The STEREO Mission: An Introduction

The twin STEREO spacecraft were launched on October 26, 2006, at 00:52 UT from Kennedy Space Center aboard a Delta 7925 launch vehicle. After a series of highly eccentric Earth orbits with apogees beyond the moon, each spacecraft used close flybys of the moon to escape into orbits about the Sun near 1 AU. Once in heliospheric orbit, one spacecraft trails Earth while the other leads. As viewed from the Sun, the two spacecraft separate at approximately 44 to 45 degrees per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of coronal mass ejection (CME) initiation and to follow the propagation of CMEs through the inner heliosphere to Earth. Researchers will use STEREO measurements to study the mechanisms and sites of energetic particle acceleration and to develop three-dimensional (3-D) time-dependent models of the magnetic topology, temperature, density and velocity of the solar wind between the Sun and Earth. To accomplish these goals, each STEREO spacecraft is equipped with an almost identical set of optical, radio and in situ particles and fields instruments provided by U.S. and European investigators. The SECCHI suite of instruments includes two white light coronagraphs, an extreme ultraviolet imager and two heliospheric white light imagers which track CMEs out to 1 AU. The IMPACT suite of instruments measures in situ solar wind electrons, energetic electrons, protons and heavier ions. IMPACT also includes a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument measures the composition of heavy ions in the ambient plasma as well as protons and alpha particles. The S/WAVES instrument uses radio waves to track the location of CME-driven shocks and the 3-D topology of open field lines along which flow particles produced by solar flares. Each of the four instrument packages produce a small real-time stream of selected data for purposes of predicting space weather events at Earth. NOAA forecasters at the Space Environment Center and others will use these data in their space weather forecasting and their resultant products will be widely used throughout the world. In addition to the four instrument teams, there is substantial participation by modeling and theory oriented teams. All STEREO data are freely available through individual Web sites at the four Principal Investigator institutions as well as at the STEREO Science Center located at NASA Goddard Space Flight Center.     

Properties of Interplanetary Coronal Mass Ejections

Interplanetary coronal mass ejections (ICMEs) originating from closed field regions on the Sun are the most energetic phenomenon in the heliosphere. They cause intense geomagnetic storms and drive fast mode shocks that accelerate charged particles. ICMEs are the interplanetary manifestations of CMEs typically remote-sensed by coronagraphs. This paper summarizes the observational properties of ICMEs with reference to the ordinary solar wind and the progenitor CMEs.     

The Origin,Early Evolution and Predictability of Solar Eruptions

Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt.     

Relationship of coronal transients to interplanetary shocks: 3D aspects

More than 1000 coronal mass ejections (CMEs) caused by different types of coronal transients have been analyzed up to now, based on the images from white light coronagraphs on board the OSO 7, Skylab, P78-1, and SMM spacecraft. In many cases, the CME images lead us to the impression of loop-like, more planar structures, similar to those of prominence structures often seen in H pictures. There is increasing evidence, though, for a three-dimensional bubble- or cloud-like structure of CMEs. In several cases, CMEs directed toward the earth (or away from it) were identified, as their outer fronts emerged on all sides of the coronagraph's occulting disk, thus suggesting a bubble-like appearance.There now appears to be unanimity about the crucial role that magnetic reconnection plays during the transient process. Recently, direct evidence was found for the pinch-off of CMEs, both from optical observations and from in situ measurements of isolated magnetic clouds' following transient shock waves. However, the detailed sequence of events during the generation of a CME is still unclear.Interplanetary shock waves associated with the CMEs are usually restricted in latitudinal extent to about the angular width of the optically observed CMEs. They may be somewhat less restricted in longitudinal extent. A nearly 1 1 association between CMEs and shock waves measured in situ from spacecraft (Helios 1 and 2, IMP 7 and 8, ISEE 3, Pioneer Venus) can be established, provided the CME and the spacecraft were in the same longitudinal and latitudinal range and the CME speed exceeds 400 km s^–1. Around the past solar activity minimum all CMEs observed were centered at solar latitudes of less than 60°. Around solar maximum, a significant fraction of CMEs also originated from the polar regions. Thus, there is a good chance that the Ulysses spaceprobe will encounter many shocks caused by both low- and high-latitude CMEs, when it finally starts its journey over the Sun's poles.     

A Historical Review of the Geomagnetic Storm-Producing Plasma Flows from the Sun

The concept of geomagnetic storm-producing solar plasma flows has evolved and advanced considerably over the last 100 years or so. This particular field of study began in an effort to understand geomagnetic disturbances and the aurora. The purpose of this paper is try to follow the ways in which early concepts evolved to later ones, not to review each concept in detail. It is fascinating to see a step-by-step buildup of these concepts, from the earliest idea of flow of solar electrons to coronal mass ejections (CMEs). The time line, though tentative, of the studies of geomagnetic storm-producing plasma flows is presented. The author hopes that this paper will serve young researchers in particular to consider how they plan to advance further this scientific field. There is still much uncertainty about geomagnetic storm-producing solar plasma flows. Some of the major questions are listed from the point of view of a geophysicist in the summary sections by grouping them in terms of the quiet-time solar wind, solar streams from corona holes and CMEs associated with solar flares.     

