Non-potential magnetic helicity ratios at the onset of eruptions |
|
Etienne Pariat Submitted: 2018-07-02 02:46
The relative magnetic helicity is a quantity that is often used to describe the level of entanglement of non-isolated magnetic fields, such as the magnetic field of solar active regions. The aim of this paper is to investigate how different kinds of photospheric boundary flows accumulate relative magnetic helicity in the corona and if and how-well magnetic helicity related quantities identify the onset of an eruption. We use a series of three-dimensional, parametric magnetohydrodynamic simulations of the formation and eruption of magnetic flux ropes. All the simulations are performed on the same grid, using the same parameters, but they are characterized by different driving photospheric flows, i.e., shearing, convergence, stretching, peripheral- and central- dispersion flows. For each of the simulations, the instant of the onset of the eruption is carefully identified by using a series of relaxation runs. We find that magnetic energy and total relative helicity are mostly injected when shearing flows are applied at the boundary, while the magnetic energy and helicity associated with the coronal electric currents increase regardless of the kind of photospheric flows. We also find that, at the onset of the eruptions, the ratio between the non-potential magnetic helicity and the total relative magnetic helicity has the same value for all the simulations, suggesting the existence of a threshold in this quantity. Such threshold is not observed for other quantities as, for example, those related to the magnetic energy.
Authors: Francesco P. Zuccarello, Etienne Pariat, Gherardo Valori, Luis Linan
Projects: None
|
Publication Status: ApJ (in press)
Last Modified: 2018-07-02 10:48
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
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
|
 
 
|
|
|
Key
|
 | Go to main E-Print page. |
 | Download Preprint. |
 | Submitters Homepage. |
 | Edit Entry. |
 | Delete abstract. |
|
|
|