Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses

Abstract

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5‐10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.

Abundances and Energy Spectra of Corotating Interaction Region Heavy Ions Observed during Solar Cycle 23

“…There is also strong observational evidence for a suprathermal seed source, due to the presence of ions in CIRs that are extremely rare in the bulk solar wind. The first such ion detected in CIRs was singly ionized He, which at 1 AU consists of 10%-20% of the total He (Hilchenbach et al 1999;Chotoo et al 2000;Möbius et al 2002), and at several AU He + dominates the CIR He abundance (Gloeckler et al 1994). During individual CIR events at 1 AU the He + abundance increases with time during the event, presumably due to improving the magnetic connection between 1 AU and increasingly efficient portions of the distant shock (Morris et al 2001).…”

“…It has long been known that there are significant differences between the heavy-ion composition of CIRs and other heliospheric energetic particle populations, such as shock-accelerated or solar energetic particles (SEPs). Furthermore, the presence of singly ionized energetic He in CIRs makes it clear that the seed population was not simply solar wind, since solar wind He + is extremely rare (e.g., Gloeckler et al 1994;Chotoo et al 2000;Möbius et al 2002). Using high-resolution spectrometers on the Advanced Composition Explorer (ACE ) spacecraft, we have surveyed CIR events over the recent solar cycle, measuring the energy spectra and composition with much higher accuracy than previously possible.…”

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

“…Especially, shocks or compression regions like Stream Interaction Regions (SIRs) with a high level of turbulence (cf. Gloeckler et al 1994;Schwadron et al 1996) can efficiently accelerate PUIs above their injection speed. In turn, this means that the He + cutoff shape and location may be significantly altered by the increased abundance of accelerated PUIs, which would contradict the assumption that the PUI cutoff VDF is comparable at all ecliptic longitudes.…”

Acceleration of Low-Energy Ions at Parallel Shocks With a Focused Transport Model

“…Thus, traditional DSA theory is not adequate to describe the acceleration process of these particles. Besides, pickup ions have been observed to be efficiently injected into the acceleration process by the forward-reverse shock pair, the bound of corotating interaction regions (CIRs; Gloeckler et al 1994;Scholer et al 1999). In CIRs, pickup ions have speeds between zero and twice the solar wind speed, and thus part of the pickup ion distribution constitutes a suprathermal particle population.…”