Heavy coronal ions in the heliosphere

II. Expected fluxes of energetic neutral He atoms from the heliosheath

Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland
e-mail: stangrze@cbk.waw.pl

Received: 25 July 2012
Accepted: 19 October 2012

Abstract

Aims. A model of heliosheath density and energy spectra of α-particles and He⁺ ions carried by the solar wind is developed. Neutralization of heliosheath He ions, mainly by charge exchange (CX) with neutral interstellar H and He atoms, gives rise to ~0.2–~100 keV fluxes of energetic neutral He atoms (He ENA). Such fluxes, if observed, would give information about plasmas in the heliosheath and heliospheric tail.

Methods. Helium ions crossing the termination shock (TS) constitute suprathermal (test) particles convected by (locally also diffusing through) hydrodynamically calculated background plasma flows (three versions of flows are employed). The He ions proceed from the TS towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations of the evolution of α- and He⁺ particle densities and energy spectra include binary interactions with background plasma and interstellar atoms (radiative and dielectronic recombinations, single and double CX, stripping, photoionization and impact ionizations), adiabatic heating (cooling) resulting from flow compression (rarefaction), and Coulomb scattering on background plasma.

Results. Neutralization of suprathermal He ions leads to the emergence of He ENA fluxes with energy spectra modified by the Compton-Getting effect at emission and ENA loss during flight to the Sun. Energy-integrated He ENA intensities are in the range ~0.05–~50 cm^-2 s^-1 sr^-1 depending on spectra at the TS (assumed kappa-distributions), background plasma model, and look direction. The tail/apex intensity ratio varies between ~1.8 and ~800 depending on model assumptions. Energy spectra are broad with maxima in the ~0.2–~3 keV range depending on the look direction and model.

Conclusions. Expected heliosheath He ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft. Data could offer insight into the heliosheath structure and improve understanding of the post-TS solar wind plasmas. HT direction and extent could be assessed.