| Observations of Pi2 pulsations at middle and low latitudes have been
explained in terms of cavity mode resonances, whereas transients associated
with field-aligned currents appear to be responsible for the high latitude
Pi2 signature.
Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on
21 January 2003, when three of the satellites were within the plasmasphere
(L=4.7, 4.5 and 4.6) while the fourth was on the plasmapause
or in the plasmatrough (L=6.6). Simultaneous pulsations at
ground observatories and the injection of particles at geosynchronous orbit
corroborate the occurrence of a substorm.
Evidence of a cavity mode resonance is established by considering the phase
relationship between the orthogonal electric and magnetic field components
associated with radial and field-aligned standing waves. The relative phase
between satellites located on either side of the geomagnetic equator
indicates that the field-aligned oscillation is an odd harmonic. Finite
azimuthal Poynting flux suggests that the cavity is effectively open ended
and the azimuthal wave number is estimated as m~13.5. |