Distinguishing Among Mechanisms That Determine Pi 2 Pulsation Period

Pi2 pulsations are an integral component of substorms, with their association with magnetic field dipolarization, particle injection, auroral brightening, and intensification of field-aligned currents. An important question about Pi2 pulsations is how their periodicity is established. Two possible mechanisms are forcing of the inner magnetosphere by periodic variations of the near-Earth plasma bulk flows, and the cavity mode response of the plasmasphere to impulsive or irregular disturbances propagating from the tail. We address this question using observations of four Pi2 pulsations that occurred in a 2-hour time span on 29 July 2013. These events were observed by THEMIS, Van Allen Probes (RBSP), and geostationary GOES and ETS-VIII, while some of these spacecraft were nearly radially aligned in the evening sector at radial distances between 2 and 10 Earth radii. Electron density data are available from THEMIS and RBSP to determine the spacecraft location relative to the plasmapause. We examine the time delay of oscillations among the spacecraft and the local wave properties such as polarization and phase delay between the electric and magnetic field components. We compare the observations with ULF wave simulations in a dipole magnetosphere to evaluate which of the two possible Pi2 generation mechanisms was more effective.