Browsing by Author "Ogunjobi, Olakunle."
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Item Outer electron radiation belt dropouts : Geosynchronous and ionospheric responses.(2011) Ogunjobi, Olakunle.; Collier, Andrew Blaine.The study of outer radiation belt dynamics has been ongoing for over 5 decades. Outer radiation belt dropouts involve the rapid lost of electron fluxes at the main phase of a storm and subsequent recovery. The characteristics of the dropouts are many and varied. This study examines the Geosynchronous Earth Orbit (GEO) and the ionospheric responses during Stream Interface (SI) andMagnetic Cloud (MC) events, using a combination of ground based and satellite instruments. SI- and MC-driven dropout events were inspected from summary plots of the Synchronous Orbit Particle Analyser (SOPA) instruments from the year 1996 to 2007. Comprehensive studies were done on six selected events. Analysis of the data from the instruments indicate that SIs and MCs are important lost drivers with significant differences in GEO and ionospheric environment. To validate the data and test for consistent response of the events, the Superposed Epoch Analysis (SEA) technique was used. The ground based measurements also revealed how the absorption peaks in the ionosphere correlated with dropouts and geophysical activity. Ionospheric absorption during SI associated dropouts was enhanced for 5 < L < 6, while significant peaks in ionospheric absorption extended to lower L during MC driven dropouts. Wave-particle interactions and southward Interplanetary Magnetic Field (IMF-Bz) are apparent causes for the precipitation. This analysis showed that, within the confines of the selected events, SI driven dropouts were more dominant at the declining phase of the solar cycle while the MC driven dropouts were more dominant during solar maximum.Item Studies on the influence of magnetic cloud, stream interface and polar mesospheric summer echoes in the mesosphere and lower thermosphere (MLT) region using model calculations and observations.Ogunjobi, Olakunle.; Venkataraman, Sivakumar.The response of Mesosphere and Lower Thermosphere (MLT) temperature to energetic particle precipitation over the Earth's polar regions is not uniform due to complex phenomena within the MLT environment. Nevertheless, the modification of MLT temperatures may require an event based study to be better observed. Three Geospace events examined in this study are: Magnetic Clouds (MC), solar wind Stream Interfaces (SI) and Polar Mesospheric Summer Echoes (PMSE). The MC is a transient ejection in the solar wind defined as the region between the preceding half of the z{ component of southward interplanetary magnetic field (IMF{Bz) and the trailing half, which contained strong northward peak or vice versa, with an accompanied large density enhancement that strongly compressed the magnetosphere. The varied instrumentation which is located not only in South African National Antarctic Expedition (SANAE) IV, but also at Halley, a same radial distance (L ~ 4) location in the Southern hemisphere, and at the vicinity of conjugate location in Northern hemisphere provide an opportunity to test the theories applied to high latitude heating rates on arrival of MC. The Halley riometer is used to monitor coincidences of absorption with arrival of a fortuitous MC that was observed on 8 November 2004. Using Monte Carlo Energy Transport Model (MCETM), the corresponding altitude of electron and proton energy distribution indicates the importance of MC triggered geomagnetic storms on mesospheric dynamics. At the arrival of SI near the Earth's bow nose, compressional streams propagate into the inner magnetosphere, where they fueled magnetospheric storms. A number of SI events were obtained close in time to the pass of temperature retrieval onboard the Thermosphere Ionosphere Mesosphere Energetic and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) over SANAE IV. The relationship between the ionospheric absorption measured by riometer and the layer of energetic particle precipitation from National Oceanic and Atmospheric Administration/Polar Orbiting Environmental Satellites (NOAA/POES) was examined during these events. Here, a superposed epoch technique is described and implemented to obtain average temperature profiles during SI{triggered particle precipitation. Prior to SI onset, there is no signifIcant temperature decrease below 100 km, whereas at the onset of the SI{triggered precipitation, we observe an immediate superposed average temperature decrease of about 35 K around 95 km. Results indicate that, cooling effects due to the production of mesospheric odd hydrogen might be major contributors to temperature decrease under compressed solar wind stream. PMSE, as a one{type Geospace event, exists because the electrically charged{ice particles reduce the mobility of mesospheric free electrons. In this study, first long term observations of PMSE, with Super Dual Auroral Radar Network (SuperDARN) SANAE IV HF radar (hereafter in this thesis referred as SuperDARN{PMSE), is presented. An extraction algorithm is described and implemented to obtain SuperDARN{PMSE during the summer period of years from 1998 to 2007. We examined the SuperDARN{PMSE occurrence rate in relation to geomagnetic activity and ow of mesospheric winds. Furthermore, the temperature changes in relation to SuperDARN{PMSE has been studied. The SuperDARN{PMSE peaks coincide with lower summer mesopause temperature and higher geomagnetic activity. Analysis of neutral wind variations, in relation to SuperDARN{PMSE peaks, indicates the importance of pole to pole mesospheric transport circulations. In addition, the mid{latitudes thermospheric zonal winds variations from the year 2005 to 2008 and its relevance to magnetospheric activity are examined. These studies could be directed towards achieving a more self{consistent interpretation of how the MLT energy budget is affected by magnetospheric processes.Item Study on extreme geomagnetic events and ionospheric response.(2018) Tire, Adila Wamisho.; Venkataraman, Sivakumar.; Ogunjobi, Olakunle.Extreme geomagnetic events are geophysical phenomena that result from the violent eruptive nature of the Sun. One type Geospace event is magnetic cloud (MC), which is an attendant of coronal mass ejection (CME). MC-triggered storms can cause injection of particles into the ionosphere. This can result in an enhance ionization and conductivity of upper and middle atmosphere. MC can be identi ed based on geomagnetic parameters and solar wind conditions which show high magnetic eld magnitudes, low ratio of plasma to magnetic pressure, low proton temperature, and smooth rotation of the magnetic eld vector. MC events that occurred on 29 April 2014, 17 March 2015, 31 December 2015 and 13 October 2016 were selected for the study. The hourly average of particle dropouts, precipitation, local ionospheric response and magnetometer variations in the region over South Africa (33:3oS, 26:5oE) are examined during geomagnetic storms triggered by MC. The Geostationary Operational Environmental Satellites (GOES) were inspected for radiation belt particle dropouts during MC events. Energetic particle precipitation associated with MC events are obtained from National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites (POES). Results show that particle dropouts and precipitation vary with the arrival of MC. A closer look of the ground based magnetometer and the time history of available daytime E-layer critical frequency from ionosonde indicate that lower ionosphere respond to MC-driven storm.