SMEI was used for CME tracking (Tappin et al., 2004; Webb et al., 2006; Howard et al., 2006, 2007), space weather forecasting (Howard et al., 2006; Webb et al., 2009; Howard and Tappin, 2010), and 3-D reconstruction (Tappin and Howard, 2009; Jackson et al., 2010b). SMEI observations have been compared with coronagraph and in-situ spacecraft measurements (Tappin et al., 2004; Tappin, 2006; Howard et al., 2006, 2007; Howard and Simnett, 2008; Webb et al., 2009) and compared with IPS observations (Jackson et al., 2008b; Bisi et al., 2008). While SMEI observed the entire sky beyond 20° elongation, its field of view was often obscured by energetic particle saturation during its passage through the magnetospheric polar caps and the South Atlantic Anomaly, and by hot pixel degradation.
In October 2006, the twin STEREO spacecraft were launched carrying the Heliospheric Imagers (HIs) (Howard et al., 2008a; Eyles et al., 2009). The HIs are part of the SECCHI suite of imaging telescopes on each spacecraft and view the inner heliosphere starting at an elongation of 4° from the Sun. HI-1 has a FoV of 20°, from 4 – 24° elongation (), and HI-2 of 70°, from 19 – 89° elongation (). There is a 5.3° overlap between the outer HI-1 and inner HI-2 FoVs. The HIs do not cover the entire position angle (PA) range around the Sun, but observe up to a 90° range in PA, usually centered on the ecliptic and viewing either east (HI-A) or west (HI-B) of the Sun. They do not suffer the same problems with particle saturation as SMEI did, but are constrained by their fields of view about the ecliptic plane. Combined with the coronagraphs, the HIs do provide for the first time a continuous view from the Sun to around 1 AU and the stereoscopic viewpoints enable the possibility for 3-D reconstruction using the coronagraphs and HI-1.
The STEREO spacecraft share similar 1 AU orbits about the Sun as the Earth but separate from the Sun-Earth line by 22.5° per year. STEREO-A (Ahead) leads the Earth in its orbit, while STEREO-B (Behind) lags. Figure 33 is a schematic showing the fields of view of the SECCHI telescopes. Figure 34 is a movie that illustrates these views from all the telescopes during a series of CMEs in early April 2010 that produced several geomagnetic storms at Earth (Davis et al., 2011). The bottom shows the B and A views of the EUVI disk, COR1 and COR2 coronagraph imagers out to , and the upper set shows the HI-1 and -2 fields viewing east (left, HI-A) and west (right, HI-B) of the Sun beyond the EUVI, COR1, COR2 set shown to scale. Most of the early work involving the STEREO-HIs and CMEs have focused on their detection and tracking, and comparison with in-situ spacecraft. Publications include Harrison et al. (2008); Davies et al. (2009); and DeForest et al. (2011).
As shown in Figures 33 and 34, the STEREO/SECCHI instrument suite provides an uninterrupted view from the Sun to around 90° elongation. While they do not have the full PA coverage of SMEI, their location outside the Earth’s magnetosphere removes noise sources that decreased the quality of SMEI images, such as energetic particle saturation from the cusp and South-Atlantic anomaly, glare from the moon, and the aurora. A number of large-scale solar wind transients have been tracked through the SECCHI field of view, including CMEs (e.g., Harrison et al., 2008; Davis et al., 2009; Möstl et al., 2010), corotating interaction regions (e.g., Sheeley Jr et al., 2008; Rouillard et al., 2008; Tappin and Howard, 2009), and solar wind “puffs” (e.g., Rouillard et al., 2010) and “blobs” (e.g., Sheeley Jr et al., 2009; Sheeley Jr and Rouillard, 2010).
The most recent scientific developments using SECCHI data involve a processing pipeline that reduces many sources of noise (starfield, F corona) from the dataset. This has permitted the tracking and measurement of features that were previously inaccessible. Analyses of these pipeline data are still in the preliminary stages, but early results include observations and measurements of CME flux ropes (Howard and DeForest, 2012a) and disconnection events (DeForest et al., 2012). Figure 35 shows an image from a HI-2 movie from DeForest et al. (2011), the movie is also included in this paper.
The important difference between heliospheric imagers and coronagraphs is that 3-D information is available in heliospheric imagers that is not available in coronagraphs. This is because the assumptions imposed on coronagraphs (Thomson scattering assumptions, low angles) are not adequate at large elongations and across large distances. This increases the difficulty of the analysis, but makes available additional information on the structure and kinematics of the CME. This thereby removes the need for auxiliary data to provide this information. The theory describing this ability is developed by Howard and Tappin (2009). More recently, papers are beginning to emerge that consider the 3-D structure of the CME, including Wood and Howard (2009), Lugaz et al. (2009, 2010), and Howard and Tappin (2009, 2010). Techniques involving the extraction of 3-D properties from heliospheric image data are reviewed by Howard (2011a).
Living Rev. Solar Phys. 9, (2012), 3
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