The IBEX-Lo Sensor

2012

A&A

Aims. A model of heliosheath density and energy spectra of α-particles and He + ions carried by the solar wind is developed. Neutralization of heliosheath He ions, mainly by charge exchange (CX) with neutral interstellar H and He atoms, gives rise to ∼0.2-∼100 keV fluxes of energetic neutral He atoms (He ENA). Such fluxes, if observed, would give information about plasmas in the heliosheath and heliospheric tail. Methods. Helium ions crossing the termination shock (TS) constitute suprathermal (test) particles convected by (locally also diffusing through) hydrodynamically calculated background plasma flows (three versions of flows are employed). The He ions proceed from the TS towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations of the evolution of α-and He + particle densities and energy spectra include binary interactions with background plasma and interstellar atoms (radiative and dielectronic recombinations, single and double CX, stripping, photoionization and impact ionizations), adiabatic heating (cooling) resulting from flow compression (rarefaction), and Coulomb scattering on background plasma. Results. Neutralization of suprathermal He ions leads to the emergence of He ENA fluxes with energy spectra modified by the Compton-Getting effect at emission and ENA loss during flight to the Sun. Energy-integrated He ENA intensities are in the range ∼0.05-∼50 cm −2 s −1 sr −1 depending on spectra at the TS (assumed kappa-distributions), background plasma model, and look direction. The tail/apex intensity ratio varies between ∼1.8 and ∼800 depending on model assumptions. Energy spectra are broad with maxima in the ∼0.2-∼3 keV range depending on the look direction and model. Conclusions. Expected heliosheath He ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft. Data could offer insight into the heliosheath structure and improve understanding of the post-TS solar wind plasmas. HT direction and extent could be assessed.

Section: Final Discussion and Conclusionmentioning

confidence: 99%

Heavy coronal ions in the heliosphere

Grzędzielski

Swaczyna

2012

A&A

“…The temperature and vector of inflow velocity of neutral interstellar helium (NIS He) on the heliosphere were measured by two direct-sampling experiments, GAS onboard of Ulysses (Witte et al 1992(Witte et al , 1993, and IBEX-Lo (Fuselier et al 2009;Möbius et al 2009b), which returned different results. Based on measurements carried out from mid-1990s to 2001, GAS obtained a J2000 longitude λ = 255.4 • ± 0.5 • , a latitude β = 5.2 • ± 0.2 • , a speed v = 26.3 ± 0.4 km s −1 , and a temperature T = 6300 ± 340 K (Witte 2004; see also Witte et al 1996Witte et al , 2004, while IBEX found λ = 259.2 • or 259.0 • , with an uncertainty of ∼3 • , and β = 5.1 • or 4.9 • , v = 22.8 or 23.5 km s −1 , T = 6200 or 5300−9000 K by Bzowski et al (2012) and by Möbius et al (2012), respectively from measurements carried out in 2009 and 2010.…”

Section: Motivationmentioning

confidence: 99%

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

Kubiak

Hlond

et al. 2014

A&A

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.

“…Such a method has recently become available with the launch of the Interstellar Boundary Explorer (IBEX) satellite McComas et al (2009), which owing to its time of flight (TOF) mass spectrometer IBEX-Lo (Fuselier et al 2009), is able to sample Appendices are available in electronic form at http://www.aanda.org neutral interstellar (NIS) H, He, Ne, and O at Earth's orbit (Möbius et al 2009;Bochsler et al 2012).…”

Section: Introductionmentioning

confidence: 99%

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

Sokół

Kubiak

et al. 2013

A&A

Self Cite

111

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.