Jupiter's auroral ovals on the planet's night side were captured by the Cassini narrow angle camera on January 13, 2001 when the spacecraft was approximately 16.5 million kilometers from the planet and about 2.5 degrees below the equator; the smallest features are about 100 kilometers across. The images, taken 13 hours apart through a narrow spectral filter centered on an emission line of hydrogen known as 'H alpha,' have been processed to remove scattered light from the overexposed crescent of the planet. (Hydrogen is a major constituent of the Jovian atmosphere.) The Jovian magnetic rotation axis is tilted relative to the rotation axis and offset from the center of the planet. The result is that the magnetic pole is offset from the rotational pole in the north but more closely coincident with the rotational pole in the south. Energetic magnetospheric particles are constantly streaming towards Jupiter on magnetic field lines which intersect the planet's atmosphere on rings centered around the magnetic poles, causing the emission of light, or aurorae. In the north (upper) image, the rotational pole is located at the convergence of the blue longitude lines and falls to the left and out of the frame. The auroral oval encircling the north magnetic pole consequently appears like a draped necklace crossing lines of constant latitude on the planet. In the south (lower image), no significant offet is visible since the magnetic and rotational poles are closer there. The cusp of Jupiter's crescent, saturated in this frame, is seen on the left.
These Cassini imaging observations are the first to capture Jupiter's southern aurora on the planet's night side.
Jupiter's aurorae are influenced by processes inside the Jovian magnetosphere as well as the Sun. The planet's aurorae are dominated by processes internal to its magnetosphere, principally along field lines connected to the plasma sheet near 10 Jupiter radii. Auroral excitations correlated with solar wind fluctuations have also been seen, and aurorae at the feet of magnetic flux tubes of the Galileo satellites are almost always present.
It is not understood why the auroral oval rings are so thin. Cassini's many images of this phenomenon will help unravel what brings about the narrow nature and other features of the aurorae, such as the break in the northern oval visible in the upper image.
[Caption updated on February 12, 2008]
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.