During periods of disturbances, small (0.1 to 0.001 gamma in size) pulsations of the field often occur. On the charts, the pulsations are seen as either a jagged irregular trace (called Pi) or a smooth continuous, quite oscillatory form (called Pc). Some pulsations arise in the magnetosphere as a result of the arrival of the solar-terrestrial storm particles and fields. Other pulsations are associated with the bundling of the electrons that make up the field-aligned currents (described previously) and cause pulsating auroras (Figure 4.22). Some field pulsations are associated with the motion of the conducting ionosphere (Figure 4.16). Other pulsations, upon arriving at high latitudes, travel rapidly around the Earth in a duct of the high ionosphere. Scientists are still studying the initiation and resonance processes that produce all these pulsation phenomena (see Section 5.2.6, p. 131) with the hope that someday the recordings can monitor the continuing physical change of magnetospheric regions far from a recording station at the Earth's surface.

FIGURE 4.21 ► A seasonal preference of major magnetic storms is shown as the percent of 1129 events between 1932 and 1991 for the Ap index values greater than 40. Figure from J. H. Allen and D. C. Wilkinson of NGDC/NOAA.

Byrd N Photometer

Byrd N-S Micropulsation

1150 1155 1200 UT

FIGURE 4.22 ► Geomagnetic field pulsations (peak values of about 0.2 gamma) and the photometer response to auroral light intensity (nitrogen emissions) recorded at Byrd Station, Antarctica, on 23 August, 1966.

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