Important results on eruption prediction from the HéliSol project.

See the detail about this result on the dedicated HéliSol webpage. The manuscript can be obtained from the ArXiv webpage.

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Article on the benchmark of the Twist Number method to measure magnetic helicity accepted

More information on the HéliSol webpage!

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Study on the magnetic properties of circular-ribbon confined flares accepted

A study on a confined solar flare, its evolution in the EUV domain and its magnetic evolution has been accepted and can be consulted on ArXiv.

 

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New article on magnetic helicity: first benchmark of volume methods

The first article in link with the HélSol project, “Magnetic helicity estimations in models and observations of the solar magnetic field. Part I: Finite volume methods”,  has just been accepted in Space Science Reviews. More info can be found on the webpage of the HéliSol project. This article is the results of the ISSI team on magnetic helicity measurements that I’m co-leading with Gherardo Valori.

The pdf can be downloaded on the Publications page of the HéliSol website .

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New article on simulation of solar jets accepted in A&A

Our new article on numerical simulation of solar coronal jets, entitled “A model for straight and helical solar jets: II. Parametric study of the plasma beta”  has just been accepted in Astronomy & Astrophysics.

This paper is the follow up study of our 2015 work. This new study presents the results of parametric studies of jets with different surrounding plasma beta parameter. It’s the outcome of numerical experiments carried on the OCCIGEN computer at the CINES French HPC centre over several years. This work has been done in collaboration with my former PhD student, Kevin Dalmasse, now at NCAR in Boulder USA, and with Rick DeVore, Spiro Antiochos and Judy Karpen from the NASA Goddard space flight center.

A preview of the paper (pdf format) can be downloaded here.

Abstract:

Context: Jets are dynamic, impulsive, well-collimated plasma events that develop at many different scales and in different layers of the solar atmosphere.

Aims: 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.

Methods: 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 $\beta$ on the generation and properties of solar-like jets.

Results: The parametric study validates our model of jets for plasma $\beta$ 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 $\beta \le 1$. This study introduces the new result that the plasma $\beta$ 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.

Conclusions: 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.

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Review article on solar jets

The review article on solar coronal jets, outcome of our ISSI team on “Understanding Solar Jets and their Role in Atmospheric Structure and Dynamics co-lead by N.-E. Raouafi and myself, has been accepted and will be published in Space Science Reviews. It can be found here: http://link.springer.com/article/10.1007/s11214-016-0260-5

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Public Conference for the “Société Astronomique de France” on June 8th

I’ll be giving a public conference for the Société Astronomique de France at Agro Paris Tech on June 8th at 19h.

The title of my conference is: Où en sommes-nous de la prévision de l’activité solaire et de ses impacts sur la Terre ? La mission Solar Orbiter

Registration starts on May 12th. More information on the SAF website here: http://www.planetastronomy.com/special/SAF/conf-mens.htm

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Public Conference for the “Société Astronomique de France” postponed to June 8th

In the wake of the terrorist attacks that striked Paris, the public conference I was supposed to give on November 18th for the “Société Astronomique de France” has been cancelled because of security reasons.The Agro ParisTech does not wish to host a public event while the secutity threat is so high. The conference has been postponed to June 8th 2016

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Paper on helicity conservation accepted!

The paper on magnetic helicity and its conservation has been accepted in Astronomy & Astrophysics :

“Testing magnetic helicity conservation in a solar-like active event” (pdf)

E. Pariat, G. Valori, P. Démoulin, K. Dalmasse:

Abstract:

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.

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Paper on topology of solar flares accepted

The first paper of a serie of 2 (for the moment) has been accepted by the Astrophysical Journal. It’s a work done in collaboration with Antonia Savcheva from the Center for Astrophysics in Harvard that we started 3 years ago following our 2012 paper. This new paper can be found in ArXiv for the moment

“The Relation between Solar Eruption Topologies and Observed Flare Features I: Flare Ribbons”

A. Savcheva, E. Pariat, S. McKillop, P. McCauley, E. Hanson, Y. Su, E. Werner, E. E. DeLuca

Abstract:

In this paper we present a topological magnetic field investigation of seven two-ribbon flares in sigmoidal active regions observed with Hinode, STEREO, and SDO. We first derive the 3D coronal magnetic field structure of all regions using marginally unstable 3D coronal magnetic field models created with the flux rope insertion method. The unstable models have been shown to be a good model of the flaring magnetic field configurations. Regions are selected based on their pre-flare configurations along with the appearance and observational coverage of flare ribbons, and the model is constrained using pre-flare features observed in extreme ultraviolet and X-ray passbands. We perform a topology analysis of the models by computing the squashing factor, Q, in order to determine the locations of prominent quasi-separatrix layers (QSLs). QSLs from these maps are compared to flare ribbons at their full extents. We show that in all cases the straight segments of the two J-shaped ribbons are matched very well by the flux-rope-related QSLs, and the matches to the hooked segments are less consistent but still good for most cases. In addition, we show that these QSLs overlay ridges in the electric current density maps. This study is the largest sample of regions with QSLs derived from 3D coronal magnetic field models, and it shows that the magnetofrictional modeling technique that we employ gives a very good representation of flaring regions, with the power to predict flare ribbon locations in the event of a flare following the time of the model.

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