Intermittent heating of the solar corona by MHD turbulence

As the dissipation mechanisms considered for the heating of the solar corona would be sufficiently efficient only in the presence of small scales, turbulence is thought to be a key player in the coronal heating processes: it allows indeed to transfer energy from the large scales to these small scales. While Direct numerical simulations which have been performed to investigate the properties of magnetohydrodynamic turbulence in the corona have provided interesting results, they are limited to small Reynolds numbers. We present here a model of coronal loop turbulence involving shell-models and Alfvén waves propagation, allowing the much faster computation of spectra and turbulence statistics at higher Reynolds numbers. We also present first results of the forward-modelling of spectroscopic observables in the UV

Nonlinear diffusion equations for anisotropic MHD turbulence with cross-helicity

Nonlinear diffusion equations of spectral transfer are systematically derived for anisotropic magnetohydrodynamics in the regime of wave turbulence. The background of the analysis is the asymptotic Alfv\'en wave turbulence equations from which a differential limit is taken. The result is a universal diffusion-type equation in ${\bf k}$-space which describes in a simple way and without free parameter the energy transport perpendicular to the external magnetic field ${\bf B_0}$ for transverse and parallel fluctuations. These equations are compatible with both the thermodynamic equilibrium and the finite flux spectra derived by Galtier et al. (2000); it improves therefore the model built heuristically by Litwick \& Goldreich (2003) for which only the second solution was recovered. This new system offers a powerful description of a wide class of astrophysical plasmas with non-zero cross-helicity.Comment: 20 pages, 3 figure

Flows at the Edge of an Active Region: Observation and Interpretation

Upflows observed at the edges of active regions have been proposed as the source of the slow solar wind. In the particular case of Active Region (AR) 10942, where such an upflow has been already observed, we want to evaluate the part of this upflow that actually remains confined in the magnetic loops that connect AR10942 to AR10943. Both active regions were visible simultaneously on the solar disk and were observed by STEREO/SECCHI EUVI. Using Hinode/EIS spectra, we determine the Doppler shifts and densities in AR10943 and AR10942, in order to evaluate the mass flows. We also perform magnetic field extrapolations to assess the connectivity between AR10942 and AR10943. AR10943 displays a persistent downflow in Fe XII. Magnetic extrapolations including both ARs show that this downflow can be connected to the upflow in AR10942. We estimate that the mass flow received by AR10943 areas connected to AR10942 represents about 18% of the mass flow from AR10942. We conclude that the upflows observed on the edge of active regions represent either large-scale loops with mass flowing along them (accounting for about one-fifth of the total mass flow in this example) or open magnetic field structures where the slow solar wind originates.Comment: 20 pages, 9 figures, accepted for publication in Astrophys.

Intermittent turbulent dynamo at very low and high magnetic Prandtl numbers

Context: Direct numerical simulations have shown that the dynamo is efficient even at low Prandtl numbers, i.e., the critical magnetic Reynolds number Rm_c necessary for the dynamo to be efficient becomes smaller than the hydrodynamic Reynolds number Re when Re -> infinity. Aims: We test the conjecture (Iskakov et al. 2007) that Rm_c actually tends to a finite value when Re -> infinity, and we study the behavior of the dynamo growth factor \gamma\ at very low and high magnetic Prandtl numbers. Methods: We use local and nonlocal shell-models of magnetohydrodynamic (MHD) turbulence with parameters covering a much wider range of Reynolds numbers than direct numerical simulations, but of astrophysical relevance. Results: We confirm that Rm_c tends to a finite value when Re -> infinity. The limit for Rm -> infinity of the dynamo growth factor \gamma\ in the kinematic regime behaves like Re^\beta, and, similarly, the limit for Re -> infinity of \gamma\ behaves like Rm^{\beta'}, with \beta=\beta'=0.4. Conclusion: Comparison with a phenomenology based on an intermittent small-scale turbulent dynamo, together with the differences between the growth rates in the different local and nonlocal models, indicate a weak contribution of nonlocal terms to the dynamo effect.Comment: 5 pages, 6 figure

