Kinetic parameters of interstellar neutral helium

Abstract

Abstract. The GAS-instrument onboard the space probe Ulysses is designed to measure the local angular distribution of the flow of interstellar neutral He-atoms within ≈3 AU distance from the sun. It allows to infer the kinetic parameters (velocity vector, temperature and density) of these particles outside the heliosphere ("at infinity"). During three observational periods, 1990/1991, shortly after launch and during the two fast latitude scans of Ulysses, from 9/1994 to 8/1996 and from 9/2000 to 8/2002, more than 300 measurements of the distributions were obtained and analyzed in detail. Known issues that relate to the pointing accuracy of the detector and to the efficiency calibration along with their impact on the results are addressed in this paper. The average values, derived from these observations and their refined analysis, are the bulk speed (v He∞ = 26.3 ± 0.4 km s −1 ), the flow direction (ecliptic longitude λ ∞ = 74.7and temperature (T He∞ = 6300 K ± 340 K). From 1990 to 2002, covering a complete solar cycle, no significant temporal variations of these parameters were observed, nor variations with solar latitude. In contrast, in the density n He∞ , derived from the series of local observations along the Ulysses-orbit, substantial apparent variations were seen. After a first attempt to explain these by an inadequate energy calibration of the efficiency they are now interpreted as variations in the loss processes (predominantly photo-ionization), which the neutral gas experiences along its trajectory to the observer. While the temporal variations of the ionization rate were taken into account using the measured solar EUV irradiance (from CELIAS/SEM on SOHO), the residual latitudinal variation has been attributed to a dependence of the solar irradiance on latitude, which thus far has not been accessible to direct observations. As a result of a simple model that includes these effects a density n He∞ = 0.015 ± 0.003 cm −3 has been deduced, which is in good agreement with an independent result from pickup ion observations.

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

“…First, remote measurements of neutral hydrogen (Bertaux & Blamont 1971) and helium (Weller & Meier 1974) were performed using backscattered solar UV observations. Later on, in situ measurements using pickup ions (PUIs) (Möbius et al 1995) and direct neutral particle observations (Witte 2004) complemented the previous studies. A more extensive summary of the history of the investigation of the interstellar flow can be found in Möbius et al (2004).…”

The Galactic Environment of the Sun: Interstellar Material Inside and Outside of the Heliosphere

“…Interstellar helium has been observed in the heliosphere in four different ways, through fluorescence of solar 584 Å emissions (Weller and Meier 1974;, direct particle counting by the GAS detector on Ulysses (Witte 2004), as PUIs formed from ionized interstellar He convected radially outwards by the solar wind (Sect. 2.2, Möbius et al 1985;Geiss et al 1995;Gloeckler et al 2000), and as ACRs generated from accelerated PUIs (Sect.…”

“…Since He o passes almost unimpeded through the outer heliosphere, these data give a CHISM velocity of −26.3 ± 0.5 km s −1 , a temperature of 6306 ± 390 K, and He o density n(He • ) = 0.0148 ± 0.0020 cm −3 ). The downwind direction for interstellar He o determined from Ulysses data and given in B1950 coordinates by Witte (2004) (Weller and Meier 1974). The more precise He o and H • (Lallement et al 2005) data now available indicate an offset angle of ∼ 4.9 • ± 1.0 • (Frisch 2007(Frisch , 2008a.…”

UV Capabilities to Probe the Formation of Planetary Systems: From the ISM to Planets

“…Although still controversial, it is generally accepted that the Earth's original atmosphere was accumulated simultaneously with the planet's formation. However, the heating of the atmosphere by the young Sun's UV and X-ray flux led to the hydrodynamical escape of this primary atmosphere (as is observed on HD 209458b, Vidal-Madjar et al, 2003, 2004. Tectonics, volcanism and the planet out-gasing then formed the secondary atmosphere in which we now live.…”

“…The maximum column density through the LIC is log N (H I) = 18.3, the LIC's maximum dimension is about 6.8 pc, and its mass is about 0.32 M . The main evidence for the Sun being located inside the LIC is that neutral helium flowing into the heliosphere, which is not influenced by the solar wind, has the same temperature and flow vector as the LIC (Bertin et al, 1993;Witte, 2004). Slavin and Frisch (2002) computed the ionisation of many elements in the LIC taking into account UV and EUV radiation from the most important ionising source, the star CMa, hot white dwarfs and other stars, the diffuse UV background, and the estimated radiation from the putative conductive boundary between the warm clouds and the hot gas of the Local Bubble.…”