2.2 Space-based stereoscopy and tomography

Helios 1 and Helios 2 were the first solar-observing spacecraft (1974 – 1980) that were sent as close as 0.3 AU to the Sun and, thus, provided a stereoscopic vantage point. A first 3D reconstruction of a coronal mass ejection (CME) was accomplished with a computer assisted tomography (CAT) technique by Jackson and Froehling (1995), using the zodiacal light photometers on the spacecraft Helios 2 (0.3 AU from the Sun) and white-light coronagraph images from Solwind (near Earth).

After the launch of Skylab in 1973, a number of solar-dedicated space missions came along with imaging capabilities in extreme ultraviolet and soft X-rays (SMM, Yohkoh, SOHO), besides white-light coronagraphs (onboard SMM and SoHO). However, all these solar-dedicated space missions had near-Earth orbits, so that no true stereoscopy was feasible, but solar rotation-based stereoscopy could be carried out for reasonably stable structures in the solar corona. Such rotation-based stereoscopy was applied to coronal loops using Skylab images (Berton and Sakurai, 1985Jump To The Next Citation Point), to produce quasi-stereoscopic images of the soft X-ray-emitting corona using Yohkoh images (Batchelor, 1994), to perform 3D tomography of a coronal hole using a 2-week’s worth of Yohkoh images (Hurlburt et al., 1994Jump To The Next Citation Point), or to reconstruct the 3D geometry of prominences from synoptic SOHO/EIT images (Foullon, 2003Jump To The Next Citation Point). Since the plasma dynamics in individual coronal loops is relatively fast (hours) compared with the required time interval to measure a significant parallax effect (about a day), while the confining magnetic field is changing on a longer time scale, a special dynamic stereoscopy method has been developed to determine the 3D geometry of planar loops using SOHO/EIT images (Aschwanden et al., 1999Jump To The Next Citation Point, 2000Jump To The Next Citation Point).

The LASCO white-light coronagraph onboard SOHO was especially designed to detect and track CMEs. A tomographic inversion method was developed by Quémerais and Lamy (2002Jump To The Next Citation Point) to invert the 2D line-of-sight integrated electron density distribution in the solar corona from a single spacecraft (LASCO-C2), using additional constraints of spherical or axial symmetry. Another tomographic method was developed by Moran and Davila (2004), Moran et al. (2010), and Dere et al. (2005) to reconstruct the 3D density distribution from the combination of intensity and polarized brightness images from LASCO. Combining LASCO images over a 14-day period, Frazin et al. (2007Jump To The Next Citation Point) accomplished a 3D tomographic reconstruction of the entire corona. The spatial resolution of the tomographic inversion is ultimately limited by the smearing due to coronal dynamics.

In 2003, the Solar Mass Ejection Imager (SMEI) was launched, which is designed to map large-scale variations of the electron density in the heliospheric solar wind or in CMEs, by observing Thompson-scattered sunlight from a near-Earth orbit (Jackson et al., 2004). Using the additional constraint of a 3D kinematic heliospheric model, the 3D density distribution can be reconstructed with tomographic quality, a CAT method that is also called corotational tomography (Jackson and Hick, 2002, 2004).

Some serendipitous measurements with true stereoscopy using the Earth and a single interplanetary spacecraft, or with multiple interplanetary spacecraft were actually also carried out before the STEREO mission. The STEREO-1 Experiment used simultaneous radio observations from the Nançay radioheliograph and the Soviet planetary probe Mars-3 to measure the directivity pattern of solar type III bursts (Caroubalos and Steinberg, 1974). Multi-spacecraft observations from the International Sun Earth Explorer (ISEE-3), Pioneer Venus Orbiter (PVO), Helios 2, the High-Energy Astrophysical Observatory A (HEAO-A), and Ulysses were used to measure the directivity of solar flare hard X-ray bursts (Kane, 1981; Kane et al., 1992, 1998), in conjunction with the near-Earth spacecraft Geostationary Operational Environmental Satellites (GOES) or Yohkoh.

View Image

Figure 1: Left: Artists concept of the STEREO twin spacecraft 3D perspective on a coronal mass ejection launched from the Sun. Right: Orbits of STEREO-A (red) and STEREO-B (blue) with respect to the Earth’s annual orbit (green) (credit: NASA).

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