Investigating the Onset of MHD Turbulence in the Corona: A Step Towards Better Solar Wind Forecasting

One of many elusive mysteries that affect our ability to understand the impacts of space weather events on society lies with understanding the stream of charged particles from the Sun that fill the solar system; the solar wind. The solar wind speed has extremes of a few hundred kilometers per second in benign conditions, to several thousand kilometers per second when storms are taking place, although a good average of the “fast” solar wind is about seven hundred kilometers per second. For many decades, since the pioneering work of Skylab scientists in the 1970s, we have associated slow and fast solar wind streams as originating in magnetically closed and open regions of the Sun’s corona. The former from active regions and the latter from regions that were dubbed “coronal holes” in the original work. Despite this correspondence, the means by which material gets into the wind, or how it can reach such ridiculous speeds, still eludes solar scientists.

Much like hurricane forecasting on Earth, a critical factor in determining storm path and arrival time lies in determining the wind structures that the storm is embedded in. In recent years HAO scientists have studied observations that may hold the key to the acceleration problem in coronal holes and also help in the closed regions. By measuring magnetic waves running through the Sun’s atmosphere called Alfven waves, named after the Nobel Prize winning Swedish scientist, HAO scientists have possibly revealed the onset of a critical turbulent process that takes place on both open and closed magnetic structures. Turbulence, in this case, is required to take the energy from the magnetic waves and use it to accelerate the particles away from the Sun.

Alfven waves observed by CoMP image
Alfven waves observed by CoMP

Much like sound and water waves, these Alfvenic waves refract (bend) and reflect depending on the material they are traveling through. Two critical observations have been made using the Coronal Multichannel Polarimeter (CoMP) at MLSO that demonstrate that conditions exist where Alfvenic waves can travel through each other - driving interference and turbulence - and also where they are travelling at high speed through slowly moving clumps material that likely produce multiple, persistent, wave reflections such that the waves, again, travel through one another gradually losing energy to the clumps of material as they both travel outward.

In the figure (right) we see Alfven waves observed by CoMP (top) traveling along an open magnetic structure at 750km/s as indicated by an inclined lines in these “space-time plots.” Those lines can be compared (bottom) to those produced by slower (150 km/s), moving (more inclined) clumps of material observed by the AIA instrument of the Solar Dynamics Observatory on the same solar structure. With the upgrade to CoMP in 2017 we will be able to trace this behavior in many more wavelengths and also to heights further away from the Sun, in which case we will see if the clumps of material gain speed with height, that is, they’re being accelerated, by the fast moving waves rippling through them. Should this come to pass, we will be able to study the solar wind acceleration process directly, and use that information to better improve forecasts of the solar wind conditions that greatly affect the predictions of solar storm arrival at Earth.