Study of the effects of space weather on low-latitude ionosphere during declining phase of solar cycles

Abstract

This thesis seeks to understand the impact of geomagnetic storms on the low-latitude ionosphere in the declining phase of solar cycles. The study of the effects of Coronal Mass Ejection (CME) and Corotating Interaction Region (CIR) induced geomag netic storms on low-latitude ionization during 2016 through 2019 were observed from stations in the Indian longitude sector. Enhancements in the Total Electron Content (TEC) (in excess of 40 TECU over quiet-time values) were noted and explained in terms of the thermospheric compositions, neutral wind interaction with the ions, and variation in the equatorial electrojet strength. The events of October 2016 and May 2017 geomagnetic storms validated that even CIR-driven events could affect the low-latitude ionization in a manner very similar to that of a stronger CME-induced geomagnetic storm. Observations from NavIC and GNSS (GPS, GLONASS, and Galileo) under different geomagnetic storm-time conditions in the declining phase (2017-2018) of solar cycle 24 were compared with standard ionospheric models, namely, International Reference Ionosphere (IRI)-2016, NeQuick2 and IRI-extended to Plasmasphere (IRI-Plas)-2017. This study brought out the fact that the IRI Plas-2017 model showed better correspondence (of the order of about 3–5 TECU) with the observed values from NavIC and GPS. The integration of NavIC data into the IRI-P model would provide better predictions over the Indian subcontinent during geomagnetically disturbed conditions. The ionospheric irregularities gener ated as a result of Prompt Penetration of Electric Field (PPEF) during the main phase of storms (in the declining phase of solar cycle 23) were studied with global in-situ ion density measurements from the sun-synchronous Defence Meteorological Satellite Program (DMSP) spacecraft. It was observed that the ionospheric irregu larities were generated within three hours of the northward-to-southward transition of Interplanetary Magnetic Field (IMF) clock angle at a longitude sector where the local time was dusk. Ionospheric irregularities studies were performed using the C/No observations recorded from NavIC and GPS L5 signal by the power spectral density analysis. Irregularities scale sizes of the order of a few hundred metres to few km were observed to be present under weak-to-strong geomagnetic storm-time conditions during the declining phase of solar cycle 24.