2.1 Ground-based stereoscopy and tomography

Ground-based solar observations are limited to optical and radio wavelengths. Since there is no appreciable parallax effect on global distances, stereoscopic or tomographic methods can only be carried out by taking advantage of the solar rotation. However, because of the huge brightness contrast, the solar corona can only be imaged during total eclipses or using (disk-occulting) coronagraphs.

The first attempts of a tomographic reconstruction of the 3D density distribution of the solar corona, based on a daily series of ground-based coronagraph images and potential-field extrapolations from magnetograms, is described in Altschuler (1979Jump To The Next Citation Point), which shows multiple coronal streamers as viewed from above the solar north pole.

A first stereoscopic triangulation of ray-like coronal structures at ≈ 1.2 – 3.0 solar radii was achieved from two white-light pictures during the total solar eclipse of 1991 July 11, observed 3 hours apart from sites at Hawaii, Mexico, and Brazil (Koutchmy and Molodenskii, 1992), which yields a parallax angle of ≈ 1.6°. Using similar white-light images taken with a radial filter from another eclipse, the 3D density structure of streamers extending into heliospheric layers was reconstructed by Koutchmy et al. (1997) and Vedenov et al. (2000), visualized as stereoscopic image pairs.

High-resolution solar images in radio wavelengths became readily available with radio interferometers such as the Very Large Array (VLA) at frequencies of >∼ 1 GHz around 1980. Reasonably stable radio-emitting structures in the solar corona, such as the optically-thick layers of free-free emission in active regions or the gyro-resonance layers above sunspots, can be imaged day by day, which allows to measure the altitudes from the parallax effect of the radio source centroids. Synthesizing these altitude measurements at multiple radio frequencies allows then to construct a 3D model of the magnetic field and/or plasma density, which is called the method of radio stereoscopy (Aschwanden et al., 1992Jump To The Next Citation Point; Aschwanden and Bastian, 1994aJump To The Next Citation Point,bJump To The Next Citation Point; Aschwanden et al., 1995Jump To The Next Citation Point).

Another approach to use radio-emitting coronal sources for coronal 3D density reconstruction is frequency tomography, which was pioneered at the Russian RATAN radio interferometer (Bogod and Grebinskij, 1997Jump To The Next Citation Point; Gelfreikh, 1998Jump To The Next Citation Point; Grebinskij et al., 2000Jump To The Next Citation Point). However, the multi-frequency imaging maps of optically thin plasma require a free-free opacity model to convert the radio frequencies into a geometric altitude.

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