Resolving magnetic ORIGINs of the hot solar atmosphere

Project Information

ORIGIN

Grant agreement ID: 101039844

DOI

10.3030/101039844

EC signature date 11 February 2022

Start date 1 June 2022

End date 31 May 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 488 649,00

EU contribution

€ 1 488 649,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 1 - ORIGIN (Resolving magnetic ORIGINs of the hot solar atmosphere)

Reporting period: 2022-06-01 to 2024-11-30

Summary of the context and overall objectives of the project

The outer atmospheric layer of the Sun, the corona, is an enigma. Coronal plasma temperatures soar over a million degrees Kelvin, some 200–500 times that of the visible surface, the photosphere. It is well known that the corona is governed and structured by magnetic fields, that are generated in the sub-surface layers. Convective motions at the surface churn the magnetic fields, and the resulting Poynting flux is channelled into higher atmospheric layers, where it is dissipated to heat the plasma. The resulting coronal structures and loops trace the magnetic lines of force, and are best observed in the extreme ultraviolet (EUV) and x-rays. But the exact processes that channel the Poynting flux through the solar atmosphere and the nature of magnetic energy release responsible for coronal heating are poorly understood. From this enigmatic solar atmosphere, streams of charged particles accelerate away to form the solar wind. The generation and acceleration processes of the solar wind are actively debated. The phenomena of coronal heating and the solar wind formation are both thought to be intricately linked through magnetic reconnection and magnetohydrodynamic (MHD) waves, two fundamental processes in astrophysical plasmas.

The research goal of Project ORIGIN is to advance our understanding of the magnetic processes responsible for coronal heating and the generation of the solar wind. To this end, we use novel observations from the Extreme Ultraviolet Imager (EUI) and Polarimetric and Helioseismic Imager (PHI), two instruments onboard the Solar Orbiter spacecraft, to investigate intricate magnetic coupling between the photosphere and the corona. In particular, we study how photospheric motions, including processes of emergence and cancellation of magnetic fields at the surface, power the corona. To gain insights into the physical processes at play, we complement these observational investigations with state-of-the-art computer MURaM models that self-consistently simulate the solar atmosphere from the near-surface layers through the corona.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Photospheric convective motions randomly move and slowly stress coronal magnetic fields, leading to a warping or braiding of field lines around each other. Energy released during untangling of magnetic braids is an often hypothesised process to explain solar coronal heating. We investigated for the evidence and operation of magnetic braiding and subsequent untangling and plasma heating in different active regions loops using observations from the Solar Orbiter spacecraft. We did find evidence for the operation of magnetic braiding and intense localised plasma heating in the corona. We also found evidence for the operation of gentle and explosive types of magnetic reconnection in the corona. However, an intriguing result is that signatures of untangling magnetic braids are rather uncommon. We placed these results in the context of coronal heating models and discussed various possibilities for the lack of observable braiding in a majority of the coronal loops.

Similar to the hot corona, the solar wind is another important characteristic of the Sun. The solar wind is formed by streams of charged particles that escape into interplanetary space, inflating heliosphere to an extent of approximately 120 astronomical units, from the Sun. We investigated the nature of the origin of the solar wind. To this end, we used high-resolution observations of a south polar coronal hole recorded by the EUI. Our observations revealed widespread jet activity on small spatial scales of a few 100 km and on timescales of about 20–60 s. We estimated the kinetic energy content of these jets and found it to be in the range of a picoflare (10^21–10^24 erg). Based on their frequency in our observations, we suggested that the picoflare jets could be a substantial source of mass and energy flux into the solar wind throughout the solar cycle.

As a next step in our quest to understanding the origin of the solar wind, we studied coronal hole observations from the Solar Orbiter spacecraft to probe the role of picoflare jets in the process. We demonstrated that these small-scale jets originate from narrow magnetic network lanes in the photosphere. Based on global magnetic modelling, we found that the solar wind measured by in situ instruments on Solar Orbiter could be traced back to the coronal regions harbouring these picoflare jets. Our study provides a new comprehensive picture of the solar wind origins from small-scale picoflare jets.

Surface magnetic activity generally characterises the solar corona. The magnetically closed quiet-Sun corona outside active regions is populated with arch-like plasma loops. Although some numerical models that self-consistently include surface magnetoconvection lead to an ~1 MK hot corona, the observational correspondence on the connection between surface magnetic fields and coronal loops is not fully understood and explored. We investigated the nature of magnetic landscape at the base of coronal loops in the quiet-Sun. We utilised unprecedented coordinated observations of a quiet-Sun region recorded by PHI and EUI, at nearly the same high spatial resolution of ~200 km (on the Sun). Our observations revealed that surface magnetic field concentrations that evolve on shorter timescales of less than 5 minutes could be crucial to understanding the coronal dynamics and heating. Our study emphasised the key role of small-scale magnetic field in the structuring of the quiet-Sun corona.

Magnetic reconnection is a fundamental process in the universe. Intense plasma heating triggered by magnetic reconnection is emitted by ionised gas in an extended wavelength range. Particles accelerated from the reconnection site can interact with the ambient gas and produce hard X-ray emission. Typically, the site of particle acceleration is thought to be in the tenuous hot corona. To this end, we identified an a usual magnetic reconnection event with hard X-ray emission located in the solar chromosphere, a cooler and denser dynamic layer at the base of the solar corona. Although the presence of hard X-ray emission indicates gas heated to over 10 MK, we found some additional unusual characteristics, in particular with the extreme ultraviolet radiation emerging from the event, which shows only limited indications of hot plasma. Our study highlights the crucial gaps in our understanding of the multi-thermal magnetic reconnection heating events in the solar corona.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our work published on the topic of the solar wind linked to the tenuous small-scale jets from a polar coronal hole has direct implications to the understanding of the generation of the heliosphere and how sun-like stars might lose mass. Unexpectedly, we identified that the source region of particle acceleration, that is typically thought to be located in the solar corona, could be embedded in the high-dense chromosphere. This has implications for understanding of the nature of particle acceleration in astrophysical plasmas.

Rapidly revolving surface magnetic footpoints at the base of the solar corona.

Finely-structured jets emerging from a polar coronal hole.

Observations of small-scale braids in the solar corona.