Heliospheric Physics: Linking the Sun to the Magnetosphere

Research into the heliospheric structure and its relation to the solar boundary is at an impasse. After successful predictions by Parker about the zeroth-order behavior of the heliospheric magnetic field and the solar wind, the heliospheric community struggles to make substantive progress toward a predictive model describing the connections between the Sun and its space environment, between the closed corona and the open corona extending to the planets. This is caused by our lack of understanding of the basic processes heating the corona and transporting open magnetic field. We detail the models used to describe this connectivity, from potential field source surface models to full MHD techniques. We discuss the current limitations of both approaches. Finally, we address a recent attempt to advance our understanding beyond these limitations. At this point in time the proposed theory remains controversial in the community, but it addresses important shortcomings of current approaches outlined above.     

Magnetic Fields in the Inner Heliosphere

The structure of Heliospheric Magnetic Field (HMF) is a function of both the coronal conditions from which it originates and dynamic processes which take place in the solar wind. The division between the inner and outer regions of the heliosphere is the result of dynamic processes which form large scale structures with increasing heliocentric distance. The structure of the HMF is normally described in the reference frame based on Parker's geometric model, but is better understood as an extension of potential field models of the corona. The Heliospheric Current Sheet (HCS) separates the two dominant polarities in the heliosphere; its large scale geometry near solar minimum is well understood but its topology near solar maximum remains to be investigated by Ulysses. At solar minimum, Corotating Interaction Regions (CIRs) dominate the near-equatorial heliosphere and extend their influence to mid-latitudes; the polar regions of the heliosphere are dominated by uniform fast solar wind streams and large amplitude, long wavelength, mostly transverse magnetic fluctuations. Coronal Mass Ejections (CMEs) introduce transient variability into the large scale heliospheric structure and may dominate the inner heliosphere near solar maximum at all latitudes.     

Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)

The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a five telescope package, which has been developed for the Solar Terrestrial Relation Observatory (STEREO) mission by the Naval Research Laboratory (USA), the Lockheed Solar and Astrophysics Laboratory (USA), the Goddard Space Flight Center (USA), the University of Birmingham (UK), the Rutherford Appleton Laboratory (UK), the Max Planck Institute for Solar System Research (Germany), the Centre Spatiale de Leige (Belgium), the Institut d’Optique (France) and the Institut d’Astrophysique Spatiale (France). SECCHI comprises five telescopes, which together image the solar corona from the solar disk to beyond 1 AU. These telescopes are: an extreme ultraviolet imager (EUVI: 1–1.7 R_⊙), two traditional Lyot coronagraphs (COR1: 1.5–4 R_⊙ and COR2: 2.5–15 R_⊙) and two new designs of heliospheric imagers (HI-1: 15–84 R_⊙ and HI-2: 66–318 R_⊙). All the instruments use 2048×2048 pixel CCD arrays in a backside-in mode. The EUVI backside surface has been specially processed for EUV sensitivity, while the others have an anti-reflection coating applied. A multi-tasking operating system, running on a PowerPC CPU, receives commands from the spacecraft, controls the instrument operations, acquires the images and compresses them for downlink through the main science channel (at compression factors typically up to 20×) and also through a low bandwidth channel to be used for space weather forecasting (at compression factors up to 200×). An image compression factor of about 10× enable the collection of images at the rate of about one every 2–3 minutes. Identical instruments, except for different sizes of occulters, are included on the STEREO-A and STEREO-B spacecraft.     

The Role of Magnetic Reconnection in CME Acceleration

Observations carried out from the coronagraphs on board space missions (LASCO/SOHO, Solar Maximum and Skylab) and ground-based facilities (HAO/Mauna Loa Observatory) show that coronal mass ejections (CMEs) can be classified into two classes based on their kinematics evolution. These two classes of CMEs are so-called fast and slow CMEs. The fast CME starts with a high initial speed that remains more or less constant; it is also called the constant-speed CME. On the other hand, the slow CME starts with a low initial speed, but shows a gradual acceleration; it is also called the accelerated and slow CME. Low and Zhang [Astrophys. J. 564, L53–L56, 2002] suggested that these two classes of CMEs could be a result of a difference in the initial topology of the magnetic fields associated with the underlying quiescent prominences. A normal prominence magnetic field topology will lead to a fast CME, while an inverse quiescent prominence results in a slow CME, because of the nature of the magnetic reconnection processes. In a recent study given by Wu et al. [Solar Phys. 225, 157–175, 2004], it was shown that an inverse quiescent prominence magnetic topology also could produce a fast CME. In this study, we perform a numerical MHD simulation for CMEs occurring in both normal and inverse quiescent prominence magnetic topology. This study demonstrates three major physical processes responsible for destabilization of these two types of prominence magnetic field topologies that can launch CMEs. These three initiation processes are identical to those used by Wu et al. [Solar Phys. 225, 157–175, 2004]. The simulations show that both fast and slow CMEs can be initiated from these two different types of magnetic topologies. However, the normal quiescent prominence magnetic topology does show the possibility for launching a reconnection island (or secondary O-line) that might be thought of as a “CME’’.     

