Browsing by Author "Tarun Kumar Pant"

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Signature of the 27-day variation in hemispheric sunspot activity and asymmetry during 2010-2015
(IOP Publishing Ltd, 2021) Prithvi Raj Singh; Ahmad Islam Saad Farid; Tarun Kumar Pant; Abhay Kumar Singh
In the present work, we study the time evolution, significance of the N-S asymmetry excesses presented as a function of the solar cycle and prominent rotational periods (∼27 d) separately for the northern and southern hemispheres. We have investigated short-term variations of the hemispheric solar activity (sunspot numbers and sunspot areas) during the time period 2010-2015, which covers the ascending and the maximum phase of solar cycle 24. We have implemented the Lomb-Scargle periodogram and continuous wavelet transform power spectrum techniques to study the time evolution and dominant rotational periods separately for the northern and southern hemispheres, and whole solar disk. Our results showed that the northern hemisphere exhibited longer solar synodic periods than the southern hemisphere, indicating that the northern hemisphere has a lower rotation rate. Moreover, the northern hemisphere was found to be dominant before transferring to the southern hemisphere during mid-2013. Also, the sunspot areas clearly demonstrated a two-peak structure of solar activity in the northern and southern hemispheres respectively during 2012 and 2014. The statistical significance of the southern hemisphere affirmed enhanced excess during the maximum phase of solar cycle 24. © 2021 National Astronomical Observatories, CAS and IOP Publishing Ltd..
Solar wind plasma variations with interplanetary magnetic field during solar cycles 22–24
(Springer, 2023) Prithvi Raj Singh; Abhay Kumar Singh; Tarun Kumar Pant
In this paper, we have studied the relationship between the monthly variations of solar wind plasma velocity, interplanetary magnetic field (IMF) and geomagnetic activity index (Ap) during solar cycles 22–24 (1986–2020). The modulation parameter (ζ = V * B) is proportional to the product of solar wind plasma velocity, V and strength of the interplanetary magnetic field, B. We have investigated the periodicities and their evolution during solar cycles 22–24 using Fast Fourier Transform (FFT), RobPer periodogram and Continuous Wavelet Transformation methods. The significant periods present in the modulation parameter include the Rieger type, semi-annual period, annual period and quasi-biennial period (mid-term). In this study, we have found that the rotation rate at the base of the convection zone is ~1.30 years. Quasi-biennial oscillations (QBOs) of solar wind plasma velocity, IMF, modulation parameters and Ap have been compared. The modulation parameter appears to be a better representative of the geomagnetic changes than the other two. © 2023, Indian Academy of Sciences.