Pulsations

The curious and beautiful (to scientists) resonances that appear as pulsations in the field are continually studied to establish what exact particle and field conditions in space are responsible for their generation. Both the Pi and Pc geomagnetic pulsations (Section 4.1.6, p. 119) occur at magnetospherically disturbed times; let us look at one way these fields are analyzed and how similar they appear at conjugate locations.

When rapid variations of the geomagnetic field are studied, it is typical to carry out a spectral analysis. That means the data are investigated for the intensity of field at different periods (or frequencies) of the oscillations. Figure 5.11 shows the simultaneous appearance of a spectral analysis of pulsations that arrive at conjugate (opposite ends of an Earth field line) high-latitude stations. In this figure, the vertical axis represents frequency in cycles per sec-

AWAY

AWAY

FIGURE 5.9 ► The view looking down on the Northern Hemisphere geomagnetic pole shows the quiet-time field variations (in local time) as if they came from ionospheric currents. There is a shift in position of this polar current system vortex depending on whether the field attending the wind of plasma from the Sun is directed toward (T) or away (A) from the Sun as it arrives at the magnetosphere. Note the difference in position of the vortex center of approximately 2.5 hours in local time for the two conditions. These polar fields are responding to the interaction of the solar wind with the tail region of the magnetosphere.

FIGURE 5.9 ► The view looking down on the Northern Hemisphere geomagnetic pole shows the quiet-time field variations (in local time) as if they came from ionospheric currents. There is a shift in position of this polar current system vortex depending on whether the field attending the wind of plasma from the Sun is directed toward (T) or away (A) from the Sun as it arrives at the magnetosphere. Note the difference in position of the vortex center of approximately 2.5 hours in local time for the two conditions. These polar fields are responding to the interaction of the solar wind with the tail region of the magnetosphere.

ond; the horizontal axis represents the time in hours, and darkening represents the amplitude (natural signal strength) of the pulsations at each frequency.

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