The Cool Alter-Ego of the Hot Solar Corona

Lead Research Organisation: University of St Andrews
Department Name: Mathematics and Statistics

Abstract

The Sun's corona or outer atmosphere has a staggeringly high temperature of several million degrees, over 200 times hotter than its surface. Explaining this feature, which is also present in most stars in the universe, represents one of the most outstanding unsolved puzzles in physics and astronomy.
However, coronal heating has a cool alter-ego. The corona is highly inhomogeneous and upholds a large amount of cold material, a cooling counterpart that is integral to coronal heating and through which unique insight can be gained. On Earth, the hotter and denser the air, the more it rains. Similar heating (evaporation) and cooling (condensation) cycles permeate the solar corona. Coronal cooling is mainly driven by the fundamental process of thermal instability, a mechanism that has recently been recognised as playing a major role in the solar corona but whose characteristics are still poorly understood. This cooling process leads to partially ionised, dense, multi-thermal and clumpy plasma that can drain back down to the solar surface as 'coronal rain', or remain magnetically suspended in the corona to form massive cool structures called prominences, whose destabilisation and eruption constitute the most hazardous phenomena for human exploration of space.
In this project I shall use my expertise in modelling waves and instabilities and my extensive experience in coordinated multi-wavelength observations with cutting edge instruments to investigate the coronal heating mechanisms and the formation of prominences and their eruption. I will strategically address coronal heating by investigating the atmospheric response to the heating in the form of cooling.

The characteristics of the thermal instability by-products, prominences and coronal rain, will be investigated by combining high resolution instruments for space (Hinode, IRIS, SDO) and ground (SST, ALMA) that allow for the first time the full temperature coverage of coronal cooling by thermal instability due to improved spatial, temporal and spectral resolution. The amount in the corona over time, the morphology and dynamics, will allow to develop quantitative models both of coronal heating and of coronal rain / prominence formation and eruption, and elucidate the solar atmospheric mass and energy cycle.

Numerical simulations (with Lare3D, AMR-VAC and Bifrost) and forward modelling (with FoMo and RH) will determine the observational signatures of the major heating candidate mechanisms, such as Alfvénic waves and magnetic reconnection in current sheets, which will then be compared with the observational results. The formation of thermal instability by-products will be investigated analytically and numerically and the cool chromospheric nature of this material will be exploited to achieve the highest resolution probe for the coronal magnetic field topology. This will provide a measure for elemental coronal structure and determine the spatial and temporal scales of the heating. The tracing of cold material and determination of heating mechanism signatures will allow to detect and quantify these mechanisms in action.

The loss of stability of prominences will be addressed by investigating novel ideas such as the MHD avalanche model, through which a kink instability of the small elemental structure can play a crucial role in the overall stability. The conversion of initial mutual magnetic helicity to self-helicity during the reconnection process will be investigated as a solution to the observed puzzling increase of twist during the eruption of prominences.

The St Andrews host institution is a world leader in the core subjects of this project: instabilities, waves and reconnection. Active collaboration and excellent project development opportunities are thus expected. My expertise gained through this project will be essential to fully exploit the capabilities for approved future solar projects such as Solar Orbiter, DKIST and Parker Probe.

Publications

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