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Relative field line helicity of a large eruptive solar active region  

Kostas Moraitis   Submitted: 2021-03-07 22:21

Context. Magnetic helicity is a physical quantity of great importance in the study of astrophysical and natural plasmas. Although a density for helicity cannot be defined, a good proxy for it is field line helicity. The appropriate quantity for use in solar conditions is relative field line helicity (RFLH). Aims. This work aims to study in detail the behaviour of RFLH, for the first time, in a solar active region (AR). Methods. The target active region is the large, eruptive AR 11158. In order to compute RFLH and all other quantities of interest we use a non-linear force-free reconstruction of the AR coronal magnetic field of excelent quality. Results. We find that the photospheric morphology of RFLH is quite different than that of the magnetic field or of the electrical current, and this is not sensitive to the chosen gauge in the computation of RFLH. The value of helicity experiences a large decrease, 25% of its pre-flare value, during an X-class flare of the AR, a change that is also depicted in the photospheric morphology of RFLH. Moreover, the area of this change coincides with the area that encompasses the flux rope, the magnetic structure that later erupted. Conclusions. The use of RFLH can provide important information about the value and location of the magnetic helicity expelled from the solar atmosphere during eruptive events.

Authors: K. Moraitis, S. Patsourakos, A. Nindos
Projects: SDO-AIA,SDO-HMI

Publication Status: accepted by Astronomy & Astrophysics
Last Modified: 2021-03-09 15:34
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Magnetic helicity and eruptivity in active region 12673  

Kostas Moraitis   Submitted: 2019-07-15 23:29

Context. In September 2017 the largest X-class flare of Solar Cycle 24 occurred from the most active region (AR) of this cycle, AR 12673. The AR attracted much interest because of its unique morphological and evolution characteristics. Among the parameters examined in the AR was magnetic helicity, but either only approximately, and/or intermittently. Aims. This work is interested in studying the evolution of the relative magnetic helicity and of the two components of its decomposition, the non-potential, and the volume-threading one, in the time interval around the highest activity of AR 12673. Special emphasis is given on the study of the ratio of the non-potential to total helicity, that was recently proposed as an indicator of ARs eruptivity. Methods. For these, we first approximate the coronal magnetic field of the AR with two different optimization-based extrapolation procedures, and choose the one that produces the most reliable helicity value at each instant. Moreover, in one of these methods, we weight the optimization by the uncertainty estimates derived from the Helioseismic and Magnetic Imager (HMI) instrument, for the first time. We then follow an accurate method to compute all quantities of interest. Results. The first observational determination of the evolution of the non-potential to total helicity ratio seems to confirm the quality it has in indicating eruptivity. This ratio increases before the major flares of AR 12673, and afterwards it relaxes to smaller values. Additionally, the evolution patterns of the various helicity, and energy budgets of AR 12673 are discussed and compared with other works.

Authors: K. Moraitis, X. Sun, E. Pariat, L. Linan
Projects: None

Publication Status: accepted by A&A
Last Modified: 2019-07-16 12:05
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Relative magnetic field line helicity  

Kostas Moraitis   Submitted: 2019-02-27 23:52

Context. Magnetic helicity is an important quantity in studies of magnetized plasmas as it provides a measure of the geometrical complexity of the magnetic field in a given volume. A more detailed description of the spatial distribution of magnetic helicity is given by the field line helicity that expresses the amount of helicity associated to individual field lines, rather than in the full analysed volume. Aims. Magnetic helicity is not a gauge-invariant quantity in general, unless it is computed with respect to a reference field, yielding the so called relative magnetic helicity. The field line helicity corresponding to the relative magnetic helicity has only been examined under specific conditions so far. This work aims to define the field line helicity corresponding to relative magnetic helicity in the most general way. In addition to its general form, we provide the expression for the relative magnetic field line helicity in a few commonly used gauges, and reproduce known results as a limit of our general formulation. Methods. By starting from the definition of relative magnetic helicity, we derive the corresponding field line helicity, and we note the assumptions it is based on. Results. We check that the developed quantity reproduces relative magnetic helicity by using three different numerical simulations. For these cases we also show the morphology of field line helicity in the volume, and on the photospheric plane. As an application to solar situations, we compare the morphology of field line helicity on the photosphere with that of the connectivity-based helicity flux density in two reconstructions of an active region's magnetic field. We discuss how the derived relative magnetic field line helicity has a wide range of applications, notably in solar physics and magnetic reconnection studies.

Authors: K. Moraitis, E. Pariat, G. Valori, K. Dalmasse
Projects: None

Publication Status: accepted by A&A
Last Modified: 2019-02-28 12:48
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Computation of Relative Magnetic Helicity in Spherical Coordinates  

Kostas Moraitis   Submitted: 2018-06-10 22:14

Magnetic helicity is a quantity of great importance in solar studies because it is conserved in ideal magneto-hydrodynamics. While many methods to compute magnetic helicity in Cartesian finite volumes exist, in spherical coordinates, the natural coordinate system for solar applications, helicity is only treated approximately. We present here a method to properly compute relative magnetic helicity in spherical geometry. The volumes considered are finite, of shell or wedge shape, and the three-dimensional magnetic field is considered fully known throughout the studied domain. Testing of the method with well-known, semi-analytic, force-free magnetic-field models reveals that it has excellent accuracy. Further application to a set of nonlinear force-free reconstructions of the magnetic field of solar active regions, and comparison with an approximate method used in the past, indicates that the proposed methodology can be significantly more accurate, thus making our method a promising tool in helicity studies that employ the spherical geometry. Additionally, the range of applicability of the approximate method is determined and discussed.

Authors: Moraitis, K., Pariat, E., Savcheva, A., Valori, G.
Projects: None

Publication Status: accepted by Solar Physics
Last Modified: 2018-06-13 12:39
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Abstracts by Author
Relative field line helicity of a large eruptive solar active region
Magnetic helicity and eruptivity in active region 12673
Relative magnetic field line helicity
Computation of Relative Magnetic Helicity in Spherical Coordinates

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