Magnetosphere‐ionosphere coupling of global Pi2 pulsations

Abstract

Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/ high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This "super" global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfvén and fast modes, field line resonance, and possibly a forced cavity-type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.

Initial deflection of middle‐latitude Pi2 pulsations in the premidnight sector: Remote detection of oscillatory upward field‐aligned current at substorm onset

“…It is noted that the low‐latitude H and low‐ to high‐latitude D components of Pi2 pulsations are generated by different physical processes. Kepko et al [], Uozumi et al [, ], and Keiling et al [] suggested that the driving sources of δ B SCW and δ B FW are closely coupled with each other. The present analysis further support their suggestions.…”

MESSENGER observations of Alfvénic and compressional waves during Mercury's substorms

“…As mentioned in section 1, many models have been proposed for Pi2 pulsations during substorm at Earth, including plasmaspheric cavity mode [e.g., Southwood and Kivelson , ; Lin et al ., ], ballooning waves [ Solovyev et al ., ; Keiling , ], bouncing Alfvén waves [e.g., Maltsev et al ., ; Lester et al ., ; Baumjohann and Glaßmeier , ], interchange oscillations in the plasma flow [ Wolf et al ., ; Keiling et al ., ; Panov et al ., ], and the braking of quasi‐periodic flow bursts in the plasma sheet [ Kepko and Kivelson , ; Kepko et al ., ]. Mercury is a very slow rotating planet (i.e., rotation period of ~ 58.6 days).…”

“…The Alfvén waves studied here could also be due to mode conversion near the equatorial region [e.g., Keiling et al ., ; Kim et al ., ]. In this model, the compressional waves generated by flow braking in the near equatorial region undergo mode conversion to Alfvén waves, which would propagate along the field line toward the nightside polar region.…”

Ultralow Frequency Waves Deep Inside the Inner Magnetosphere Driven by Dipolarizing Flux Bundles

“…This frequency range of DFB‐driven waves was revealed and explored by simultaneous observations of DFBs and magnetic oscillations of such frequencies (e.g., Kepko & Kivelson, ; Shiokawa et al, ). In situ magnetospheric observations showed that the DFB‐driven oscillations cover L shells extending from the GEO to the plasma sheet (e.g., Keiling et al, ; Runov et al, ). At the same time as DFB observations, ground magnetometers recorded Pi2‐band oscillations throughout 40°–70° latitudes, consistent with the spacecraft's wave observations (e.g., Keiling et al, ; Kepko, Kivelson, & Yumoto, ).…”