Particle acceleration at the Sun and in the heliosphere

Energetic particles are accelerated in rich profusion at sites throughout the heliosphere. They come from solar flares in the low corona, from shock waves driven outward by coronal mass ejections (CMEs), from planetary magnetospheres and bow shocks. They come from corotating interaction regions (CIRs) produced by high-speed streams in the solar wind, and from the heliospheric termination shock at the outer edge of the heliospheric cavity. We sample many populations near Earth, but can distinguish them readily by their element and isotope abundances, ionization states, energy spectra, angular distributions and time behavior. Remote spacecraft have probed the spatial distributions of the particles and examined new sources in situ. Most acceleration sources can be ‘seen’ only by direct observation of the particles; few photons are produced at these sites. Wave-particle interactions are an essential feature in acceleration sources and, for shock acceleration, new evidence of energetic-proton-generated waves has come from abundance variations and from local cross-field scattering. Element abundances often tell us the physics of the source plasma itself, prior to acceleration. By comparing different populations, we learn more about the sources, and about the physics of acceleration and transport, than we can possibly learn from one source alone. This revised version was published online in August 2006 with corrections to the Cover Date.     

A two-Type Classification of Lasco Coronal Mass Ejection

The causes and origins of Coronal Mass Ejections (CMEs) remain among the outstanding questions in Space Physics. The observations of CMEs by the LASCO coronagraphs on SOHO suggest that there are two distinct types of CMEs. The two types of events can be most easily distinguished by examining height-time plots. The Type A (Acceleration) events produce curved plots that often indicate a constant acceleration. These events are usually associated with pre-existing helmet-streamers, and are often associated with prominence eruptions or filament disappearance. The Type C (Constant speed) events show a constant speed. These events are usually brighter, larger, and faster than Type A events and may be associated with X-ray flares. While the two types of events can be distinguished in other ways, the height-time plots are a simple and unambiguous way to make this identification.     

The heliospheric magnetic field

The heliospheric magnetic field (HMF) is an important component of the heliospheric medium. It has been the subject of extensive studies for the past thirty five years. There is a very large observational data base, mostly from the vantage point of the ecliptic plane, but now also from the solar polar regions, from the Ulysses mission. This review aims to present its most important large scale characteristics. A key to understand the HMF is to understand the source functions of the solar wind and magnetic fields close to the sun. The development of new modelling techniques for determining the extent and geometry of the open magnetic field regions in the corona, the sources of the solar wind and the HMF has provided a new insight into the variability of the source functions. These are now reasonably well understood for the state of the corona near solar minimum. The HMF at low-to-medium heliolatitudes is dominated, near solar minimum, by the Corotating Interaction Regions (CIRs) which arise from the interaction of alternating slow and fast solar wind streams, and which, in turn, interact in the outer heliosphere to form the large scale Merged Interaction Regions. The radial component of the HMF is independent of heliolatitude; the average direction is well organised by the Parker geometry, but with wide distributions around the mean, due, at high latitudes, to the presence of large amplitude, Alfvénic fluctuations. The HMF at solar maximum is less well understood, due in part to the complexity of the solar source functions, and partly to the lack of three dimensional observations which Ulysses is planned to remedy at the next solar maximum. It is suggested that the in-ecliptic conditions in the HMF, largely determined by the dynamics of transients (Coronal Mass Ejections) may also be found at high latitudes, due to the wide latitude distribution of the CMEs.     

Interplanetary Coronal Mass Ejections Observed in the Heliosphere: 3. Physical Implications

We conclude the heliospheric image series with this third and final instalment, where we consider the physical implications of our reconstruction of interplanetary coronal mass ejections from heliospheric imagers. In Paper 1 a review of the theoretical framework for the appearance of ICMEs in the heliosphere was presented and in Paper 2 a model was developed that extracted the three-dimensional structure and kinematics of interplanetary coronal mass ejections directly from SMEI images. Here we extend the model to include STEREO Heliospheric Imager data and reproduce the three-dimensional structure and kinematic evolution of a single Earth-directed interplanetary coronal mass ejection that was observed in November 2007. These measurements were made with each spacecraft independently using leading edge measurements obtained from each instrument. We found that when data from the three instruments was treated as a single collective, we were able to reproduce an estimate of the ICME structure and trajectory. There were some disparities between the modelled ICME and the in situ data, and we interpret this as a combination of a slightly more than spherically curved ICME structure and a corotating interaction region brought about by the creation of a coronal hole from the CME eruption. This is the first time evidence for such a structure has been presented and we believe that it is likely that many ICMEs are of this nature.     

