The density of the Io plasma torus ribbon

We investigate two contradictory determinations of the density of the “ribbon” feature within the Io plasma torus. A value of ∼2000 cm−3 was measured by the Voyager 1 Plasma Science and Planetary Radio Astronomy instruments. Volwerk et al. [1997] recently suggested a value at least 5 times higher, and possibly much greater. Their upward revision is based on rapid EUV intensity variations observed by the Voyager Ultraviolet Spectrometers: Short radiative cooling times are only possible with higher plasma densities. We have modeled emissions from both the Voyager-based torus model and the high-density case suggested by Volwerk, and show that the latter is inconsistent with the intensity and morphology of numerous observations. We suggest that the observations by Volwerk et al. [1997] can be explained by a closer examination of the radiative cooling process and possibly by short-term temperature variations caused by variability of the dawn-dusk electric field in the Jovian magnetosphere.