Multi-scale analysis of coronal Fe xiv emission: The role of mid-range periodicities in the Sun–heliosphere connection

• The combination of EMD and MWT techniques is used to study the mid-term periodicities in coronal green emission.
• The rotation periods of 26 and 29 days correspond to the two principal components of differential rotation in the solar corona.
• The Rieger-type periodicity near 150 days is not found in time series of coronal index.
• the MTQPs around 1.3 and 1.7 years indeed exist in the solar coronal region.

Two relatively modern analysis techniques, including the empirical mode decomposition and Morlet wavelet transform, are combined to investigate the short- and mid-term periodicities of daily coronal index from 1 January 1939 to 31 December 2008. Apart from the well-known periodicities of annual-oscillation variation, 11-year Schwabe cycle, 17-year full-disk activity cycle and 22-year magnetic cycle, the combined procedure reveals a variety of mid-term quasi-periodicities (MTQPs) in this solar-activity indicator, especially the periodicities near the 1.3 and 1.7 years. The prominent results show the followings: (1) the synodic rotation periodicities of 26 and 29 days correspond to the two principal components (the fast and slow types) of differential rotation in the solar corona; (2) the Rieger-type periodicity near 150 days is not found in time series of coronal index, which may be due to the intermittent character of its quasi-periodic pattern; (3) the MTQPs with a shorter periodicity around 1.3 years and a longer one around 1.7 years indeed exist in the solar coronal region. Our analysis results suggest that the coronal dynamics should be a connecting agent among the oscillations in the tachocline layer (Howe et al., 2000), in the lower atmospheric layers (Krivova and Solanki, 2002) and in the interplanetary space (Mursula and Vilppola, 2004).