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 similar to 2 000 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 upwa rd revision is based on rapid EW intensity variations observed by the Voyager Ultraviolet Spectrometers: Short radiative cooling times are o nly possible with higher plasma densities. We have modeled emissions f rom both the Voyager-based torus model and the high-density case sugge sted by Volwerk, and show that the latter is inconsistent with the int ensity and morphology of numerous observations. We suggest that the ob servations by Volwerk et al. [1997] can be explained by a closer exami nation of the radiative cooling process and possibly by short-term tem perature variations caused by variability of the dawn-dusk electric fi eld in the Jovian magnetosphere.