Lights in the Night Sky
Toward the end of the first encounter period, Voyager 1 flew behind Jupiter, and the spacecraft’s wide-angle camera scanned the northern hemisphere on the nightside of the planet, searching for aurorae and lightning bolts. The most impressive darkside feature found was a tremendous aurora in the north polar region. But this was not the first time Jovian aurorae had been detected. Very-high-energy auroral emissions resulting from ultraviolet glows of atomic and molecular hydrogen had been detected prior to encounter on the bright side of Jupiter by the ultraviolet spectrometer. The ultraviolet observations indicate that atmospheric temperatures in the auroral regions are at least 1000 K. In both the visible and the ultraviolet spectra, the aurorae are confined to the polar regions and result from charged magnetospheric particles striking the upper atmosphere. The ultraviolet aurorae are created when high-energy particles from the Io plasma torus spiral in toward Jupiter on magnetic field lines.
Several meteor trails were also evident in the darkside pictures of Jupiter’s atmosphere. Traveling at roughly 60 kilometers per second as they entered, these fireballs brightened quickly and seemed to survive for about 1000 kilometers before they died.
Clusters of lightning bolts—indicative of electrical storms—were also discovered on Jupiter’s nightside. This particular phenomenon does not seem to depend on latitude. The Voyager 1 photograph that captured the huge Jovian aurora also caught the electrical discharges of 19 superbolts of lightning, and Voyager 2 photographs located eight additional flashes. Radio emission (whistlers) from lightning discharges were also detected by the Voyager radio astronomy receivers and the plasma wave instrument.
The night side of Jupiter is not dull. A large aurora (northern light) arcs across the northern horizon, while farther south about twenty large bolts of lightning illuminate electrical storms in the clouds. Similar pictures also revealed fireballs, or large meteors, burning up in the atmosphere of Jupiter. [P-21283B/W]
Magnetic Field
Deep in the interior of Jupiter, the pressures are so great that hydrogen becomes an electrical conductor, like a metal. Currents driven by the rapid rotation of the planet are thought to flow in this metallic core. The result is a magnetic field that penetrates the space around Jupiter.
Direct measurements of the Jovian magnetic field were first made by the Pioneers, and Voyager results generally confirm the initial findings. The strength of the Jovian field is about 4000 times greater than that of the Earth. The dipolar axis is not at the center of Jupiter, but offset by about 10 000 kilometers and tipped by 11 degrees from the axis of rotation. Each time the planet spins, the field wobbles up and down, carrying with it the trapped plasma of the radiation belts. The Voyager particles and fields instruments concentrated not on the planetary magnetic field but on the processes taking place in the magnetosphere.
