**Figure 1:** Power spectrum of the magnetic field observed at DU
between 06:03 and 06:24 UT on May 9, 1979, and
the cross-phase spectrum for the station pair DU-LL for the
same time interval.
Also included is the wave power of the *H* component at DU for a time
interval (0002-0023 UT on the same day) when no pulsation activity was
observed.

**Figure 2:** Eigenfrequencies identified by the cross-phase spectra for
the *H* component of five pairs of adjacent stations.
The latitude of the resonant field line is assumed to be at
the center of the station pair.
If a continuum of field line resonances is not present, this assumption will not be valid.
A set of curves representing the first six harmonics following the calculation
by *Schulz* [1996].

**Figure 3:** The cross-phase spectra for the *H* component of DU and other
lower-latitude stations at similar longitude. The separation of
station pairs ranges from 175 km (DU-LL) to 800 km (DU-HA).
The phase change generally increases with increasing separation.

**Figure 4:** Schematic diagram showing the cross-phase spectrum
interpreted in terms of
field line resonance theory. The top panel shows the phase as a
function of latitudes for three examples of resonant *L*-shells with
resonant frequencies , , . The phase difference between
the signals observed at the two stations is shown in the bottom panel.
The phase difference becomes greater as the separation of stations increases.