Icarus 160, 336–349 (2002) doi:10.1006/icar.2002.6977 Monte Carlo Radiative Transfer Modeling of Lightning Observed in Galileo Images of Jupiter U. A. Dyudina NASA/Goddard Institute for Space Studies, 2880 Broadway, New York, New York 10025 E-mail: ulyana@gps.caltech.edu A. P. Ingersoll 150-21, Geological and Planetary Sciences, Caltech, Pasadena, California 91125 A. R. Vasavada Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567 S. P. Ewald 150-21, Geological and Planetary Sciences, Caltech, Pasadena, California 91125 and the Galileo SSI Team Received October 25, 2001; revised August 10, 2002 We study lightning on Jupiter and the clouds illuminated by the lightning using images taken by the Galileo orbiter. The Galileo images have a resolution of ∼25 km/pixel and are able to resolve the shape of single lightning spots, which have half widths (radii) at half the maximum intensity in the range 45–80 km. We com- pare the shape and width of lightning flashes in the images with simulated flashes produced by our 3D Monte Carlo light-scattering model. The model calculates Monte Carlo scattering of photons in a 3D opacity distribution. During each scattering event, light is partially absorbed. The new direction of the photon after scattering is chosen according to a Henyey–Greenstein phase function. An image from each direction is produced by accumulating photons emerging from the cloud in a small range (bins) of emission angles. The light source is modeled either as a point or a vertical line. A plane-parallel cloud layer does not always fit the data. In some cases the cloud over the light source appears to resemble cumu- lus clouds on Earth. Lightning is estimated to occur at least as deep as the bottom of the expected water cloud. For the six flashes studied, we find that the clouds above the lightning are optically thick (τ> 5). Jovian flashes are more regular and circular than the largest terrestrial flashes observed from space. On Jupiter there is nothing equivalent to the 30–40-km horizontal flashes that are seen on Earth. c  2002 Elsevier Science (USA) Key Words: Jupiter, atmosphere; radiative transfer; image pro- cessing; atmospheres, structure. 1. INTRODUCTION Lightning in the atmosphere of Jupiter acts as a natural probe of the cloud structure. The light scattered from the lightning penetrates the clouds, gets scattered, and brings us informa- tion about the shape, size, and opacity of the clouds, and about the lightning itself. The observed optical energy of lightning helps to determine the total energy associated with lightning on Jupiter (Borucki and Williams 1986, Borucki and Magalh˜ aes 1992, Little et al. 1999). The presence and high energy of the lightning suggests strong moist convection (Yair et al. 1995), which may play an important role in the large-scale dynamics of the planet (Gierasch et al. 2000, Ingersoll et al. 2000). In this study we use Galileo images showing diffuse spots of lightning seen through optically thick clouds. The spatial resolution is about 25 km/pixel, which is better than the 37, 100, and 60 km/pixel resolution of Voyager 1, Voyager 2, and Cassini, respectively. The light diffuses up from the source to the cloud tops, and we model the lightning spot size to derive the lightning depth. Because the lightning spots observed by Galileo are spatially well resolved, we can accurately determine the shape of the brightness falloff in the lightning spots. Our model combines two features: it reproduces images of lightning at various emission angles [previously modeled by Borucki and Williams (1986) and Little et al. (1999)] and it can model three- dimensional, non-plane-parallel clouds [previously modeled by 336 0019-1035/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.