5.3 Heliospheric imagers

Today’s heliospheric imagers are the successors to the zodiacal-light photometers (Leinert et al., 1975) on the twin Helios spacecraft flown in solar orbits in the 1970s and early 1980s. SMEI, in particular, was designed to exploit the heliospheric remote sensing capability demonstrated by that instrument (Jackson, 1985; Webb and Jackson, 1990). Unlike Helios, which could only observe a few narrow strips across the sky, this new generation of imager could observe large areas simultaneously. SMEI was the first such imager, developed as a proof-of-concept U.S. Air Force experiment for operational forecasting. Launched in January 2003 on the Coriolis spacecraft, SMEI imaged nearly the entire sky in white light once per 102-minute spacecraft orbit, using three baffled camera systems. Individual frames are mapped into ecliptic coordinates to produce a nearly complete sky map (Figure 31View Image). SMEI was deactivated in September 2011 and over its 8.5 year lifetime it observed nearly 400 CMEs (e.g., Webb et al., 2006Jump To The Next Citation Point; Howard and Simnett, 2008Jump To The Next Citation Point) many of which were Earth-directed (e.g., Tappin et al., 2004Jump To The Next Citation Point; Howard et al., 2006Jump To The Next Citation Point; Webb et al., 2009Jump To The Next Citation Point) allowing the comparison with in-situ spacecraft and prediction of arrival times and speeds. Unlike with in-situ spacecraft, however, SMEI enabled the comparison with coronagraph events in any direction, enabling large-scale tracking and 3-D reconstruction. Figure 32Watch/download Movie is a movie of a halo-type CME that was tracked by SMEI until it produced a major geomagnetic storm at Earth.
View Image

Figure 31: A composite all-sky image from SMEI taken in February 2003. An equal-area Hammer–Aitoff projection centered on the Sun with North and South ecliptic poles at top and bottom. The dark circle is a zone of exclusion 20° in radius usually centered on the Sun. The inset box shows a large, loop CME in May 2003 superimposed on the all-sky image. CMEs can only be detected in the SMEI data by careful subtraction of backgrounds that include particle contamination because of its Earth orbit (for details see, e.g., Webb et al., 2006Jump To The Next Citation Point).

Get Flash to see this player.


Figure 32: avi-Movie (10682 KB) Orbit difference images of an Earthward halo from SMEI. Halo was visible as an arc over ≥ 150° of sky (arrows). Blacked-out areas are due to shuttering of bright sunlight and CCD noise from particles in Coriolis’ 840 km circular Earth orbit.

SMEI was used for CME tracking (Tappin et al., 2004Jump To The Next Citation Point; Webb et al., 2006; Howard et al., 2006Jump To The Next Citation Point, 2007Jump To The Next Citation Point), space weather forecasting (Howard et al., 2006Jump To The Next Citation Point; Webb et al., 2009Jump To The Next Citation Point; Howard and Tappin, 2010Jump To The Next Citation Point), and 3-D reconstruction (Tappin and Howard, 2009Jump To The Next Citation Point; 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, 2007Jump To The Next Citation Point; 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 (∼ 12– 85 R⊙), and HI-2 of 70°, from ∼ 19 – 89° elongation (∼ 68– 216R ⊙). 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 33View Image is a schematic showing the fields of view of the SECCHI telescopes. Figure 34Watch/download Movie 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 15 R ⊙, 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. (2008Jump To The Next Citation Point); Davies et al. (2009); and DeForest et al. (2011Jump To The Next Citation Point).

As shown in Figures 33View Image and 34Watch/download Movie, 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., 2008Jump To The Next Citation Point; 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 35Watch/download Movie shows an image from a HI-2 movie from DeForest et al. (2011Jump To The Next Citation Point), the movie is also included in this paper.

View Image

Figure 33: The fields of view of the STEREO SECCHI HI telescopes flanking that of the SOHO/LASCO C3 instrument. The SECCHI EUVI, COR1, and COR2 telescopes are Sun-pointed like LASCO but the COR2 field extends to only half that of C3. Image adapted from Harrison et al. (2008).

Get Flash to see this player.


Figure 34: mpeg-Movie (28364 KB) Combined views from all of the STEREO SECCHI telescopes during a series of CMEs in early April 2010. The bottom panels show the ST-B and ST-A views of the EUVI disk, COR1 and COR2 imagers out to 15 R⊙, 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. From the online data at: External Linkhttp://secchi.nrl.navy.mil/index.php?p=movies.

Get Flash to see this player.


Figure 35: avi-Movie (70269 KB) STEREO/HI2-A images following the latest data processing pipeline for SECCHI (DeForest et al., 2011).

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 (2009Jump To The Next Citation Point). 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).


  Go to previous page Go up Go to next page