Energetic characterisation and statistics of solar coronal brightenings 

Eric Buchlin Submitted: 20160512 02:09
Context: To explain the high temperature of the corona, much attention has been paid to the distribution of energy in dissipation events. Indeed, if the event energy distribution is steep enough, the smallest, unobservable events could be the largest contributors to the total energy dissipation in the corona. Previous observations have shown a wide distribution of energies but remain inconclusive about the precise slope. Furthermore, these results rely on a very crude estimate of the energy. On the other hand, more detailed spectroscopic studies of structures such as coronal bright points do not provide enough statistical information to derive their total contribution to heating.
Aims: We aim at getting a better estimate of the distributions of the energy dissipated in coronal heating events using highresolution, multichannel Extreme UltraViolet (EUV) data.
Methods: To estimate the energies corresponding to heating events and deduce their distribution, we detect brightenings in five EUV channels of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We combine the results of these detections and we use maps of temperature and emission measure derived from the same observations to compute the energies.
Results: We obtain distributions of areas, durations, intensities, and energies (thermal, radiative, and conductive) of events. These distributions are powerlaws, and we find also powerlaw correlations between event parameters.
Conclusions: The energy distributions indicate that the energy from a population of events like the ones we detect represents a small contribution to the total coronal heating, even when extrapolating to smaller scales. The main explanations for this are how heating events can be extracted from observational data, and the incomplete knowledge of the thermal structure and processes in the coronal plasma attainable from available observations.
Authors: V. Joulin, E. Buchlin, J. Solomon, C. Guennou
Projects: SDOAIA

Publication Status: Accepted for publication in A&A
Last Modified: 20160516 09:09



Intermittent turbulent dynamo at very low and high magnetic Prandtl numbers 

Eric Buchlin Submitted: 20111109 15:48
Context: Direct numerical simulations have shown that the dynamo is efficienteven at low Prandtl numbers, i.e., the critical magnetic Reynolds number Rm_cnecessary for the dynamo to be efficient becomes smaller than the hydrodynamicReynolds number Re when Re > infinity. Aims: We test the conjecture (Iskakovet 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 andhigh magnetic Prandtl numbers. Methods: We use local and nonlocal shellmodelsof magnetohydrodynamic (MHD) turbulence with parameters covering a much widerrange of Reynolds numbers than direct numerical simulations, but ofastrophysical relevance. Results: We confirm that Rm_c tends to a finite valuewhen Re > infinity. The limit for Rm > infinity of the dynamo growth factorgamma in the kinematic regime behaves like Re^eta, and, similarly, thelimit for Re > infinity of gamma behaves like Rm^{eta'}, witheta=eta'=0.4. Conclusion: Comparison with a phenomenology based on anintermittent smallscale turbulent dynamo, together with the differencesbetween the growth rates in the different local and nonlocal models, indicate aweak contribution of nonlocal terms to the dynamo effect.
Authors: E. Buchlin
Projects: None

Publication Status: A&A Letters (published)
Last Modified: 20111110 10:58



Profiles of heating in turbulent coronal magnetic loops 

Eric Buchlin Submitted: 20070228 12:45
Context: The location of coronal heating in magnetic loops has been the subject of a longlasting controversy: does it occur mostly at the loop footpoints, at the top, is it random, or is the average profile uniform?
Aims: We try to address this question in model loops with MHD turbulence and a profile of density and/or magnetic field along the loop.
Methods: We use the ShellAtm MHD turbulent heating model described in Buchlin & Velli (2006), with a static mass density stratification obtained by the HydRad model (Bradshaw & Mason 2003). This assumes the absence of any flow or heat conduction subsequent to the dynamic heating.
Results: The average profile of heating is quasiuniform, unless there is an expansion of the flux tube (nonuniform axial magnetic field) or the variation of the kinetic and magnetic diffusion coefficients with temperature is taken into account: in the first case the heating is enhanced at footpoints, whereas in the second case it is enhanced where the dominant diffusion coefficient is enhanced.
Conclusions: These simulations shed light on the consequences on heating profiles of the complex interactions between physical effects involved in a nonuniform turbulent coronal loop.
Authors: E. Buchlin, P. J. Cargill, S. J. Bradshaw, M. Velli
Projects: None

