Glamorous Display

As precipitating energetic particles of field-aligned currents encounter the denser upper atmosphere of the Earth, near the 100- to 300-km-high (63- to 190-mile) ionosphere at high latitudes, they hit and break apart oxygen and nitrogen molecules. The resulting ions release the excess energy from the collision as auroral lights (Plate 5) with red, yellow, green, and purple colors that are characteristic of the air molecules and the excitation energy (Figures 4.6 and 4.7). This process is somewhat similar to the glowing of gas in a neon sign as electrons of the electric current bombard the special gas within the tube.

During the equinoxes, when the high-latitude northern and southern hemispheres have similar darkness periods, photographs show that the disturbance particles arriving from the far magnetosphere are about equally likely to spill into both hemispheres of the Earth. The almost mirror-image patterns in the two high-latitude regions are called Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights).

In addition to the auroras, other effects of this magnetic storm are magnetospheric shape change, growth and decay of intense magnetospheric tail currents, strong partial ring currents, powerful field-aligned currents, high-latitude thermospheric (atmosphere above 90 km or 56 miles in altitude) heating, thermospheric pressure waves, intense ionospheric currents, and induced currents in the Earth. Let us look at some of these effects as detected at the Earth's surface observatories.

The Cause of the Aurora

FIGURE 4.6 ► Electrons, arriving in the upper atmosphere as part of the magnetospheric field-aligned current, bombard the local oxygen and nitrogen atoms, which release their newly acquired extra energy as auroral lights (wavelengths given in nanometers, nm). Figure adapted from L. Combs and R. Viereck of NOAA Space Environment Center.

FIGURE 4.6 ► Electrons, arriving in the upper atmosphere as part of the magnetospheric field-aligned current, bombard the local oxygen and nitrogen atoms, which release their newly acquired extra energy as auroral lights (wavelengths given in nanometers, nm). Figure adapted from L. Combs and R. Viereck of NOAA Space Environment Center.

FIGURE 4.7 ► Auroral display over the Antarctic region photographed May 1985 from Spacelab Challenger. Photo by R. Overmyer of NASA.
0 0

Post a comment