Mid-term periodicities and heliospheric modulation of coronal index and solar flare index during solar cycles 22–23

Abstract

We applied fast Fourier transform techniques and Morlet wavelet transform on the time series data of coronal index, solar flare index, and galactic cosmic ray, for the period 1986–2008, in order to investigate the long- and mid-term periodicities including the Rieger ( $${\sim }130$$ to $${\sim }190$$ days), quasi-period ( $${\sim }200$$ to $${\sim }374$$ days), and quasi-biennial periodicities ( $${\sim }1.20$$ to $${\sim }3.27$$ years) during the combined solar cycles 22–23. We emphasize the fact that a lesser number of periodicities are found in the range of low frequencies, while the higher frequencies show a greater number of periodicities. The rotation rates at the base of convection zone have periods for coronal index of $${\sim }1.43$$ years and for solar flare index of $${\sim }1.41$$ year, and galactic cosmic ray, $${\sim }1.35$$ year, during combined solar cycles 22–23. In relation to these two solar parameters (coronal index and solar flare index), for the solar cycles 22–23, we found that galactic cosmic ray modulation at mid cut-off rigidity ( $$\hbox {Rc} = 2.43\hbox {GV}$$ ) is anti-correlated with time-lag of few months.