Solar Wind Helium and Neon from Metallic Glass Flown on Genesis – Preliminary Bulk and Velocity- Dependent Data

Introduction: We present noble gas data from the metallic glass (BMG) [1] flown on Genesis [2]. This target sampled the bulk solar wind (SW) at all velocities during the entire 887 days collection period of Genesis. The main motivation for selecting the BMG has been to look for the putative solar energetic particle (SEP) component reported to be present in lunar and asteroidal regolith samples [3]. The BMG is ideal for a depth and thus energy dependent gas release by closed-system stepwise etching (CSSE) [4] to distinguish the SEP component from the lower energetic SW. Here we report He and Ne data measured up to date. Experimental: We have analysed He and Ne isotopes in five BMG aliquots with three different extraction techniques. For total gas release three pieces of 7 to 9 mm each were melted by pyrolysis. Additionally, one sample was extracted with an UV-eximer laser (λ = 248 nm) at seven spots of 0.006 to 0.033 mm. The released He and Ne was analysed at ETH Zürich with an ultra-sensitive mass spectrometer equipped with a compressor source [5]. Finally, to determine the depth distribution of He and Ne one sample of 11 mm was extracted by CSSE in 9 major steps with HNO3 as the etching agent. Results from a second CSSE experiment will be presented at the meeting. Prior to CSSE the sample surface had to be cleaned by SF6-plasma etching to remove a molecular film which was deposited in space. Tests discussed below showed that the cleaning procedure did not lead to any gas loss. In contrast to CSSE, no surface cleaning was needed for total extraction experiments. To correct for losses of ions due to backscattering during the irradiation in space we calculated correction factors for all Ne isotopes with the SRIM code [6]. The same code was used to determine the theoretical depth distribution of Ne isotopes within the BMG. Backscatter correction factors for He isotopes were determined experimentally [7]. Results and Discussion: The three different extraction methods yield slightly different He isotopic ratios. The mean He/He value obtained for laser ablation is (4.62±0.04)×10 compared to (4.38±0.01)×10 as measured by pyrolysis and (4.49±0.03)×10 by CSSE. On the other hand, Ne/Ne ratios deduced by all methods are in very good agreement with each other: 14.03±0.09 for laser ablation, 14.00±0.05 for pyrolysis and 14.08±0.11 for CSSE. The uncertainties are given as 2σ of the mean and the adopted backscatter correction factors are 1.02 for He/He and 1.01 for Ne/Ne. The ratios for all methods are slightly higher than the mean values of the SWC foils given by Geiss et al. (He/He =(4.26±0.22)×10 and Ne/Ne = 13.7±0.3) [8]. However, regarding the large variation of the single SWC values, results of both missions are consistent with each other. Also elemental ratios and abundances determined by all three methods agree well with each other, as can be seen in Fig. 1. The He/Ne ratios show very little variation, ranging from 490±10 for CSSE to 530±22 for pyrolysis. They are slightly lower than, but consistent with the SWC mean He/Ne ratio of 570±70. The variation for the He flux is even smaller with perfectly agreeing mean values of (8.29±0.15)×10 atoms/cms for CSSE compared to (8.51±0.4)×10 atoms/cms for pyrolysis and (8.73±0.6)×10 atoms/cms for laser ablation. The obtained numbers for the Ne flux agree similarly well (adopted backscatter correction factors are shown in Fig. 1).