not known but the presence of its material was detected as high as over the ring plane. The inner boundary of the halo is relatively sharp and located at the radius (), but some material is present further inward to approximately . Thus the width of the halo ring is about . Its shape resembles a thick torus without clear internal structure. In contrast to the main ring, the halo's appearance depends only slightly on the viewing geometry. The halo ring appears brightest in forward-scattered light, in which it was extensively imaged by Galileo. While its surface brightness is much less than that of the main ring, its vertically (perpendicular to the ring plane) integrated photon flux is comparable due to its much larger thickness. Despite a claimed vertical extent of more than , the halo's brightness is strongly concentrated towards the ring plane and follows a power law of the form z−0.6 to z−1.5, where z is altitude over the ring plane. The halo's appearance in the back-scattered light, as observed by Keck and HST, is the same. However its total photon flux is several times lower than that of the main ring and is more strongly concentrated near the ring plane than in the forward-scattered light. The spectral properties of the halo ring are different from the main ring. The flux distribution in the range 0.5–2.5 μm is flatter than in the main ring; the halo is not red and may even be blue. Origin of the halo ring The optical properties of the halo ring can be explained by the hypothesis that it comprises only dust with particle sizes less than 15 μm. Parts of the halo located far from the ring plane may consist of submicrometre dust. This dusty composition explains the much stronger forward-scattering, bluer colors and lack of visible structure in the halo. The dust probably originates in the main ring, a claim supported by the fact that the halo's optical depth is comparable with that of the dust in the main ring. The large thickness of the halo can be attributed to the excitation of orbital inclinations and eccentricities of dust particles by the electromagnetic forces in the Jovian magnetosphere. The outer boundary of the halo ring coincides with location of a strong 3:2 Lorentz resonance. As Poynting–Robertson drag causes particles to slowly drift towards Jupiter, their orbital inclinations are excited while passing