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Relative magnetic helicity as a diagnostic of solar eruptivity  

Etienne Pariat   Submitted: 2017-03-31 01:06

The discovery of clear criteria that can deterministically describe the eruptive state of a solar active region would lead to major improvements on space weather predictions. Using series of numerical simulations of the emergence of a magnetic flux rope in a magnetized coronal, leading either to eruptions or to stable configurations, we test several global scalar quantities for the ability to discriminate between the eruptive and the non-eruptive simulations. From the magnetic field generated by the three-dimensional magnetohydrodynamical simulations, we compute and analyse the evolution of the magnetic flux, of the magnetic energy and its decomposition into potential and free energies, and of the relative magnetic helicity and its decomposition. Unlike the magnetic flux and magnetic energies, magnetic helicities are able to markedly distinguish the eruptive from the non-eruptive simulations. We find that the ratio of the magnetic helicity of the current-carrying magnetic field to the total relative helicity presents the highest values for the eruptive simulations, in the pre-eruptive phase only. We observe that the eruptive simulations do not possess the highest value of total magnetic helicity. In the framework of our numerical study, the magnetic energies and the total relative helicity do not correspond to good eruptivity proxies. Our study highlights that the ratio of magnetic helicities diagnoses very clearly the eruptive potential of our parametric simulations. Our study shows that magnetic-helicity-based quantities may be very efficient for the prediction of solar eruptions.

Authors: E. Pariat, J. E. Leake, G. Valori, M. G. Linton, F. P. Zuccarello, K. Dalmasse
Projects: None

Publication Status: A&A, in press
Last Modified: 2017-04-02 19:07
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A model for straight and helical solar jets: II. Parametric study of the plasma beta  

Etienne Pariat   Submitted: 2016-09-28 03:20

Jets are dynamic, impulsive, well-collimated plasma events that develop at many different scales and in different layers of the solar atmosphere. Jets are believed to be induced by magnetic reconnection, a process central to many astrophysical phenomena. Within the solar atmosphere, jet-like events develop in many different environments, e.g., in the vicinity of active regions as well as in coronal holes, and at various scales, from small photospheric spicules to large coronal jets. In all these events, signatures of helical structure and/or twisting/rotating motions are regularly observed. The present study aims to establish that a single model can generally reproduce the observed properties of these jet-like events. In this study, using our state-of-the-art numerical solver ARMS, we present a parametric study of a numerical tridimensional magnetohydrodynamic (MHD) model of solar jet-like events. Within the MHD paradigm, we study the impact of varying the atmospheric plasma β on the generation and properties of solar-like jets. The parametric study validates our model of jets for plasma β ranging from 10-3 to 1, typical of the different layers and magnetic environments of the solar atmosphere. Our model of jets can robustly explain the generation of helical solar jet-like events at various β ≤ 1. This study introduces the new result that the plasma β modifies the morphology of the helical jet, explaining the different observed shapes of jets at different scales and in different layers of the solar atmosphere. Our results allow us to understand the energisation, triggering, and driving processes of jet-like events. Our model allows us to make predictions of the impulsiveness and energetics of jets as determined by the surrounding environment, as well as the morphological properties of the resulting jets.

Authors: E. Pariat, K. Dalmasse, C.R. DeVore, S.K. Antiochos, J.T. Karpen
Projects: None

Publication Status: Accepted in Astronomy and Astrophysics
Last Modified: 2016-09-28 10:47
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Testing magnetic helicity conservation in a solar-like active event  

