Quasi-periodic pulsations (QPP) in solar flares
The first observations of QPP in solar flares occured decades ago, though they remain poorly understood. Pulsations in flares are an important study because they imply the presence of some fundamental flaring physics. Why do some flares exhibit QPP and others not? What is the mechanism which generates them, and how does it stay periodic?
Image: An example of quasi-periodic pulsations in a microwave signal detected by the Nobeyama Radioheliograph at 17 GHz. The signal corresponds to an M-class flare from 8th May 1998.
Pulsations in solar flares are also notoriously difficult to quantify. Most examples of QPP are far less clear to the human eye than the example shown above. The duration of such events is also typically short; examples of QPP which exhibit more than four or five complete oscillations are rare. Since pulsations occur against the backdrop of a powerful flaring energy release, understanding and compensating for the background emission spectrum is also difficult.
A list of currently known flaring QPP events is available here.
Theory of QPP?
At present there are two main theories that could explain the presence of QPP in a flaring event. The first of these postulates that the observed pulsations are the product of magnetohydrodynamic (MHD) wave modes that have been generated in the flaring plasma. This is an attractive description which is also used to describe commonly observed oscillations in non-flaring coronal loops. Consider a cylindrical magnetic structure in the coronal plasma. For such a structure to be stable the pressure must be balanced at the cylinder's boundary. Using this condition, combined with the linearised MHD equations, it is possible to derive a wave equation describing the behaviour of the plasma. This derivation was well ilustrated by Edwin and Roberts (1983). This wave equation has oscillatory solutions inside the cylinder, and it is these which we consider in the context of QPP.
However, some flares cannot be easily explained by MHD wave modes. Another explanation is that the observed pulsations are caused by a periodic regime of magnetic reconnection occuring in the flare. Magnetic reconnection is thought to be a primary source of energy release in flares, however a mechanism which could drive reconnection periodically is not well established.