E-Print Archive

There are 3783 abstracts currently viewable.


Search:

Advanced Search
Options
Main Page Add New E-Print Submitter
Information
Feedback
News Help/FAQ About Preferences
Manage Key Phrase
Notification
Energetic characterisation and statistics of solar coronal brightenings  

Eric Buchlin   Submitted: 2016-05-12 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 high-resolution, multi-channel Extreme Ultra-Violet (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) on-board 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 power-laws, and we find also power-law 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: SDO-AIA

Publication Status: Accepted for publication in A&A
Last Modified: 2016-05-16 09:09
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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

Eric Buchlin   Submitted: 2012-04-09 14:21

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.

Authors: C. Boutry, E. Buchlin, J.-C. Vial, S. Régnier
Projects: Hinode/EIS

Publication Status: Accepted for publication in ApJ
Last Modified: 2012-04-10 11:29
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Intermittent turbulent dynamo at very low and high magnetic Prandtl numbers  

Eric Buchlin   Submitted: 2011-11-09 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 shell-modelsof 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 Rmeta', witheta=eta'=0.4. Conclusion: Comparison with a phenomenology based on anintermittent small-scale 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: 2011-11-10 10:58
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Turbulence in the sub-Alfvénic solar wind driven by reflection of low-frequency Alfvén waves  

Eric Buchlin   Submitted: 2010-09-07 04:13

We study the formation and evolution of a turbulent spectrum of Alfvén waves driven by reflection off the solar wind density gradients, starting from the coronal base up to 17 solar radii, well beyond the Alfvénic critical point. The background solar wind is assigned and 2D shell models are used to describe nonlinear interactions. We find that the turbulent spectra are influenced by the nature of reflected waves. Close to the base, these give rise to a flatter and steeper spectrum for the outgoing and reflected waves respectively. At higher heliocentric distance both spectra evolve toward an asymptotic Kolmogorov spectrum. The turbulent dissipation is found to account for at least half of the heating required to sustain the background imposed solar wind and its shape is found to be determined by the reflection-determined turbulent heating below 1.5 solar radii. Therefore reflection and reflection-driven turbulence are shown to play a key role in the acceleration of the fast solar wind and origin of the turbulent spectrum found at 0.3 AU in the heliosphere.

Authors: Verdini, A, Velli, M., Buchlin, E.
Projects: None

Publication Status: Published (Astrophys.J.700:L39-L42,2009)
Last Modified: 2010-09-07 06:56
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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

Eric Buchlin   Submitted: 2009-06-09 03:30

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 1016 m^-3 at T = 2 105 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.

Authors: E. Buchlin, J.-C. Vial
Projects: SoHO-SUMER

Publication Status: Accepted for publication in A&A
Last Modified: 2009-06-09 04:03
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Profiles of heating in turbulent coronal magnetic loops  

Eric Buchlin   Submitted: 2007-02-28 12:45

Context: The location of coronal heating in magnetic loops has been the subject of a long-lasting 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 quasi-uniform, unless there is an expansion of the flux tube (non-uniform 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 non-uniform turbulent coronal loop.

Authors: E. Buchlin, P. J. Cargill, S. J. Bradshaw, M. Velli
Projects: None

Publication Status: Astron. Astrophys., accepted
Last Modified: 2007-03-01 11:05
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Shell-models of RMHD turbulence and the heating of solar coronal loops  

Eric Buchlin   Submitted: 2007-02-28 12:43

A simplified non-linear numerical model for the development of incompressible magnetohydrodynamics (MHD) in the presence of a strong magnetic field B0 and stratification, nicknamed Shell-Atm, is presented. In planes orthogonal to the mean field, the non-linear incompressible dynamics is replaced by 2D shell-models 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 shell-models 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 non-linearly 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 power-laws. Cross-correlations 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: 2007-03-01 11:05
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Multi-scale Hall-MHD turbulence in the solar wind  

Eric Buchlin   Submitted: 2007-02-28 12:40

Solar wind magnetic fluctuation spectra exhibit a significant power law steepening at frequencies f>1Hz. The origin of this multi-scaling is investigated through dispersive Hall magnetohydrodynamics. We perform three-dimensional numerical simulations in the framework of a highly turbulent shell model and show that the large-scale magnetic fluctuations are characterized by a k^-5/3-type spectrum which steepens at scales smaller than the ion inertial length d_i, to k^-7/3 if the magnetic energy overtakes the kinetic energy, or to k^-11/3 in the opposite case. These results are in agreement both with a heuristic description a la Kolmogorov, and with the range of power law indices found in the solar wind.

Authors: S. Galtier, E. Buchlin
Projects: None

Publication Status: Astrophysical Journal, 656, 560-566, 2007
Last Modified: 2007-03-01 11:05
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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

Eric Buchlin   Submitted: 2006-05-18 04:19

We analyze a series of full-Sun 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 line-of-sight 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 power-law 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: SoHO-SUMER

Publication Status: Astron. Astrophys. 451 (2006) 1091-1099
Last Modified: 2006-05-18 09:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Influence of the definition of dissipative events on their statistics  

Eric Buchlin   Submitted: 2006-05-18 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 one-dimensional data set such as a time-series of energy dissipation. Each definition is then applied to a time-series of energy dissipation issued from simulations of a shell-model of magnetohydrodynamic turbulence as defined in Giuliani and Carbone (1998), or from a new model of coupled shell-models designed to represent a magnetic loop in the solar corona. We obtain distributions of the peak dissipation power, total energy, duration and waiting-time 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, power-law 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, 355-362 (2003)
Last Modified: 2006-05-18 09:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

A simplified numerical model of coronal energy dissipation based on reduced MHD  

Eric Buchlin   Submitted: 2006-05-18 03:52

A 3D model intermediate between cellular automata (CA) models and the reduced magnetohydrodynamic (RMHD) equations is presented to simulate solar impulsive events generated along a coronal magnetic loop. The model consists of a set of planes distributed along a magnetic loop between which the information propagates through Alfvén waves. Statistical properties in terms of power-laws for energies and durations of dissipative events are obtained, and their agreement with X-ray and UV flares observations is discussed. The existence of observational biases is also discussed.

Authors: E. Buchlin, V. Aletti, S. Galtier, M. Velli, G. Einaudi, J.-C. Vial
Projects: None

Publication Status: Astron.Astrophys. 406 (2003) 1061-1070
Last Modified: 2006-05-18 09:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 


Key
Go to main E-Print pageGo to main E-Print page.
Download PreprintDownload Preprint.
Submitter's HomepageSubmitters Homepage.
Edit EntryEdit Entry.
Delete AbstractDelete abstract.

Abstracts by Author
Energetic characterisation and statistics of solar coronal brightenings
Flows at the Edge of an Active Region: Observation and Interpretation
Intermittent turbulent dynamo at very low and high magnetic Prandtl numbers
Turbulence in the sub-Alfvénic solar wind driven by reflection of low-frequency Alfvén waves
Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity
Profiles of heating in turbulent coronal magnetic loops
Shell-models of RMHD turbulence and the heating of solar coronal loops
Multi-scale Hall-MHD turbulence in the solar wind
A statistical study of SUMER spectral images: events, turbulence, and intermittency
Influence of the definition of dissipative events on their statistics
A simplified numerical model of coronal energy dissipation based on reduced MHD

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2003 Solar Physics Group - Montana State University