Etienne Pariat   Submitted: 2015-06-30 04:01

Magnetic helicity has the remarkable property of being a conserved quantity of ideal magnetohydrodynamics (MHD). Therefore, it could be used as an effective tracer of the magnetic field evolution of magnetised plasmas. Theoretical estimations indicate that magnetic helicity is also essentially conserved with non-ideal MHD processes, e.g. magnetic reconnection. This conjecture has however been barely tested, either experimentally or numerically. Thanks to recent advances in magnetic helicity estimation methods, it is now possible to test numerically its dissipation level in general three-dimensional datasets. We first revisit the general formulation of the temporal variation of relative magnetic helicity on a fully bounded volume when no hypothesis on the gauge are made. We introduce a method to precisely estimate its dissipation independently of the type of non-ideal MHD processes occurring. In a solar-like eruptive event simulation, using different gauges, we compare its estimation in a finite volume with its time-integrated flux through the boundaries, hence testing the conservation and dissipation of helicity. We provide an upper bound of the real dissipation of magnetic helicity: It is quasi-null during the quasi-ideal MHD phase. Even when magnetic reconnection is acting the relative dissipation of magnetic helicity is also very small (<2.2%), in particular compared to the relative dissipation of magnetic energy (>30 times larger). We finally illustrate how the helicity-flux terms involving velocity components are gauge dependent, hence limiting their physical meaning. Our study paves the way for more extended and diverse tests of the magnetic helicity conservation properties. Our study confirms the central role that helicity can play in the study of MHD plasmas. For instance, the conservation of helicity can be used to track the evolution of solar magnetic fields, from its formation in the solar interior until their detection as magnetic cloud in the interplanetary space.

Authors: E. Pariat; G. Valori; P. Démoulin & K. Dalmasse
Projects: None

Publication Status: Accepted in Astronomy and Astrophysics
Last Modified: 2015-06-30 10:45
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A model for straight and helical solar jets: I. Parametric studies of the magnetic field geometry  

Etienne Pariat   Submitted: 2014-11-28 04:38

Jets are dynamic, impulsive, well collimated plasma events developing at many different scales and in different layers of the solar atmosphere. Jets are believed to be induced by magnetic reconnection, a process central to many astrophysical phenomena. Studying their dynamics can help us to better understand the processes acting in larger eruptive events (e.g., flares and coronal mass ejections) as well as mass, magnetic helicity and energy transfer at all scales in the solar atmosphere. The relative simplicity of their magnetic geometry and topology, compared with larger solar active events, makes jets ideal candidates for studying the fundamental role of reconnection in energetic events. In this study, using our state-of-the-art numerical solver ARMS, we present several parametric studies of a three-dimensional numerical magneto-hydrodynamic model of solar jet-like events. We study the impact of the magnetic field inclination and photospheric field distribution on the generation and properties of two morphologically different types of solar jets, straight and helical, which can account for the observed so-called ``standard'' and ``blowout'' jets. The present parametric studies validate our model of jets for different geometric properties of the magnetic configuration. We find that a helical jet is always triggered for the range of parameters that we tested. This demonstrates that the 3D magnetic null-point configuration is a very robust structure for the energy storage and impulsive release characteristic of helical jets. In certain regimes determined by the magnetic geometry, a straight jet precedes the onset of a helical jet. We show that the reconnection occurring during the straight jet phase influences the triggering of the helical jet. Our results allow us to better understand the energization, triggering, and driving processes of straight and helical jets. Our model predicts the impulsiveness and energetics of jets in terms of the surrounding magnetic field configuration. Finally, we discuss the interpretation of the observationally defined standard and blowout jets in the context of our model, and the physical factors that determine which type of jet will occur.

Authors: E. Pariat, K. Dalmasse, C. R. DeVore, S. K. Antiochos, J. T. Karpen
Projects: None

Publication Status: A&A accepted
Last Modified: 2014-12-03 15:04
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Estimation of the squashing degree within a three-dimensional domain  

Etienne Pariat   Submitted: 2012-03-05 03:01

The study of the magnetic topology of magnetic fields aims at determining the key sites for the development of magnetic reconnection. Quasi-separatrix layers (QSLs), regions of strong connectivity gradients, are topological structures where intense-electric currents preferentially build-up, and where, later on, magnetic reconnection occurs. QSLs are volumes of intense squashing degree, Q; the field-line invariant quantifying the deformation of elementary flux tubes. QSL are complex and thin three-dimensional (3D) structures difficult to visualize directly. Therefore Q maps, i.e. 2D cuts of the 3D magnetic domain, are a more and more common features used to study QSLs. We analyze several methods to derive 2D Q maps and discuss their analytical and numerical properties. These methods can also be used to compute Q within the 3D domain. We demonstrate that while analytically equivalent, the numerical implementation of these methods can be significantly different. We derive the analytical formula and the best numerical methodology that should be used to compute Q inside the 3D domain. We illustrate this method with two twisted magnetic configurations: a theoretical case and a non-linear force free configuration derived from observations. The representation of QSL through 2D planar cuts is an efficient procedure to derive the geometry of these structures and to relate them with other quantities, e.g. electric currents and plasma flows. It will enforce a more direct comparison of the role of QSL in magnetic reconnection.

