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

The imaging telescope on board the Transition Region and Coronal Explorer (TRACE) spacecraft observed the decaying transversal oscillations of a long [(130 +/- 6) x 10(6) meters], thin [diameter (2.0 +/- 0.36) x 10(6) meters], bright coronal loop in the 171 angstrom Fe(IX) emission line. The oscillations were excited by a solar flare in the adjacent active region. The decay time of the oscillations is 14.5 +/- 2.7 minutes for an oscillation with a frequency 3.90 +/- 0.13 millihertz. The coronal dissipation coefficient is estimated to be eight to nine orders of magnitude larger than the theoretically predicted classical value. The larger dissipation coefficient may solve existing difficulties with wave heating and reconnection theories.

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.

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We obtained high-resolution extreme-ultraviolet (EUV) spectra of solar active regions, quiet-Sun areas, and off-limb areas during 1991 May 7 and 1993 August 17 flights of NASA/Goddard Space FUght Center's Solar EUV Rocket Telescope and Spectrograph (SERTS). The 1991 flight was the first time a multilayer coated diffraction grating was ever used in space. Emission fines from the eight ionization stages of iron between Fe +9 (Fe x) and Fe + 16 (Fe xvn) were observed. Values of numerous density-and temperature-insensitive fine intensity ratios agree with their corresponding theoretical values.Intensity ratios among various fines originating in a common stage of ionization provide measurements of coronal electron density. Numerous density-sensitive ratios are available for Fe xm, and they yield active region density (cm -3 ) logarithms of 9.66 + 0.49 and 9.60 + 0.54 for the 1993 and 1991 flights, respectively, and a quiet-Sun density of 9.03 + 0.28 for the 1993 flight. Filling factors, calculated from the derived densities assuming a path length of 1 x 10 9 cm, range from several thousandths to nearly unity.Intensity ratios among fines originating in different ionization stages of iron yield measurements of coronal electron temperature in the isothermal approximation. The fine ratios yield temperatures ranging from 1.1 x 10 6 to 5.2 x 10 6 K for the active regions, and 1.0 x 10 6 to 2.1 x 10 6 K for the quiet Sun, depending upon the ionization stages used. The derived temperature diminishes with decreasing ionization stages. Fe xvn emission, detected in the active regions but not in the quiet areas, accounts for the higher maximum active region temperature. Derived active region temperatures are greater than their quiet-Sun counterparts for ratios that include fines from Fe xrv through Fe xvi; however, the derived active region and quiet-Sun temperatures are not statistically significantly different for line intensity ratios that involve only Fe x through Fe xm. The latter similarity in derived temperatures suggests the presence of similar thermal structures in all the areas observed, although the active regions also harbor hotter material.Differential emission measure (DEM) distributions were constructed for the active region and quiet-Sun observations obtained during both flights. The two quiet-Sun DEM curves and the 1993 active region DEM curve all show peaks between log T = 6.1 and 6.2. The 1993 active region DEM has a second peak between log T = 6.6 and 6.7, and the 1991 active region DEM has only one peak, between log T = 6.5 and 6.6. Thus, the 1993 active region DEM curve appears, in some sense, to be a composite of the quiet-Sun DEM curve and the 1991 active region DEM curve. The 1991 active region exhibited flaring activity, yielded higher fine ratio temperatures, and contained greater photospheric magnetic fields than the 1993 active region.