NEUTRAL INTERSTELLAR HELIUM PARAMETERS BASED ON IBEX-Lo OBSERVATIONS AND TEST PARTICLE CALCULATIONS

Abstract

Because of its high ionization potential and weak interaction with hydrogen, Neutral Interstellar Helium is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. This second most abundant species 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 newlyestablished flow direction is ecliptic longitude 79.2 • , latitude −5.1 • , the velocity is ∼ 22.8 kms −1 , 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.

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

“…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. The parameter values obtained from IBEX are tightly correlated with the inflow longitude, and the acceptable parameter ranges form long alleys in the parameter space.…”

“…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 simulations were carried out using a specially adapted version of the Warsaw test particle model (Tarnopolski & Bzowski 2009) that was used by Bzowski et al (2012) for analysis of the IBEX observations reported in Möbius et al (2009a) and Bzowski et al (2012). 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).…”

“…Similarly to the analysis by Bzowski et al (2012), the simulations were carried out in the spacecraft inertial frame. The first step was to calculate the incident NIS He flux on a dense mesh (see Fig.…”

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

“…Since 2008, these neutrals are measured by IBEX amongst others, from which one can derive u LISM and the temperature of the interstellar neutral flow, T ISN , through the hyperbolic trajectory equation (see Bzowski et al 2012;Möbius et al 2012). The inflow vector can also be expressed by the inflow longitude, λ flow ; latitude, β flow ; and speed, V ISN .…”