Space Science Reviews

, Volume 161, Issue 1–4, pp 63–148 | Cite as

Review of Pi2 Models

  • Andreas KeilingEmail author
  • Kazue Takahashi


More than half a century after the discovery of Pi2 pulsations, Pi2 research is still vigorous and evolving. Especially in the last decade, new results have provided supporting evidence for some Pi2 models, challenged earlier interpretations, and led to entirely new models. We have gone beyond the inner magnetosphere and have explored the outer magnetosphere, where Pi2 pulsations have been observed in unexpected places. The new Pi2 models cover virtually all magnetotail regions and their coupling, from the reconnection site via the lobes and plasma sheet to the ionosphere.

In addition to understanding the Pi2 phenomenon in itself, it has also been important to study Pi2 pulsations in their role as transient manifestations of the coupling between the magnetosphere and the ionosphere. The transient Pi2 is an integral part of the substorm phenomenon, especially during substorm onset. Key questions about the workings of magnetospheric substorms are still awaiting answers, and research on Pi2 pulsations can help with those answers. Furthermore, the role of Pi2 pulsations in association with other dynamic magnetospheric modes has been explored in the last decade. Thus, the application of Pi2 research has expanded over the years, assuring that Pi2 research will remain active in this decade and beyond.

Here we review recent advances, which have given us a new understanding of Pi2 pulsations generated at various places in the magnetosphere during different magnetospheric modes. We review seven Pi2 models found in the literature and show how they are supported by observations from spacecraft and ground observatories as well as numerical simulations. The models have different degrees of maturity; while some enjoy wide acceptance, others are still speculative.


ULF wave Pi2 pulsation Magnetosphere Substorm 



Bursty bulk flow


Bursty bulk flow Pi2


Ballooning mode Pi2


Central plasma sheet


Equivalent ionospheric current


Field-aligned current


Field line resonance


Inertia current


Kinetic Alfvén wave


Kinetic ballooning instability


Kelvin-Helmholtz instability


Low Earth orbit




Poleward boundary intensification


Plasmaspheric cavity resonance


Plasmaspheric cavity resonance Pi2


Plasma sheet boundary layer


Plasmaspheric surface mode


Plasmapause surface mode Pi2


Plasmaspheric virtual resonance


Plasmaspheric virtual resonance Pi2


Pulsed reconnection Pi2


Substorm current wedge


Transient response


Transient response Pi2


Ultra-low frequency


Westward traveling surge


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© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  1. 1.Space Sciences LaboratoryUniversity of California-BerkeleyBerkeleyUSA
  2. 2.Applied Physics LaboratoryJohns Hopkins UniversityLaurelUSA

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