The Science with the Interstellar Heliopause Probe H. Fichtner, B. Heber, M. Leipold To cite this version: H. Fichtner, B. Heber, M. Leipold. The Science with the Interstellar Heliopause Probe. Astrophysics and Space Sciences Transactions, Copernicus Publications, 2006, 2 (1), pp.33-43. hal-00297478 HAL Id: hal-00297478 https://hal.archives-ouvertes.fr/hal-00297478 Submitted on 15 Jun 2006 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Astrophys. Space Sci. Trans., 2, 33–43, 2006 www.astrophys-space-sci-trans.net/2/33/2006/ © Author(s) 2006. This work is licensed under a Creative Commons License. Astrophysics and Space Sciences Transactions The Science with the Interstellar Heliopause Probe H. Fichtner1, B. Heber2, and M. Leipold3 1Institut fur¨ Theoretische Physik IV, Ruhr-Universitat¨ Bochum, 44780 Bochum, Germany 2Institut fur¨ Experimentelle und Angewandte Physik, Christian-Albrechts-Universitat¨ Kiel 24118 Kiel, Germany 3Kayser-Threde GmbH, Space Technology and Utilization Section, 81379 Munich, Germany Received: 22 December 2005 – Revised: 14 May 2006 – Accepted: 19 May 2006 – Published: 15 June 2006 Abstract. After the exciting in-situ observations of the ter- Termination Shock mination shock and the entry of the Voyager 1 spacecraft in the heliosheath, there is a growing awareness of the signifi- Bow Shock cance of the physics of the outer heliosphere. Its understand- ing helps to clarify the structure of our immediate interstellar neighbourhood, contributes to the clarification of fundamen- Cosmic tal astrophysical processes like the acceleration of charged Rays particles at a steller wind termination shock, and also sheds light on the question to what extent interstellar-terrestrial re- Interstellar IHP lations are important for the environment of and on the Earth. Medium Consequently, there are new seriously discussed suggestions for sending a modern spacecraft into the heliosheath and be- yond. One of those candidates is the Interstellar Heliopause Probe (IHP) that has been studied in a Technology Reference Study by ESA/ESTEC. Here, we discuss the science objec- tives and expected scientific performance of this mission. Heliopause (adapted from 'Astronomy' [2005]) 1 Introduction Fig. 1. A sketch of the structure of the heliopsheric interface (in the restframe of the Sun) and the trajectories of the Voyager spacecraft After the first period of concept formulation regarding the and the Interstellar Heliopause Explorer (IHP). The interface con- large-scale structure of the solar cavity (Davis, 1955), nowa- sists of three discontinuities, namely the termination shock where days called heliosphere according to a suggestion by Dessler the solar wind is slowed from supersonic to subsonic speed, the (1967), a period of numerical modelling began in the late heliopause separating the solar and the interstellar plasma, and the 1970’s (Baranov et al., 1979) and is being continued since bow shock where the interstellar plasma possibly undergoes a tran- then, see the review by Zank (1999) and the book by Florin- sition to subsonic speed. In addition, there exists a hydrogen wall, ski et al. (2004a). The resulting ‘heliospheric’ paradigm i.e. a pile-up of neutral hydrogen between the heliopause and the claims that the heliosphere is a plasma bubble formed by bow shock as a consequence of its charge-exchange coupling to the interstellar plasma flow that is disturbed by the obstacle heliosphere the interaction of the solar wind with the local interstellar (sketch adopted from Astronomy (2005)). medium (LISM). It has a basic drop-like shape resulting from the relative motion of the Sun and the LISM as sketched in Fig. 1. This paradigm has been corroborated by the direct or indi- termination shock by Voyager 1 at 94 AU, see Burlaga et al. rect observations: (2005), Decker et al. (2005), Gurnett and Kurth (2005), and (1) Most excitingly is the recent discovery of the solar wind Stone et al. (2005). (2) Radio measurements with the two Voyager spacecraft Correspondence to: H. Fichtner ([email protected]) (Gurnett et al., 2003; Kurth and Gurnett, 2003) can be Published by Copernicus GmbH on behalf of the Arbeitsgemeinschaft Extraterrestrische Forschung e.V. 34 H. Fichtner et al.: Science with IHP interpreted as signatures from the interaction of a global essary to get in-situ observations from the heliopause and merged interaction region (GMIR) with the heliopause the region beyond. Therefore, a ’Heliopause Explorer’ mis- Mitchell et al. (2004). sion (Leipold et al., 2003), now called Interstellar Heliopause (3) There is overwhelming evidence from the analysis of in- Probe (IHP, see Falkner (2005), Lyngvi et al. (2005a) and terstellar absorption lines for the existence of a hydrogen wall Lyngvi