Tachocline Instabilities and Solar Activity: a Fifty-Six-Year Personal Perspective

Abstract:

There are several hydrodynamic and magnetohydrodynamic instabilities likely to be at work in the solar tachocline. Theoretical models of these instabilities have been developed over the past 25 years, ever since the tachocline was discovered. My primary focus will be on instabilities that arise from rotation and differential rotation, and/or toroidal magnetic fields, and have global scale at least in longitude. I will give a personal perspective on these instabilities, their physical significance, and how they may help determine evidence of magnetic activity we observe at the solar surface. I will also discuss their possible helioseismic signatures, and what they might contribute to the workings of the solar dynamo. A few possible paradoxes will be outlined, as well as how the theories could be made physically more realistic.