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The Infant Bow Shock: a New Frontier at a Weak Activity Comet

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The Infant Bow Shock: a New Frontier at a Weak Activity Comet A&A 619, L2 (2018) Astronomy https://doi.org/10.1051/0004-6361/201834225 & c ESO 2018 Astrophysics LETTER TO THE EDITOR The infant bow shock: a new frontier at a weak activity comet Herbert Gunell1,2?, Charlotte Goetz3;?, Cyril Simon Wedlund4, Jesper Lindkvist2, Maria Hamrin2, Hans Nilsson5, Kristie Llera6, Anders Eriksson7, and Mats Holmström5 1 Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium e-mail: [email protected] 2 Department of Physics, Umeå University, 901 87 Umeå, Sweden 3 Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany e-mail: [email protected] 4 Department of Physics, University of Oslo, Box 1048 Blindern, 0316 Oslo, Norway 5 Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden 6 Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA 7 Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, 751 21 Uppsala, Sweden Received 12 September 2018 / Accepted 15 October 2018 ABSTRACT The bow shock is the first boundary the solar wind encounters as it approaches planets or comets. The Rosetta spacecraft was able to observe the formation of a bow shock by following comet 67P/Churyumov–Gerasimenko toward the Sun, through perihelion, and back outward again. The spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. Rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets. Key words. comets: general – comets: individual: 67P/Churyumov–Gerasimenko – plasmas – shock waves 1. Introduction boundaries (Nilsson et al. 2015), followed by the development of plasma boundaries as a result of the interaction between Shocks are known to form at the interface between plasmas the comet’s atmosphere and the solar wind (Mandt et al. 2016). in relative motion: bow shocks upstream of planets and other Between April and December 2015 when the comet was less objects immersed in stellar winds, termination shocks at the than 1.8 AU from the Sun, no solar wind ions were observed by edge of the solar system, and shocks in other astrophysical Rosetta at its position close to the nucleus, showing that the solar settings (Treumann 2009). Early observations of Earth’s bow wind was deflected upstream of the spacecraft (Nilsson et al. shock showed a sharp increase in the magnetic field as the 2017). This placed the inner coma in a solar wind cavity that has solar wind passes the shock, followed by a number of oscilla- been discussed in terms of a single-particle model (Behar et al. tions in the downstream region (Heppner et al. 1967). At comets, 2017). At about the same time as the solar wind disappeared, bow shocks were observed during the flybys of 21P/Giacobini– the diamagnetic cavity was detected for the first time, and it Zinner (Jones et al. 1986), 1P/Halley (Gringauz et al. 1986a), was then observed frequently until February 2016 (Goetz et al. and 26P/Grigg–Skjellerup (Neubauer et al. 1993). These bow 2016a,b). Until now, it has been thought that no bow shock was shocks were found to be wider and more gradual than the sharp encountered by Rosetta during the mission (Mandt et al. 2016; shocks seen at planets due to solar wind mass loading, and Simon Wedlund et al. 2017). We present the development of a in some instances they have been called “bow waves” instead cometary bow shock from 7 March 2015 when the comet was (Neubauer et al. 1993). These observations were made when the at a heliocentric distance of 2.2 AU on its way toward perihelion comets were close to perihelion and their bow shocks were and from the outward journey on 24 February 2016 at 2.4 AU. At fully developed. At comet 1P/Halley additional boundaries were these times we can identify an infant bow shock, which we inter- found: a cometopause, which separates the solar wind from the pret as the structure that will be continuously transformed into a cometary ion environment (Gringauz et al. 1986b), and a dia- fully developed bow shock at higher comet activity stages. The magnetic cavity (Neubauer et al. 1986). infant bow shock is asymmetric and it is wider – relative to the In the course of its journey, the Rosetta spacecraft distance from shock to nucleus – than fully developed cometary (Glassmeier et al. 2007a) followed comet 67P/Churyumov– bow shocks. This early stage in bow shock development had Gerasimenko for two years, providing unprecedented opportu- never been observed before Rosetta’s