1. The multiscale magnetic pattern and the roots of solar activity F. Berrilli, S. Scardigli, D. Del Moro Department of Physics, University of Rome Tor Vergata, Italy (berrilli@roma2.inf.it) 11th Edition of the European Space Weather Week, 17-21nd November 2014, Liège, Belgium

2. The nature of the multiscale convective pattern observed on the solar surface remains a long-standing puzzle in solar physics. Size of a realistic MHD simulation domain HR MDI Mag The convective dynamo is the source of solar magnetism that modulates the activity of the Sun, creates active regions and drives Space Weather. The analysis of solar magnetic pattern provides a way to investigate all the organization scales of convection, from granulation to the global circulation.

3. If we observe the Sun we see that the magnetic fields do not fill photosphere uniformly but instead are clustered in a network with large voids (relatively empty regions) between them. The inspection of photospheric magnetograms that were taken at the limits of the available resolution reveals regions where magnetic fields are weak and very inclined (commonly named voids). A division of such voids in classical scales (i.e., granulation, meso-granulation, super- granulation) is probably of historical and not physical origin (Nordlund et al., 2009).

4. We study the void size distribution (VSD) to determine whether it reveals distinct flow scales or it is smoothly distributed. The analysis was performed on a series of SOHO/MDI LOS magnetograms acquired during the solar activity minimum between cycles 23 and 24 and the Hinode/SOT/SP LOS magnetogram observed at disk center on 10 March 2007. We have analyzed a dataset of 511 high- resolution quiet Sun MDI magnetograms selected to cover a period of 18 months from 1/1/2008. The void-detection algorithm singled out 252 488 and 1951 voids in MDI and HINODE magnetograms, respectively.

5. Conclusions VSD shows a quasi-exponential decay in the observed ranges. The lack of features in the 2–60 Mm range supports the multiscale hypothesis of convective motion flows at the solar surface (e.g., Nordlund et al. 2009; Rieutord et al. 2010, Yelles Chaouche et al. 2011; Berrilli et al. 2013). Berrilli F., Scardigli S., Giordano S., Multiscale Magnetic Underdense Regions on the Solar Surface: Granular and Mesogranular Scales, Solar Physics, 282, 2013 Berrilli F., Scardigli S., Del Moro D., Magnetic pattern at super-granulation scale: the void size distribution, A&A 568, 2014