1,328 research outputs found
Modulation of neutral interstellar He, Ne, O in the heliosphere. Survival probabilities and abundances at IBEX
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 and processes in
the heliosphere interface.}{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.}{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 develop 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 calculate densities of these species along the Earth's
orbit and simulate the fluxes of NIS species as observed by IBEX. We study
pairwise ratios of survival probabilities, densities and fluxes of NIS species
at IBEX to calculate correction factors for inferring the abundances at
TS.}{The analytic method to calculate 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. Electron impact ionization is surprisingly
important for NIS O. Interpreting the IBEX observations using the time
dependent model yields the LIC Ne/O abundance of . The uncertainty
is mostly due to uncertainties in the ionization rates and in the NIS gas flow
vector.}{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.Comment: Astronomy & Astrophysics, in press. Language and editing corrections
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Neutral interstellar He parameters in front of the heliosphere 1994--2007
Analysis of IBEX measurements of neutral interstellar He flux brought the
inflow velocity vector different from the results of earlier analysis of
observations from GAS/Ulysses. Recapitulation of results on the helium inflow
direction from the past ~40 years suggested that the inflow direction may be
changing with time. We reanalyze the old Ulysses data and reprocess them to
increase the accuracy of the instrument pointing to investigate if the GAS
observations support the hypothesis that the interstellar helium inflow
direction is changing. We employ a similar analysis method as in the analysis
of the IBEX data. We seek a parameter set that minimizes reduced chi-squared,
using the Warsaw Test Particle Model for the interstellar He flux at Ulysses
with a state of the art model of neutral He ionization in the heliosphere, and
precisely reproducing the observation conditions. We also propose a
supplementary method of constraining the parameters based on cross-correlations
of parameters obtained from analysis of carefully selected subsets of data. We
find that the ecliptic longitude and speed of interstellar He are in a very
good agreement with the values reported in the original GAS analysis. We find,
however, that the temperature is markedly higher. The 3-seasons optimum
parameter set is lambda = 255.3, beta = 6, v = 26.0 km/s, T = 7500 K. We find
no evidence that it is varying with time, but the uncertainty range is larger
than originally reported. The originally-derived parameters of interstellar He
from GAS are in good agreement with presently derived, except for the
temperature, which seems to be appreciably higher, in good agreement with
interstellar absorption line results. While the results of the present analysis
are in marginal agreement with the earlier reported results from IBEX, the most
likely values from the two analyses differ for reasons that are still not
understood.Comment: submitted for publication in Astronomy & Astrophysic
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