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

Franco Rappazzo   Submitted: 2018-06-28 17:45

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 tp to the Alfvén crossing time along a loop tA. It is shown that for tp/tA ≳ 10-24 the heating rate and maximum temperature are largest and approximately independent of tp/tA, leading to a strong emission in X-rays and EUV. On the opposite decreasing tp/tA to smaller values leads to lower heating rates and plasma temperatures, and consequently fading high-energy radiative emission once tp/tA ≾ 1-3. The average volumetric loop heating rate is shown to scale as 𝓁p up B02/4π L2, where 𝓁p and up are respectively the convective granule length-scale and velocity, B0 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.

Authors: A. F. Rappazzo, R. B. Dahlburg, G. Einaudi, M. Velli
Projects: None

Publication Status: Published in Monthly Notices of the Royal Astronomical Society, Volume 478, Issue 2, 1 August 2018, Pages 2257?2266, https://doi.org/10.1093/mnras/sty1132
Last Modified: 2018-06-28 18:57
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Coronal Heating Topology: The Interplay of Current Sheets and Magnetic Field Lines  

Franco Rappazzo   Submitted: 2017-08-04 21:07

The magnetic topology and field line random walk (FLRW) properties of a nanoflare-heated and magnetically confined corona are investigated in the reduced magnetohydrodynamic regime. Field lines originating from current sheets form coherent structures, called current sheet connected (CSC) regions, which extend around them. CSC FLRW is strongly anisotropic, with preferential diffusion along the current sheets' in-plane length. CSC FLRW properties remain similar to those of the entire ensemble but exhibit enhanced mean square displacements and separations due to the stronger magnetic field intensities in CSC regions. The implications for particle acceleration and heat transport in the solar corona and wind, and for solar moss formation are discussed.

Authors: A. F. Rappazzo, W. H. Matthaeus, D. Ruffolo, M. Velli, S. Servidio
Projects: None

Publication Status: ApJ 844, 87 (2017)
Last Modified: 2017-08-07 12:04
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Interchange reconnection in a turbulent Corona  

Franco Rappazzo   Submitted: 2012-09-25 02:52

Magnetic reconnection at the interface between coronal holes and loops, so-called interchange reconnection, can release the hotter, denser plasma from magnetically confined regions into the heliosphere, contributing to the formation of the highly variable slow solar wind. The interchange process is often thought to develop at the apex of streamers or pseudo-streamers, near Y and X-type neutral points, but slow streams with loop composition have been recently observed along fanlike open field lines adjacent to closed regions, far from the apex. However, coronal heating models, with magnetic field lines shuffled by convective motions, show that reconnection can occur continuously in unipolar magnetic field regions with no neutral points: photospheric motions induce a magnetohydrodynamic turbulent cascade in the coronal field that creates the necessary small scales, where a sheared magnetic field component orthogonal to the strong axial field is created locally and can reconnect. We propose that a similar mechanism operates near and around boundaries between open and closed regions inducing a continual stochastic rearrangement of connectivity. We examine a reduced magnetohydrodynamic model of a simplified interface region between open and closed corona threaded by a strong unipolar magnetic field. This boundary is not stationary, becomes fractal, and field lines change connectivity continuously, becoming alternatively open and closed. This model suggests that slow wind may originate everywhere along loop-coronal hole boundary regions, and can account naturally and simply for outflows at and adjacent to such boundaries and for the observed diffusion of slow wind around the heliospheric current sheet.

Authors: A. F. Rappazzo, W. H. Matthaeus, D. Ruffolo, S. Servidio, M. Velli
Projects: None

Publication Status: ApJ Letters (accepted)
Last Modified: 2012-09-25 10:26
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Turbulent Coronal Heating Mechanisms: Coupling of Dynamics and Thermodynamics  

Franco Rappazzo   Submitted: 2012-08-14 01:16

Context. Photospheric motions shuffle the footpoints of the strong axial magnetic field that threads coronal loops giving rise to turbulent nonlinear dynamics characterized by the continuous formation and dissipation of field-aligned current sheets where energy is deposited at small-scales and the heating occurs. Previous studies show that current sheets thickness is orders of magnitude smaller than current state of the art observational resolution (~700 km). Aim. In order to understand coronal heating and interpret correctly observations it is crucial to study the thermodynamics of such a system where energy is deposited at unresolved small-scales. Methods. Fully compressible three-dimensional magnetohydrodynamic simulations are carried out to understand the thermodynamics of coronal heating in the magnetically confined solar corona. Results. We show that temperature is highly structured at scales below observational resolution and nonhomogeneously distributed so that only a fraction of the coronal mass and volume gets heated at each time. Conclusions. This is a multi-thermal system where hotter and cooler plasma strands are found one next to the other also at sub-resolution scales and exhibit a temporal dynamics.

Authors: R. B. Dahlburg, G. Einaudi, A. F. Rappazzo, M. Velli
Projects: None

Publication Status: A&A (in press)
Last Modified: 2012-08-14 06:39
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Shear Photospheric Forcing and the Origin of Turbulence in Coronal Loops  

Franco Rappazzo   Submitted: 2010-03-23 12:46

We present a series of numerical simulations aimed at understanding the nature and origin of turbulence in coronal loops in the framework of the Parker model for coronal heating. A coronal loop is studied via reduced magnetohydrodynamics simulations in Cartesian geometry. A uniform and strong magnetic field threads the volume between the two photospheric planes, where a velocity field in the form of a 1D shear flow pattern is present. Initially the magnetic field which developes in the coronal loop is a simple map of the photospheric velocity field. This initial configuration is unstable to a multiple tearing instability which develops islands with X and O points in the plane orthogonal to the axial field. Once the nonlinear stage sets in the system evolution is characterized by a regime of MHD turbulence dominated by magnetic energy. A well developed power law in energy spectra is observed and the magnetic field never returns to the simple initial state mapping the photospheric flow. The formation of X and O points in the planes orthogonal to the axial field allows the continued and repeated formation and dissipation of small scale current sheets where the plasma is heated. We conclude that the observed turbulent dynamics are not induced by the complexity of the pattern that the magnetic field-lines footpoints follow but they rather stem from the inherent nonlinear nature of the system.

Authors: A. F. Rappazzo, M. Velli, G. Einaudi
Projects: None

Publication Status: ApJ, 722, 65 (2010)
Last Modified: 2010-10-16 05:13
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Coronal Heating, Weak MHD Turbulence and Scaling Laws  

Franco Rappazzo   Submitted: 2007-02-06 14:32

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 B0, which depends on the spectral index of total energy for given loop parameters, must vary from B0^3/2 for weak fields to B0^2 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.

Authors: A.F. Rappazzo, M. Velli, G. Einaudi, R.B. Dahlburg
Projects: None

Publication Status: accepted for publications on ApJ Letters
Last Modified: 2007-02-06 14:32
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Abstracts by Author
Subresolution Activity in Solar and Stellar Coronae from Magnetic Field Line Tangling
Coronal Heating Topology: The Interplay of Current Sheets and Magnetic Field Lines
Interchange reconnection in a turbulent Corona
Turbulent Coronal Heating Mechanisms: Coupling of Dynamics and Thermodynamics
Shear Photospheric Forcing and the Origin of Turbulence in Coronal Loops
Coronal Heating, Weak MHD Turbulence and Scaling Laws

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