Icarus 307 (2018) 17–24 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus An upper limit on Pluto’s ionosphere from radio occultation measurements with New Horizons D.P. Hinson a,b,∗ , I.R. Linscott b , D.F. Strobel c , G.L. Tyler b , M.K. Bird d,e , M. Pätzold d , M.E. Summers f , S.A. Stern g , K. Ennico h , G.R. Gladstone i , C.B. Olkin g , H.A. Weaver j , W.W. Woods b , L.A. Young g , the New Horizons Science Team a Carl Sagan Center, SETI Institute, Mountain View, CA 94043, USA b Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA c The Johns Hopkins University, Baltimore, MD 21218, USA d Rheinisches Institut für Umweltforschung, Universität Köln, Cologne 50931, Germany e Argelander Institut für Astronomie,Universität Bonn, Bonn 53121, Germany f George Mason University, Fairfax, VA 22030, USA g Southwest Research Institute, Boulder, CO 80302, USA h NASA Ames Research Center, Moffett Field, CA 94035, USA i Southwest Research Institute, San Antonio, TX 78238, USA j The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA a r t i c l e i n f o Article history: Received 11 August 2017 Revised 5 December 2017 Accepted 7 February 2018 Available online 7 February 2018 Keywords: Pluto Ionospheres Occultations Radio observations a b s t r a c t On 14 July 2015 New Horizons performed a radio occultation (RO) that sounded Pluto’s neutral atmo- sphere and ionosphere. The solar zenith angle was 90.2° (sunset) at entry and 89.8° (sunrise) at exit. We examined the data for evidence of an ionosphere, using the same method of analysis as in a previous investigation of the neutral atmosphere (Hinson et al., 2017). No ionosphere was detected. The measure- ments are more accurate at occultation exit, where the 1-sigma sensitivity in integrated electron content (IEC) is 2.3 × 10 11 cm −2 . The corresponding upper bound on the peak electron density at the terminator is about 1000 cm −3 . We constructed a model for the ionosphere and used it to guide the analysis and interpretation of the RO data. Owing to the large abundance of CH 4 at ionospheric heights, the domi- nant ions are molecular and the electron densities are relatively small. The model predicts a peak IEC of 1.8 × 10 11 cm −2 for an occultation at the terminator, slightly smaller than the threshold of detection by New Horizons. © 2018 Elsevier Inc. All rights reserved. 1. Introduction New Horizons sped past Pluto in July 2015 (Stern et al., 2015). Its reconnaissance of the Pluto System included radio occultation (RO) measurements at both Pluto and Charon (Tyler et al., 2008; Young et al., 2008). These observations were implemented in an uplink configuration, using signals transmitted by four antennas of the NASA Deep Space Network and received by the spacecraft (Hinson et al., 2017). The flight component of the radio science in- strument consists of two radio receivers, which are used routinely for communication and navigation (Fountain et al., 2008). Each re- ceiver also contains a specialized signal processor — known as REX ∗ Corresponding author at: Carl Sagan Center, SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, USA. E-mail address: dhinson@seti.org (D.P. Hinson). — designed specifically to support the radio occultations and other scientific observations (Tyler et al., 2008). The Pluto radio occultation addressed two objectives of the New Horizons Mission (Young et al., 2008). The primary ob- jective was to determine the atmospheric pressure at the sur- face and the temperature structure of the lower atmosphere. Gladstone et al. (2016) reported the initial results, which were later refined and extended by Hinson et al. (2017) in a more thorough analysis of the full data set. The REX observation yielded a mean surface pressure of 11.5 ± 0.7 microbar at a radius of 1189.9 ± 0.2 km and the first atmospheric temperature profiles that extend all the way to the surface. The measured air temperature adjacent to the surface was 38.9 ± 2.1 K at occultation entry and 51.6 ± 3.8 K at occultation exit. The subject of this paper is the secondary objective of the Pluto occultation, to characterize the ionosphere. We apply the method of analysis developed in the previous investigation of the neutral https://doi.org/10.1016/j.icarus.2018.02.011 0019-1035/© 2018 Elsevier Inc. All rights reserved.