Pi2 pulsations driven by ballooning instability

Abstract

With the evidence set forth in this study, we argue that ballooning modes are viable candidates for the generation of high‐latitude Pi2 pulsations. In the scenario proposed here, it is the spatial separation ballooning wave perturbations along an energized boundary in the near‐Earth plasma sheet combined with a westward ion diamagnetic drift, which imprint a temporal signature, namely, Pi2 pulsation, on conjugate, stationary ground stations. The ballooning wave perturbations are connected via field‐aligned currents to the ionosphere causing Pi2 perturbations in the ground magnetic field. The observational evidence for this mechanism was conjugate spacecraft and ground data, showing correlated diamagnetic plasma perturbations and ground magnetic field oscillations, for two spatially separated, simultaneously occurring Pi2 events on 23 March 2007. The plasma perturbations were westward‐drifting waves with azimuthal mode number, m, of approximately 40 consistent with drift ballooning modes. During the Pi2 events, particle energy and energy flux gradually increased until the onset of major particle injections, causing auroral intensifications and additional larger‐amplitude Pi2s. The ground Pi2s, also shown to be westward‐traveling, were superposed on a slowly decreasing H component which abruptly turned into a substorm bay. A ballooning‐driven, high‐latitude Pi2 differs from a high‐latitude Pi2 that starts synchronized with substorm onset and that has been attributed to the transient response mechanism in that it is the frequency of drift ballooning mode in the near‐Earth plasma sheet that determines the Pi2 frequency and not the bounce frequency of Alfvén waves along a field line. The geophysical context for this type of Pi2 is a class of substorms that develop from a plasma instability in the near‐Earth plasma sheet. If this instability does not lead to a substorm breakup, it could lead to a pseudo‐breakup. Hence, it is also likely that some Pi2s associated with pseudo‐breakup are caused by the mechanism described here.