H

N GWR ft-IOOy

N GWR ft-IOOy

GWR

MOOy

Byrd

Byrd

14 UT

FIGURE 4.14 ► During a magnetic storm day, these are the field changes in the northward (H), eastward (D), and downward (Z) directions at the conjugately located, auroral zone stations of Great Whale River (GHW), Canada, and Byrd Station, Antarctica. Scale sizes are indicated by the arrows to the right of the field traces. Hours at bottom are given in Universal Time (UT) for a March event.

Byrd

14 UT

FIGURE 4.14 ► During a magnetic storm day, these are the field changes in the northward (H), eastward (D), and downward (Z) directions at the conjugately located, auroral zone stations of Great Whale River (GHW), Canada, and Byrd Station, Antarctica. Scale sizes are indicated by the arrows to the right of the field traces. Hours at bottom are given in Universal Time (UT) for a March event.

ducing a storm-time ionization change and modifying the day-side quiet-time ionospheric dynamo currents.

Some phenomena, which are not traceable to solar sources, can also agitate the atmosphere sufficiently to cause magnetic fields. Pressure waves from volcanic explosions often reach the ionosphere and move the charged particles

2045 2055 2110

FIGURE 4.15 ► Heating from auroral currents caused this atmospheric pressure wave, which oscillates in the period range of 10 to 50 seconds. The light and dark traces indicate north-south- and east-west-directed microphones that are used to determine the arrival direction. The pressure waves, which originated in the region of auroral displays on 17 August, 1962, were detected at Fort Yukon, Alaska. The maximum amplitude shown here is approximately 3.5 dynes/cm2 (pressure scale units).

in that region, causing electric currents whose signature is seen at the Earth. A Russian high-altitude nuclear explosion above Novaya Zemlya in August 1962 initiated a pressure wave that traveled around the world at sonic speeds. That blast disturbed the ionosphere sufficiently to cause global dynamo electric currents whose fields were detected at the Earth's surface (Figure 4.16).

What the local magnetic records show for a solar-terrestrial disturbance depends on the location of the observatory because so many differing storm processes have suddenly been initiated. There are no clear latitude boundaries for the many storm effects and parts of many sources are spread globally. However, it is possible to point out some generalizations. At the po-

FIGURE 4.16 ► An atmospheric nuclear explosion in August 1962 at Novaya Zemlya, Russia, initiated an Earth-circling pressure wave that traveled at sonic speed. Upon its arrival at Fort Yukon, Alaska, the explosion pressure wave produced an infrasonic disturbance and generated small ionospheric dynamo currents that were recorded as geomagnetic field pulsations with periods of 5 to 30 seconds.

FIGURE 4.16 ► An atmospheric nuclear explosion in August 1962 at Novaya Zemlya, Russia, initiated an Earth-circling pressure wave that traveled at sonic speed. Upon its arrival at Fort Yukon, Alaska, the explosion pressure wave produced an infrasonic disturbance and generated small ionospheric dynamo currents that were recorded as geomagnetic field pulsations with periods of 5 to 30 seconds.

lar regions the effects of magnetospheric boundary currents and field-aligned currents are most important. At the high latitudes of auroral and subauroral regions, the field-aligned currents (on the same Earth side as the observatory) and auroral ionospheric currents dominate. At mid-latitudes some magneto-spheric currents, field-aligned currents, and high-latitude ionospheric currents all contribute to the magnetic recordings. At low and equatorial latitudes, the night-side fields are dominated by the magnetospheric tail current behavior and the day-side fields are dominated by ionospheric currents.

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