Pi 2 pulsations
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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
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.
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.
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.
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
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,
- Lester, M., W. J. Hughes, and H. J. Singer, Longitudinal structure in Pi 2
pulsations and the substorm current wedge, J. Geophys. Res., 89,
- 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,
- 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,
- Pytte, T., R. L. McPherron, and S. Kokubun, The ground signatures of the
expansion phase during multiple onset substorms, Planet. Space Sci., 24,
- Saito, T., Geomagnetic pulsations, Space Sci. Rev., 10,
- 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.