
the season, as well as the existence of magnetic storms. The worldwide distribution of
Global Positioning System (GPS) stations enables such studies. I analyze the
ionospheric characteristics in mid- and south- America using GPS signals to study the
effect of magnetic storm on the generation of irregularities, specifically choosing the
low-occurrence season to prevent the effects from other factors.
The study on the occurrence of F-region BSS irregularities is done through
numerical simulation using the two-dimensional fluid model simulation code. The
GPS data, which is used to study the relationship between the occurrence of
irregularities and factors such as the longitude, season, and the existence of magnetic
storms, are provided by the International GPS Service (IGS). For the formal, we select
the entire year of 1998 for analysis, while for the later we choose magnetic storms that
occurred in May-August (i.e., the low-occurrence season in mid- and south-America)
between 1997 and 2000. Utilizing the information from dual-frequency pseudo range
measurements and the carrier phase observations, the total electron content (TEC) can
be estimated. Consequently, the variation of TEC with respect to time gives the GPS
phase fluctuation due to the ionospheric irregularities. The relation between the
occurrence of irregularities and the longitude and season can be then derived from the
statistics on the number of days when irregularities are observed versus the total
number of observation days. Furthermore, the effect of magnetic storms on the
generation of irregularities can be delineated from the time sequence of Dst index and
phase fluctuations.
Our simulation results indicate that specific environment is necessary for the
occurrence of F-region BSS irregularities. When the seeding disturbance near the
bottom side of F-region moves upward due to the amplifying effects from GRT and
ExB instabilities, the dynamic instability would confine the generated irregularity
near the bottom of ionosphere if there is a jet stream (i.e., vertical wind shear)
immediately below the F-region peak of the ionosphere, resulting the F-region BSS
irregularities.
As for the relationship between the irregularities and longitude/season, our results
indicate stations in the Atlantic have high occurrence rate in winter (May-August)
than in summer (November-February). In contrast, stations in the Pacific have the
opposite pattern. Our study on the 8 magnetic storms indicates significant correlation
between the time variation of Dst index and the GPS phase fluctuations. Strong phase
fluctuations can be observed only when Dst index drops rapidly during the time of
sunset. Furthermore, the intensity of the magnetic storm is another factor that controls
the occurrence of ionospheric irregularities. | en_US |