2.3 The STEREO mission

Since about 90% of the literature on solar and heliospheric stereoscopy and tomography resulted from the STEREO mission at the time of writing, the STEREO mission is undoubtly the most important observatory for this scientific discipline.

The twin Solar TErrestrial RElations Observatory (STEREO) spacecraft were launched on October 26, 2006. At the end of January 2007 the two spacecraft separated and entered heliospheric orbits in opposite directions, STEREO-A(ahead) leading in east direction and STEREO-B(behind) trailing in west direction around the Sun, increasing their separation by ≈ 45° per year, but maintaining their average distance of ≈ 1.0 ± 0.1 AU from the Sun all the time (Figure 1View Image). At the time of writing, the two spacecraft are separated by ≈ 180°. These particular orbits provide solar data that are suitable for small-angle stereoscopy at the beginning of the mission (or whenever the separation angle is near zero or 180 degrees), while large-angle stereoscopy and tomography is feasible in the later years. The data rate is highest at the beginning of the mission, but drops continuously due to the lower telemetry rate later in the mission. The STEREO mission concept is described in Grigoryev (1993), Schmidt and Bothmer (1996), Socker et al. (1996), Davila et al. (1996), Davila (1998Jump To The Next Citation Point), Socker (1998), Rust (1998), Liewer et al. (1998), Bothmer et al. (1998), Socker et al. (2000), Mueller et al. (2003), Kaiser (2005), Kaiser et al. (2008), and Driesman et al. (2008).

Each of the two identical spacecraft contains a set of four instrument packages, including (i) the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite (Howard et al., 2002; Michels, 2002; Howard et al., 2008), which includes the Extreme UltraViolet Imager EUVI (Wülser et al., 2004; Delouille et al., 2008), the coronagraph COR-I for 1.4 – 4.0 solar radii (Thompson et al., 2003; Thompson and Reginald, 2008; Thompson et al., 2010), the coronagraph COR-II for 2 – 15 solar radii, and the Heliospheric Imagers (HI-1) for 8 – 45 solar radii and (HI-2) for 35 – 200 solar radii (Harrison et al., 2009), (ii) the In situ Measurments of PArticles and CME Transients (IMPACT) experiment (Luhmann et al., 2008), (iii) the PLAsma and SupraThermal Ion Composition (PLASTIC) experiment (Galvin et al., 2008), and (iv) the STEREO/WAVES (SWAVES) antenna (Bougeret et al., 2008). Supporting software to analyze STEREO data from the solar corona includes the Flexible Image Transport System (FITS) world coordinate system (Thompson, 2006; Thompson and Wei, 2010) and the atomic database CHIANTI for the response function of EUVI (Young and Landi, 2009).

While the STEREO mission is already 4 years underway at the time of writing, new solar-dedicated imager missions came along, such as the US-Japanese mission Hinode and the NASA mission Solar Dynamics Observatory (SDO), which provide valuable context information to STEREO with unprecedented high-resolution imagery in EUV, soft X-rays, and white-light wavelengths. Especially the combination of STEREO/EUVI-A, EUVI-B, and the Atmospheric Imaging Assembly AIA/SDO configure a powerful triple-viewpoint system for improved stereoscopy and tomography.

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