arXiv subjects

A. F. Rappazzo

Publications and source records attributed to A. F. Rappazzo.

22 records · Page 2Linked to original sources

arXiv2010

Nonlinear Processes in Coronal Heating and Slow Solar Wind Acceleration

This work consists of two parts: the first devoted to the study of the heating of the magnetically confined Solar Corona, and the second to the acceleration of the Slow Solar Wind. Direct 3D reduced MHD simulations are presented. They model the heating of coronal loops in the solar atmosphere via the tangling of coronal field lines by photospheric footpoints motions within the framework of the "Parker scenario". We have derived scalings of physical quantities with loop length, and the ratio of photospheric to coronal Alfven velocities. The development of a turbulent dynamics makes the dissipation rate independent of the Reynolds number. The dynamics in physical space are desribed by weak turbulence, which develops when an MHD system is embedded in a strong axial magnetic field. The slow wind originates in and around the coronal streamer belt. The LASCO instrument onboard the SOHO spacecraft has observed plasma density enhancements forming beyond the cusp of a helmet streamer. Previous theoretical models for the formation and initial motion of these density enhancements are improved. The average expansion suffered by a parcel of plasma propagating outward, and the diamagnetic force due to the overall magnetic field radial gradients are now included. It is found that the magnetized wake configuration is resistively unstable, that an outward accelerating magnetic island develops at the center of the streamer, and that density enhancements occur within the magnetic islands. The values of the acceleration and density contrasts can be in good agreement with LASCO observations, provided the spherical divergence of the magnetic lines starts beyond a critical distance from the Sun. This result provides a constraint on the topology of the magnetic field.

astro-ph.SR↗

arXiv2009

Turbulence, Energy Transfers and Reconnection in Compressible Coronal Heating Field-line Tangling Models

MHD turbulence has long been proposed as a mechanism for the heating of coronal loops in the framework of the Parker scenario for coronal heating. So far most of the studies have focused on its dynamical properties without considering its thermodynamical and radiative features, because of the very demanding computational requirements. In this paper we extend this previous research to the compressible regime, including an energy equation, by using HYPERION, a new parallelized, viscoresistive, three-dimensional compressible MHD code. HYPERION employs a Fourier collocation -- finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization. We show that the implementation of a thermal conduction parallel to the DC magnetic field induces a radiative emission concentrated at the boundaries, with properties similar to the chromosphere--transition region--corona system.

astro-ph.SR↗

arXiv2007

Nonlinear Dynamics of the Parker Scenario for Coronal Heating

The Parker or field line tangling model of coronal heating is studied comprehensively via long-time high-resolution simulations of the dynamics of a coronal loop in cartesian geometry within the framework of reduced magnetohydrodynamics (RMHD). Slow photospheric motions induce a Poynting flux which saturates by driving an anisotropic turbulent cascade dominated by magnetic energy. In physical space this corresponds to a magnetic topology where magnetic field lines are barely entangled, nevertheless current sheets (corresponding to the original tangential discontinuities hypothesized by Parker) are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfers energy from the scale of convective motions ($\sim 1,000 km$) down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. Current sheets constitute the dissipative structure of the system, and the associated magnetic reconnection gives rise to impulsive ``bursty'' heating events at the small scales. This picture is consistent with the slender loops observed by state-of-the-art (E)UV and X-ray imagers which, although apparently quiescent, shine bright in these wavelengths with little evidence of entangled features. The different regimes of weak and strong MHD turbulence that develop, and their influence on coronal heating scalings, are shown to depend on the loop parameters, and this dependence is quantitatively characterized: weak turbulence regimes and steeper spectra occur in {\it stronger loop fields} and lead to {\it larger heating rates} than in weak field regions.

astro-ph↗

arXiv2007

Coronal Heating, Weak MHD Turbulence and Scaling Laws

Long-time high-resolution simulations of the dynamics of a coronal loop in cartesian geometry are carried out, within the framework of reduced magnetohydrodynamics (RMHD), to understand coronal heating driven by motion of field lines anchored in the photosphere. We unambiguously identify MHD anisotropic turbulence as the physical mechanism responsible for the transport of energy from the large scales, where energy is injected by photospheric motions, to the small scales, where it is dissipated. As the loop parameters vary different regimes of turbulence develop: strong turbulence is found for weak axial magnetic fields and long loops, leading to Kolmogorov-like spectra in the perpendicular direction, while weaker and weaker regimes (steeper spectral slopes of total energy) are found for strong axial magnetic fields and short loops. As a consequence we predict that the scaling of the heating rate with axial magnetic field intensity $B_0$, which depends on the spectral index of total energy for given loop parameters, must vary from $B_0^{3/2}$ for weak fields to $B_0^{2}$ for strong fields at a given aspect ratio. The predicted heating rate is within the lower range of observed active region and quiet Sun coronal energy losses.

astro-ph↗