Spectral Properties and Abundances of Suprathermal Heavy Ions in Compression Regions near 1 au

Abstract

We have analyzed spectral properties and abundances of ∼0.02–3.0 MeV nucleon −1 suprathermal (ST) H–Fe ions in 41 stream interaction regions (SIRs) near 1 au observed by Wind and ACE spacecraft from 1995 January through 2008 December. We find that, (i) the event-averaged spectral index is γ  ∼ 2.44, with a standard deviation ( σ ) of 0.67, (ii) γ ’s are poorly correlated with the magnetic compression ratios, and 17% of the events group around γ  ∼ 1.5, (iii) γ ’s for both O and Fe at ∼0.02–0.09 MeV nucleon −1 and 0.09–0.3 MeV nucleon −1 are correlated, but do not exhibit any systematic steepening or flattening as a function of energy, (iv) the ST heavy ion abundance ratios remain constant with increasing energy, implying that the spectral rollovers, defined by the e-folding energy E 0 , are independent of the ion’s mass per charge ( M / Q ), and (v) SIR ST abundances are similar to the corresponding solar wind values, and do not exhibit any systematic behavior when plotted versus the ion’s M / Q or first ionization potential. The above results pose challenges for (1) particle acceleration models that invoke either a corotating interaction region or SIR shocks between ∼3 and 5 au, (2) particle transport models that predict M / Q -dependent spectral rollovers due to interplanetary turbulence effects, and (3) the notion that SIR ST ions originate directly from the bulk solar wind. Instead, we suggest that the SIR ST ions are accelerated out of a pool of material that includes particles accelerated in solar energetic particle events and processed or heated solar wind ions.