1. Trend of the magnetic helicity flux during the formation and the destabilization of flux ropes F. Zuccarello 1, S.L. Guglielmino 1, P. Romano 2 and F.P. Zuccarello 3,4,5 1Dipartimento di Fisica e Astronomia - Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy 2INAF -- Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy 3Centre for mathematical Plasma-Astrophysics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium. 4Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium 5 LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France

2. Abstract We present the results describing the trend of the magnetic helicity accumulation during the phases of formation and successive destabilization of the flux ropes in the Active Regions (ARs) NOAA 11318 and NOAA 11675, where a C1.4 GOES class flare associated with a Coronal Mass Ejection (CME) and an M1.9 GOES class flare occurred, respectively. Observing the two ARs by HMI/SDO and AIA/SDO since their appearance on the solar disc, we found a different behaviour in the accumulation of the magnetic helicity flux in corona, depending on the magnetic configuration and on the location of the flux ropes in the ARs. Our results suggest that the complexity and strength of the photospheric magnetic field is only a partial indicator of the real likeliness of an AR to produce the eruption of a flux rope and subsequent CME. This allows us to speculate that for the occurrence of CMEs associated with ARs, it is important not only the presence of a flux rope, but also the configuration of the surrounding magnetic field. This research work has received funding from the European Commissions Seventh Framework Programme under the grant agreements no. 284461 (eHEROES project), no. 312495 (SOLARNET project), no. 606862 (F-Chroma project). This research is also supported by the ITA MIUR-PRIN grant on “The active sun and its effects on space and Earth climate" and by Space WEather Italian COmmunity (SWICO) Research Program. The contribution of F.P.Z. has been funded by the FWO Vlaanderen through the grant agreement no 1272714N (FWO Vlaanderen).

3.  Aim: Investigate the trend of magnetic helicity flux during the formation and eruption of flux ropes.  Method:  two active regions: NOAA 11318 and NOAA 11675 have been selected (HMI/SDO and AIA/SDO data)  the evolution of both ARs at different atmospheric levels has been studied from the very beginning of their appearance on the solar disc  the photospheric magnetic configuration has been analyzed  the magnetic helicity trend has been investigated measuring its flux from the convection zone to the corona. Outline of this work

4. The AR NOAA 11318 has been studied using AIA/SDO images (171 Å, 193 Å, 304 Å, 335 Å) and HMI/SDO longitudinal magnetograms between 11 – 15 Oct 2011. It was characterized by a simple bipolar configuration. The axis of the AR rotates during the phase of flux emergence. The photospheric magnetic configuration and the continuous separation of the two polarities supports a scenario leading to the appearance of a flux rope. NOAA 11318

5. 14 Oct 18:10 UT14 Oct 20:55 UT14 Oct 22:30 UT 15 Oct 04:30 UT 15 Oct 05 05 UT 15 Oct 06:30 UT An initial destabilization of a filament is observed at 22:40 UT on 14 Oct and the filament is activated at 02:10 on 15 Oct. In this case the formation of the FR culminated in a C2.3 class flare observed on 15 Oct 2011 at 04:19 UT (peak 04:40 UT), associated to a CME observed by LASCO. NOAA 11318

6. 5:48 UT6:36 UT The CME time correlated with this flare has been observed by LASCO starting from 5:48 UT NOAA 11318

7. The AR NOAA 11675 has been studied using AIA/SDO and HMI/SDO longitudinal magnetograms between 16 – 21 Feb 2013 We also analyzed SDO/HMI (SHARPs) data. These data series provide maps of the photospheric vector magnetic field using VFISV code. The continuous separation of the two polarities supports the scenario of the appearance of a flux rope. In this case no sigmoid in coronal images is observed, but several flares occurred NOAA 11675

8. To infer the formation of a flux rope during the emergence phase of a compact bipole in the middle of the AR, we have calculated the shear angle between the observed (measured) horizontal field and the horizontal field derived through a potential field extrapolation computed using the method of Alissandrakis (1981). We note that the region between the opposite polarities of the compact bipole is characterized by high values of the shear angle, larger than 45°. NOAA 11675

9. Only a minor outflow is visible in the running differences images obtained by STEREO COR1-B after the main flare event. 16:30 UT16:50 UT NOAA 11675

10. 304 A14 Oct 20:10 UT NOAA 11318 304 A17 Feb 14:30 UT NOAA 11675 Red and blue symbols indicate positive and negative flux, respectively. The black lines indicate the total unsigned magnetic flux. t=0 corresponds to 00:00 UT on 11 Oct 2011 and 16 Feb 2013 for AR NOAA 11318 and 11675, respectively. The vertical line indicates the occurrence of flares and their thickness the flare class (thin: C-class; thick: M-class). Comparing the results

11. The different activity registered in the two ARs sheds light on the behavior of the magnetic helicity accumulation during the formation and eruption of FRs:  The sign of the magnetic helicity is consistent with the forward-S shape sigmoid in the AR 318 and by the forward-S shape filament in the AR 675.  In the first case, i.e., the bipolar AR, the FR formation corresponds to a monotonic accumulation of H.  In the second case, we see a redistribution of H in the AR and a more complex trend of H accumulation.  The B- and C-class flares preceding the main event may be interpreted as signatures of magnetic reconnection processes between the forming FR and the overlying magnetic flux system that may dissipate the magnetic free energy and H from the FR into the ambient field, reducing the amount of energy available for the eruption. Conclusions

12. Our results also highlight different conditions for the occurrence of flares and/or CMEs :  Both ARs accumulated more or less the same magnetic helicity amount during the same observing time interval, but only one of the ARs, the simplest one at photospheric level, produced a CME.  Several flares occurred during the earlier phase of observations in the more complex AR, but these events did not give rise to eruptive events in the outer corona.  We speculate that the surrounding magnetic field in the more complex AR confined the FR eruption.  For the occurrence of CMEs associated with ARs, it is important not only the presence of a FR, but also the configuration of the surrounding magnetic field (see, i.e., Kusano et al., 2004; Galsgaard et al., 2007; Kliem & Torok 2006). The two cases reported in this study provide a clear evidence that the photospheric complexity of an AR is only a partial indication of the real likeliness of an AR to produce CMEs. Conclusions