SH21D-01 INVITED 08:00h
The STEREO Mission: An Overview
In February 2006, NASA will launch the twin STEREO spacecraft 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 will use close flybys of the moon to escape into heliocentric orbits at 1 AU, with one spacecraft trailing Earth and the other leading Earth. As viewed from the sun, the two spacecraft will separate at approximately 45 degrees per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of CME initiation and to follow the propagation of CMEs through the heliosphere. Additionally, STEREO will study the mechanisms and sites of energetic particle acceleration and determine 3-D time-dependent traces of the magnetic topology, temperature, density and velocity of the solar wind between the sun and Earth. To accomplish these goals, each STEREO spacecraft will be equipped with set of optical, radio and in situ particles and fields instruments. The SECCHI suite of instruments includes two white light coronagraphs covering the range from 1.4 to 15 solar radii, an extreme ultra violet imager covering the chromosphere and inner corona, and two heliospheric white light imagers covering the outer corona from 12 solar radii to 1 AU. The IMPACT suite of instruments will measure in situ solar wind electrons in the energy range from essentially 0 to 100 keV, energetic electrons to 6 MeV, and protons and heavier ions to 100 MeV/nucleon. IMPACT also contains a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument will measure the composition of heavy ions as well as protons and alpha particles. The SWAVES instrument will use radio waves to track the location of CME-driven shocks and the 3-D topology of open field lines along which energetic particles flow. Additionally, SWAVES will measure in situ plasma waves to provide an independent estimate of the local plasma density and temperature. Each of the four instruments will produce a small real-time stream of selected data for purposes of predicting space weather events at Earth, primarily for NOAA forecasters.
SH21D-02 INVITED 08:22h
Preparing for STEREO - Revisit Helios!
Exactly 30 years ago, the first Helios solar probe was launched into an elliptical heliocentric orbit, with a perihelion of 0.3 AU. It had a set of then modern particle and field instruments on board, but no imagers. One year later, an almost identical probe was put into a very similar orbit, approaching the Sun even a bit closer. For most of their common lifetime of 4 years, the two probes were separated in longitude by no more than about 30 degrees. Further, due to their special orbits, they spent many months above the Sun's limb (as seen from Earth). In conjunction with the Earth-orbiting IMP 7&8 satellites and the Voyager 1&2, and the Pioneer 10&11 space probes, real multipoint studies covering large parts of the heliosphere could be performed successfully for the first time. The Helios mission resembled the upcoming STEREO mission in several respects and was indeed of good use for defining the STEREO science goals. For example, Helios could reveal details about the longitudinal and latitudinal solar wind stream structure, it allowed unique associations between limb CMEs and their radial propagation towards an in-situ observer, and the propagation of solar energetic particles could be studied. However, since then our understanding of the heliosphere has improved considerably, thanks to recent space missions (Ulysses, Yohkoh, SOHO, Wind, ACE) and to more and more refined theoretical models. In view of these new results, it appears worthwhile to revisit the huge Helios data sets since they certainly keep hiding some answers that future observations from STEREO might benefit from.
SH21D-03 INVITED 08:44h
What Can We Deduce about Coronal Mass Ejections from STEREO observations?
Coronal mass ejections (CMEs) have been a topic of considerable scientific interest for many years. Despite years of study, many questions remains unanswered. What is the 3D structure of CMEs? How do we relate the white-light images of CMEs near the Sun to the in situ properties of ICMEs? What initiates CMEs? The STEREO mission promises to yield an impressive array of images and in situ measurements that can be brought to bear on these questions. The challenges facing the solar and heliospheric community is to utilize this data to unravel the structure and dynamics of CMEs. In this talk we will use 3D MHD simulations of CMEs to illustrate some of the challenges we face in deducing the structure of CMEs from 3 different viewpoints. We will discuss how models may help to reduce these uncertainties. Research supported by NASA and NSF.
SH21D-04 INVITED 09:06h
3D Density Structure and LOS Observations of a Model CME
We present synthetic Thomson-scattered white-light images of a simulated coronal mass ejection (CME). The simulations are based on a 3-D MHD model of a CME propagating through a bimodal solar wind characteristic of solar minimum. The CME is driven by a 3-D Gibson-Low flux rope inserted in the helmet streamer of the steady-state corona. Synthetic coronograph images are produced that follow the evolution of the CME to 1 AU from several points of view. The white light images provide a basis for comparison with wide angle coronographs, like those of SMEI or STEREO. We find that a large amount of plasma is swept up from the solar wind by the CME-driven shock wave, which dominates the density structure far from the Sun. We also find that the shape of this compressed plasma is highly distorted by the variation in speed of the ambient solar wind. Comparisons of 2-D integrated images to the 3-D density structure show that the viewing angle severely effects the line-of-sight appearance of the CME, as well as the estimated mass of the CME from such 2D images.
SH21D-05 09:28h
On the use of STEREO Measurements for Solar Rotational Tomography
Polarized brightness measurements made by coronagraphs give the electron density ($N_e$) integrated along the line of sight (LOS). Solar rotational tomography (SRT) exploits the view angles provided by Sun's rotation to remove the effects of LOS integration and form three-dimensional images of $N_e$. Reconstructed three-dimensional $N_e$\ images based on LASCO-C2 data are presented. One difficulty with SRT is that the structure of the corona changes as the Sun rotates. This effect can be mitigated with simultaneous multiple view angles, such as those provided by the STEREO mission. In this work, SRT is applied to a time-dependent model corona with STEREO and single spacecraft geometries, and the effects of time-variability in the reconstructions are discussed.
SH21D-06 09:43h
Modeling of CME Visibility for the STEREO Mission
One of the objectives of the STEREO mission is to determine the three-dimensional configuration of CMEs. The STEREO mission consists of two identical spacecraft, one leading Earth and the other trailing Earth, which will separate from each other at the rate of about 45 degrees per year. To understand the visibility of CMEs and the ability to discern the 3D structure, we have been developing a "forward modeling" capability (RAYTRACE) described in another paper at this meeting. This capability complements the inversion technique we have also been developing. Using RAYTRACE we may compute synthetic total and polarized brightness images using the Thomson scattering formulae from an assumed electron density model. Several (geometric) models of a CME have been defined - loop, spherical shell, cylindrical shell and a graduated cylindrical shell (GCS). Since the GCS model is a reasonable simulation of a flux-rope CME, we have used it to investigate the appearance of a CME as a function of STEREO separation angle. In this model the angular size in the two directions, the height of the leading edge, the orientation of the structure in the corona and the radial electron density distribution can be specified. We present the results of this study and compare the simulations with observed CMEs from LASCO.