A survey of ³He enhancements at 2–20 MeV/nucleon: Ulysses COSPIN/LET

Abstract

We present the results of a survey of enhancements in the ratio of 3He/4 He as measured by the COSPIN/LET instrument on board the Ulysses spacecraft in the energy range 2–20 MeV/n. In the context of this study, all ratios of 3He/4 He above 0.05 are considered to be enhanced compared with the solar system value of ~0.0004. Previous studies have shown that enhanced fluxes of 3He are frequently associated with small, impulsive solar flare events in which the observer is well connected to the flare site. These events also often show enrichments in heavy ion composition compared with standard coronal values. Recent attention has also focused on 3He enrichments as evidence for re-acceleration of remnant particle populations that are present in the inner heliosphere following periods of enhanced solar activity. For this study, we have examined the Ulysses data set from launch (October 1990) up to the present, representing a full solar cycle. The spatial coverage extends from 1–5.4 AU in heliocentric distance, and effectively includes the complete range of heliomagnetic latitudes from pole to pole. We have identified 12 periods of enhanced 3He/4 He, occurring at heliocentric distances out to 5 AU, and heliolatitudes up to 48°. We investigate the characteristics of the events, many of which last for several days, in order to establish possible origins.Key words. Interplanetary physics (energetic particles) – Solar physics, astrophysics and astronomy (energetic particles; flares and mass ejections)