Periodic Reporting for period 4 - BOSS-WAVES (Back-reaction Of Solar plaSma to WAVES)
Période du rapport: 2022-04-01 au 2023-03-31
The project is concerned with the solar corona. The solar corona is the hot, outermost region of the solar atmosphere, and is located outside the visible disc of the Sun. Normally it is only visible during solar eclipses. The solar corona is much hotter (1000000 degrees) than the solar surface (6000 degrees), and it is not known why this is the case. Imagine going away from the camp fire (core of the Sun) and the temperature going up! Because of the high temperature of the solar corona, the gas is ionised and forms a plasma. Plasma is the 4th state of matter, and it responds to electric and magnetic fields. The magnetic fields organise the plasma in active regions and coronal loops.
In this project, we consider the option of heating the solar corona with transverse waves. These have been observed to be plentiful in the solar corona since the last decade. To assess this option, we perform 3D numerical modelling of coronal loops.
The heating of the solar corona is one of the great outstanding problems in astrophysics. Its impact on society is through space weather. That constitutes plasma that is outstreaming from the solar corona towards the Earth. Often solar explosions make the environment around the Earth extremely hostile. This has an effect on telecommunication and GPS satellites. In the worst cases, it can even lead to power outages. This project helps to better understanding the working of the solar corona, and thus it helps to more accurately model the resulting generation of the solar wind.
In this project, we consider the option of heating the solar corona with transverse waves. These have been observed to be plentiful in the solar corona since the last decade. To assess this option, we perform 3D numerical modelling of coronal loops.
The heating of the solar corona is one of the great outstanding problems in astrophysics. Its impact on society is through space weather. That constitutes plasma that is outstreaming from the solar corona towards the Earth. Often solar explosions make the environment around the Earth extremely hostile. This has an effect on telecommunication and GPS satellites. In the worst cases, it can even lead to power outages. This project helps to better understanding the working of the solar corona, and thus it helps to more accurately model the resulting generation of the solar wind.
In the project, we have considered two physical models for configurations in the solar corona. The first configuration is for closed coronal loops, and the other configuration is for open coronal plumes.
In the first configuration we have considered active region coronal loops, which are magnetically closed on both ends. After driving the loops with transverse waves, instabilities are formed, that resemble very much water waves generated by wind, i.e. the so-called Kelvin-Helmholtz instability. Through this instability, smaller and smaller scales are created in the magnetic field, and eventually the plasma is heated. In our current simulations, we obtain significant plasma heating, of up to 500000 degrees, comparable to radiative energy losses in the solar corona. Our latest results show a loop that can be heated by transverse wave driven turbulence, even in the presence of radiative losses. We have used the theoretical results in this project to formulate equations for simulating the whole solar coronal volume with this heating mechanism, in a parametrised way.
In the second configuration, we study the open magnetic field regions near the poles of the Sun. There the "loops" are magnetically closed on one end, but open on the other end. Thus, the generated waves do not return and continue going into outer space. Still, we have found a new way to also generate an instability, which we have called uniturbulence. We have studied how our simulations compare to the observations. There we have shown that our simulations can explain the correlation between the width of a spectral line and the Doppler shift can be self-consistently generated. We have obtained an analytic function that describes the heating in this case. This combines with the theoretical model for the first configuration to model the heating in the open field regions of the solar corona.
In the first configuration we have considered active region coronal loops, which are magnetically closed on both ends. After driving the loops with transverse waves, instabilities are formed, that resemble very much water waves generated by wind, i.e. the so-called Kelvin-Helmholtz instability. Through this instability, smaller and smaller scales are created in the magnetic field, and eventually the plasma is heated. In our current simulations, we obtain significant plasma heating, of up to 500000 degrees, comparable to radiative energy losses in the solar corona. Our latest results show a loop that can be heated by transverse wave driven turbulence, even in the presence of radiative losses. We have used the theoretical results in this project to formulate equations for simulating the whole solar coronal volume with this heating mechanism, in a parametrised way.
In the second configuration, we study the open magnetic field regions near the poles of the Sun. There the "loops" are magnetically closed on one end, but open on the other end. Thus, the generated waves do not return and continue going into outer space. Still, we have found a new way to also generate an instability, which we have called uniturbulence. We have studied how our simulations compare to the observations. There we have shown that our simulations can explain the correlation between the width of a spectral line and the Doppler shift can be self-consistently generated. We have obtained an analytic function that describes the heating in this case. This combines with the theoretical model for the first configuration to model the heating in the open field regions of the solar corona.
We go beyond the state of the art, because this project has produced a self-consistent 3D simulation that heats a coronal loop, maintaining it against radiative losses. This is the first time that this has been done. Before the project, 3D models for loops heated against radiative cooling was only possible for DC heating mechanisms, which assumed unrealistically high resistivity. With our current model, we have dropped the latter assumption, and thus our new model is more general. Moreover, it pushes ideas that only existed as cartoons or 1D models into the 3D realm, bringing the AC heating mechanisms to new levels. In this respect, the main goal of the ERC project was achieved.
Going beyond even the above proof of concept, we have modified our model to ever more realistic setups. A recent step forward is the inclusion of a chromosphere as a mass reservoir. The heating leads to evaporation of material, and can eventually compensate also for the losses via thermal conduction. In the near future, we also want to expand our model for a single loop to a full active region, encompassing many loops. This will partly be done through the analytic heating function we have obtained, but also through direct numerical simulation.
Going beyond even the above proof of concept, we have modified our model to ever more realistic setups. A recent step forward is the inclusion of a chromosphere as a mass reservoir. The heating leads to evaporation of material, and can eventually compensate also for the losses via thermal conduction. In the near future, we also want to expand our model for a single loop to a full active region, encompassing many loops. This will partly be done through the analytic heating function we have obtained, but also through direct numerical simulation.