What Causes Jupiter's Northern and Southern Lights?
8 Oct 2002
(Source: American Astronomical Society - Division for Planetary Sciences)
DPS Press Release
Embargoed until 11:30 AM, Tue., Oct. 8, 2002
Jupiter's aurora borealis (northern lights) and aurora australis (southern lights) are the subject of debate among planetary scientists. As on Earth, these aurorae are generally attributed to the interaction between the solar wind, a high-energy stream of charged particles traveling outward from the Sun, and the magnetism generated by Jupiter itself. Like winds on Earth, the solar wind waxes and wanes in both speed and intensity. Theoretical modeling predicts that when the solar wind is more intense, Jupiter's magnetic bubble (magnetosphere) is compressed, and the aurorae tend to be less bright; a less intense solar wind results in a larger magnetosphere and a less intense auroral display. Observations in ultraviolet light seemed to support the theoretic models.
This inverse relationship between solar wind intensity and auroral brightness, however, appears to be at odds with what is observed in infrared light. During the flyby of Jupiter by the Ulysses spacecraft in the spring of 1992, 15 nights of infrared observations of Jupiter's aurorae were obtained by Baron and his colleagues from NASA's Infrared Telescope Facility (IRTF) at the Mauna Kea Observatory in Hawaii. They reported their findings in Icarus 120, 437 (1996). They found that the infrared aurorae tended to brighten slightly during periods of increased solar wind pressure as measured by the Ulysses spacecraft.
Infrared studies of the Jupiter aurorae are less likely to be sensitive to rapid changes in the Jupiter atmosphere than are the ultraviolet observations typically used for auroral studies. Dr. Takehiko Satoh and his colleagues therefore chose to extend the earlier analysis using a high-resolution infrared camera (NSFCAM) on the IRTF for 14 nights in 1999.
In the accompanying figure, the variation in total brightness of the northern aurora as Jupiter rotates is shown. On any given night, the variation is smooth (sinusoidal), as would be expected for an aurora of constant brightness from the 10-degree tilt between the rotation axis and the magnetic pole of Jupiter. However, the size (amplitude) of the variation changes from night to night due to changes in the auroral brightness.
The better resolution of NSFCAM not only enabled Satoh and his colleagues to measure the total brightness, but also to pinpoint the brightness variations among the several distinct parts of the aurora on any given night. Satoh and colleague J. E. C. Connerney described those observations in a paper in Icarus 141, 236 (1996).
More recently, the total auroral brightness has been compared to the intensity of the solar wind at Jupiter on each night as computed from detailed modeling of the solar wind as described in a paper submitted to Journal of Geophysical Research by K. Hayashi and co-workers. It now appears that the Jupiter auroral brightness is not dependent solely on the intensity of the solar wind, sometimes brightening and other times dimming as the solar wind increases. In late July and early August 1999, the auroral brightness decreased as the solar wind intensity decreased. In November 1999, however, the aurora appears much brighter on the 10th than on the 8th, even though the solar wind was much weaker on the 10th. Satoh and his colleagues now believe that this behavioral change is due primarily to changes in the direction of the Sun's magnetic field near Jupiter. This conclusion has strong implications on the nature of processes near Jupiter that cause the aurorae.