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Dahlburg R.B.,U.S. Navy | Einaudi G.,Berkeley Research Assoc. Inc. | Taylor B.D.,U.S. Navy | Ugarte-Urra I.,George Mason University | And 3 more authors.
Astrophysical Journal | Year: 2016

The evolution of a coronal loop is studied by means of numerical simulations of the fully compressible three-dimensional magnetohydrodynamic equations using the HYPERION code. The footpoints of the loop magnetic field are advected by random motions. As a consequence, the magnetic field in the loop is energized and develops turbulent nonlinear dynamics characterized by the continuous formation and dissipation of field-aligned current sheets: energy is deposited at small scales where heating occurs. Dissipation is nonuniformly distributed so that only a fraction of the coronal mass and volume gets heated at any time. Temperature and density are highly structured at scales that, in the solar corona, remain observationally unresolved: the plasma of our simulated loop is multithermal, where highly dynamical hotter and cooler plasma strands are scattered throughout the loop at sub-observational scales. Numerical simulations of coronal loops of 50,000 km length and axial magnetic field intensities ranging from 0.01 to 0.04 T are presented. To connect these simulations to observations, we use the computed number densities and temperatures to synthesize the intensities expected in emission lines typically observed with the Extreme Ultraviolet Imaging Spectrometer on Hinode. These intensities are used to compute differential emission measure distributions using the Monte Carlo Markov Chain code, which are very similar to those derived from observations of solar active regions. We conclude that coronal heating is found to be strongly intermittent in space and time, with only small portions of the coronal loop being heated: in fact, at any given time, most of the corona is cooling down. © 2016. The American Astronomical Society. All rights reserved.

Velli M.,University of California at Los Angeles | Pucci F.,University of Rome Tor Vergata | Rappazzo F.,Advanced Heliophysics | Tenerani A.,University of California at Los Angeles
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2015

Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photosphericmotions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, 'tornadoes'). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection. ©2015 The Author(s) Published by the Royal Society. All rights reserved.

Rappazzo A.F.,Advanced Heliophysics
Astrophysical Journal | Year: 2015

The dynamics of magnetic fields in closed regions of solar and stellar coronae are investigated with a reduced magnetohydrodynamic (MHD) model in the framework of the Parker scenario for coronal heating. A novel analysis of reduced MHD equilibria shows that their magnetic fields have an asymmetric structure in the axial direction with variation length scale zℓ ∼ ℓB0/b, where B0 is the intensity of the strong axial guide field, b that of the orthogonal magnetic field component, and ℓ the scale of Equilibria are then quasi-invariant along the axial direction for variation scales larger than approximatively the loop length zℓ Lz, and increasingly more asymmetric for smaller variation scales zℓ ≲ Lz. The critical length zℓ ∼ Lz corresponds to the magnetic field intensity threshold b ∼ ℓB0/Lz. Magnetic fields stressed byhotospheric motions cannot develop strong axial asymmetries. Therefore, fields with intensities below such a threshold evolve quasi-statically, readjusting to a nearby equilibrium, without developing nonlinear dynamics or dissipating energy. But stronger fields cannot access their corresponding asymmetric equilibria; hence, they are out of equilibrium and develop nonlinear dynamics. The subsequent formation of current sheets and energy dissipation is necessary for the magnetic field to relax to equilibrium, since dynamically accessible equilibria have variation scales larger than the loop length zℓ Lz, with intensities smaller than the threshold b ≲ ℓB0/Lz. The dynamical implications for magnetic fields of interest to solar and stellar coronae are investigated numerically and the impact on coronalhysics discussed. © 2015. The American Astronomical Society. All rights reserved.

Tenerani A.,University of California at Los Angeles | Velli M.,University of California at Los Angeles | Rappazzo A.F.,Advanced Heliophysics | Pucci F.,University of Rome Tor Vergata
Astrophysical Journal Letters | Year: 2015

We study, by means of MHD simulations, the onset and evolution of fast reconnection via the ideal tearing mode within a collapsing current sheet at high Lundquist numbers (S ≥104 ). We first confirm that as the collapse proceeds, fast reconnection is triggered well before a Sweet¡VParker-type configuration can form: during the linear stage, plasmoids rapidly grow in a few Alfven times when the predicted ideal tearing threshold S.1/3 is approached from above; after the linear phase of the initial instability, X-points collapse and reform nonlinearly. We show that these give rise to a hierarchy of tearing events repeating faster and faster on current sheets at ever smaller scales, corresponding to the triggering of ideal tearing at the renormalized Lundquist number. In resistive MHD, this process should end with the formation of sub-critical (S≤104) Sweet-Parker sheets at microscopic scales. We present a simple model describing the nonlinear recursive evolution that explains the timescale of the disruption of the initial sheet. © 2015. The American Astronomical Society. All rights reserved.

Panasenco O.,Advanced Heliophysics | Martin S.F.,Helio Research | Velli M.,Jet Propulsion Laboratory
Solar Physics | Year: 2014

Recent high-resolution observations from the Solar Dynamics Observatory (SDO) have reawakened interest in the old and fascinating phenomenon of solar tornado-like prominences. This class of prominences was first introduced by Pettit (Astrophys. J. 76, 9, 1932), who studied them over many years. Observations of tornado prominences similar to the ones seen by SDO had already been documented by Secchi (Le Soleil, 1877). High-resolution and high-cadence multiwavelength data obtained by SDO reveal that the tornado-like appearance of these prominences is mainly an illusion due to projection effects. We discuss two different cases where prominences on the limb might appear to have a tornado-like behavior. One case of apparent vortical motions in prominence spines and barbs arises from the (mostly) 2D counterstreaming plasma motion along the prominence spine and barbs together with oscillations along individual threads. The other case of apparent rotational motion is observed in a prominence cavity and results from the 3D plasma motion along the writhed magnetic fields inside and along the prominence cavity as seen projected on the limb. Thus, the "tornado" impression results either from counterstreaming and oscillations or from the projection on the plane of the sky of plasma motion along magnetic-field lines, rather than from a true vortical motion around an (apparent) vertical or horizontal axis. We discuss the link between tornado-like prominences, filament barbs, and photospheric vortices at their base. © 2013 Springer Science+Business Media Dordrecht.

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