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Irregular, damped ULF range magnetic pulsations occurring in connection with magnetospheric substorms are called Pi 2 pulsations (T = 40 - 150 s; Saito, 1969). They are fed by the magnetic wave energy released during the impulsive magnetic field dipolarization processes that characterize substorms (Takahashi et al., 1995). As a result, substorm current wedge (SCW) forms connecting magnetosphere and auroral ionosphere. The Pi 2 activity exhibits the same temporal fine structure as many other substorm related pheonomena (Pytte et al., 1976; Takahashi et al., 1995). For a review on Pi 2, see Yumoto (1986).

Pulsations in the Pi2 range seem to have (at least) two separate, but related sources. At higher latitudes, the observed nightside magnetic pulsations are created by the SCW oscillations, while the lower latitude pulsations seem to be related to oscillations confined to plasmasphere, and they can be observed also on dayside (e.g., Li et al., 1998). Since the waves can be detected with little time delay over a wide range of latitude and longitude, they have been used for the ground based determination of the substorm onset time. Finally, note also that localized high-latitude Pi B pulsations overlap partly the Pi2 frequency range, and are related to substorm onset.

SCW oscillations

At high and mid-latitudes, the source of the observed Pi2 pulsations is the oscillating SCW (Lester et al., 1983, 1984, 1989). One important signature of this source is the rotation of polarization major axis azimuth from northeast to northwest as one moves from west to east over the source region.

Plasmasphere relation

Pi2 pulsations exhibit a secondary maximum around the plasmapause location. To explain these lower latitude pulsations, plasmapause surface waves and cavity/waveguide resonances of the inner magnetosphere have been suggested.

Substorm timing

Pi2 pulsations have been traditionally used to define the substorm onset time. However, there are several reasons to be cautios in interpreting the results. First of all, because of the fine structure of the substorms and possible pseudobreakup activity, several wave burst per substorm is typically observed. Then, time delays between different types of waves observed at different latitudes and local times complicates the situation (e.g., propagation effects). There is also a clear time difference between the pulsations and first auroral brightening (typically the waves are delayed). Finally, even with a given wave event, different methods to select the onset time from the data may provide different timings. For example, Kepko et al. (AGU Fall Meeting 1998) proposed that one should define the onset as the time the amplitude of polarized power at a nightside mid-latitude station reaches 1/3 the maximum latitude.


  • Lester, M., W. J. Hughes, and H. J. Singer, Polarization patterns of Pi 2 magnetic pulsations and the substorm current wedge, J. Geophys. Res., 88, 7958-7966, 1983.
  • Lester, M., W. J. Hughes, and H. J. Singer, Longitudinal structure in Pi 2 pulsations and the substorm current wedge, J. Geophys. Res., 89, 5489-5494, 1984.
  • Lester, M., H. J. Singer, D. P. Smits, and W. J. Hughes, Pi 2 pulsations and the substorm current wedge: Low-latitude polarization, J. Geophys. Res., 94, 17133-17141, 1989.
  • Li, Y., B. J. Fraser, F. W. Menk, D. J. Webster, and K. Yumoto, Properties and sources of low and very low latitude Pi2 pulsations, J. Geophys. Res., 103, 2343-2358, 1998.
  • Pytte, T., R. L. McPherron, and S. Kokubun, The ground signatures of the expansion phase during multiple onset substorms, Planet. Space Sci., 24, 1115-1132, 1976.
  • Saito, T., Geomagnetic pulsations, Space Sci. Rev., 10, 319-412, 1969.
  • Takahashi, K., S. Ohtani, and B. J. Anderson, Statistical analysis of Pi2 pulsations observed by the AMPTE CCE spacecraft in the inner magnetosphere, J. Geophys. Res., 100, 21929-21941, 1995.
  • Yeoman, T. K., M. Lester, D. K. Milling, and D. Orr, Polarization, propagation, and MHD wave modes of Pi 2 pulsations: SABRE/SAMNET results, Planet. Space Sci., 39, 983-998, 1991.
  • Yumoto, K., Generation and propagation mechanisms of low-latitude magnetic pulsations - A review, J. Geophys., 60, 79-105, 1986.
  • Yumoto, K., K. Takahashi, T. Saito, F. W. Menk, B. J. Fraser, T. A. Potemra, and L. J. Zanetti, Some aspects of the relation between Pi 1-2 magnetic pulsations observed at L = 1.3-2.1 on the ground and substorm-associated magnetic field variations in the near-Earth magnetotail observed by AMPTE CCE, J. Geophys. Res., 94, 3611-3618, 1989.