4 Suprathermal ions of solar and interstellar origin
As mentioned in the previous section, two species of energetic ions are commonly thought to be associated with solar energetic particle events: 1) solar wind ions, such as doubly charged 4He++, accelerated at CME-driven shocks, and 2) ions preferentially accelerated in impulsive flares such as 3He++ ions.
Advances have been made in understanding the relative contribution of each species to the energetic particle fluxes observed at CME-driven shocks (e.g. Mason et al. [1999], Tylka et al. [2001]).
In this work, it has been found that interstellar pick-up He+ are efficiently accelerated at CME-driven shocks [Bamert et al., 2002] as well. The May 1998 events were the first events analyzed in the suprathermal energy range, where He+ ions of interstellar origin have been discovered. A strong enhancement of the He+/He++ ratio in the upstream region of the CME-driven shock on April 30, 1998, was observed. The ratio depended strongly on the energy-per-mass ratio. It was higher at lower energies. Two days later, on May 2-3, 1998, measurements showed a dramatic increase of bulk solar wind He+ [Skoug et al., 1999, Gloeckler et al., 1999]. No enhancement of the He+/He++ ratio during the respective time period was observed with STOF. This is a strong indication that suprathermal He+ particles are not accelerated out of the solar wind, but they are pick-up ions of interstellar origin. Enhancements of suprathermal He+ of interstellar origin are also observed during the April 9--12, 2001, CMEs [Bamert et al., 2004b]. In particular. differences in the acceleration mechanisms of pick-up ions and solar wind He ions are evident. While solar wind ions appear to be mainly pre-accelerated - or at least present - in the turbulences downstream of the shock (see also Kallenbach et al. [2004]), the flux of interstellar 4He+ pick-up ions is particularly enhanced upstream of the shock.
The phenomenon of preferential acceleration of interstellar pick-up ions is also known from the observations of the He+/He++ ratio in suprathermal particles associated with corotating interaction regions at 1 AU [Hilchenbach et al., 1999, Chotoo et al., 2000] and at 5 AU [Gloeckler et al.,1994]
Detailed understanding of the injection and acceleration of pick-up ions at CME-driven shocks has strong implications for the study of the acceleration of Anomalous Cosmic Rays (ACRs) at the termination shock, because ACRs are thought to be accelerated pick-up ions (e.g. Fisk et al., [1974]).
The additional signature in the He spectrum associated with the April 9-12, 2001, events is identified as 3He by the mass-resolving HSTOF subsystem [Bamert et al., 2004b]. As shown in Figure 4 the 3He spectrum matches a model on stochastic acceleration in electron firehose waves in the source plasma of impulsive flares [Paesold et al., 2003] and subsequent transport through interplanetary space.
Preliminary results are available for the April 2-5, 2001, event. We give an overview in Figure 5. There are significant differences between the spectra and the spatial distribution of helium ions of different origin. At the onset of the event the 3He++ and 4He++ spectra show fluxes that increase with energy, presumably due to propagation effects. The 4He+ spectra are almost flat.
During and after the passage of the shock the 4He++ spectra have decreasing flux with increasing energy, while the 3He++ spectra are rather flat.
The 4He+ spectra are flat at the shock, and slightly decreasing in the downstream region. This suggests that 3He++ undergoes a different acceleration process than 4He++ or 4He+.
At the onset of the event, the 3He++/4He++ ratio is well above 1. With decreasing distance to the shock it successively drops to values below 0.1 and does not change very much during the shock passage.The 3He++/4He++ ratio depends on the energy per mass, and is higher for higher energies (see Figure 4).
The fluxes of all ions peak right at the shock, but their spatial variations differ. At the onset of the event 3He++ is more abundant than 4He++. The 3He++ flux does not change considerably far upstream of the shock. This indicates that 3He++ is already present in the upstream region, presumably accelerated in an impulsive flare, as a strong suprathermal population.
However, 3He++ appears to be additionally re-accelerated at the shock. 4He+ rises more rapidly than 3He++ and also more than 4He++ with decreasing distance to the shock. After the passage of the shock 4He+ drops by a larger factor than the two other ion species.
The spatial and temporal evolution of the three He species will be further investigated in an extended energy range.