Publication Status: Astron. Astrophys., accepted
Last Modified: 20070301 11:05



Shellmodels of RMHD turbulence and the heating of solar coronal loops 

Eric Buchlin Submitted: 20070228 12:43
A simplified nonlinear numerical model for the development of incompressible magnetohydrodynamics (MHD) in the presence of a strong magnetic field B0 and stratification, nicknamed ShellAtm, is presented. In planes orthogonal to the mean field, the nonlinear incompressible dynamics is replaced by 2D shellmodels for the complex variables u and b, allowing one to reach large Reynolds numbers while at the same time carrying out sufficiently long time integrations to obtain a good statistics at moderate computational cost. The shellmodels of different planes are coupled by Alfvén waves propagating along B0. The model may be applied to open or closed magnetic field configurations where the axial field dominates and the plasma pressure is low; here we apply it to the specific case of a magnetic loop of the solar corona heated via turbulence driven by photospheric motions, and we use statistics for its analysis. The Alfvén waves interact nonlinearly and form turbulent spectra in the directions perpendicular and, via propagation, also parallel to the mean field. A heating function is obtained, and is shown to be intermittent; the average heating is consistent with values required for sustaining a hot corona, and is proportional to the aspect ratio of the loop to the power 1.5; characteristic properties of heating events are distributed as powerlaws. Crosscorrelations show a delay of dissipation compared to energy content.
Authors: E. Buchlin, M. Velli
Projects: None

Publication Status: Astrophysical Journal, 660 (2007, in press)
Last Modified: 20070301 11:05



A statistical study of SUMER spectral images: events, turbulence, and intermittency 

Eric Buchlin Submitted: 20060518 04:19
We analyze a series of fullSun observations, which was performed with the SoHO/SUMER instrument between March and October 1996. Some parameters (radiance, shift and width) of the S VI 93.3 nm, S VI 94.4 nm, and Lyman Epsilon line profiles were computed on board. Radiances and lineofsight velocities in a large central region of the Sun are studied statistically: distributions of solar structures, field Fourier spectra and structure functions are obtained. The structures have distributions with powerlaw tails, the Fourier spectra of the radiance fields also display power laws, and the normalized structure functions of the radiance and velocity fields increase at small scales. These results support the idea of the existence of small scales, created by turbulence, and of intermittency of the observed fields. These properties may provide insight into the processes needed for heating the transition region, or, if confirmed in the corona, the corona itself. The difficulties encountered in this analysis, especially for the velocity data, underline the needs for sensitive ultraviolet imaging spectrometers.
Authors: E. Buchlin, J.C. Vial, P. Lemaire
Projects: SoHOSUMER

Publication Status: Astron. Astrophys. 451 (2006) 10911099
Last Modified: 20060518 09:07



Influence of the definition of dissipative events on their statistics 

Eric Buchlin Submitted: 20060518 03:57
A convenient and widely used way to study the turbulent plasma in the solar corona is to do statistics of properties of events (or structures), associated with flares, that can be found in observations or in numerical simulations. Numerous papers have followed such a methodology, using different definitions of an event, but the reasons behind the choice of a particular definition (and not another one) is very rarely discussed. We give here a comprehensive set of possible event definitions starting from a onedimensional data set such as a timeseries of energy dissipation. Each definition is then applied to a timeseries of energy dissipation issued from simulations of a shellmodel of magnetohydrodynamic turbulence as defined in Giuliani and Carbone (1998), or from a new model of coupled shellmodels designed to represent a magnetic loop in the solar corona. We obtain distributions of the peak dissipation power, total energy, duration and waitingtime associated to each definition. These distributions are then investigated and compared, and the influence of the definition of an event on statistics is discussed. In particular, powerlaw distributions are more likely to appear when using a threshold. The sensitivity of the distributions to the definition of an event seems also to be weaker for events found in a highly intermittent time series. Some implications on statistical results obtained from observations are discussed.
Authors: E. Buchlin, S. Galtier, M. Velli
Projects: None

Publication Status: Astron.Astrophys. 436, 355362 (2003)
Last Modified: 20060518 09:07




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