In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections

The heliospheric counterparts of coronal mass ejections (CMEs) at the Sun, interplanetary coronal mass ejections (ICMEs), can be identified in situ based on a number of magnetic field, plasma, compositional and energetic particle signatures as well as combinations thereof. We summarize these signatures and their implications for understanding the nature of these structures and the physical properties of coronal mass ejections. We conclude that our understanding of ICMEs is far from complete and formulate several challenges that, if addressed, would substantially improve our knowledge of the relationship between CMEs at the Sun and in the heliosphere.     

Shock Acceleration of Ions in the Heliosphere

Energetic particles constitute an important component of the heliospheric plasma environment. They range from solar energetic particles in the inner heliosphere to the anomalous cosmic rays accelerated at the interface of the heliosphere with the local interstellar medium. Although stochastic acceleration by fluctuating electric fields and processes associated with magnetic reconnection may account for some of the particle populations, the majority are accelerated by the variety of shock waves present in the solar wind. This review focuses on “gradual” solar energetic particle (SEP) events including their energetic storm particle (ESP) phase, which is observed if and when an associated shock wave passes Earth. Gradual SEP events are the intense long-duration events responsible for most space weather disturbances of Earth’s magnetosphere and upper atmosphere. The major characteristics of gradual SEP events are first described including their association with shocks and coronal mass ejections (CMEs), their ion composition, and their energy spectra. In the context of acceleration mechanisms in general, the acceleration mechanism responsible for SEP events, diffusive shock acceleration, is then described in some detail including its predictions for a planar stationary shock, shock modification by the energetic particles, and wave excitation by the accelerating ions. Finally, some complexities of shock acceleration are addressed, which affect the predictive ability of the theory. These include the role of temporal and spatial variations, the distinction between the plasma and wave compression ratios at the shock, the injection of thermal plasma at the shock into the process of shock acceleration, and the nonlinear evolution of ion-excited waves in the vicinity of the shock.     

Modelling of Solar Wind, CME Initiation and CME Propagation

Simulations of coronal mass ejections (CMEs) evolving in the interplanetary (IP) space from the Sun up to 1 AU are performed in the framework of ideal magnetohydrodynamics (MHD) by the means of a finite-volume, explicit solver. The aim is to quantify the effect of the background solar wind and of the CME initiation parameters, such as the initial magnetic polarity, on the evolution and on the geo-effectiveness of CMEs. First, three different solar wind models are reconstructed using the same numerical grid and the same numerical scheme. Then, different CME initiation models are considered: Magnetic foot point shearing and magnetic flux emergence. For the fast CME evolution studies, a very simple CME model is considered: A high-density and high-pressure magnetized plasma blob is superposed on a background steady state solar wind model with an initial velocity and launch direction. The simulations show that the initial magnetic polarity substantially affects the IP evolution of the CMEs influencing the propagation velocity, the shape, the trajectory (and thus, the geo-effectiveness).     

Interchange Reconnection: Remote Sensing of Solar Signature and Role in Heliospheric Magnetic Flux Budget

Interchange reconnection at the Sun, that is, reconnection between a doubly-connected field loop and singly-connected or open field line that extends to infinity, has important implications for the heliospheric magnetic flux budget. Recent work on the topic is reviewed, with emphasis on two aspects. The first is a possible heliospheric signature of interchange reconnection at the coronal hole boundary, where open fields meet closed loops. The second aspect concerns the means by which the heliospheric magnetic field strength reached record-lows during the recent solar minimum period. A?new implication of this work is that interchange reconnection may be responsible for the puzzling, occasional coincidence of the heliospheric current sheet and the interface between fast and slow flow in the solar wind.     

Isotopic Fractionation by Plasma Processes

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gy ago from the protosolar nebula. The solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. Data from space missions and theoretical models indicate that the isotopes of the volatile elements N, O, Ne, and Ar are fractionated by at most a few percent per atomic mass unit in different solar wind regimes. In contrast, isotopic abundances of solar and heliospheric energetic particles at energies larger than about 100 keV/amu are observed to strongly vary relative to solar abundances. Processes such as resonant acceleration or pre-acceleration by plasma waves, first-order Fermi acceleration, or propagation in the interplanetary plasma are discussed as causes for charge-to-mass dependent fractionation. This revised version was published online in August 2006 with corrections to the Cover Date.