NOAA 11283: an e-HEROIC Active Region

P. Romano1, S.L. Guglielmino2, F. Zuccarello2 and the SWICO collaboration

1 INAF - Osservatorio Astrofisico di Catania2 Dipartimento di Fisica e Astronomia - Sezione Astrofisica, Università di Catania

eHEROES 3rd annual meeting Leuven (Belgium) 7-9th of January 2015

Deliverables

D2.1. Report on the study of the photospheric flare precursors with a new tool developed to follow the internal motions and all changes in the active regions which lead to the increase of the horizontal gradient of the magnetic field.

D3.1. Analysis of CME-prolific solar active regions (ARs) during their lifetime by means of multi-wavelength space-borne and ground-based data and series of SDO/HMI magnetograms to determine their morphological evolution, the magnetic free energy, the photospheric helicity flux and coronal helicity content before and after CMEs, drawing their helicity budget.

Flares and CMEsFlares and coronal mass ejections (CMEs), which can influence the whole heliosphere, originate from reconfiguration of magnetic fields and release of free magnetic energy stored in the solar corona.

More intense magnetic field together with a more complex topology are usually associated with the most energetic events.

Flux rope (FR): a particular field topology characterized by a set of magnetic field lines that collectively wrap around a central, axial field line (see Zuccarello talk).

More recent numerical simulations have shown that an FR may not emerge bodily from below the photosphere, but reforms in the corona, i.e., the field lines wrap around a new central axis that is different from the original flux tube axis (Manchester at al. 2004; Magara 2006; Archontis & Torok, 2008; Archontis et al. 2014).

Target: NOAA 11283

Observation time interval: from Sept. 5, 2011 at 00:00 UT to Sept. 8, 2011 at 16:00 UT

Space-weather Active Region Patches (SHARPs) data (Hoeksema et al. 2014) acquired by HMI/SDO at 6173 Å with a pixel size of 0.51 arcsec and a time cadence of 96 min.

AIA/SDO (Lemen et al., 2012) images at 193 Å (Fe XII; logT = 6.1) and 304 Å(He II; logT = 4.7) with a pixel size of 0.6 arcsec and a time cadence of 96 min.

This AR has been site of several flares.

Four recurrent GOES M and X-class flares occurred in the AR during the selected observation interval and their time and class are reported in following Table.

Date Start (UT) Peak (UT) End (UT) GOES class6-9-2011 01:35 01:50 02:05 M5.3

6-9-2011 22:12 22:20 22:24 X2.1

7-9-2011 22:32 22:38 22:44 X1.8

8-9-2011 15:32 15:46 15:52 M6.7

All these events occurred at the same time of CMEs observed by LASCO/SOHO and reported in the LASCO catalogue (http://cdaw.gsfc.nasa.gov/CME_list/).

Target: NOAA 11283

Impact of the flares and associated CMEs on the near-Earth environment

Several CMEs struck our planet's magnetic field since Sept. 9 around 11:30 UT. Their impact sparked several strong (Kp = 7) geomagnetic storm.

Picture taken by Frank Olsen of Tromsø just after local midnight on Sept. 10, 2011

Estimated Planetary K index (3 hour data) since Sept. 9 at 00:00 UT.

Target: NOAA 11283

This AR has been already object of study by some authors (Jiang et al., 2013, 2014; Feng et al., 2013; Ruan et al., 2014).

Target: NOAA 11283

We determined the mean magnetogram corresponding to the average between two consecutive magnetograms of the vertical component of the magnetic field.

We measured the horizontal velocity fields by means of the Differential Affine Velocity Estimator (DAVE) method (Schuck, 2005) using a full width at half maximum of the apodization window of 11 pixels (5”.5).

The horizontal photospheric velocity field

The shear and the dip angle

We also calculated the shear between the observed (measured) horizontal field and the horizontal field derived through a potential field extrapolation (Wang et al. 1994), computed using the method described by Alissandrakis (1981).

We have also computed the dip angle, which measures the difference between the inclination angle of the observed field and that of the potential field (see, e.g., Gosain & Venkatakrishnan 2010; Petrie 2012). This is defined as:

where γ = 90°−arctan (Br/Bh) is the inclination angle derived in both cases.

The dip angle can be understood in terms of azimuthal currents, in the same way as the shear angle is understood in terms of axial currents.