voyage. Moreover, on 28 nities to observe first, a low-activity state with no identified March 2015, no diamagnetic cavity was detected in a close flyby, down to a 15 km cometocentric distance at a 1.99 AU heliocen- ? These authors contributed equally to this work. tric distance (Goetz et al. 2016a), and that is closer to the Sun Article published by EDP Sciences L2, page 1 of5 A&A 619, L2 (2018) Fig. 1. Rosetta data from 7 March 2015. Panel a: RPC-ICA ion energy spectrum. Panel b: RPC-IES ion energy spectrum. Panel c: RPC-IES electron energy spectrum. The spectra in panels a–c are summed over all viewing directions and the colour-coded quantities ΓICA, ΓIES;i, and ΓIES;e represent the differential particle flux in units of s−1 sr−1 m−2 eV−1. Panel d: magnetic flux density in CSEQ coordinates measured by RPC-MAG: Bx (blue), By (green), and Bz (red). Panel e: magnitude jBj of the B-field. Panel e: estimate of the cosine of the angle between the solar wind + 2+ + + electric field vector and the nucleus to spacecraft direction projected onto the y–z plane. Approximate E=q values for H , He , He , and H2O are indicated on the right-hand side of panels a and b. The cyan ticks in each panel indicate the times referred to in the text. than the 2.2 AU and 2.4 AU heliocentric distance of the infant oscillations in the observed field are expected due to the rel- bow shock observations. Since the cavity expands with decreas- ative motion of the spacecraft and the inhomogeneous mag- ing heliocentric distance, this suggests that during the observa- netic field associated with the shock. In order to emphasise the tions presented here no diamagnetic cavity had formed and that low-frequency part of the spectrum the magnetic field has been the infant bow shock was the only plasma boundary separating low-pass filtered with a −3 dB cut-off frequency of 10 mHz. At the comet and solar wind environments. the first event (05:53), deceleration of the protons is seen as a decreased E=q value in Fig.1 b. The direction of the magnetic 2. Rosetta observations field also changed significantly at this time (Fig.1 d). The RPC- ICA and RPC-IES instruments are mounted on the spacecraft so Figure1 shows ion spectra collected by the Ion Composi- that their fields of view complement each other. The solar wind tion Analyzer of the Rosetta Plasma Consortium (RPC-ICA; alpha particle beam was present throughout the day, but it shifted Nilsson et al. 2018), both ion and electron spectra measured between the RPC-ICA and RPC-IES fields of view; for exam- by the Ion and Electron Sensor (RPC-IES; Burch et al. 2007), ple, at 15:54 the RPC-ICA alpha particle signal decreased, while and magnetic flux density from observations by the RPC-MAG the corresponding RPC-IES signal increased. A similar shift was instrument (Glassmeier et al. 2007b). During the period on 7 seen for the protons at 05:53, when they only appeared in the March 2015 shown in Fig.1 , Rosetta was located between 77 RPC-IES signal. The E=q value in Fig.1 b is seen to decrease and 88 km from the cometary nucleus, a short distance upstream and the width of the proton distribution increase in the minutes at x ≈ 50 km in the body-Centred Solar EQuatorial (CSEQ) following that time, showing the deceleration and heating of the coordinate system, where the x-axis points toward the Sun, the proton distribution. A widening of the proton spectrum is also z-axis is the component of the Sun’s rotation axes orthogonal to seen for the event starting at 15:54 in Figs.1 a and b. During the the x-axis, and the y-axis completes the right-handed frame of rest of the day the proton flux in the two instruments varied in reference. unison indicating a varying proton density. In the ion spectra (Figs.1 a and b) four ion species can be dis- 2 tinguished: water group ions at E=q . 10 V, protons with E=q 3. Observations compared to simulations values up to about 103 V, alpha particles at E=q ≈ 2:5 × 103 V, and He+ ions around E=q ≈ 5 × 103 V. These E=q values are In support of the Rosetta observations, we have used a hybrid shown on the right-hand side of the panels. Three times dur- plasma model to simulate the interaction between the comet and + + ing the day – near 05:53, 10:38, and 15:54, indicated by cyan the solar wind. Cometary H2O ions and solar wind H and ticks in Fig.1 – the flux of suprathermal ∼200 eV electrons sud- He2+ ions are included as particles, and the electrons are treated denly increased and remained high for 1–2 h (Fig.1 c).
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