Radial evolution of ejecta characteristics and transient shocks: Ulysses in‐ecliptic observations

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Abstract

During its travel to Jupiter, Ulysses detected 25 ejecta and 32 transient forward shocks (TFS), and in the first 3 months after the Jupiter flyby Ulysses detected 9 ejecta and 5 TFS. Half of these ejecta (17) were associated with TFS. We identified the ejecta using bidirectional streaming of suprathermal electrons accompanied by other plasma cloud signatures. This data set of 34 ejecta at low latitudes (±10°) complements previous statistical studies of ejecta and TFS within 1 AU and allows us to study their heliocentric evolution from 1 to 5.4 AU. We used solar wind plasma data to analyze dynamic aspects of these events. In general, ejecta associated with TFS and ejecta without shocks had similar characteristics. However, ejecta associated with TFS had fronts propagating faster than the ambient solar wind, while ejecta not associated with shocks had fronts propagating at about the same speed as the ambient solar wind. The ejecta radial width did not present a clear statistical tendency to expand with heliocentric distance, and the ejecta mean flux density did not decrease more quickly than the square of the distance from the Sun. These two results might suggest that the rate of expansion of ejecta from 1 to 5 AU is less than the rate of expansion within 1 AU. However, a limitation of this analysis is the ambiguity of comparing ejecta parameters that can vary depending on différent aspects and with heliocentric distance. Only about 40% of the ejecta associated with TFS had leading edges propagating supermagnetosonically with respect to the ambient solar wind. Ejecta with large radial widths were faster than small ejecta. Stronger TFS (higher Mach numbers) were followed by larger ejecta.

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