Full disk
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The solar disk
Solar and Heliosperic Observatory
SoHO is a spacecraft launched in 1995 and has been observing the Sun ever since. It is located in a halo orbit around the Earth-Sun L1 Lagrange point, which allows the satellite to study the Sun 24 hours a day, 365 days a year without interruptions. When observing from space our images do not get disturbed by the Earth's atmosphere. The MDI instrument on board SoHO takes full disk images of the Sun in the Ni II 6767 Å line, both in continuum intensity and magnetograms (maps of the solar magnetic fields).
This animation is made from MDI images taken on September 27, 2003, and alternates between intensity images and magnetograms recorded as close in time as possible. The field of view is the same, except that sometimes we see the image in intensity, and sometimes the magnetogram. Bundles of magnetic field break through the surface from the interior of the Sun, sometimes staying for weeks on the solar surface. When we look at the polarisation of light we can compute magnetic field maps. In those maps we observe both polarities of the emerged fields, positive (in white) and negative (in black). However, when we look in intensity we see dark and bright features superimposed on the solar surface. The reason why sunspots are dark in intensity images is because the enormous magnetic fields that form sunspots locally inhibit the convection of the Sun, thus not letting the heat from below rise to the surface. Other areas are bright because they are less opaque and we can see deeper in the Sun which is hotter (and thus brighter). Note the spatial coincidence between high magnetic fields (either positive or negative) with the presence of sunspots.
To download the movie, click HERE
Movie credit: SOHO (ESA, NASA)
Text credit: Ada Ortiz (ITA, University of Oslo)