Hudson (2000) conjectured that the free energy is stored in non-potential magnetic loops that are stretched upward and that the free-energy release during the flare must be accompanied by a sudden shrinkage or implosion in the field.

It is predicted that after the flare the field should become more horizontal (Hudson et al. 2008).

It was described by Venkatakrishnan (1990) that in force-free fields a high NP implies a weaker magnetic tension, which in turn implies a larger vertical extension of the field due to a lower magnetic pressure gradient.

Conversely, the release of the free magnetic energy during the flare implies loss of magnetic NP leading to a decrease in the vertical extension of the field or shrinkage (Forbes & Acton 1996).

The closer the post-flare field approaches the potential field configuration, the smaller is the value of the inclination difference Δγ expected.

This may give an explanation for the decrease usually observed in the dip angle Δγ after a flare.

The shear and the dip angle

The horizontal component of the magnetic field

The green line indicates the main polarity inversion line.

The white background includes points with total fields lower than 200G that are not represented. The red (blue) contours indicate a longitudinal field of +500G (−500 G).

The shear angle

The twist shear is dependent on sub-photospheric/photospheric forces, so the twist shear continue to increase independent of coronal processes such as flares

The white background includes points with total fields lower than 200G that are not represented. The red (blue) contours indicate a longitudinal field of +500G (−500 G).

The dip angle

The plasma β decreases rapidly above the photosphere, and thus there is no non-magnetic force or shear that is strong enough to change the inclination of the field lines. Hence, inclination should be more responsive to coronal processes.

The magnetic helicity flux

We estimated the magnetic helicity flux using Equation (18) of Pariat et al. (2005):

where r is the vector between two photospheric points x and x’ and, consequently, (dϑ(r)/dt ) is the relative rotation rate of these points, Bn and Bn’ = Bn’(x’) are the normal components of the photospheric magnetic field, and S’ is the integration surface.

In our case, we used the radial component of the HMI vector magnetograms as Bnand Bn’.

We computed the magnetic helicity accumulation in the AR since the beginning of the observations

Finally, we applied the labelling algorithm YAFTA (Welsch & Longcope, 2003) on the helicity maps to group contiguous pixels into features.

The red vertical lines indicate the flare/CME occurrence. t = 0 corresponds to 00:00 UT on Sept. 5, 2011.

The magnetic helicity flux

14:24 UT Sept. 8

The saturation levels are +/- 1.5 x 1018 Mx2 cm-2 s-1

DiscussionFrom our horizontal velocity maps we found a persistent peculiar motions of the main sunspots involved in the event.

According to Ruan et al. (2014), before the X2.1 class flare we noted a clockwiserotation of the main negative sunspot. Moreover, from our analysis we detected a counter-clockwise rotation of the emerging positive polarity sunspot near the previous one during the whole observation time interval.

The main result of these rotations seems the high values of shear near the polarity inversion line (PIL).

It is worth of note that the shear angle remains at high values after all the events, which means that the horizontal displacements are able to maintain the shear.

The dip angle maintains its correlation with the shear angle after each event. This means that the flare/CME was not able to remove all the stored free energy (“eHEROIC” character of this AR).

Probably, the rotation of the two sunspots of opposite polarities is also responsible of the opposite sign of magnetic helicity flux at the sides of the PIL, where two main patches of helicity flux of opposite polarity were close to each other.

The magnetic helicity distribution seems in agreement with some numerical simulations (Linton et al., 2001; Mok et al., 2001 and Kusano et al., 2004) where the interaction between magnetic fluxes with opposite helicity allows the release of the free energy through magnetic reconnection processes.

These reconnection processes are probably able to reconfigure the system to a state similar to the initial one.

The sequence of homologous eruptions may be attributed to the continuous horizontal displacement, which contributes to the monotonic injection of magnetic helicity in corona and the reconfiguration of the magnetic system.

Conclusion

Date Start (UT) Peak (UT) End (UT) GOES class6-9-2011 01:35 01:50 02:05 M5.3

6-9-2011 22:12 22:20 22:24 X2.1

7-9-2011 22:32 22:38 22:44 X1.8

8-9-2011 15:32 15:46 15:52 M6.7

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 work is partly supported by the Italian MIUR-PRIN grant 2012P2HRCR on "The active Sun and its effects on Space and Earth climate" and by Space Weather Italian COmmunity (SWICO) Research Program.

Thank you!