Prof. Paul Cally is currently full Professor of Solar Physics in the School of Mathematical Sciences at Monash University (Melbourne, Australia), position he has held since 2003. He is currently President of the International Astronomical Union Commission on Solar Activity. He obtained his PhD in Applied Mathematics from Monash in 1980, and held postdoctoral positions in the UK before returning to Monash as a lecturer in 1984. He has been an Affiliate Scientist at the High-Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado, USA since 1998. Prof Cally's research mainly concerns waves and instabilities in the solar plasma. He is a foremost expert on magnetohydrodynamics mode conversion and applications to solar activity. Paul has made fundamental contributions to magnetohydrodynamic (MHD) wave and instability theory, with particular reference to the Sun: 

• Definitive theory of leaky tube waves; 

• New interpretation of MHD phase mixing as a cascade in wavenumber space; 

• Solution of the sunspot p-mode absorption problem in terms of fast-to-slow mode conversion (both analytic and the first numerical simulations confirming the process); 

• Identification of several new instabilities in toroidal tachocline magnetic field; 

• development of a generalized ray theory that quantitatively models mode conversion; 

• Identification of fast-to-Alfvén mode conversion as an important process in wave behavior in active region atmospheres, and to local seismology.

During his visit to the Instituto de Astrofísica de Canarias (IAC), Prof. Cally will collaborate with Dr. E. Khomenko’s group to model some of the processes mentioned above. The overall objective of this collaborative project is to gain insights into how the solar atmosphere is heated by means of magnetohydrodynamic (MHD) waves. Mode conversion is one of the fundamental processes that affects energy propagation through the solar atmosphere and the heating of its upper layers: chromosphere and corona. Thanks to this process, acoustic p-modes generated by solar convection beneath the surface can transfer their energy to other types of waves (magneto-acoustic, Alfvén), which are able to reach the upper layers with relatively mild dissipation and are not affected by the acoustic cut-off. Although mode conversion theory was initially developed under the MHD approximation, it is now known that the solar plasma contains a large fraction of neutral atoms. Dr. Khomenko’s group is leading the international effort to investigate the influence of partial ionization on solar atmospheric heating processes. Our previous joint work has shown that neutrals can significantly degrade the energy flux of fast magneto-acoustic waves around the conversion region, thereby decreasing the flux of Alfvén waves reaching the corona. During Prof. Cally’s stay, we will work on developing a new and more robust theory of mode conversion: fast magnetoacoustic-to-Alfvén mode conversion, within a two-fluid (plasma-neutral) mathematical framework.