Elena Khomenko and Manuel Collados, both of them at the IAC, have published a review article retailing the latest advances, both theoretical and observations, in research in the waves which propagate in the Sun’s magnetic field. The article, with the title “Oscillations and waves in Sunspots” has recently been published in the annual journal Living Review in Solar Physics.
Sunspots are regions of intense magnetic field which act to channel magnetohydrodynamic waves (waves that can propagate through fluids which conduct electricity in the presence of magnetic fields). These waves come in different types, and their properties change, both horizontally and vertically, strongly influenced by the topology of the magnetic field. As with any type of waves, they undergo reflection and refraction and can be transformed into one another when conditions allow.
In this article a number of their properties are discussed, potencial mechanisms for exciting them, their propagation and conditions for the transformation of one type to another, as well as whether there are preferred frequencies in different places.
However, in spite of the considerable progress in this field, there are still doubts to be resolved. For example, we need to explain the origins of the different modes of oscillation, and the influence they may have on the find structure in the umbra (the dark central zone of a sunspot) or the penumbra (a ring of smaller filaments which surround it). Also it is still not determined whether the origin of the wave modes is outside the sunspots or if they are the result of weak magnetoconvection inside them, nor do we yet know how much energy the different modes of oscillation can transport, which is then dissipated in heating the higher layers of the solar atmosphere, which also needs quantification.
Questions such as these will stimulate further studies of our local star, as there is still much to learn about the complex mechanisms governing the interaction between the magnetic field, the sunspots, and the surrounding plasma.
Article: "Oscillations and Waves in Sunspots", by Elena Khomenko y Manuel Collados, Living Review. Solar Phys. 12, (2015), 6. DOI: 10.1007/lrsp-2015-6
Manuel Collados: mcv [at] iac.es