Researchers led by PhD candidate ZHANG Xiaojuan and Prof. DENG Linhua from the Yunnan Observatories of the Chinese Academy of Sciences (CAS) investigated the temporal variation of the hemispheric asymmetry of high-latitude and low-latitude coronal mass ejections (CMEs) during the time interval from 1996 to 2020.

This work was published in Monthly Notices of the Royal Astronomical Society on Feb. 20.

The hemispheric asymmetry phenomenon of solar magnetic field structures is a manifestation of the evolution of solar activity cycle. Research on solar activity indicators (sunspot numbers, sunspot areas, flare index, filament and prominence, etc.) reveals that the magnetic activity in the southern and northern hemispheres shows amplitude asymmetry and phase in each solar cycle is out of synchronization.

However, hemispheric variations of CMEs at different latitudes, in different cycles, and for different types (regular and specific events) are still unclear.

In this study, the researchers studied the hemispheric asymmetry of high-latitude and low-latitude CMEs in time interval from January 1996 to December 2020, based on the list of white-light CMEs in the Coordinated Data Analysis Web catalogue.

They found that the regular CMEs were significantly correlated with solar activity expressed by international sunspot numbers, particularly for the low latitudes. However, the specific CMEs were not correlated with solar activity. The main reason for the hemispheric asymmetry of the CMEs was attributable to specific CMEs, not regular CMEs. The hemispheric asymmetry of high-latitude CMEs appeared to have little connection to that of low-latitude CMEs.

"For the total and specific CMEs, the relationship between the absolute asymmetry index at high and low latitudes has a positive correlation prior to the cycle maximum, but a negative correlation after the cycle maximum," said ZHANG.

The researchers also found that the dominant hemisphere, the cumulative trend, and the amplitude of the total, specific and regular CMEs in cycle 23 differed from those in cycle 24.

"Our study can help to understand the cyclical variation of the magnetic free energy during different solar cycles, and provide insight into more physical processes responsible for the solar-terrestrial relationship," said Prof. DENG.