Mitigation of Industrial Hazards by Water Spray Curtains

PresentationA Nowadays, the water spray curtain is recognized as a useful technique to mitigate major industrial hazards. It combines attractive features such as simplicity of use, efficiency and adaptability to different types of risks. In case of accidental toxic gas releases, the spray curtain may be used as a direct-contact reactor exchanging momentum, heat and mass with the gas phase. The cloud is diluted, warmed, and if toxic, some of its toxic content can be absorbed by the droplets to which chemical reactants can be added. In case of fire, water sprays can provide thermal shielding to maintain the integrity of storage tanks. The curtain behaves as a filter and can produce significant attenuation of the incident radiation that impinges on crucial structures such as petro-chemical storage tanks. Both of these applications have been thoroughly investigated at the von Karman Institute The outcomes of these research projects is a comprehensive engineering code simulating on the one hand the forced dispersion, heating and physico-chemical inhibition of cold pollutant clouds and on the other hand the thermal shielding performance of a water curtain. The numerical approach is supported by laboratory (including wind tunnel tests) and field tests dedicated to investigate the effects of numerous operating parameters on the water spray curtain efficiency and to build a data base for model validation. The paper gives an overview of the main features of the modeling and on practical industrial applications with a special focus on the adequate water curtain operating conditions and the influence of environmental factors

Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity

Context: The sharp temperature and density gradients in the coronal transition region are a challenge for models and observations. Aims: We set out to get linearly- and quadratically-weighted average electron densities in the region emitting the S VI lines, using the observed opacity and the emission measure of these lines. Methods: We analyze SoHO/SUMER spectroscopic observations of the S VI lines, using the center-to-limb variations and radiance ratios to derive the opacity. We also use the Emission Measure derived from radiance at disk center. Results: We get an opacity at S VI line center of the order of 0.05. The resulting average electron density is 2.4 10^16 m^-3 at T = 2 10^5 K. This value is higher than the values obtained from radiance measurements. Conversely, taking a classical value for the density leads to a too high value of the thickness of the emitting layer. Conclusions: The pressure derived from the Emission Measure method compares well with previous determinations and implies a low opacity of 5 10^-3 to 10^-2. The fact that a direct derivation leads to a much higher opacity remains unexplained, despite tentative modeling of observational biases. Further measurements need to be done, and more realistic models of the transition region need to be used.Comment: 11 pages, 9 figure

IRIS Observations of Spicules and Structures Near the Solar Limb

We have analyzed IRIS spectral and slit-jaw observations of a quiet region near the South Pole. In this article we present an overview of the observations, the corrections, and the absolute calibration of the intensity. We focus on the average profiles of strong (Mg ii h and k, C ii and Si iv), as well as of weak spectral lines in the near ultraviolet (NUV) and the far ultraviolet (FUV), including the Mg ii triplet, thus probing the solar atmosphere from the low chromosphere to the transition region. We give the radial variation of bulk spectral parameters as well as line ratios and turbulent velocities. We present measurements of the formation height in lines and in the NUV continuum, from which we find a linear relationship between the position of the limb and the intensity scale height. We also find that low forming lines, such as the Mg ii triplet, show no temporal variations above the limb associated with spicules, suggesting that such lines are formed in a homogeneous atmospheric layer and, possibly, that spicules are formed above the height of 2 arc sec. We discuss the spatio-temporal structure near the limb from images of intensity as a function of position and time. In these images, we identify p-mode oscillations in the cores of lines formed at low heights above the photosphere, slow moving bright features in O i and fast moving bright features in C ii. Finally, we compare the Mg ii k and h line profiles, together with intensity values of the Balmer lines from the literature, with computations from the PROM57Mg non-LTE model developed at the Institut d'Astrophysique Spatiale and estimated values of the physical parameters. We obtain electron temperatures in the range of $\sim8000$ K at small heights to $\sim20000$ K at large heights, electron densities from $1.1 \times 10^{11}$ to $4 \times 10^{10}$ cm$^{-3}$ and a turbulent velocity of $\sim24$km/s.Comment: Accepted for publication in Solar Physic