Programme by Session

The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. 45 years ago, Chen and Hasegawa proposed that the reflection of surface waves by the ionosphere might result in a standing wave or eigenmode of the magnetopause surface on the dayside. While many potential impulsive upstream drivers which might excite this mode exist, only recently has strong observational evidence of its existence been discovered, which we present.

Inspired by observational criteria for other MHD eigenmodes, we employ multipoint and polarisation observations using the THEMIS spacecraft at Earth. During a rare isolated magnetosheath jet event, we show that the broadband impulsive driver excited narrowband oscillations of the magnetopause location and associated magnetospheric ultra-low frequency waves. Through comparing the observations with theoretical expectations for several possible mechanisms, we conclude that the isolated jet excited the magnetopause surface eigenmode – like how hitting a drum once reveals the sounds of its normal modes.

Using global MHD simulations, we discuss the implications of this eigenmode on other MHD wave modes, energy propagation and radiation belt interactions throughout Earth’s magnetosphere. Finally we discuss the possible existence of this mode at other planetary magnetospheres across the solar system.