Periodic Reporting for period 1 - ELISITY (fiEld Line helIcity and Solar eruptivITY)
Reporting period: 2021-03-01 to 2023-02-28
The Sun, although responsible for maintaining life on Earth, it often exhibits explosive behaviour which may have important consequences on our planet. Solar eruptivity consists of solar flares, intense, localized electromagnetic bursts, and Coronal Mass Ejections (CMEs), expulsions of magnetized solar plasma to the interplanetary space. When solar radiation, magnetic fields and particles hit Earth, they affect its space environment and can cause damages to human technology, and even life. Predicting solar activity and its effects, Space Weather in short, is thus very important, with a lot of effort being put into this direction.
It is well known that solar activity is of magnetic origin. The constant photospheric motions twist and tangle the magnetic field lines, resulting in the build up of energy into the magnetic field. Once this gets distorted enough it can experience a sudden rearrangement of its topology, known as magnetic reconnection, which releases the stored energy in an explosive manner.
A quantity that can measure the geometrical complexity of a magnetic field is magnetic helicity, which, moreover, is conserved in ideal magneto-hydrodynamic (MHD), the theoretical description of the solar atmosphere. A proxy for the density of relative magnetic helicity, the appropriate helicity in the case of the Sun, is relative field line helicity (RFLH), a quantity that can highlight the locations where helicity is more important. Before the start of this project, studies of RFLH were mostly restricted to approximations and/or MHD simulations of the Sun.
The main goal of this project was thus to study in detail the behaviour of RFLH in solar active regions (ARs) during the production and early stages of solar eruptions. Additionally, to determine whether RFLH could be used to define quantities which indicate solar eruptivity.
It is well known that solar activity is of magnetic origin. The constant photospheric motions twist and tangle the magnetic field lines, resulting in the build up of energy into the magnetic field. Once this gets distorted enough it can experience a sudden rearrangement of its topology, known as magnetic reconnection, which releases the stored energy in an explosive manner.
A quantity that can measure the geometrical complexity of a magnetic field is magnetic helicity, which, moreover, is conserved in ideal magneto-hydrodynamic (MHD), the theoretical description of the solar atmosphere. A proxy for the density of relative magnetic helicity, the appropriate helicity in the case of the Sun, is relative field line helicity (RFLH), a quantity that can highlight the locations where helicity is more important. Before the start of this project, studies of RFLH were mostly restricted to approximations and/or MHD simulations of the Sun.
The main goal of this project was thus to study in detail the behaviour of RFLH in solar active regions (ARs) during the production and early stages of solar eruptions. Additionally, to determine whether RFLH could be used to define quantities which indicate solar eruptivity.
The work of the project started with the decision of a suitable AR sample to be examined. This was chosen to be as diverse as possible regarding the level of activity and morphology of the individual ARs, and it consisted initially from nine ARs. After downloading the observational magnetometric data from the Helioseismic and Magnetic Imager (HMI) instrument of the Solar Dynamics Observatory (SDO), the next, and most demanding computationally, task was the reconstruction of the 3D coronal magnetic field for all ARs and all snapshots considered. This was done with a widely-used non-linear force-free extrapolation method and by using a high-performance computing facility in Greece. Starting from the 3D magnetic field, three more vector fields were computed: the appropriate potential magnetic field, and the respective vector potentials that generate the original and the potential magnetic fields. The final computational step in order to obtain RFLH was the field line integrations along the two magnetic fields. Since RFLH is generally a gauge-dependent quantity, its computation process was repeated with a different gauge setup.
Apart from RFLH, many other physical parameters of the ARs were estimated, such as various energies and magnetic helicities. The quality of the reconstructed coronal magnetic fields was also assessed with many metrics and only those with high levels of solenoidality were considered in the subsequent study.
The analysis of RFLH was restricted to the times around 28 big flares (above M-class) from seven ARs, and to the regions known as magnetic polarity inversion lines (PILs) where the magnetic field changes sign. The use of RFLH allowed us to estimate the relative magnetic helicity in these PILs, in addition to the other physical parameters. The comparison of the relative helicity of the PILs with their magnetic flux, a quantity known as the R-parameter and whose high values are related with strong solar flares, showed that the helicity-based R values are as good as the magnetic-field-based ones, and sometimes, outperform the traditional ones. Apart from these quantities, many other PIL-deduced parameters were examined as well, but they did not show similar potential in indicating solar eruptivity.
The results of the project were (and will be) published in two international peer-reviewed scientific journals. They were also disseminated through oral and poster presentations, physically or virtually, in eight international scientific conferences, both in Greece and abroad.
The researcher of the project also participated in outreach activities, at schools or with the general public, such as the online ‘Science is Wonderful! 2021’ event, or the ‘2022 European Researchers’ Night’ in Athens, Greece. He also gained experience in teaching activities at the host University and in familiarization with solar observations. More details for the project can be found at its webpage, at the address ‘myweb.uoi.gr/k.moraitis/elisity.html’, where links to individual results are given.
Apart from RFLH, many other physical parameters of the ARs were estimated, such as various energies and magnetic helicities. The quality of the reconstructed coronal magnetic fields was also assessed with many metrics and only those with high levels of solenoidality were considered in the subsequent study.
The analysis of RFLH was restricted to the times around 28 big flares (above M-class) from seven ARs, and to the regions known as magnetic polarity inversion lines (PILs) where the magnetic field changes sign. The use of RFLH allowed us to estimate the relative magnetic helicity in these PILs, in addition to the other physical parameters. The comparison of the relative helicity of the PILs with their magnetic flux, a quantity known as the R-parameter and whose high values are related with strong solar flares, showed that the helicity-based R values are as good as the magnetic-field-based ones, and sometimes, outperform the traditional ones. Apart from these quantities, many other PIL-deduced parameters were examined as well, but they did not show similar potential in indicating solar eruptivity.
The results of the project were (and will be) published in two international peer-reviewed scientific journals. They were also disseminated through oral and poster presentations, physically or virtually, in eight international scientific conferences, both in Greece and abroad.
The researcher of the project also participated in outreach activities, at schools or with the general public, such as the online ‘Science is Wonderful! 2021’ event, or the ‘2022 European Researchers’ Night’ in Athens, Greece. He also gained experience in teaching activities at the host University and in familiarization with solar observations. More details for the project can be found at its webpage, at the address ‘myweb.uoi.gr/k.moraitis/elisity.html’, where links to individual results are given.
The project fulfilled the objectives set out at the time of its proposal. During the project the behaviour of RFLH in a large sample of ARs, and at many individual instances, was examined, for the first time to such extent. Moreover, the examination of whether RFLH-derived parameters can be used as indicators of upcoming solar activity led to the definition of the helicity-based R parameter. This turned out to be as good as, and in some cases even better than, the respective magnetic-field-based parameter.
The results of the project have the potential to be further exploited in Space Weather applications that aim at the prediction of solar activity. The work of this project, by showcasing the helicity-based R parameter as a viable eruptivity indicator, is a first step towards this direction.
The results of the project have the potential to be further exploited in Space Weather applications that aim at the prediction of solar activity. The work of this project, by showcasing the helicity-based R parameter as a viable eruptivity indicator, is a first step towards this direction.