Julia K. Thalmann¹, Kostas Moraitis², Luis Linan², Etienne Pariat², Gherardo Valori³, and Kevin Dalmasse⁴
1 University of Graz, Institute of Physics/IGAM, A-8010 Graz, Austria
2 LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Universite de Paris, 5 place Jules Janssen, 92195 Meudon, France
3 Mullard Space Science Laboratory, University College London, Holmbury St.\ Mary, Dorking, Surrey RH5 6NT, UK
4 IRAP, Universite de Toulouse, CNRS, CNES, UPS, 31028 Toulouse, France

A full understanding of the physics behind explosive solar phenomena is, to date, still missing. Related research still searches for reliable indicators of upcoming flare activity, and also for the respective potential to evolve into a coronal mass ejection (CME). The latter is essential for any attempt to predict the induced space weather on Earth. Until now, no observation- or model-based parameter has been singled out as a promising candidate to unambiguously characterize the flaring potential of solar active regions. A magnetic helicity based parameter attracted recent attention, however.

Magnetic helicity is related to the level of entanglement of magnetic field lines in a magnetized plasma^[1], and is thus uniquely determined by the 3D distribution of the magnetic field. For application to the solar corona, the gauge-invariant formulation of the so-called relative helicity is used to employ physically meaningful quantities^[2]. Then, the total relative helicity, H_v, of a finite coronal volume can be decomposed into two contributions, one of which (H_J) is associated with the current-carrying (i.e., non-potential) part of the magnetic field^[3]. Based on these quantities, a parameter which quantifies the fraction of non-potential relative magnetic helicity associated to the present electric currents can be defined as |H_J|/|H_v|, the so-called helicity ratio. The helicity ratio has been recently proposed as a highly sensitive proxy for the eruptivity of active-region like magnetic configurations based on, e.g., three-dimensional MHD model experiments^[4].

Figure 1| Temporal evolution of the helicity ratio for AR 11158 (left) and AR 12192 (right). Black curves represent mean values, computed from the results based on three different finite-volume helicity computation methods. The shaded area represents the spread of the individual solutions. Vertical dashed and solid lines mark the GOES peak times of major M- and X-class flares, respectively. Horizontal dotted line indicates a characteristic level of |H_J|/|H_v| prior to the eruptive flares in AR 11158. Adapted from Fig. 3 of Thalmann J. K., Moraitis, K., Linan, L., et al. 2019, ApJ, 887, 64.

We applied the above concepts to two solar active regions (ARs) prolific in either major eruptive, i.e., CME-associated (AR 11158) or confined, i.e., CME-less (AR 12192) flaring, and analyzed the temporal evolution of the helicity ratio with respect to the observed major flares (GOES class M5.0 and larger). Based on the vector magnetic field deduced from the polarization measurements of the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we employ 3D nonlinear force-free (NLFF) coronal magnetic field models with a high degree of solenoidality, essential in order to guarantee reliable helicity estimates^[5].

Based on observations of two solar ARs we check the sensitivity of the helicity ratio with respect to the eruptivity of the active-region corona. Our results support that the helicity ratio indeed is highly sensitive to the eruptive potential of solar active regions. In particular, we find:

(1) The helicity ratio increases strongly before the major eruptive flares in AR 11158 (left panel in Fig. 1), but shows only little variations prior to the major confined flares in AR 12192 (right panel in Fig. 1).

(2) We find distinctly different characteristic pre-flare levels of the helicity ratio prior to the eruptive flares in AR 11158 (|H_J|/|H_v|> 0.15; left panel in Fig. 1) and confined flares in AR 12192 (|H_J|/|H_v|<0.1; right panel in Fig. 1). The helicity ratio, however, appears not to be indicative to the magnitude of the upcoming flares.

(3) Pronounced responses of the helicity ratio timely related to the occurrence of flares are only found for the major eruptive flares in AR 11158 (indicated by the vertical lines in the left panel of Fig. 1). In other words, only for major eruptive flares are corresponding decreases of the helicity ratio found, representing the signature of the ejection of magnetized plasma carrying strong electric currents from the active-region corona via the associated CMEs.

In summary, our study provides additional confirmation that the helicity ratio shows a strong ability to indicate the eruptive potential of solar active regions, and therefore represents a promising candidate for forecasting solar eruptions.

For details of this work, please refer to our original publication:

Thalmann, J. K., Moraitis, K., Linan, L., Pariat, E., Valori, G., Dalmasse, K. 2019, ApJ, 887, 64.

References

[1] Moffat, H. K., 1969, JFM, 35, 117
[2] Berger, M. A., Field, G. B., 1984, JFM, 147, 133
[3] Berger, M. A., 1999, PPCF, 41, B167
[4] Pariat, E., Leake, J. E., Valori, G., et al. 2017, A&A, 601, A125
[5] Valori, G., Demoulin, P., Pariat, E., et al. 2013, A&A, 553, A38