Relationship Between Low‐Latitude Pi2 Pulsations and Cavity Mode Oscillations

Abstract On the ground, Pi2 magnetic pulsations are detected at low latitudes at all magnetic local times (MLTs), unlike in the inner magnetosphere. To gain insight into the mechanism for the global appearance, we study the MLT dependence of the properties of low‐latitude ground Pi2 pulsations detected at four longitudinally separated stations. The pulsation properties are defined with respect to compressional magnetic field oscillations detected by Van Allen Probes at = 2.5–6.5 within 2 hr of midnight. Up to two peaks between 6.7 and 40 mHz found in the spectrum are selected as possible signatures of the source of ground Pi2 pulsations. For each spectral peak, we compute the coherence of the ground horizontal northward component with , and those events exhibiting high coherence are used in statistical analyses. The radial mode structure of the oscillations indicates they are fundamental or second harmonics of cavity mode oscillations (CMOs). Ground pulsations appear primarily in the component with time delays of less than a few seconds and amplitudes comparable relative to the oscillations in the low‐ region. The observations suggest that, if the dayside ground Pi2 pulsations are driven by ionospheric currents as previously proposed, the current must be coupled to the CMOs, not to the currents flowing on field lines connected to the auroral zone.