Solar wind charge exchange in laboratory – Observation of forbidden X-ray transitions Naoki Numadate a,⇑ , Hirofumi Shimaya a , Takuya Ishida a , Kunihiro Okada b , Nobuyuki Nakamura c , Hajime Tanuma a a Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan b Department of Physics, Sophia University, 7-1 Kioicho, Chiyoda, Tokyo 102-8554, Japan c Institute of Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-0021, Japan article info Article history: Received 29 November 2016 Received in revised form 13 April 2017 Accepted 25 April 2017 Available online xxxx Keywords: Solar wind charge exchange Forbidden transitions Soft X-ray emissions ECR ion source Kingdon ion trap abstract We have reproduced solar wind charge exchange collisions of hydrogen-like O 7+ ions with He gas at collision energies of 42 keV in the laboratory and observed the forbidden transition of 1s 2 1 S 0 –1s2s 3 S 1 in helium-like O 6þ ions produced by single electron capture. The measured soft X-ray spectrum had a peak at 560 eV which corresponds to the energy of the forbidden 1s 2 1 S 0 –1s2s 3 S 1 transition in the O 6þ ion, and a reasonable energy difference between peak positions of the forbidden and resonance lines was found, which ensured that we succeeded in observing the forbidden transition of O 6þ ions. The dom- inant electron capture level in the collision of O 7+ ions with He can be estimated to be a principal quan- tum number n ¼ 4 by the classical over barrier model and the two-center atomic orbital close coupling method. After the charge exchange, the population of the 1s2s state becomes large due to cascade tran- sitions from the higher excited states, so the long-lived forbidden transition to the 1s 2 1 S 0 ground state is one of main features observed in the charge exchange spectra. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction The Roentgen satellite, ROSAT, which was launched in 1990 as an X-ray observatory satellite, observed soft X-ray emission whose intensity varied in cycles of several days during the all-sky survey [1]. This emission came from areas where there was no particular hot object, so it remained mysterious. In 1996, the ROSAT also observed the soft X-ray emission from Comet C/Hyakutake 1996 B2 approaching to Earth [2]. This was also mysterious and surpris- ing, because the comet which was composed mainly of ice and dust was too cold to emit soft X-rays. Following this initial observation, soft X-ray emission was subsequently observed from various comets. According to Cravens’ suggestion [3], it has been recog- nized that part of the soft X-ray background emission stems from charge exchange collisions between the solar wind ions and the neutrals in the heliosphere, and this phenomenon is called Solar Wind Charge eXchange (SWCX). The SWCX emission has attracted much attention, because it appears in soft X-ray spectra obtained by observatory satellites as foreground emission when astronomi- cal objects are observed, and can act as a new probe of low density neutrals in the solar system. According to observations by the Suzaku satellite, the forbidden 1s 2 1 S 0 –1s2s 3 S 1 transition in meta- stable O 6þ ions produced by charge exchange was one of main fea- tures in the SWCX [4,5]. In addition to the SWCX, attention has been also paid to discoveries of soft X-ray emission caused by charge exchange in outer rims of the supernova remnants [6]. In order to analyze the observed spectra and to construct a model of the soft X-ray emission, the absolute values of emission cross sections are required by astrophysicists [7,8]. Fig. 1 shows a schematic level diagram of the n ¼ 1; 2 levels of helium-like O 6þ ion. The dominant decay paths, their associated theoretical transition energies and lifetimes are indicated [9,10]. The forbidden 1s 2 1 S 0 –1s2s 3 S 1 transitions following SWCX had not been observed in ground based experiments. The solar wind ions travel several hundred meters during the forbidden transi- tions due to their velocities of 300–800 km/s [11] and the transi- tion lifetimes of milliseconds. This makes it difficult to observe the forbidden lines derived from the SWCX in the laboratory. In our laboratory, we have observed soft X-ray emission in the SWCX process for electric dipole allowed transitions with very short lifetimes [12,13]. In order to observe the forbidden transi- tions in our laboratory, we have developed a Kingdon ion trap sys- tem for trapping externally injected highly charged ions [14]. At http://dx.doi.org/10.1016/j.nimb.2017.04.083 0168-583X/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: numadate-naoki@ed.tmu.ac.jp (N. Numadate). Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb Please cite this article in press as: N. Numadate et al., Solar wind charge exchange in laboratory – Observation of forbidden X-ray transitions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.083