How the Sun’s corona is heated to million Kelvin temperatures remains an open and fundamental problem in solar physics, despite decades of intensive research. This dynamic region of the solar atmosphere sits above the photosphere, which has a temperature of (only) around 6000 K, meaning complex physics must be continuously occurring to transport energy upwards where it can be dissipated into the local plasma. One of the most promising theories proposed to explain coronal heating is the ubiquitous occurrence of small-scale magnetic reconnection events in the tenuous upper atmosphere, known as ‘nanoflares’. In the nanoflare model, the constant buffeting of the photosphere inputs stress into the coronal magnetic field (for example in the form of twisting and braiding) making it highly non-potential. At some point, the magnetic field is predicted to become too stressed, after which it spontaneously restructures to a lower energy state, releasing (some of) the accumulated non-potential energy to heat the local plasma. Potential multi-thermal signatures of magnetic reconnection (including, but not limited to, solar flares, Ellerman bombs, IRIS bursts, Explosive Events) have been observed in different magnetic environments (e.g. quiet-Sun, active regions), over a range of spatial and temporal scales. However, if this process is to be important in widespread coronal heating, then it must display signatures in wavelengths sampling high temperatures (MK) with a very high frequency down to (and even below) the spatial resolutions of the best telescopes available to researchers. One of the key early results from the European Space Agency’s (ESA) Solar Orbiter satellite was the discovery of thousands of localised transient EUV brightenings, colloquially referred to as ‘campfires’, on previously undetectable spatial scales throughout the quiet-Sun corona (close to 1 MK). The frequency and spatial coverage of these potential magnetic reconnection driven features has caused excitement in the solar physics community, with numerous high-impact follow-up studies providing intriguing new information about these events. We now know, for example, that many EUV brightenings form around 1-5 Mm above the solar photosphere and that potentially 70% overlie magnetic bipoles observed in the solar photosphere. Despite this progress, many important questions need to be answered before we can fully assess the role of these events in coronal heating. This international team contains experts in both solar observing and numerical magnetohydrodynamic (MHD) simulations and aims to drive rapid improvements in our understanding of EUV brightenings. This aim will be achieved by answering three important and complementary overarching questions, namely: (A) What is the connectivity between the ubiquitous localised EUV brightenings in data sampled by Solar Orbiter and previously reported bursts?; (B) What is the magnetic connectivity of EUV brightenings and what does this tell us about their formation mechanisms?; And, (C) can we reconcile the observational signatures of EUV brightenings, across a wide range of spectral windows, with numerical simulations of energy release in the solar atmosphere?
Team Members:
Chris J. Nelson (Team Lead) – European Space Agency
Lakshmi Pradeep Chitta (Co-Team Lead) – Max Planck Institute for Solar System Research
David Berghmans – Royal Observatory Belgium
Sanja Danilovic – Institutionen för astronomi, Stockholm
Viggo Hansteen – Bay Area Environmental Research Institute
Louise Harra – PMOD/WRC & ETH Zürich, Switzerland
Zhenghua Huang – Institute of Space Sciences, Shandong University
Lei Ni – Yunnan Observatories, Chinese Academy of Sciences
Susanna Parenti – IAS, Université Paris-Saclay
Hardi Peter – Max Planck Institute for Solar System Research
Sami Solanki – Max Planck Institute for Solar System Research
Yingjie Zhu (early-career member) – PMOD/WRC
Yajie Chen (early-career member) – Max Planck Institute for Solar System Research
Nancy Narang (affiliate member) – Royal Observatory Belgium