An Investigation On Ionospheric Disturbances During Different Phases Of Geomagnetic Storms

The ionosphere is an important layer of the Earth that plays a vital role in satellite communication and navigation.The ionosphere is known to vary regularly under quiet condition.Apart from the regular variations there are often superposed external disturbances that can make the ionosphere to behave irregularly from the usual quiet time condition.Depending on their sources,these disturbances are classed under solar activities(solar flares and radio bursts),geomagnetic activities(geomagnetic storms)and meteorological activities(gravity,tidal and planetary waves).Chief of these classes known to have profound impact on the ionosphere are geomagnetic storms.When geomagnetic storms occur the ionosphere can experience perturbations in total electron content(TEC)or/and irregularities of plasma density and structures.The consequent variation in the ionosphere during disturbances are of importance to space physics research,spacecraft orbital design and in practical application of Global Navigation Satellite Systems(GNSS)navigation and radio communication.More importantly is the scintillation of radio signals near the equatorial region,which is the cause of positioning errors in GNSS signals.Hence,to investigate the ionosphere during disturbed period is of much benefit both for scientific research and practical purposes.Previous studies have also shown that the topside ionosphere may sometimes respond differently from the bottomside/F2 region during geomagnetic storms.In this thesis,the interest is to investigate the characteristics of TEC perturbations and ionospheric irregularities at the F2 and topside regions of the ionosphere.These were achieved by analyzing the results from space-based and ground-based observations,which include Beidou Geostationary satellite(GEO)TEC,GPS uplooking TEC from GRACE,SWARM mission,TerraSAR-X and MetOp-A satellites and in-situ electron density from Langmuir probes aboard GRACE and SWARM mission.The main results are summarized as follows:(1)Characteristics of the topside ionospheric response to the 7-8 September 2017 storm.An individual case study of topside ionospheric response to 7-8 September geomagnetic storm over the Asian-Australian sector was examined using topside observation in comparison with an earlier study on ground-based observations.The storm time perturbation in TEC was determined by subtracting the TEC on the storm day from average TEC of a number of quiet days close to the storm time.The storm time associated ionospheric irregularities was characterized by using a derived index from uplooking TEC called rate of TEC index(ROTI)and a similar index to it,derived from in situ electron density(RODI).Significant TEC enhancements were observed during the first main phase of the storm,the uplooking TEC did not show unusual enhancements at the morning and evening local times in the Asian-Australian sector during the recovery phase of the storm.Meanwhile,prominent TEC hemispheric asymmetries at the middle and high latitudes were observed at the day and night sectors.Long-duration recovery of topside TEC with respect to the prestorm condition was also detected in this event.As for observation relating to irregularities,nighttime ROTI enhancements with wide latitudinal range from the equator to the poles during the main phases of the storm were observed.In addition,the ionospheric electric field disturbances associated with IMF-Bz fluctuations was suggested to have played a very important role in triggering ionospheric irregularities during the relatively weak geomagnetic activity on 7 September,which implies that ionospheric irregularities do not necessarily occur under the severe geomagnetic conditions only.(2)Recurrence of daytime TEC enhancements during recovery phase of geomagnetic storms.A statistical study on positive daytime ionospheric response over the Asian sector during 2016-2018 was carried out by analyzing Beidou GEO TEC.Particularly,the occurrence of TEC enhancements during the different phases of several geomagnetic storms were examined in order to ascertain the dominant forcing of the observed enhancements.In this study the recurrence in TEC enhancements during the recovery phase of geomagnetic storms at the northern hemisphere stations were observed.This is unique,because TEC is known to recover to the initial condition from the storm-time effects during the recovery phase.Furthermore,the occurrence rates during the main phases of the geomagnetic storms were well above the baseline occurrence observed at individual station and were attributed to penetrating effect of electric fields and equator-ward neutral wind.Substantial decrease in the number of occurrences during the initial and recovery phases were observed,and the percentage occurrences were close to or below the baselines.In general,the effects of both chemical composition changes during the post-storms period and possible effects of modulating forcing from the lower atmosphere were inferred.(3)Observation of large number of irregularities during the recovery phase.Another statistical study was done on the occurrence of topside ionospheric irregularities during 24 geomagnetic storm events(Dst<-50 nT)in 2015,with the aim of characterizing the topside ionospheric irregularities using the topside ROTI technique.The local time,altitudinal and seasonal characteristics were examined using four LEO satellites(GRACE,SWARM-C,SWARM-B and TerraSAR-X)during the post-midnight to the pre-noon local time sector.Interestingly,the largest number of ionospheric irregularities occurred during the recovery phase while the percentage occurrences were pronounced during the main phases.The observed ionospheric irregularities showed altitudinal,local time and seasonal dependences,with the two lower satellites showing more relative percentage number of observed irregularities to number of storm events in solstice months than in equinox months and vice versa for the two higher altitude satellites.In conclusion,the results of this thesis are relevant in improving the understanding of F2/topside ionospheric responses during geomagnetic storms,and more importantly the possibilities of TEC enhancements and the triggering of irregularities during the recovery phases of geomagnetic storms.The knowledge obtained can find applications in the effort to mitigate errors associated with GNSS signals.