THEMIS Observations of Tangential Discontinuity-Driven Foreshock Bubbles
Monday, 14 December 2015
Poster Hall (Moscone South)
Foreshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the magnetosphere-ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have no normal magnetic field component, prohibiting access of field-aligned particles upstream. Thus, they have not been previously considered as drivers of FBs. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multi-point observations from Advanced Composition Explorer (ACE) and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD-driven FBs identified by using classical FB selection criteria. We applied multi-point THEMIS observations on TD-driven FBs to study their spatial structures and time evolutions. We also statistically studied the formation condition of TD-driven FBs. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.