Authors: E. Pariat & P. Demoulin
Projects:

Publication Status: accepted in A&A
Last Modified: 2012-03-07 07:39
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Three-Dimensional Modeling of Quasi-Homologous Solar Jets  

Etienne Pariat   Submitted: 2010-03-31 05:46

Recent solar observations (e.g., obtained with Hinode and STEREO) have revealed that coronal jets are a more frequent phenomenon than previously believed. This higher frequency results, in part, from the fact that jets exhibit a homologous behavior: successive jets recur at the same location with similar morphological features. We present the results of three-dimensional (3D) numerical simulations of our model for coronal jets. This study demonstrates the ability of the model to generate recurrent 3D untwisting quasi-homologous jets when a stress is constantly applied at the photospheric boundary. The homology results from the property of the 3D null-point system to relax to a state topologically similar to its initial configuration. In addition, we find two distinct regimes of reconnection in the simulations: an impulsive 3D mode involving a helical rotating current sheet that generates the jet, and a quasi-steady mode that occurs in a 2D-like current sheet located along the fan between the sheared spines. We argue that these different regimes can explain the observed link between jets and plumes.

Authors: Pariat E., Antiochos S.K. , DeVore C.R.
Projects: None

Publication Status: ApJ, in press
Last Modified: 2010-03-31 08:12
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Current build-up in emerging serpentine flux tubes  

Etienne Pariat   Submitted: 2009-07-06 09:52

The increase of magnetic flux in the solar atmosphere during active-region formation involves the transport of the magnetic field from the solar convection zone through the lowest layers of the solar atmosphere, through which the plasma beta changes from >1 to <1 with altitude. The crossing of this magnetic transition zone requires the magnetic field to adopt a serpentine shape also known as the ''sea-serpent'' topology. In the frame of the resistive flux-emergence model, the rising of the magnetic flux is believed to be dynamically driven by a succession of magnetic reconnections which are commonly observed in emerging flux regions as Ellerman bombs. Using a data-driven, three-dimensional (3D) magnetohydrodynamic numerical simulation of flux emergence occurring in active region 10191 on November 16-17, 2002, we study the development of 3D electric current sheets. We show that these currents build-up along the 3D serpentine magnetic-field structure as a result of photospheric diverging horizontal line-tied motions that emulate the observed photospheric evolution. We observe that reconnection can not only develop following a ''pinching'' evolution of the serpentine field line, as usually assumed in 2D geometry, but can also result from 3D shearing deformation of the magnetic structure. In addition, we report for the first time on the observation in the UV domain with the Transition Region and Coronal Explorer (TRACE) of extremely transient loop-like features, appearing within the emerging flux domain, which link several Ellermam bombs with one another. We argue that these loop transients can be explained as a consequence of the currents that build up along the serpentine magnetic field.

Authors: E. Pariat, S. Masson, G. Aulanier
Projects: TRACE

Publication Status: ApJ (in press)
Last Modified: 2009-07-09 08:13
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A Model for Solar Polar Jets  

Etienne Pariat   Submitted: 2008-10-16 06:59

We propose a model for the jetting activity that is commonly observed in the Sun's corona, especially in the open-field regions of polar coronal holes. Magnetic reconnection is the process driving the jets and a relevant magnetic configuration is the well known null-point and fan-separatrix topology. The primary challenge in explaining the observations is that reconnection must occur in a short-duration energetic burst, rather than quasi-continuously as is implied by the observations of long-lived structures in coronal holes, such as polar plumes. The key idea underlying our model for jets is that reconnection is forbidden for an axisymmetric null-point topology. Consequently, by imposing a twisting motion that maintains the axisymmetry, magnetic stress can be built up to high levels until an ideal instability breaks the symmetry and leads to an explosive release of energy via reconnection. Using three-dimensional magnetohydrodynamic simulations, we demonstrate that this mechanism does produce massive, high-speed jets driven by nonlinear Alfvén waves. We discuss the implications of our results for observations of the solar corona.

