Subresolution Activity in Solar and Stellar Coronae from Magnetic Field Line Tangling

Abstract

The heating of coronal loops is investigated to understand the observational consequences in terms of the thermodynamics and radiative losses from the Sun as well as the magnetized coronae of stars with an outer convective envelope. The dynamics of the Parker coronal heating model are studied for different ratios of the photospheric forcing velocity timescale $t_p$ to the Alfvén crossing time along a loop $t_A$. It is shown that for $t_p/t_A\gtrsim$ 10--24 the heating rate and maximum temperature are largest and approximately independent of $t_p/t_A$, leading to a strong emission in X-rays and EUV. On the opposite decreasing $t_p/t_A$ to smaller values leads to lower heating rates and plasma temperatures, and consequently fading high-energy radiative emission once $t_p/t_A\lesssim$ 1--3. The average volumetric loop heating rate is shown to scale as ${\ell }_p{u}_p{B}_0^2/4\pi L^2$, where ${\ell }_p$ and $u_p$ are respectively the convective granule length-scale and velocity, $B_0$ is the intensity of the strong magnetic field threading the loop, and $L$ the loop length. These findings support a recent hypothesis explaining ultracool dwarf observations of stars with similar magnetic field strength but radically different topologies displaying different radiative emission.

arXiv comment11 pages, 5 figures, MNRAS (in press)

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Cited by in corpus (5)

• The Properties of the Solar Corona and Its Connection to the Solar Wind
• Nonlinear firehose relaxation and constant-B field fluctuations
• Energetics and 3-D Structure of Elementary Events in Solar Coronal Heating
• Resistively controlled primordial magnetic turbulence decay
• Magnetic field line twisting by photospheric vortices: energy storage and release