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

Abstract

Context. A recent analysis of IBEX measurements of the neutral interstellar He flux showed that the inflow velocity vector is different from the results of an earlier analysis of observations from the GAS instrument onboard Ulysses. A recently published compilation of published results on the helium inflow direction from the past ∼40 years suggested that the inflow direction may be changing with time. Aims. We reanalyzed the old Ulysses data and reprocessed them to increase the accuracy of the instrument pointing to investigate whether the GAS/Ulysses observations, carried out during almost two solar cycles, support the hypothesis that the direction of the interstellar helium inflow is changing. Methods. We employed a similar analysis method as had been used in the analysis of the IBEX data. We sought a parameter set that minimizes the reduced χ 2 , 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 that precisely reproduces the observation conditions. We also propose a supplementary method of constraining the parameters based on cross-correlations of parameters obtained from an analysis of carefully selected subsets of data. Results. We find that the ecliptic longitude and speed of interstellar He agree very well with the values reported in the original GAS analysis. We find, however, that the temperature is markedly higher. The three-seasons best-fit parameter set is λ = 255.3 • , β = 6 • (J2000), v = 26.0 km s −1 , and T = 7500 K. We do not find evidence that these parameters are varying with time, but their uncertainty range is wider than originally reported. Conclusions. The originally derived parameters of interstellar He from direct sampling on GAS/Ulysses agree well with those currently derived, except for the temperature, which seems to be appreciably higher; this agrees well with interstellar absorption line results. While the results of our analysis agree marginally with the previously reported results from IBEX, the most likely values from the two analyses differ for reasons that are still not understood.

DETERMINATION OF INTERSTELLAR He PARAMETERS USING FIVE YEARS OF DATA FROM THE IBEX : BEYOND CLOSED FORM APPROXIMATIONS

“…Our second analysis includes all of the data from 2009 through 2013 excluding outliers (outside 3.5 standard deviation). Results from both 2 c minima are listed in the last two rows of Table 4 along with results from previous ISN He studies including Ulysses data first analyzed by Witte et al (2004) and re-analyzed by Bzowski et al (2014) and Wood et al (2015). Additionally, we compare these results along the parameter tube in Figure 7.…”

“…T ISN (kK) Spacecraft Witte et al (2004) 75.4 ± 0.5 26.3 ± 0.4 −5.2 ± 0.2 6.30 ± 0.34 Ulysses Bzowski et al (2014) 75.3 + 1.2(−1.1) 26.0 + 1.0(−1.5) −6.0 ± 1.0 7.5 + 1.5(−2.0) Ulysses Wood et al (2015) 75. Table 1, the total uncertainties lie along the parameter tube and are therefore dependent on one another.…”

INTERSTELLAR NEUTRAL HELIUM IN THE HELIOSPHERE FROM IBEX OBSERVATIONS. II. THE WARSAW TEST PARTICLE MODEL (WTPM)

“…nWTPM is a heavy-duty version for mass-scale calculations, needed to fit the model parameters to the data, and includes fully time-and latitudedependent ionization losses. nWTPM is a strongly optimized and refined version of the WTPM model used by Bzowski et al (2012Bzowski et al ( , 2013aBzowski et al ( , 2014, Kubiak et al (2013Kubiak et al ( , 2014, Rodríguez Moreno et al (2013Moreno et al ( , 2014, Park et al (2014), andMcComas et al (2015a) in their analyses of various species of interstellar gas in the heliosphere, observed by IBEX or Ulysses. aWTPM was used by Sokół et al (2015) and Galli et al (2015) in the search for the fall peak in ISN He and discussion of the expected low-level "haze" in the sky due to extended wings of the Warm Breeze and ISN He populations.…”

“…Subsequently, the model was tailored to accommodate ISN He observed by IBEX (Bzowski et al 2012). It was also used by Bzowski et al (2014) to re-analyze observations from GAS/Ulysses, including the first analysis of the data from the last Ulysses orbit in 2007, which had previously not been analyzed. This analysis brought a flow vector similar to the original analysis by Witte (2004), but a temperature higher by at least ∼1000 K. It was also used by Bzowski et al (2013a) and Park et al (2014) to analyze the abundance of Ne/O ratio in the LIC based on IBEX-Lo measurements, and by Kubiak et al (2014) to discover the additional ISN He population detected by IBEX-Lo dubbed the Warm Breeze, which is very likely the secondary heliospheric population of ISN He.…”

Heliospheric Structure as Revealed by the 3–88 keV H ENA Spectra

“…This assumption has been commonly made in the heliospheric physics. The temperature and flow velocity of the VLISM we used are robust against independent analyses of observations from Ulysses (Bzowski et al 2014;Wood et al 2015) and the inflow direction obtained from pickup ion observations (Gloeckler et al 2004;Möbius et al 2015a). The density of interstellar neutral H at the upwind point of the termination shock was obtained based on two independent estimates: (1) the magnitude of slowdown of the SW expansion speed due to mass-loading by charge exchange with interstellar neutral H, observed in situ by Voyager 2 , and (2) in situ measurement by Ulysses of the production rate of pickup ions at the boundary of the density cavity of interstellar H .…”