Statistical study of global modes outside the plasmasphere

Abstract

Global modes, trapped fast mode magnetohydrodynamic (MHD) waves in the Earth's magnetosphere, may exhibit a monochromatic frequency spectrum even in the presence of a driver with a broadband frequency spectrum; they can in turn drive standing Alfvén waves at discrete frequencies via field line resonance (FLR). Direct observations of global modes are limited to a few case studies due to unique challenges associated with detecting them in situ. In this study, we use electric field, magnetic field, and plasma data from multiple THEMIS spacecraft as well as ground-based observations to identify and characterize global modes outside the nominal plasmapause location. We establish a lower bound of 1.0% for the global mode occurrence rate in the 3-20 mHz frequency range in that region and show that global modes occur more frequently during high-speed solar wind intervals. We also show that global modes with frequency between 10 and 20 mHz occur preferentially in the noon local time sector and are likely driven by processes in the ion foreshock. Key PointsThe lower bound for the global mode occurrence rate is 1.0%Global modes occur more frequently near the noon local time sectorThe ion foreshock drives global modes with frequency ≤ 10 mHz © 2013. American Geophysical Union. All Rights Reserved.