Coronal Heating, Weak MHD Turbulence, and Scaling Laws

<p style='text-indent:20px;'>We introduce the concept of intermittency dimension for the magnetohydrodynamics (MHD) to quantify the intermittency effect. With dependence on the intermittency dimension, we derive phenomenological laws for intermittent MHD turbulence with and without the Hall effect. In particular, scaling laws of dissipation wavenumber, energy spectra and structure functions are predicted. Moreover, we are able to provide estimates for energy spectra and structure functions which are consistent with the predicted scalings.</p>

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Can magnetized turbulence set the mass scale of stars?

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1883 ◽

2020 ◽

Vol 496 (4) ◽

pp. 5072-5088 ◽

Cited By ~ 2

Author(s):

Dávid Guszejnov ◽

Michael Y Grudić ◽

Philip F Hopkins ◽

Stella S R Offner ◽

Claude-André Faucher-Giguère

Keyword(s):

Star Formation ◽

Scaling Laws ◽

Initial Conditions ◽

Mass Function ◽

Stellar Mass ◽

Mass Scale ◽

Initial Mass Function ◽

Mhd Turbulence ◽

Numerical Resolution ◽

Stellar Masses

ABSTRACT Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the gizmo code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs ($\mathcal {M} \sim 10\!-\!50$). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M50 becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find $M_\mathrm{50} \,\, \buildrel\propto \over \sim \,\,M_\mathrm{0} \mathcal {M}^{-3} \alpha _\mathrm{turb}\, \mathrm{SFE}^{1/3}$ for GMC mass M0, sonic Mach number $\mathcal {M}$, virial parameter αturb, and star formation efficiency SFE = M⋆/M0. This fit agrees well with previous IMF results from the ramses, orion2, and sphng codes. Although M50 has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M50 to be $\gt 20 \, \mathrm{M}_{\odot }$, an order of magnitude larger than observed ($\sim 2 \, \mathrm{M}_\odot$), together with an excess of brown dwarfs. Moreover, M50 is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF.

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An Observational Test for Coronal Heating Models

Symposium - International Astronomical Union ◽

10.1017/s0074180900182476 ◽

2004 ◽

Vol 219 ◽

pp. 473-477

Author(s):

Lidia van Driel-Gesztelyi ◽

Pascal Démoulin ◽

Cristina H. Mandrini ◽

Louise K. Harra ◽

James A. Klimchuk

Keyword(s):

Magnetic Flux ◽

Flux Density ◽

Scaling Laws ◽

Emission Measure ◽

Coronal Heating ◽

Mhd Waves ◽

Magnetic Flux Density ◽

Coronal Loops ◽

Coronal Emission ◽

The Mean

We correlate the evolution of the mean X-ray flux, emission measure and temperature (Yohkoh SXT & BCS) with the magnetic flux density (SOHO/MDI) in active region NOAA 7978 from its birth throughout its decay, for five solar rotations. We show that these plasma parameters together with other quantities deduced from them, such as the density and the pressure, follow power-law relationships with the mean magnetic flux density (B). We derive the dependence of the mean coronal heating rate on the magnetic flux density. We use the obtained scaling laws of coronal loops in thermal equilibrium to derive observational estimates of the scaling of the coronal heating with B. These results are used to test the validity of coronal heating models. We find that models based on the dissipation of stressed, current-carrying magnetic fields are in better agreement with the observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona. This confirms, with smaller error bars, previous results obtained for individual coronal loops, as well as for the global coronal emission of the Sun and cool stars.

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