7 research outputs found
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
Neutral interstellar helium parameters based on IBEX-Lo observations and test particle calculations
Neutral Interstellar Helium (NISHe) is almost unaffected at the heliospheric
interface with the interstellar medium and freely enters the solar system. It
provides some of the best information on the characteristics of the
interstellar gas in the Local Interstellar Cloud. The Interstellar Boundary
Explorer (IBEX) is the second mission to directly detect NISHe. We present a
comparison between recent IBEX NISHe observations and simulations carried out
using a well-tested quantitative simulation code. Simulation and observation
results compare well for times when measured fluxes are dominated by NISHe (and
contributions from other species are small). Differences between simulations
and observations indicate a previously undetected secondary population of
neutral helium, likely produced by interaction of interstellar helium with
plasma in the outer heliosheath. Interstellar neutral parameters are
statistically different from previous in situ results obtained mostly from the
GAS/Ulysses experiment, but they do agree with the local interstellar flow
vector obtained from studies of interstellar absorption: the newly-established
flow direction is ecliptic longitude 79.2 deg, latitude -5.1 deg, the velocity
is \sim 22.8 km/s, and the temperature is 6200 K. These new results imply a
markedly lower absolute velocity of the gas and thus significantly lower
dynamic pressure on the boundaries of the heliosphere and different orientation
of the Hydrogen Deflection Plane compared to prior results from Ulysses. A
different orientation of this plane also suggests a new geometry of the
interstellar magnetic field and the lower dynamic pressure calls for a
compensation by other components of the pressure balance, most likely a higher
density of interstellar plasma and strength of interstellar magnetic field.Comment: Data from IBEX-Lo on Interstellar Boundary Explorer analyzed; 40
pages, 24 figures, 2 table