On the Generation of Pi2 Pulsations due to Plasma Flow Patterns Around Magnetosheath Jets

We report observations of a magnetosheath jet followed by a period of decelerated background plasma. During this period, THEMIS‐A magnetometer showed abrupt disturbances which, in the wavelet spectrum, appeared as prominent and irregular pulsations in two frequency bands (7.6–9.2 and 12–17 mHz) within the Pi2 range. The observations suggest—for the first time to our knowledge—that these pulsations were locally generated by the abrupt magnetic field changes driven by the jet’s interaction with the ambient magnetosheath plasma. Furthermore, similar pulsations, detected by THEMIS‐D inside the magnetosphere with a 140 s time‐lag (which corresponds to the propagation time of a disturbance traveling with Alfvénic speed), are shown to be directly associated with the ones in the magnetosheath, which raises the question of how exactly these pulsations are propagated through the magnetopause. Plain Language Summary The Earth’s magnetosheath is particularly a turbulent region even during steady solar wind conditions. It is permeated both by waves that have been transmitted through the bow shock, as well as by fluctuations that have been generated locally. Jets in the magnetosheath can drive various wave species, especially via interaction with the magnetopause, and can have a significant magnetospheric effect. In this work, we report—for the first time—the generation of ultralow frequency waves generated, not by the jet itself but, with the jet’s interaction with the ambient magnetosheath plasma. We further show that similar pulsations, which are detected inside the magnetosphere, are directly associated with the ones in the magnetosheath, which raises the question of how exactly these pulsations are propagated through the magnetopause. Key Points Pi2 pulsations in two frequency bands (7.6–9.2 and 12–17 mHz) detected in the magnetosheath These pulsations were locally generated by the Jet’s After‐Flow Similar pulsations inside the magnetosphere are directly associated with the ones in the magnetosheath