Institute of Geophysics and Planetary Physics, University of California Riverside, Riverside CA 92521, USA

Institute of Geophysics and Planetary Physics, University of California Riverside, Riverside CA 92521, USA

Institute of Astronomy, Department of Physics and Centre for Complex Systems, National Central University, Chung-Li, 32054, Taiwan, Republic of China

work carried out on leave at: IGPP, University of California Riverside, CA, USA

School of Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa

Institute of Geophysics and Planetary Physics, University of California Riverside, Riverside CA 92521, USA

School of Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa

Institute of Geophysics and Planetary Physics, University of California Riverside, Riverside CA 92521, USA