Heating

Strong field-aligned currents flowing between the magnetosphere and the high-latitude ionosphere cause intense heating. This process (called Joule heating) arises in much the same way as the heating from strong electric currents flowing through a given resistance in the wires of home electrical heaters. The sudden Joule heating during storms expands the atmospheric gasses in the auroral storm current region with two effects. A faint pressure wave travels through the atmosphere at the sonic speed of about 300 m/sec (675 miles/hr) to be detected at the Earth's surface with ultrasensitive barometers (Figure 4.15). Another stronger pressure wave, at just under sonic speed, travels equator-ward in the thermosphere (mainly just above the ionosphere maximum) from the conjugate auroral zones. This wave disrupts the high ionosphere, pro-

Count of Bombarding Electrons from Space r--1-r

Absorption of Cosmic Noise by Ionosphere

Absorption of Cosmic Noise by Ionosphere

Northward Magnetic Field Change

Northward Magnetic Field Change

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Field Pulsations with Periods of 5 to 30 seconds

Field Pulsations with Periods of 5 to 30 seconds

Local Time (hours)

FIGURE 4.13 ► For a geomagnetic storm measured at Fairbanks, Alaska, the traces, top to bottom, illustrate the simultaneous occurrence of the arriving auroral electrons, the increase in ionospheric density causing absorption of radiowaves, the field from the westward auroral electrojet current, and the increase in geomagnetic field pulsations.

Local Time (hours)

FIGURE 4.13 ► For a geomagnetic storm measured at Fairbanks, Alaska, the traces, top to bottom, illustrate the simultaneous occurrence of the arriving auroral electrons, the increase in ionospheric density causing absorption of radiowaves, the field from the westward auroral electrojet current, and the increase in geomagnetic field pulsations.

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