Authors: E. Pariat, S. K. Antiochos & C. R. DeVore
Projects: None

Publication Status: ApJ, in press
Last Modified: 2008-10-16 08:32
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Subject will be restored when possible  

Etienne Pariat   Submitted: 2008-01-18 06:45

Magnetic flux emergence corresponds to the mechanism leading to the establishment of magnetized structures in the solar atmosphere. The magnetic flux emergence is directly traced on the solar surface (in visible-white light) by the presence of dark, mainly round-shaped areas, called sunspots, surrounded by brighter regions called plages. Measurements of magnetic fields at the solar surface shows that sunspots tend to be grouped in pairs, one with positive and one with negative magnetic polarity. Such a group of sunspots forms what is called an active region. The occurrence of large-scale magnetic flux emergence follows the solar cycle periodicity and is governed by the solar dynamo process. The solar surface is also covered by the so-called magnetic carpet with small magnetic bipoles scattered everywhere over the solar surface. These bipoles are due to the recycling of magnetic field by convection (granules and supergranules). This process is beyond the scope of the present discussion of magnetic flux emergence. This article deals with observable properties of magnetic field emergence in emerging flux regions (EFR) in the solar atmosphere and is complementary with the article on numerical simulations of magnetic field emergence.

Authors: B. Schmieder; E. Pariat
Projects: None

Publication Status: Scholarpedia (edited)
Last Modified: 2008-01-18 09:35
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Spectrophotometric analysis of Ellerman Bombs in the Ca II, Hα and UV range  

Etienne Pariat   Submitted: 2007-07-03 09:24

Even if Ellerman bombs have been observed in the Hα line withinemerging magnetic flux region since the early 20th century, their origin andthe mechanisms which lead to their formation has been strongly debated.Recently new arguments in favor of chromospheric magnetic reconnection havebeen advanced. Ellerman bombs seem to be the signature of reconnections thattake place during the emergence of the magnetic field. We have observed anactive region presenting emergence of magnetic flux. We detected and studiedEllerman bombs in two chromospheric lines: Ca II 8542 Å and Hα . Weinvestigated the link between Ellerman bombs and other structures andphenomena appearing in an emerging active region: UV Bright points, ArchFilament Systems and magnetic topology. On August 3rd, 2004, we performedmulti-wavelength observations of the active region NOAA 10655. This activeregion was the target of the SoHO Joint Observation Program 157. SoHO/MDIand TRACE (195 A & 1600 A) were used. Simultaneously, we observed in the CaII and Na D1 lines with the spectro-imager MSDP mode of THEMIS.Alternatively to the MSDP, we used the MTR spectropolarimeter on THEMIS toobserve in Hα and in the Fe I doublet at 6302 A. We derived themagnetic field vectors around some Ellerman Bombs. We presented the firstimages of EBs in the Ca II line. We confirmed that Ellerman bombs can indeedbe observed in the Ca II line, presenting the same `moustache'' geometryprofiles, as in the Hα line, but with a narrower central absorption inthe Ca II line, in which the peaks of emission are around ± 0.35 A. Wenoticed that the Ellerman bombs observed in the wings of Ca II line have anelongated shape - the length being about 50 % larger than the width. Wederived mean semi-axis lengths of 1.4'' x 2.0''. In the UV time profiles ofthe Ellerman bombs, we noticed successive enhanced emissions. Thedistribution of lifetimes of these individual impulses presents a strongmode around 210 s. Studying the magnetic topology we found that 9 out of the13 studied EBs are located on the inversion line of the longitudinal fieldand that some typical examples might be associated with a Bald Patchtopology. We provided new arguments in favor of the reconnection origin forEllerman bombs. The different individual impulses observed in UV may berelated to a bursty mode of reconnection. We also showed that this Ca II8542 Å chromospheric line is a good indicator to detect Ellerman bombs andcan bring new pieces of information about these phenomena.

Authors: E. Pariat, B. Schmieder, A. Berlicki, Y. Deng, N. Mein, A. Lopez Ariste & S. Wang
Projects: None

Publication Status: A&A (in press)
Last Modified: 2007-07-03 09:43
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What is the spatial distribution of magnetic helicity injected in a solar active region?  

