Pioneering Insights into Giant Solar Twists
The puzzling and dramatic increase in the solar temperature from approximately 6000 degrees on the visible surface of the Sun (photosphere) to well over a million degrees as one travels higher into the overlaying tenuous ubiquitously magnetised solar corona, has remained at the forefront of astrophysical research for over half a century. An international collaboration of solar physicists between the University of Sheffield, Queen´s University Belfast and California State University (Northridge), have found unambiguous evidence for a new breed of wave phenomena under solar circumstances. The detailed findings of this important discovery will be published in the illustrious Science magazine on Friday, March 20th 2009.
|
Figure 1: Alfvén (or purely magnetic torsional) waves propagating upwards in a magnetic flux tube, the building block of the solar atmosphere.
|
Observations, based on some theoretical predictions and calculations led by Professor Erdélyi (Head of the Solar Physics and Space Plasma Research Centre -SP2RC-, Dept of Applied Mathematics, Univeristy of Sheffield) were designed by scientists from Queen´s University and have been carried out, by using the Swedish Solar Telescope in the Canary Islands, to detect for the first time these waves in the Sun´s lower atmosphere. The specific quest was to search for large-scale, purely magnetic oscillations in the turbulent and dynamic solar atmosphere. These unique magnetic oscillations, also called Alfvén waves, are fundamental for the energy household of the solar atmosphere. Alfvén waves may also crucially determine Space Weather, and offer the much-sought-after solution to the almost century-long problem of solar and stellar atmospheric heating.
Meticulous data analysis, lead by colleagues from Queen´s University, together with the theoretical interpretation lead by a scientist from the University of Sheffield, resulted in the discovery of these puzzling Alfvénic waves in magnetic plasma concentrations, also known as magnetic flux tubes, in the solar atmosphere. A magnetic flux tube may be realized as a vertical elastic rod twisted at its footpoints by shear motion (see Fig. 1). It was found that these purely magnetic waves, created by the torsional motion of the flux tube, propagate upwards from the solar surface with an average speed of over 20 km/s and may carry enough energy to heat the plasma to well over a few million degrees.
|
Figure 2: An expanding solar magnetic structure sandwiched between the solar surface (photosphere) and its middle atmosphere (chromosphere). Torsional Alfvén waves will be generated by the twisting and swirling of such structures, often bigger in size than the British Isles.
|
The existence of purely magnetic waves in highly magnetized environments was first proposed by Hannes Alfvén in 1942, who received a Nobel Prize a few decades later for his pioneering work in this area of research. Unequivocal evidence for the presence of these torsional Alfvén waves in the lower solar atmosphere, had until recently, remained elusive.
Purely magnetic torsional Alfvén waves are caused by the twisting and swirling of structures in the Sun´s atmosphere (Fig. 2), and can be detected by the periodic velocity signals emitted, as suggested by Prof Erdélyi (see Science, 318, 1572, 2007). The Alfvén waves discovered in this study were found to be associated with a large magnetic field concentration on the surface of the Sun, approximately twice the size of the British Isles. These strong magnetic fields manifest as bright features (Fig. 3), often with lifetimes exceeding one hour. The Swedish Solar Telescope is the largest solar telescope in Europe and produces some of the sharpest images currently available. Bearing in mind that the Sun is 150 million kilometres away, the measurements carried out are equivalent to reading the time on Big Ben in London from Tokyo, Japan.
Dr. Jess, who is the lead author of the influential Science paper, said: "Often, waves can be visualized by the rippling of water when a stone is dropped into a pond, or by the motions of a guitar string when plucked. However, Alfvén waves cannot be seen so easily. In fact, they are completely invisible to the naked eye! Only by examining the motions of structures and their corresponding velocities in the Sun´s turbulent atmosphere could we find, for the first time, the presence of these elusive Alfvén waves." Dr. David Jess has recently been awarded a prestigious STFC Postdoctoral Fellowship to pursue his research in this field over the next 3 years in collaborations between QUB and Sheffield.
|
Figure 3 Image captured by the Swedish Solar Telescope of a region on the surface of the sun approximately 8000 km x 8000 km in size. The brighter area near the middle of the image indicates the presence of Alfvén waves, which are invisible to the unaided eye. Note: NEVER look directly into the Sun!
|
Prof. Mathioudakis, the leader of the QUB Solar Group, said: "Understanding solar activity and its influence on the Earth´s climate is of paramount importance for human kind. The Sun is not as `quiet´ as many people think. The solar corona, visible from Earth only during a total solar eclipse, is a very dynamic environment which can erupt suddenly, releasing more energy than 10 billion atomic bombs. Our study makes a major advancement in the understanding of how the million-degree corona manages to achieve this feat."
Prof. Erdélyi, who led the theoretical interpretation of Alfvén waves said: "Heat was on to find evidence for the existence of Alfvén waves. International space agencies have invested considerable resources trying to find purely magnetic oscillations of plasmas in space, particularly in the Sun. These waves, once detected, can be used to determine the physical conditions in the invisible regions of the Sun and other stars, through the technique of magneto-seismology. It was a real thrilling experience to interpret the data found by my colleagues at Queen´s University."
|
