IMAGING JUPITER AURORA AT VISIBLE WAVELENGTHS

On November 9, 1996 and again on April 2, 1997, the Galileo spacecraft 's Solid State Imaging (SSI) camera targeted the northern auroral regi on of Jupiter, These observations represent (i) the first spatially re solved images of the jovian auroral oval either at visible wavelengths or on the nightside of the planet, (ii) the first image at visible wa velengths of an auroral footprint of the Io Flux Tube (IFT), (iii) the first unambiguous detection at visible wavelengths of auroral emissio n on the jovian limb, and (iv) the first images of the aurora with spa tial resolution below 100 km per pixel (46 and 35 km, respectively). R elative to many prior expectations, the visible aurora is (i) lower in altitude, (ii) associated with magnetic field lines that cross the eq uator closer to the planet, and (iii) more variable in time and space. The 1996 images used a clear (broadband) filter, while the 1997 image s used both the clear filter and five narrower filters over wavelength s ranging from violet to 968 nm. The filtered images imply that the vi sible auroral emission contains atomic hydrogen lines, although there is also a continuum component. We were able to position the aurora in three-dimensional space and found the limb emission to be similar to 2 40 km above the surface of a standard (P approximate to 1 bar) referen ce ellipsoid. Our most accurate analysis of the equatormost part of th e oval placed it at 54.5 degrees planetocentric latitude and 168 degre es west longitude. Combined with the latest magnetic field models, our results imply that the particles that cause the aurora originate in J upiter's equatorial plane similar to 13 R-J from the center of the pla net. The oval was brighter and wider in the 1996 images than in the 19 97 images. The broadband radiance of a typical place on the oval as se en directly overhead varied from similar to 80 kR in 1997 to similar t o 300 kR in 1996. Our estimates of the full width of the oval varied f rom under 500 km to over 8000 km, partly depending on the signal-to-no ise ratio of the image. The radiated power per unit length along the o val ranged from similar to 60 to similar to 700 W/m, with the associat ed radiated power from the entire oval varying from similar to 10(9) t o similar to 9 x 10(10) W, Appreciable auroral emission also occurred both north and south of the main oval. One image contains the northern footprint of the IFT, which appears as a central ellipse with a tail of emission that lies downstream with respect to the plasma flow past Io. The central ellipse is similar to 1200 km downstream by similar to 500 km cross stream. The IFT is comparable in brightness to the nearb y auroral oval (similar to 250 kR) and has a total radiated power of s imilar to 3 x 10(8) W. (C) 1998 Academic Press.