Properties of magnetic loops responsible for solar atmospheric heating and solar wind generation
Dynamic Formation and Associated Heating of a Magnetic Loop. II. A Characteristic of an Emerging Magnetic Loop with the Effective Footpoint Heating Source
We investigated an emerging magnetic loop dynamically formed on the Sun, which has the effective footpoint heating source that may play a key role in heating a solar atmosphere with free magnetic energy in it. It is suggested that the heating source could be related to local compression of a plasma in the emerging loop by means of Lorentz force, which converts the magnetic energy to the internal energy of the plasma that is used to reaccelerate a decelerated downflow along the loop, eventually generating the source when the kinetic energy of the downflow is thermalized. By analyzing very high-cadense data obtained from a magnetohydrodynamic simulation, we demonstrate how the local compression is activated to trigger the generation of the heating source. This reveals a characteristic of the emerging loop that experiences a dynamic loop-loop interaction, which causes the local compression and makes the plasma gain the internal energy converted from the magnetic energy in the atmosphere. What determines the characteristic that could distinguish an illuminated emerging loop from a nonilluminated one is discussed.
Dynamic Formation and Associated Heating of a Magnetic Loop
To seek an atmospheric heating mechanism operating on the Sun we investigated a heating source generated by a downflow, both of which may arise in a magnetic loop dynamically formed on the Sun via flux emergence. Since an observation shows that the illumination of evolving magnetic loops under the dynamic formation occurs sporadically and intermittently, we performed a magnetohydrodynamic simulation of flux emergence to obtain a high-cadence simulated data, where temperature enhancement was identified at the footpoint of an evolving magnetic loop. Unlike a rigid magnetic loop with a confined flow in it, the evolving loop in a low plasma $\beta$ atmosphere is subjected to local compression by the magnetic field surrounding the loop, which drives a strong supersonic downflow generating an effective footpoint heating source in it. This may introduce an energy conversion system to the magnetized atmosphere of the Sun, in which the free magnetic energy causing the compression via Lorentz force is converted to the flow energy, and eventually reduced to the thermal energy. Dynamic and thermodynamic states involved in the system are explained.
Twist and Expansion Profiles of Coronal Magnetic Loops in an Emerging Flux Region
The generation of outflows from the Sun known as solar winds is coupled with the
heating of the solar corona, and both processes are operated in magnetic structures
formed on the Sun. To study the magnetic configuration responsible for these processes,
we use three-dimensional magnetohydrodynamic simulations to reproduce magnetic
structures via flux emergence and investigate their configurations. We focus on two key
quantities characterizing a magnetic configuration: the force-free parameter α and the flux
expansion rate fex, the former of which represents how much a magnetic field is twisted
while the latter represents how sharply a magnetic field expands.We derive distributions
of these quantities in an emerging flux region. Our result shows that an emerging flux
region consists of an outer part where a magnetic loop takes a large flux expansion rate
but a small value of α at their photospheric footpoints, and an inner part occupied by
those loops where a strong electric current flows. We also investigate the expansion
profile of a magnetic loop comprising an emerging flux region. The profile is given
by an exponential expansion type near the solar surface while it is given by a quadratic
expansion type in an outer atmosphere. These detailed magnetic configurations obtained
by this study contribute to developing a realistic model for the coronal heating and solar
wind generation.
