More than 50 years ago astronomers noticed that the Sun’s outermost layer, the corona, contains more silicon, iron, magnesium, and other elements with a low first-ionization potential (FIP) than does the photosphere, the layer from which most of the Sun’s light emerges. When modelers are simulating the corona and the solar wind, the so-called FIP effect provides an experimental constraint.
A compelling explanation for the elemental variation came in 2004, when physicist Martin Laming first put forward the idea that the effect could be caused by Alfvén waves. When the magnetohydrodynamic oscillations reflect or refract in the chromosphere, which lies between the corona and the photosphere, they separate the ions from neutral particles. Now a new finding by Deborah Baker of University College London and her colleagues strongly supports the theoretical prediction. For the first time, they simultaneously observed Alfvénic perturbations in the chromosphere of a large sunspot and the chemical composition in the overlying corona.
Baker and her colleagues began measuring the AR12546 sunspot on 20 May 2016. They used several instruments—namely the Extreme Ultraviolet Imaging Spectrometer (EIS) aboard Japan’s Hinode probe, the terrestrial Interferometric Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope in New Mexico, and the Atmospheric Imaging Assembly (AIA) from NASA’s Solar Dynamics Observatory satellite. The video below from the recent paper shows two days of observations from the AIA. The larger and smaller boxes indicate the respective fields of view for the EIS and IBIS. The right-hand video shows magnetogram data.
Baker and her colleagues analyzed the chemical composition of the coronal loops of arcing plasma directly above the sunspot. By modelling the magnetic field, they linked the loops containing an overabundance of Si to regions of Alfvén waves in the chromosphere. The waves give rise to the nonlinear ponderomotive force, which moves charged particles in an inhomogeneous electromagnetic field toward areas of weaker field strength. Other processes then transport the separated ions to the corona and produce a composition distinct from that of the underlying photosphere. (D. Baker et al., Astrophys. J. 907, 16, 2021.)