et al. (2005b)), can represent the next step in the ex- ahead of the heliosphere (Linsky and Wood, 1996; Wood et ploration of the outer boundaries of the heliosphere. al., 2004). Such hydrogen walls have been detected for other stars as well (Izmodenov et al., 1999; Wood et al., 2004). (4) Some information about the local interstellar magnetic 2 The scientific objectives of IHP field appears to be available from measurements of starlight polarization and energetic particles, see Frisch (2003) and 2.1 The basic questions Gloeckler et al. (1997), respectively. During recent years the study of the heliosphere as such An in-situ exploration of the heliopause and the heliospheric has been extended towards its treatment as an astrosphere be- interface will allow us to answer a number of basic scientific ing embedded in a changing galactic environment. There is questions that arose during the last 40 years of heliospheric growing evidence that, in particular, cosmic rays are likely research: to play a role for the long-term conditions of the Earth’s environment and on Earth itself (Shaviv, 2003; Shaviv and • What are the characteristics of the solar wind termina- Veizer, 2003), and that the heliosphere serves to some extent tion shock and the heliopause, where are these struc- as a protecting shield against the interstellar environment as tures located and how do their characteristics and loca- reviewed by Scherer et al. (2002). tions change with time? Parallel to the development of the theory of the heliosphere and its direct interstellar neighborhood, the LISM, space mis- • How does the solar wind termination shock act as an sions have shed light on many aspects of this paradigm that astrophysical accelerator of (anomalous) cosmic rays? has emerged over the years. In addition to the measurement of the solar wind, the solar radiation back-scattered from • How does the heliosphere shield the Earth from galactic neutral atoms, the direct detection of neutral atoms as well cosmic rays and the interstellar neutral gas? as so-called interstellar pick-up ions, especially the observa- tions of cosmic rays have contributed to our present under- • What is the state and composition of the local interstel- standing, see the reviews by Fichtner (2001), Heber (2001), lar medium beyond the heliopause? and Fichtner (2005). Crucial for the further understanding is a direct in-situ ob- In view of the recently revived interest in the so-called ‘Pio- servation of the heliospheric interface that can be defined neer Anomaly’ (Anderson et al., 2002) a fifth question could as the region between the supersonically expanding solar be added: wind and the (probably) supersonically streaming local in- terstellar medium (Fahr, 2000; Alexashov and Izmodenov, • Is the ‘Pioneer Anomaly’ real and is Newton’s law of 2005). It consists of the inner heliosheath between the he- gravitation to be modified? liospheric shock terminating the supersonic expansion of the solar wind, the heliopause separating the solar and the in- As there is another mission being discussed in the context terstellar plasma, and the outer heliosheath extending from of this question (Nieto et al., 2005), we do not address it the heliopause to the bow shock marking the transition of the further here, but note that an extension of the scientific pay- interstellar plasma flow from supersonic to subsonic speed. load discussed below would allow to add a test of the Pioneer The bow shocks of other stars have been detected (Brown Anomaly to the scientific objectives. and Bomans, 2005). Because all mentioned particle components, i.e. anoma- The only hope for an in-situ observation of the helio- lous and galactic cosmic rays (ACRs, GCRs) and the in- spheric interface in near future is based on the two Voyager terstellar gas are interacting with the heliospheric interface spacecraft launched in the 1970’s. With a power supply last- (Florinski et al., 2004b; Langner and Potgieter, 2005; Scherer ing for perhaps another 15 years these deep space probes will and Ferreira, 2005) and are likely to influence the Earth’s serve to explore the interface up to heliospheric distances of environment their understanding is of fundamental interest about 130–140 AU. This means that it is unclear whether or (Scherer et al., 2002). This interest manifests itself in the not they will observe the heliopause (named as such by Bhat- recent development of models that try to incorporate the ki- nagar and Fahr (1972)) for which the distance estimates vary netic cosmic ray transport into a dynamical description of between about 110 and 160 AU (Gurnett et al., 2003; Webber the heliosphere, see, e.g. Florinski et al. (2003), Ferreira et and Lockwood, 2004). In order to explore the structure and al.