Sigmoids in the solar corona have been studied for many years, but little explanation of their formation or why they are often the seed of powerful solar flares have been forthcoming. Using high-resolution X-ray images from the Japanese-led solar mission Hinode (originally Solar-B), solar physicists have known that these very hot S-shaped structures are composed of many highly stressed magnetic flux tubes filled with energized plasma (also known as ‘fibrils’), but until now, little was known about the formation and flare eruption processes that occur in sigmoids.
Now, a team of solar physicists from the University of St Andrews believe they have found an answer using powerful magnetohydrodynamic (MHD) computer models, aiding our understanding of coronal dynamics and getting us one step closer to forecasting space weather…
The solar corona is one of the most mysterious regions in the Solar System. The corona is basically the atmosphere of the Sun, dominated by the solar magnetic field and tenuous (yet very, very hot) solar plasma. This is one of the most compelling questions hanging over solar research: why is the corona so hot?
In a nutshell, the corona is too hot. We know that the solar photosphere has a temperature of approximately 6000K, but only a short distance above the solar “surface”, solar plasma increases in temperature through the chromosphere and then through the transition region. From the photosphere to the corona (where the transition region is the lower limit), solar plasma is heated from 6000 K to over 1,000,000 K.
The mechanisms behind this coronal plasma heating has occupied solar physicists for decades (it even occupied me from 2002 to 2006). Is there a coronal heating mechanism? Is there a chromospheric plasma energization mechanism? Do acoustic waves, Alfven waves or nanoflares have a role to play?
Many of these questions are slowly being answered or at least debated (my research focused on modelling coronal heating by Alfven wave interactions along coronal loops) with the eventual goal to help us predict coronal dynamics. The corona is what drives solar flares, coronal mass ejections (CMEs) and solar wind, otherwise known as “space weather”. The more we understand about space weather, the better we can prepare ourselves for the onset of solar maximum and being hit by a barrage of high energy particles (the same particles that drive the beautiful aurorae are also responsible for damaging satellites and overloading national power grids on the ground).
So, to this end, Professor Alan Hood and Dr Vasilis Archontis from the Mathematical Institute at St. Andrews University, Scotland, will present their team’s results at the European Week of Astronomy and Space Science conference at the University of Hertfordshire, UK, from April 20th-23rd.
Hood and Archontis have studied high-resolution XRT (X-Ray Telescope) observations of the eruption phase of a distinctive sigmoid, revealing a complex bundle of individual X-ray emitting fibrils. The S-shape is actually caused by two oppositely-oriented J-like bundles of magnetic flux and heated plasma. Interestingly, toward the end of the sigmoid evolution, the region erupted as a solar flare.
Many models have been proposed over the years, but this event provided the St. Andrews solar physicists with a unique opportunity to test an improved MHD simulation. It looks like they may be close to explaining the dynamics that drive a flaring sigmoid.
“Sigmoids work as ‘mangers’ or ‘cocoons’ for solar eruptions. There is a high probability that they will result in powerful eruptions and other explosive events. Our model helps scientists understand how this happens.” — Dr Vasilis Archontis, University of St. Andrews.
Prof. Hood gos so far as directly linking the dynamics of sigmoids with how the Sun affects life on Earth. “Sigmoids are among the most interesting features for scientists trying to forecast the solar eruptions – events that can disrupt telecommunications, damage satellites and affect the way navigation systems are operated,” Hood added.
Looking at the output from the Hood and Archontis model, it would appear they may be close to understanding how the complex, and tightly wound magnetic field of the Sun may be influencing the dynamics of the corona. Now we have a better understanding about the underlying physics of sigmoids, perhaps we are approaching an era of better space weather prediction.