Heliolatitude dependence of interplanetary heavy ions

Abstract

In its rapid traversal of 80° of heliolatitude from the southern solar pole to the equator, the HI-SCALE instrument on the Ulysses spacecraft measured the composition of ions accelerated at the co-rotating interaction regions (CIRs) that it crossed. We compare ion composition measurements as a function of heliolatitude during this traversal to those made on the ascent to the southern solar pole. The C/O ratio (∼1) in the fast traverse at ∼1.4 AU is found to be about twice the value (∼0.5) found previously in the ecliptic as well as at intermediate latitudes beyond ∼2 AU. For the CIR-associated events in the southerly high speed solar wind stream, the ion intensity enhancements are found to decrease monotonically with increasing heliolatitude once Ulysses crossed ∼20°S. Thus, the optimum region for CIR-related particle acceleration in solar minimum conditions occurs near the boundary of the current sheet at distances between the orbits of Mars and Jupiter. These results, together with HI-SCALE results on the heliolatitude dependence of anomalous oxygen suggest that the lower value of the C/O ratio arises from the enhanced (relative to 1 AU) abundance of low energy anomalous O at heliolatitudes beyond the current sheet and distances beyond ∼2 AU.