ausblenden:
Schlagwörter:
Ionosphere, GNSS measurements, TEC; thermosphere-ionosphere coupling, MSTIDs; AGWs
Zusammenfassung:
The Earth’s upper atmosphere – a part of it, the ionosphere- is a dynamic partly ionized region
with temporal and spatial variations under different phases of solar activity. The ionosphere being
a dispersive medium causes signal strength fluctuation, propagation delay, signal attenuation, and
signal degradation. These have constituted significant threats to both communication and
navigation systems operating in microwave band which is due to the presence of high electron
density and its irregularities. The key parameter of the ionosphere which is closely related to most
of these delay effects on radio signals is the electron density and density gradients, in particular -
its vertical integral, the Total Electron Content (TEC) which can be estimated from the Global
Positioning System (GPS) data. The estimated TEC profiles, and TEC perturbation are studied to
gain insights into the occurrence of irregular structures in the ionosphere and their distribution.
One of the ionospheric irregularities located within the F region, and E region top side are
Traveling ionospheric disturbances (TIDs). TIDs are propagating perturbations in the ionospheric
electron density as a consequence of Atmospheric Gravity Waves (AGWs) passage. The AGWs
originate in the troposphere or stratosphere, and exhibit neutral wind perturbations propagating to
the F region heights (i.e. ionospheric heights), where the neutral wind perturbations interact with
the plasma via collisions, carrying it along the magnetic field lines (i.e. ion-neutral collision). This
entire process in the ionosphere is manifested as oscillations of the ionospheric electron density,
resulting in a TID. However, TIDs vary in scale sizes ranging within a few hundred kilometers
(km) to over one thousand km, and based on this, they are categorized as either medium-scale
TIDs (MSTIDs) or large scale TIDs (LSTIDs). In this thesis, we focus only on MSTIDs as one of
the major and frequent ionospheric irregularity phenomena which may degrade positioning
systems and could cause a delay in GPS signal transmission between a satellite and the GPS
receiver.
Multiple studies of ionospheric irregularities with the main focus on MSTIDs over different
regions and continents around the world have been carried out, but studies of MSTIDs over the
African region have neither been carried out nor reported probably due to lack of GPS data set,
and the question of what drives its occurrence in the region which is not yet documented.
The objective of this thesis is to study and describe for the first time the occurrence of MSTIDs
and its characteristics over the African region under quiet geomagnetic condition (Kp ≤ 3) during
the years 2008 – 2016. In addition, this thesis presents novel results of the time series of MSTIDs percentage occurrence rate (POR) during daytime and nighttime, and seasonal occurrence. Ionneutral
coupling processes like the connection between AGW and MSTIDs are also discussed in
the study. Observational TEC data used in this thesis are obtained from ground-based GPS
networks within the African region and nearby stations. Additionally, temperature data from
COSMIC radio occultation and SABER satellite observations for some case studies were used to
validate AGWs passage as a driving source of MSTIDs, especially during the daytime.
Consequently, regional MSTIDs distribution maps have been generated to capture the latitudinal,
seasonal, and local time extent of the MSTID occurrence. Investigation of regional ionospheric
irregularities over Africa (IRIA) gives a novel result of a climatological view of MSTIDs over
Northern and Southern hemispheres in the African region.
