The suprathermal seed population for corotating interaction region ions at 1 AU deduced from composition and spectra of H+, He++, and He+ observed on Wind

Academic Article


  • We have measured H+, He++, and He+ distribution functions over the solar wind through the suprathermal energy range during two corotating interaction region (CIR) events observed by the STICS, MASS, and STEP instruments on board the Wind spacecraft at 1 AU during April and May 1995. The major properties we find are as follows: In the suprathermal energy range (∼10–500 keV/nucleon), the particle intensities peak inside the CIR itself, in the compressed and decelerated fast solar wind, in contrast to the situation at MeV energies, where the peak intensities are observed outside the CIR in the fast solar wind. The distribution functions of solar wind H+ and He++ change smoothly from the core at solar wind speeds to a power law or exponential form at higher energies, with no turnover observed at intermediate energies. CIR He+ is observed with an abundance ratio He+/He++ ∼ 16–17%, orders of magnitude higher than that in the bulk solar wind but nevertheless lower than that observed in CIRs at 4.5 AU. The H+, He++, and He+ spectra have similar slopes above speeds of ∼2.5–3 times the solar wind speed (Vsw) in the spacecraft frame. The ion speed at which the CIR He++/H+ ratio changes from typical solar wind values of 4–5% to the higher (>10%) value typical of CIRs is ∼1.5–1.7 Vsw, measured in the space‐craft reference frame. Analyzing these observations in the context of previous global observations and simple models of CIR acceleration and transport [Fisk and Lee, 1980], we conclude the following: (1) Suprathermal CIR ions at 1 AU originated close (within ∼0.5 AU) to the point of observation, not in the outer heliosphere; (2) the injection/acceleration mechanism is not especially sensitive to charge‐to‐mass ratio over the range 0.25–1.0; (3) since the particles are locally accelerated, the low‐energy ion populations we observe contain the seed population; (4) the bulk solar wind itself is not the source of the energetic ions; rather, the source is in the suprathermal tail, with an injection threshold in the spacecraft frame of ∼1.8–2.5 times the solar wind speed; and (5) in at least one of these CIRs, suprathermal particle acceleration is not shock associated and must therefore be associated with a statistical mechanism or compression in the solar wind.
  • Authors

  • Chotoo, K
  • Schwadron, Nathan
  • Mason, GM
  • Zurbuchen, TH
  • Gloeckler, G
  • Posner, A
  • Fisk, LA
  • Galvin, AB
  • Hamilton, DC
  • Collier, MR
  • Status

    Publication Date

  • October 1, 2000
  • Published In

    Digital Object Identifier (doi)

    Start Page

  • 23107
  • End Page

  • 23122
  • Volume

  • 105
  • Issue

  • A10