The helium parameters of the very local interstellar medium as derived from the distribution of He^+^ pickup ions in the solar wind.

Details The helium parameters of the very local interstellar medium as derived from the distribution of He^+^ pickup ions in the solar wind. The helium parameters of the very local interstellar medium as derived from the distribution of He^+^ pickup ions in the solar wind.

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...304..505m&link_type=abstract

Astronomy and Astrophysics, v.304, p.505

Astronomy and Astrophysics

Astrophysics

48

Interplanetary Medium, Ism: General, Abundances, Methods: Data Analysis

Scientific paper

Until very recently the parameters of the local interstellar hydrogen and helium could only be determined from measurements of solar UV light back-scattered by interstellar neutral gas. In this paper the parameters (density, temperature and bulk velocity) characterizing the helium component in the Very Local Inter-Stellar Medium (VLISM) are derived from pickup helium ion distributions obtained with the time-of-flight spectrometer SULEICA on AMPTE/RM in 1985. Because of the relative stability of the Extreme Ultra-Violet (EUV) flux, which is the major contributor to the ionization of the interstellar helium, this solar minimum period provides the ideal conditions for inferences based on measurements in the inner heliosphere. First the time period was investigated for variations of the pickup ion fluxes with interplanetary parameters. For example, radial interplanetary magnetic field conditions showed extremely low fluxes, while during subsequent turns of the field extremely high fluxes were observed compared with the average values. After such non-representative cases were excluded from the analysis, the pickup ion fluxes near and in the focusing cone on the downwind side of the Sun were compared with simulations based on a hot model of the interstellar helium distribution in interplanetary space. A total ionization rate for He of 5.5-6.5x10^-8^s^-1^ was inferred from the level of the 10.7cm solar radio flux during the observations. The best agreement between the data and the model was obtained for a VLISM parameter set which brackets the range N_He{infinity}_=0.0067-0.0087cm^-3^, T_He_=9000-15000K, and v_b_=19-25km/s. These values take into account a correction for the spatial diffusion and convection of the ions after pickup by the solar wind, which leads to a wider cone in the pickup ion distribution compared with that of the neutral gas.

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