1. School of Mathematics and Statistics Ningbo University Ningbo 315211 Zhejiang China
2. Solar Physics and Space Plasma Research Centre, School of Mathematics and Statistics University of Sheffield Hicks Building, Hounsfield Road Sheffield S3 7RH UK
3. Institute for Advanced Study Shenzhen University Shenzhen 518060 Guangdong China
4. Department of Astronomy Eötvös University Pázmány P. sétány 1/A Budapest H‐1117 Hungary