Final Report Summary - ASTRODYN (Astrophysical Dynamos)
Magnetic fields are widespread in astrophysics and of significance also to cosmology and fusion plasmas. In the Sun and in the Galaxy, magnetic fields are produced by a self-excited dynamo through plasma motions converting kinetic energy into magnetic. The AstroDyn project has led to important advances regarding the following three important aspects of solar magnetism.
Firstly, using numerical simulations of strongly stratified rotating convection, we have demonstrated for the first time that in the nonlinear stage of the evolution, there is equatorward migration of dynamo waves at low latitudes. This is similar to what is observed in the Sun. The cycle period is also comparable to that of the solar dynamo, which is 22 years. Our model therefore supports the interpretation that the solar dynamo works throughout the entire convection zone and that the bottom of the convection zone does not play a crucial role, as is often assumed.
Secondly, we have put forward a new approach to the origin of sunspots. Previous thinking interpreted them as the endpoints of long flux ropes emerging from deep inside the Sun, while our new simulations show them as being self-consistently generated at the solar surface. This work has been a major mile stone since our first numerical detection of a so-called negative effective magnetic pressure instability (NEMPI) in 2011. Among the many subsequent papers in this field, we have shown that magnetic spots can be produced whose energy density can exceed that of the turbulent pressure near the top of the atmosphere. We have also shown that bipolar magnetic spots can be generated if we allow for an outer corona. While our work was designed to establish NEMPI as a new physical mechanism, it is not yet sufficiently realistic to allow proper comparison with sunspots. There is now ongoing work devoted to the investigation of the relation between NEMPI and real sunspots.
Thirdly, we have demonstrated that the outer corona plays an integral part in the solar dynamo, because it allows magnetic twist from the interior of the Sun to escape. Without it, the dynamo would not work efficiently. We have thereby developed the first model that couples the dynamo to coronal mass ejections emanating from the Sun. Using observations from the Ulysses spacecraft, we were able to determine the sign of magnetic twist in the solar wind separately at large and small scales. Our results showed a sign reversal at intermediate length scales, which is now understood and in agreement with our simulations. The ejecta from the Sun are known for affecting space weather and the radiation load in the upper Earth atmosphere. They are therefore of concern to airlines, because they need to reroute their flights to more southern latitudes away from the poles where the radiation is strongest during days of high solar activity.
Firstly, using numerical simulations of strongly stratified rotating convection, we have demonstrated for the first time that in the nonlinear stage of the evolution, there is equatorward migration of dynamo waves at low latitudes. This is similar to what is observed in the Sun. The cycle period is also comparable to that of the solar dynamo, which is 22 years. Our model therefore supports the interpretation that the solar dynamo works throughout the entire convection zone and that the bottom of the convection zone does not play a crucial role, as is often assumed.
Secondly, we have put forward a new approach to the origin of sunspots. Previous thinking interpreted them as the endpoints of long flux ropes emerging from deep inside the Sun, while our new simulations show them as being self-consistently generated at the solar surface. This work has been a major mile stone since our first numerical detection of a so-called negative effective magnetic pressure instability (NEMPI) in 2011. Among the many subsequent papers in this field, we have shown that magnetic spots can be produced whose energy density can exceed that of the turbulent pressure near the top of the atmosphere. We have also shown that bipolar magnetic spots can be generated if we allow for an outer corona. While our work was designed to establish NEMPI as a new physical mechanism, it is not yet sufficiently realistic to allow proper comparison with sunspots. There is now ongoing work devoted to the investigation of the relation between NEMPI and real sunspots.
Thirdly, we have demonstrated that the outer corona plays an integral part in the solar dynamo, because it allows magnetic twist from the interior of the Sun to escape. Without it, the dynamo would not work efficiently. We have thereby developed the first model that couples the dynamo to coronal mass ejections emanating from the Sun. Using observations from the Ulysses spacecraft, we were able to determine the sign of magnetic twist in the solar wind separately at large and small scales. Our results showed a sign reversal at intermediate length scales, which is now understood and in agreement with our simulations. The ejecta from the Sun are known for affecting space weather and the radiation load in the upper Earth atmosphere. They are therefore of concern to airlines, because they need to reroute their flights to more southern latitudes away from the poles where the radiation is strongest during days of high solar activity.