Modulation of neutral interstellar He, Ne, O in the heliosphere. Survival probabilities and abundances at IBEX

Abstract

Context. Direct sampling of neutral interstellar (NIS) atoms by the Interstellar Boundary Explorer (IBEX) can potentially provide a complementary method for studying element abundances in the Local Interstellar Cloud (LIC) and processes in the heliosphere interface. Aims. We set the stage for abundance-aimed in-depth analysis of measurements of NIS He, Ne, and O by IBEX and determine systematic differences between abundances derived from various calculation methods and their uncertainties.Methods. Using a model of ionization rates of the NIS species in the heliosphere, based on independent measurements of the solar wind and solar EUV radiation, we developed a time-dependent method of calculating the survival probabilities of NIS atoms from the termination shock (TS) of the solar wind to IBEX. With them, we calculated densities of these species along the Earth's orbit and simulated the fluxes of NIS species as observed by IBEX. We studied pairwise ratios of survival probabilities, densities, and fluxes of NIS species at IBEX to calculate correction factors for inferring the abundances at TS. Results. The analytic method of calculating the survival probabilities gives acceptable results only for He and Ne during low solar activity. For the remaining portions of the solar cycle, and at all times for O, a fully time-dependent model should be used. Electronimpact ionization is surprisingly important for NIS O. Interpreting the IBEX observations using the time-dependent model yields the LIC Ne/O abundance of 0.16 ± 40%. The uncertainty is mostly due to uncertainties in the ionization rates and in the NIS gas flow vector. Conclusions. The Ne/He, O/He, and Ne/O ratios for survival probabilities, local densities, and fluxes scaled to TS systematically differ and thus an analysis based only on survival probabilities or densities is not recommended, except the Ne/O abundance for observations at low solar activity.

Neutral interstellar He parameters in front of the heliosphere 1994–2007

“…In addition to the data analyzed originally by Witte (2004), we process the data from the last Ulysses season in 2007, less than two years before the IBEX launch. The analysis is carried out in a similar way to the analysis presented by Bzowski et al (2012), using the same numerical code, that was adapted for the needs of GAS analysis, and the same model of He ionization A&A 569, A8 (2014) (right-hand scale) illustrates the helium photoionization rate used in this study, adopted from Bzowski et al (2013a). The two vertical bars mark the ionization rates (with uncertainties) obtained by Witte (2004) in his original analysis of the GAS observations.…”

“…The boundaries of these arcs were carefully selected so that the heliolatitude ranges match between the orbits. (Bzowski et al 2013a). Other assumptions are also identical, including the important assumption that the whole signal can be approximated by a single, homogeneously distributed population of interstellar gas given by the Maxwell-Boltzmann distribution in front of the heliosphere.…”

“…The model is a kinetic test-particle code equivalent to the hot-model of interstellar gas, with a finite temperature and time-and latitude-dependent ionization rate (for details of the hot-model of neutral interstellar gas in the heliosphere see Thomas (1978) or Fahr (1978), for details of the effects of time dependence in the ionization rate on the distribution of interstellar He in the heliosphere (see Ruciński et al 2003). In the simulations, we used a time-and heliolatitude-dependent model of photoionization and electron-impact ionization rates based on actual observations of the solar EUV radiation and solar wind (Bzowski et al 2013a). The actually used photoionization rate for the solar equator is shown in Fig.…”

“…The photoionization rate is quasi-elliptic as a function of heliolatitude, with polar rates lower by approximately 20% than the equatorial rates. In addition to photoionization, we took into account charge exchange with solar wind protons and alpha particles (negligible most of the time) and electron-impact ionization according to Bzowski et al (2013a), which vary with heliolatitude proportionally to the solar wind density (Sokół et al 2013).…”

Low Energy Neutral Atoms From the Heliosheath

“…Uncertainties in the flux at 100 AU are a combination of flux uncertainties at 1 AU and uncertainties in the survival. For a given survival probability, the uncertainty is actually asymmetric with larger negative uncertainty than positive uncertainty (Bzowski et al 2013a). In addition, the flux at about 0.1 keV has a larger negative uncertainty on the basis of the assessment of the background between 0.1 and 0.2 keV in the previous section.…”