Space Sci Rev (2008) 136: 117–184 DOI 10.1007/s11214-007-9170-x STEREO IMPACT Investigation Goals, Measurements, and Data Products Overview J.G. Luhmann · D.W. Curtis · P. Schroeder · J. McCauley · R.P. Lin · D.E. Larson · S.D. Bale · J.-A. Sauvaud · C. Aoustin · R.A. Mewaldt · A.C. Cummings · E.C. Stone · A.J. Davis · W.R. Cook · B. Kecman · M.E. Wiedenbeck · T. von Rosenvinge · M.H. Acuna · L.S. Reichenthal · S. Shuman · K.A. Wortman · D.V. Reames · R. Mueller-Mellin · H. Kunow · G.M. Mason · P. Walpole · A. Korth · T.R. Sanderson · C.T. Russell · J.T. Gosling Received: 17 October 2006 / Accepted: 8 March 2007 / Published online: 1 June 2007 © Springer Science+Business Media, Inc. 2007 Abstract The IMPACT (In situ Measurements of Particles And CME Transients) investiga- tion on the STEREO mission was designed and developed to provide multipoint solar wind and suprathermal electron, interplanetary magnetic field, and solar energetic particle infor- mation required to unravel the nature of coronal mass ejections and their heliospheric con- sequences. IMPACT consists of seven individual sensors which are packaged into a boom suite, and a SEP suite. This review summarizes the science objectives of IMPACT, the in- struments that comprise the IMPACT investigation, the accommodation of IMPACT on the STEREO twin spacecraft, and the overall data products that will flow from the IMPACT measurements. Accompanying papers in this volume of Space Science Reviews highlight the individual sensor technical details and capabilities, STEREO project plans for the use of IMPACT data, and modeling activities for IMPACT (and other STEREO) data interpre- tation. Keywords STEREO · Coronal mass ejection · Solar energetic particles · Suprathermal electrons · Solar wind electrons · Interplanetary magnetic field · Multipoint heliospheric measurements J.G. Luhmann () · D.W. Curtis · P. Schroeder · J. McCauley · R.P. Lin · D.E. Larson · S.D. Bale Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA e-mail: [email protected] J.-A. Sauvaud · C. Aoustin CESR, BP 4346, 31028 Toulouse Cédex 4, France R.A. Mewaldt · A.C. Cummings · E.C. Stone · A.J. Davis · W.R. Cook · B. Kecman California Institute of Technology, Pasadena, CA 91125, USA M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA 91109, USA T. von Rosenvinge · M.H. Acuna · L.S. Reichenthal · S. Shuman · K.A. Wortman · D.V. Reames NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA 118 J.G. Luhmann et al. List of acronyms ACR Anomalous Cosmic Rays CIR Corotating Interaction Region CME Coronal Mass Ejection GCR Galactic Cosmic Rays ICME Interplanetary Coronal Mass Ejection IDPU Instrument Data Processing Unit IMF Interplanetary Magnetic Field HET High Energy Telescope LET Low Energy Telescope MAG Magnetometer SEP Solar Energetic Particles SEPT Solar Electron and Proton Telescope SIT Suprathermal Ion Telescope STE Suprathermal Electron Telescope STEREO Solar Terrestrial Observatory SWEA Solar Wind Electron Analyzer 1 Introduction to the IMPACT Investigation The primary mission of the STEREO (Solar TERrestrial RElations Observatory) twin space- craft is to understand the 3-dimensional nature of the Sun’s corona, including the eruptions known as coronal mass ejections (CMEs), from their birth in the corona to their impact at the Earth. The IMPACT (In situ Measurements of Particles and CME Transients) inves- tigation focuses on the invisible particles and fields that affect Earth’s space weather by contributing solar wind and suprathermal electron, interplanetary magnetic field, and solar energetic particle (SEP) measurements toward this goal. IMPACT measurements are also essential for other STEREO objectives, especially for relating the 3D corona and solar wind structure, for advancing knowledge of the sources and physics controlling solar energetic particle acceleration and propagation, and for unraveling the relationship between CMEs R. Mueller-Mellin · H. Kunow University of Kiel, 24118 Kiel, Germany G.M. Mason Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA P. Walpole University of Maryland, College Park, MD 20742, USA A. Korth Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany T.R. Sanderson Research and Scientific Support Division, ESA, Noorwijk, The Netherlands C.T. Russell Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095, USA J.T. Gosling Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA STEREO IMPACT Investigation Goals, Measurements, and Data 119 and the solar magnetic cycle (see the overall STEREO mission goals described by Kaiser et al., this volume, 2007). IMPACT includes seven scientifically related in situ sensors on each spacecraft, a tele- scoping boom, and an instrument data processing unit that