Nanoflares: Explosive Heating of our Sun's Atmosphere

The Sun is one of the most important objects for humankind, with solar activity driving "space weather" and having a profound effect on the Earth's environment. We can directly see the effects of the Sun's powerful radiation through fascinating sights on Earth, such as the aurora borealis. However, it is the paradoxical nature of our Sun's temperature structure that continues to frustrate scientists. One of the greatest scientific problems plaguing physicists is the fact that the outer atmosphere of our Sun is much hotter than its surface. Common sense leads us to believe that the local temperature will decrease as we move away from the Sun's 6000 K surface temperature. However, the corona, an atmospheric layer a few thousand km above the surface, radiates with a temperature exceeding one million degrees. Efforts to understand the heating processes responsible have remained at the forefront of observational and theoretical research for over 50 years, producing a popular class of theory known as flare heating. This mechanism suggests that turbulent plasma processes cause the magnetic field lines embedded in the Sun's atmosphere to become twisted and stretched. The process of magnetic reconnection results in these strained field lines returning to a more stable configuration, but releasing huge quantities of energy in the process. A large-scale solar flare can release in excess of 10^25 Joules of energy during a single event; the equivalent of over 5 million times more than the total combined energy of all atomic bombs ever detonated. However, these large events are too rare to support the continuously elevated temperatures in our Sun's outer atmosphere. Instead, it is believed that small-scale flares, or "nanoflares" with an equivalent energy of a single modern atomic bomb, may occur with such regularity that they can provide a continuous basal background heating.

It is my desire to help improve our understanding of the physical processes at work within the Sun's atmosphere, an object that is so influential to life on Earth. A natural consequence of understanding the effects of solar flares will be the ability to predict solar activity, something that will ultimately allow us to protect ourselves from fierce outbursts of space weather. To pursue this crucial agenda, we need to observe and model these explosive processes occurring in the Sun's atmosphere on their intrinsic scales. A new breed of highly sensitive scientific cameras will allow for the first time fundamental processes associated with the release of magnetic energy to be studied at an unprecedented level of detail. With an STFC Research Grant, a post-doctoral researcher will employ one of these modern pieces of equipment to image a variety of magnetic structures in the Sun's atmosphere with frame rates approaching 100 per second. The intensities of all structures will be monitored, with the characteristics of small-scale nanoflares evaluated. An in-depth examination of the regions where nanoflare activity is omnipresent will allow the processes at work to be compared precisely to the underlying magnetic field configurations. Fundamental parameters deduced from high-resolution observations will be incorporated into advanced computer simulations. Large computer clusters, often exceeding 200 CPUs, will be used to examine the effects of sub-resolution nanoflare activity on the intensity profiles extracted from the high-resolution observations. A direct comparison between the simulations and the observations will be undertaken, with key nanoflare characteristics determined, including the reconnection rates, the total energy released, and the plasma relaxation time scales. For the first time, the conditions promoting nanoflares will be understood, with the specific role they play in the heating of our Sun's atmosphere evaluated.

The proposed programme of research will have significant impact worldwide, including important aspects within the scientific community, beneficiaries within the commercial sector, and assisting with the public understanding of science. A brief overview of how this research will contribute is outlined below.

Commercial Sector:
Current instrumentation on ground-based telescopes, including the STFC-supported Rapid Oscillations in the Solar Atmosphere (ROSA; Jess et al. 2010, Sol. Phys., 261, 363) camera system, typically employ 1 megapixel CCD cameras for scientific imaging at frame rates of 30 per second. Recently, the UK has benefitted from a major industrial drive to provide high sensitivity, fast readout, and scientifically calibrated imaging solutions for the upcoming Advanced Technology Solar Telescope and the European Solar Telescope, which are due to see first light in 2018 and 2025, respectively. The next-generation sCMOS detectors house much larger 5.5 megapixel chips, and when combined with superior read noise characteristics and high quantum efficiencies, will not only result in a major increase in the size of the captured field-of-view (550% increase), but also in higher frame rates (up to 100 per second) to allow dynamic phenomena to be captured with unprecedented accuracy. The proposed programme of research will pioneer the use of these cutting-edge cameras which have been designed for research projects that require large format detectors operating at high frame rates, hence producing very large data rates. For example, a new sCMOS camera will generate approximately 30 TB of data during a typical observing run of 8 hours. ANDOR Technology, a world leader in scientific imaging and spectroscopy, has identified large format high-speed sCMOS detectors as a growing market, especially with the next generation of astronomical telescopes requiring such detectors. Over the next number of years, I plan to provide expertise to ANDOR to assist their international competitiveness by creating:

1. Custom design solutions for large area, low noise cameras operating at high frame rates. Key to this will be the fundamental requirement to maintain respectable levels of sensor cooling (thus eliminating noise associated with dark current), a particular challenge for large area detectors.

2. Investigations into the design of mosaic platforms.

3. Answers to the issue of multi-port readout functionality - i.e. how to handle the data generated simultaneously by numerous high-speed detectors.

Public Understanding of Science:
The Sun has recently attracted a wealth of media attention due to its heightened activity in the form of flares, coronal mass ejections, and associated aurora in the Earth's upper atmosphere. This media fascination is likely to continue for a number of years until the Sun passes through a maximum in its solar cycle. There is always a high demand for information that the general public can readily grasp and understand. Due to the close proximity of the Sun to the Earth, and its profound impact on life, I feel the expected major outcomes of the proposed programme of research will be suitable for widespread media involvement. To date, I have extensively publicised my research activities through both professional and public outreach activities, including invited seminars at amateur astronomy meetings, and participation in high-school open days. In order to promote public understanding of this work, and indeed stir the imagination of all in the field of science, it is imperative to continue public outreach activities. My work has received widespread media attention in recent years through reports, press releases and radio interviews in conjunction with the British Broadcasting Corporation (BBC), NASA, local and national newspapers, and high-impact international magazines including National Geographic. I envisage this project will maintain a high level of media linkage over the next number of years.