ROAM: a Radialbasisfunction Optimization Approximation Method for diagnosing the threedimensional coronal magnetic field 

Kevin Dalmasse Submitted: 20160712 22:17
The Coronal Multichannel Polarimeter (CoMP) routinely performs coronal polarimetric measurements using the Fe XIII 10747 Å and 10798 Å lines, which are sensitive to the coronal magnetic field. However, inverting such polarimetric measurements into magnetic field data is a difficult task because the corona is optically thin at these wavelengths and the observed signal is therefore the integrated emission of all the plasma along the line of sight. To overcome this difficulty, we take on a new approach that combines a parameterized 3D magnetic field model with forward modeling of the polarization signal. For that purpose, we develop a new, fast and efficient, optimization method for modeldata fitting: the Radialbasisfunctions Optimization Approximation Method (ROAM). Modeldata fitting is achieved by optimizing a userspecified loglikelihood function that quantifies the differences between the observed polarization signal and its synthetic/predicted analogue. Speed and efficiency are obtained by combining sparse evaluation of the magnetic model with radialbasisfunction (RBF) decomposition of the loglikelihood function. The RBF decomposition provides an analytical expression for the loglikelihood function that is used to inexpensively estimate the set of parameter values optimizing it. We test and validate ROAM on a synthetic test bed of a coronal magnetic flux rope and show that it performs well with a significantly sparse sample of the parameter space. We conclude that our optimization method is wellsuited for fast and efficient modeldata fitting and can be exploited for converting coronal polarimetric measurements, such as the ones provided by CoMP, into coronal magnetic field data.
Authors: K. Dalmasse, D. W. Nychka, S. E. Gibson, Y. Fan, N. Flyer
Projects: None

Publication Status: Accepted in Frontiers in Astronomy and Space Sciences
Last Modified: 20160713 09:18



The origin of net electric currents in solar active regions 

Kevin Dalmasse Submitted: 20150720 09:34
There is a recurring question in solar physics about whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using threedimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Another source of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net vs. neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose linetied, quasistatic, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both direct and return currents, (2) induce very weak compression currents  not observed in 2.5D  in the ambient field present in the close vicinity of the currentcarrying field, and (3) can generate forcefree magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line  a special condition rarely observed. We conclude that, as magnetic flux emergence, photospheric flows can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on preeruption magnetic fields possessing a net coronal current.
Authors: K. Dalmasse, G. Aulanier, P. Demoulin, B. Kliem, T. Torok, E. Pariat
Projects: None

Publication Status: Accepted in The Astrophysical Journal
Last Modified: 20150720 10:42



Can we explain nontypical solar flares? 

Kevin Dalmasse Submitted: 20141030 18:38
We used multiwavelength highresolution data from ARIES, THEMIS, and SDO instruments, to analyze a nonstandard, C3.3 class flare produced within the active region NOAA 11589 on 2012 October 16. Magnetic flux emergence and cancellation were continuously detected within the active region, the latter leading to the formation of two filaments.
Our aim is to identify the origins of the flare taking into account the complex dynamics of its close surroundings.
We analyzed the magnetic topology of the active region using a linear forcefree field extrapolation to derive its 3D magnetic configuration and the location of quasiseparatrix layers (QSLs) which are preferential sites for flaring activity. Because the active region's magnetic field was nonlinear forcefree, we completed a parametric study using different linear forcefree field extrapolations to demonstrate the robustness of the derived QSLs.
The topological analysis shows that the active region presented a complex magnetic configuration comprising several QSLs. The considered data set suggests that an emerging flux episode played a key role for triggering the flare. The emerging flux likely activated the complex system of QSLs leading to multiple coronal magnetic reconnections within the QSLs. This scenario accounts for the observed signatures: the two extended flareribbons developed at locations matched by the photospheric footprints of the QSLs, and were accompanied with flare loops that formed above the two filaments which played no important role in the flare dynamics.
This is a typical example of a complex flare that can apriori show standard flare signatures that are nevertheless impossible to interpret with any standard model of eruptive or confined flare. We find that a topological analysis however permitted to unveil the development of such complex sets of flare signatures.
Authors: K. Dalmasse, R. Chandra, B. Schmieder, G. Aulanier
Projects: None

Publication Status: Accepted in A&A
Last Modified: 20141031 09:40



First observational application of a connectivitybased helicity flux density 

Kevin Dalmasse Submitted: 20130718 11:18
Measuring the magnetic helicity distribution in the solar corona can help in understanding the trigger of solar eruptive events because magnetic helicity is believed to play a key role in solar activity due to its conservation property. A new method for computing the photospheric distribution of the helicity flux was recently developed. This method takes into account the magnetic field connectivity whereas previous methods were based on photospheric signatures only. This novel method maps the true injection of magnetic helicity in active regions. We applied this method for the first time to an observed active region, NOAA 11158, which was the source of intense flaring activity. We used highresolution vector magnetograms from the SDO/HMI instrument to compute the photospheric flux transport velocities and to perform a nonlinear forcefree magnetic field extrapolation. We determined and compared the magnetic helicity flux distribution using a purely photospheric as well as a connectivitybased method. While the new connectivitybased method confirms the mixed pattern of the helicity flux in NOAA 11158, it also reveals a different, and more correct, distribution of the helicity injection. This distribution can be important for explaining the likelihood of an eruption from the active region. The connectivitybased approach is a robust method for computing the magnetic helicity flux, which can be used to study the link between magnetic helicity and eruptivity of observed active regions.
Authors: K. Dalmasse, E. Pariat, G. Valori, P. Démoulin, L. M. Green
Projects: SDOHMI

Publication Status: A&A (published, Vol. 555, L6)
Last Modified: 20130718 18:46




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