Etienne Pariat   Submitted: 2006-03-06 02:38

Magnetic helicity is suspected to play a key role in solar phenomena such as flares and coronal mass ejections. Several investigations have recently computed the photospheric flux of magnetic helicity in active regions. The derived spatial maps of the helicity flux density, called GA, have an intrinsic mixed-sign patchy distribution. Pariat et al. (2005) recently showed that GA is only a proxy of the helicity flux density, which tends to create spurious polarities. They proposed a better proxy, G heta. We investigate here the implications of this new approach on observed active regions. The magnetic data are from MDI/SoHO instrument and the photospheric velocities are computed by local correlation tracking. Maps and temporal evolution of GA and G heta are compared using the same data set for 5 active regions. Unlike the usual GA maps, most of our G heta maps show almost unipolar spatial structures because the nondominant helicity flux densities are significantly suppressed. In a few cases, the G heta maps still contain spurious bipolar signals. With further modelling we infer that the real helicity flux density is again unipolar. On time-scales larger that their transient temporal variations, the time evolution of the total helicity fluxes derived from GA and G heta show small differences. However, unlike GA, with G heta the time evolution of the total flux is determined primarily by the predominant-signed flux while the nondominant-signed flux is roughly stable and probably mostly due to noise. Our results strongly support the conclusion that the spatial distribution of helicity injected into active regions is much more coherent than previously thought: on the active region scale the sign of the injected helicity is predominantly uniform. These results have implications for the generation of the magnetic field (dynamo) and for the physics of both flares and coronal mass ejections.

Authors: Pariat E., Nindos A., Démoulin P., Berger M.A.
Projects: None

Publication Status: A&A accepted
Last Modified: 2006-03-06 02:38
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Resistive flux emergence in undulatory flux tubes  

Etienne Pariat   Submitted: 2004-06-22 01:19

During its January 2000 flight, the {em Flare Genesis Experiment} observed the gradual emergence of a bipolar active region, by recording a series of high resolution photospheric vector magnetograms and images in the blue wing of the Hα line. Previous analyses of these data revealed the occurence of many small scale transient Hα brightenings identified as Ellerman bombs (EBs). They occure during the flux emergence, and many of them are located near moving magnetic dipoles in which the vector magnetic field is nearly tangential to the photosphere. A linear force-free field extrapolation of one of the magnetograms was performed to study the magnetic topology of small-scale EBs and their possible role in the flux emergence process. We found that 23 out of 47 EBs are co-spatial with Bald Patches (BPs), while 15 are located at the footpoint of very flat separatrix field lines passing through a distant BP. We conclude that EB can be due to magnetic reconnection, not only at BP locations, but also along their separatrices, occurring in the low chromosphere. The topological analysis reveals, for the first time, that many EBs/BPs are linked by a hierarchy of elongated flux tubes showing aperiodic spatial undulations, whose wavelengths are typically above the threshold of the Parker instability. These findings suggest that arch filament systems and coronal loops do not result from the smooth emergence of large-scale Omega-loops from below the photosphere, but rather from the rise of undulatory flux tubes whose upper parts emerge due to the Parker instability, and whose dipped lower parts emerge due to magnetic reconnection. Ellerman bombs are then the signature of this resistive emergence of undulatory flux tubes.

Authors: E. Pariat, G. Aulanier, B. Schmieder, M.K. Georgoulis, D.M. Rust and P.N. Bernasconi
Projects: None,TRACE

Publication Status: ApJ (in press)
Last Modified: 2004-06-22 01:19
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Abstracts by Author
Relative magnetic helicity as a diagnostic of solar eruptivity
A model for straight and helical solar jets: II. Parametric study of the plasma beta
Testing magnetic helicity conservation in a solar-like active event
A model for straight and helical solar jets: I. Parametric studies of the magnetic field geometry
Estimation of the squashing degree within a three-dimensional domain
Three-Dimensional Modeling of Quasi-Homologous Solar Jets
Current build-up in emerging serpentine flux tubes
A Model for Solar Polar Jets
Subject will be restored when possible
Spectrophotometric analysis of Ellerman Bombs in the Ca II, Hα and UV range
What is the spatial distribution of magnetic helicity injected in a solar active region?
Resistive flux emergence in undulatory flux tubes

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