On the dynamics in evaporating cloud envelopes

Giuliani, J. L.

doi:10.1086/161731

Cited by 34 publications

(34 citation statements)

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“…where μ = 1.265 is the mean atomic mass (assuming solar metal abundances; Anders & Grevesse 1989), m H is the mass of the hydrogen atom, n H is the hydrogen number density, p t is the total pressure, that is, the sum of the thermal pressure and the magnetic pressure (the factor 1/ √ 4π is absorbed in the definition of B), E is the total energy density, that is, the sum of the thermal energy density (ρ ), the kinetic energy density and the magnetic energy, u is the plasma velocity, g is the solar gravity, Λ(T ) is the radiative loss function for optically thin plasma, F c is the conductive flux, H is a heating function whose only role is to keep the unperturbed atmosphere in energy equilibrium, c s is the sound speed for an isothermal plasma, Φ is a free parameter (<1, Giuliani 1984) that determines the degree of saturation of the thermal conduction; we set Φ = 0.9, which corresponds to quite an efficient conduction. The radiative losses were computed according to version 7 of the CHIANTI code (Landi et al 2012), assuming a density of 10 9 cm −3 and ionization equilibrium according to Dere (2009).…”

Section: Modelmentioning

confidence: 99%

MHD modelling of coronal loops: injection of high-speed chromospheric flows

Petralia

Reale

Orlando

et al. 2014

A&A

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Context. Observations reveal a correspondence between chromospheric type II spicules and bright upward-moving fronts in the corona observed in the extreme-ultraviolet (EUV) band. However, theoretical considerations suggest that these flows are probably not the main source of heating in coronal magnetic loops. Aims. We investigate the propagation of high-speed chromospheric flows into coronal magnetic flux tubes and the possible production of emission in the EUV band. Methods. We simulated the propagation of a dense 10 4 K chromospheric jet upward along a coronal loop by means of a 2D cylindrical MHD model that includes gravity, radiative losses, thermal conduction, and magnetic induction. The jet propagates in a complete atmosphere including the chromosphere and a tenuous cool (∼0.8 MK) corona, linked through a steep transition region. In our reference model, the jet initial speed is 70 km s −1 , its initial density is 10 11 cm −3 , and the ambient uniform magnetic field is 10 G. We also explored other values of jet speed and density in 1D and different magnetic field values in 2D, as well as the jet propagation in a hotter (∼1.5 MK) background loop. Results. While the initial speed of the jet does not allow it to reach the loop apex, a hot shock-front develops ahead of it and travels to the other extreme of the loop. The shock front compresses the coronal plasma and heats it to about 10 6 K. As a result, a bright moving front becomes visible in the 171 Å channel of the SDO/AIA mission. This result generally applies to all the other explored cases, except for the propagation in the hotter loop. Conclusions. For a cool, low-density initial coronal loop, the post-shock plasma ahead of upward chromospheric flows might explain at least part of the observed correspondence between type II spicules and EUV emission excess.

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Section: Modelmentioning

confidence: 99%

MHD modelling of coronal loops: injection of high-speed chromospheric flows

Petralia

Reale

Orlando

et al. 2014

A&A

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“…where c s is the isothermal sound speed, and φ is a number of the order of unity; we set φ = 1 according to the values suggested for stellar coronae (Giuliani 1984;Borkowski et al 1989;Fadeyev et al 2002, and references therein). We solve the MHD equations using cylindrical coordinates in the plane (r, z), assuming axisymmetry and the stellar surface lying on the r axis (see Fig.…”

Section: Mhd Modelingmentioning

confidence: 99%

X-ray emitting MHD accretion shocks in classical T Tauri stars

Orlando¹,

Sacco

Argiroffi

et al. 2010

A&A

103

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Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β 1). Aims. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β > ∼ 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results. The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β > 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β: for β > 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β ≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

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“…We set φ = 0.3 according to the values suggested for a fully ionized cosmic gas: 0.24 < φ < 0.35 (Giuliani 1984;Borkowski et al 1989;Fadeyev et al 2002, and references therein); we assumed that electron and ion temperatures are equal 3 . In order to trace the motion of the cloud material, we consider a passive tracer associated with the cloud.…”

Section: The Model Equationsmentioning

confidence: 99%

Crushing of interstellar gas clouds in supernova remnants

Orlando¹,

Peres

Reale

et al. 2005

A&A

112

148

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We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature T ≈ 1.7 × 10 6 K and ≈4.7 × 10 6 K, respectively), we explore two very different physical regimes: for M = 30, the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud, which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries.

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On the dynamics in evaporating cloud envelopes

Cited by 34 publications

References 0 publications

MHD modelling of coronal loops: injection of high-speed chromospheric flows

MHD modelling of coronal loops: injection of high-speed chromospheric flows

X-ray emitting MHD accretion shocks in classical T Tauri stars

Crushing of interstellar gas clouds in supernova remnants

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