Study of Ionospheric response to intense  Solar Flares in the ascending half of the solar cycle 25 

Well organized and systematic study of sun-earth connection is vital. The fact that the state and conditions of space are influenced by solar activity, makes the space weather domain a field of vibrant research. Solar flares are rapid expulsions of electromagnetic radiation from the Sun's active regions. These complex transient excitations, mainly in soft X-rays (0.1 – 10nm), and extreme ultraviolet (10 – 121.6 nm) resulting in ionospheric response, have been a subject of keen interest over decades. Studies show a clear indication of Coronal Mass Ejections (CMEs) associated with flares and prominences. This is of prime importance, as the research on flare associated CMEs does have some underlying impacts to be revealed. The sudden enhancement of X-ray and extreme ultraviolet irradiance during flares raises the density of the ionosphere through enhanced photoionization. Sudden ionospheric disturbances due to the enhancement of plasma density is crucial and the total electron content (TEC) is a potent measure of the ionospheric response. Present study focuses on the analysis of ionospheric plasma irregularities and TEC variation due to M and X class solar flares in the beginning of solar cycle 25. We considered intense flares in the period 2019 - 2024, due to the solar activity growth at the ascending part of the solar cycle 25. Out of these, 15 M class and 15 X class flares are chosen to study plasma instabilities and TEC variations. On the basis of multiple observations from GNSS receivers and satellite missions, we present how flare characteristics affect flare responses in the ionosphere and the formation of large-scale travelling  ionospheric disturbances, during intense solar flares. The estimation of enhanced TEC (ΔTEC) shows that the peak enhancement in TEC is highly correlated with peak enhancement in X-ray flux during solar flares. Plasma density shows significant escalations on flare days than on non-flare days. More intense X-class flare provoked a more significant response in the ionosphere than the less intense M class flare. In addition to this, our study also expands in relating the same to flare associated CMEs in the given solar cycle. The CMEs whose source regions are known, can be used to draw out valid conclusions on CME - flare association and how this impacts the ionospheric responses. The growing space weather effects has also led to an increase in space weather research that aims to enumerate the sun-earth connection more precisely. The investigation on variation of both TEC and plasma density leads to better understanding of the ionospheric response to flare activity to a remarkable extent.