On ultraviolet emission observed on the flanks of Io
 Two very bright ultraviolet (UV) radiation sources (equatorial spots) which are located on the limb of Io near its equator have been detected in a series of observations with the Hubble space telescope. In this paper, we propose the mechanism that provides the sufficient energy of the equatorial spots to explain their high brightness in the UV wavelength range. According to the proposed model, this UV radiation is generated due to electrons which are formed as a result of additional ionization of the atmosphere in the front part of the satellite. These secondary electrons in crossed electric and magnetic fields are shifted downstream into Io’s flanks. The optical depth of the source increases on the flanks of Io’s atmosphere (from the vantage point of the observer), and we therefore observe the brightest UV radiation in this region, the value of which is in good agreement with the measured values. Also, a reasonable explanation is given for the main observed properties of the UV equatorial spots, such as (1) a correlation between the brightness of the emission and the magnetic longitude of Io and as a result, Io’s distance from the plasma torus centrifugal equator; (2) a correlation between the equatorial spot location and the planetary magnetic field orientation; and (3) the excess of the brightness of anti-Jovian UV source over the brightness of sub-Jovian source.