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Velli, M.

Publications and source records attributed to Velli, M..

9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

A Model for Magnetically Coupled Sympathetic Eruptions

Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another remain poorly understood. We present a three-dimenSional MHD simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for the so-called twin filament eruptions. The suggested mechanisms are also applicable for sympathetic eruptions occurring in other magnetic configurations.

Torok, T.↗
NASA NTRS2007

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 the 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, which depends on the spectral index of total energy for given loop parameters, must vary from B3/2 for weak fields to B2 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.

corona↗
NASA NTRS2006

Magnetic effects change our view of the Heliosheath

In this paper we review these recent results, and present an additional simulation with constant neutral atom background. In this case the jet is still present but with reduced intensity. Further study, e.g., including neutrals and the tilt of the solar rotation from the magnetic axis, is required before we can definitively address how the heliosheath behaves. Already we can say that this region presents remarkable dynamics, with turbulent flows, indicating that the heliosheath might be very different from what we previously thought.

turbulent flows↗
NASA NTRS1995

Coronal heating, nanoflares and MHD turbulence

Coronal heating is at the origin of the 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 photospheric motions. Here we focus on theories which rely on magnetic fields and electric currents both for the energy transfer and storage in the corona. The dissipation of this energy, whether in the form of reconnection in current sheets (nanoflare?) or the dissipation of MHD waves, depends crucially on the development of extremely small scales in the coronal magnetic field, where kinetic effects are likely to be fundamental. The question of whether coronal heating and flares may be viewed respectively as the macroscopic, low-energy average and the high-energy, temporally intermittent aspect of the same underlying driven, dissipative, turbulent system is also addressed, with emphasis placed on the main observational and theoretical stumbling blocks in the way of a confinement or disproof of such a conjecture.

Velli, M.↗
NASA NTRS1995

What determines the direction of minimum variance of the magnetic field fluctuations in the solar wind?

The solar wind is not an isotropic medium; two symmetry axis are provided, first the radial direction (because the mean wind is radial) and second the spiral direction of the mean magnetic field, which depends on heliocentric distance. Observations show very different anisotropy directions, depending on the frequency waveband; while the large-scale velocity fluctuations are essentially radial, the smaller scale magnetic field fluctuations are mostly perpendicular to the mean field direction, which is not the expected linear (WkB) result. We attempt to explain how these properties are related, with the help of numerical simulations.

Grappin, R.↗
NASA NTRS1995

High amplitude waves in the expanding solar wind plasma

We simulated the 1-D nonlinear time-evolution of high-amplitude Alfven, slow and fast magnetoacoustic waves in the solar wind propagating outward at different angles to the mean magnetic (spiral) field, using the expanding box model. The simulation results for Alfven waves and fast magnetoacustic waves fit the observational constraints in the solar wind best, showing decreasing trends for energies and other rms-quantities due to expansion and the appearance of inward propagating waves as minor species in the wind. Inward propagating waves are generated by reflection of Alfven waves propagating at large angles to the magnetic field or they coincide with the occurrence of compressible fluctuations. In our simulations, fast and slow magnetoacoustic waves seem to have a level in the density-fluctuations which is too high when we compare with the observations. Furthermore, the evolution of energies for slow magnetoacoustic waves differs strongly from the evolution of fluctuation energies in situ.

Schmidt, J. M.↗