provides the primary interface between the sensors and the spacecraft. Detailed descriptions of the different IMPACT sen- sors can be found in companion papers by Mewaldt et al. (this volume, 2007), Mueller- Mellin et al. (this volume, 2007), von Rosenvinge et al. (this volume, 2007), Mason et al. (this volume, 2007), Acuna et al. (this volume, 2007), Sauvaud et al. (this volume, 2007), and Lin et al. (this volume, 2007), and the novel boom design is described by Ullrich et al. (this volume, 2007). A previous brief summary of the complete IMPACT investigation can be found in Luhmann et al. (2005). In this paper we provide more details about the overall makeup and science goals of IMPACT including its data plans and products. The IMPACT team is committed to the regular integration of IMPACT data with the other STEREO ob- servations to take full advantage of the information obtained by the observatories. Com- plementary PLASTIC investigation ion composition measurements, and SWAVES plasma wave measurements are described in accompanying papers by Galvin et al. and Bougeret et al., respectively. Joint observing and data analysis plans involve these other in situ mea- surements, as well as the SECCHI investigation imaging (Howard et al., this volume, 2007) and SWAVES radio remote sensing results (Bougeret et al., this volume, 2007). The IMPACT investigation was designed to fulfill many of the in situ measurement requirements for STEREO, illustrated by Fig. 1. The strength and orientation of the in- Fig. 1 Illustration of the interaction of a flux rope-like interplanetary CME (ICME) structure inferred from pre-STEREO measurements with the ambient solar wind. Counterstreaming suprathermal electron beams within the ejecta have been used to deduce its structure and size. An interplanetary shock is formed when the ejected mass of plasma and field moves sufficiently fast with respect to the ambient solar wind ahead. This shock often is a source of solar energetic particles that race ahead of it along the spiral interplanetary field lines to arrive at 1 AU well before the ICME and shock. The inset shows how the instruments of the IMPACT investigation on STEREO, described later in the text, are configured to make in situ measurements of the structures and particle populations in this picture at each spacecraft location 120 J.G. Luhmann et al. terplanetary magnetic field at the two different STEREO spacecraft, combined with the suprathermal (several 100 s of eV) electron measurements help determine whether the lo- cal magnetic fields of the interplanetary CME, the ICME, are rooted at the Sun at one or both ends or disconnected. Such magnetic topology information tells us about the coro- nal origins of the CME/ICME and also about the role of processes such as reconnection at the Sun and in the interplanetary medium. Higher energy electron, and also ion, de- tectors provide information on whether the local field connects to a flaring active region. Detection of SEP events at the two locations, with high time resolution and directional in- formation over the energy range from 10 s of keV to ∼100 MeV, allow remote sensing of the CME-initiated shock location and strength as it travels into the heliosphere. SEP ion composition measurements make it possible to distinguish between electron and heavy ion-rich flare-accelerated particles and solar wind particles accelerated at the interplanetary shock. The lowest energy SEP ions also provide information on the ambient suprathermal ion “seed” populations that may feed the higher energy SEP production at the CME re- lated shocks. With the radio and plasma waves investigation, opportunities exist for in situ plasma microphysics diagnostics on local shock structures and waves, as well as compar- isons of radio remote sensing and SEP remote sensing of coronal shocks. The in situ data integration with images and ground based images and magnetograph observations through Sun-to-Earth modeling (e.g. Aschwanden et al., this volume, 2006) enables a STEREO era of regular cradle-to-grave analyses of the CME phenomenon (e.g. Fox et al. 1998; Webb et al. 2000). In particular, details of the conditions on the Sun that lead to specific 1 AU consequences can be investigated, and used to refine observational strategies for future space weather missions such as Solar Dynamics Observatory, SDO, and the Heliospheric Sentinels, and for formulating updated science questions for Solar Orbiter and Solar Probe. Most solar wind structures can be categorized as either corotating or transient, with ex- amples being long-lived high-speed streams and ICMEs, respectively. In both cases, iden- tical measurements of the same structures at the two STEREO locations flanking