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Magnetic field generation on long time scales Axel Brandenburg (Nordita/Stockholm) Kemel+12 Ilonidis+11Brandenburg+11Warnecke+11 Käpylä+12
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White light image of yesterday 3 Tips of icebergs: Magnetic flux concentrations in magnetogram!
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The thin flux tube paradigm 4 Caligari et al. (1995)Charbonneau & Dikpati (1999)
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5 Standard dynamo wave Differential rotation (faster inside) Cyclonic convection; Buoyant flux tubes Equatorward migration New loop - effect
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6 Alternative proposal: Conveyor belt model Dikpati et al. (2006)
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7 Simulations of the solar dynamo? Tremendous stratification –Not only density, also scale height change Near-surface shear layer (NSSL) not resolved Contours of cylindrical, not spoke-like (i) Rm dependence (catastrophic quenching) –Field is bi-helical: to confirm for solar wind (ii) Location: bottom of CZ or distributed –Shaped by NSSL (Brandenburg 2005, ApJ 625, 539) –Formation of active regions near surface
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Brun, Brown, Browning, Miesch, Toomre 8 Brown et al. (2011) ASH code: anelastic spherical harmonics
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9 Cycle now common! Activity from bottom of CZ but at high latitudes Ghizaru, Charbonneau, Racine, … Racine et al. (2011)
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Pencil code Started in Sept. 2001 with Wolfgang Dobler High order (6 th order in space, 3 rd order in time) Cache & memory efficient MPI, can run PacxMPI (across countries!) Maintained/developed by ~80 people (SVN) Automatic validation (over night or any time) 0.0013 s/pt/step at 1024 3, 2048 procs http://pencil-code.googlecode.com Isotropic turbulence –MHD, passive scl, CR Stratified layers –Convection, radiation Shearing box –MRI, dust, interstellar –Self-gravity Sphere embedded in box –Fully convective stars –geodynamo Other applications –Chemistry, combustion –Spherical coordinates
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11 Dynamo wave from simulations Kapyla et al (2012)
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Type of dynamo? 12 Use phase relation Closer to 2 dynamo Wrong for dyn. Mitra et al. (2010) Oscillatory 2 dynamo
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13 Remaining aspects (i)Bi-helical fields inverse cascade (ii)Solar wind also bi-helical field (iii)Formation of active regions at solar surface
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(i) Dynamo produces bi-helical field Magnetic helicity spectrum Pouquet, Frisch, & Leorat (1976)
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15 Helicity fluxes to alleviate catastrophic quenching Brandenburg (2005, ApJ) 10 46 Mx 2 /cycle
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Magnetic helicity flux EMF and resistive terms still dominant Fluxes import at large Rm ~ 1000 Rm based on k f Smaller by 2 16
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Magnetic helicity flux EMF and resistive terms still dominant Fluxes import at large Rm ~ 1000 Rm based on k f Smaller by 2 17 Gauge-invariant in steady state! Del Sordo, Guerrero, Brandenburg (2013, MNRAS 429, 1686)
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18 This is how it looks like… Coronal mass ejections from helical structures Gibson et al. (2002)
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19 (ii) Helicity from solar wind Matthaeus et al. (1982) Measure correlation function In Fourier space, calculate magnetic energy and helicity spectra Should be done with Ulysses data away from equatorial plane
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20 Measuring 1-D correlation tensor Taylor hypothesis:
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21 Bi-helical fields from Ulysses Taylor hypothesis Broad k bins Southern latitude with opposite sign Small/large distances Positive H at large k Break point with distance to larger k
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22 Comparison Field in solar wind is clearly bi-helical...but not as naively expected Need to compare with direct and mean- field simulations Recap of dynamo bi-helical fields HelicityLSSS Dynamo +- Solar wind -+
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23 Dynamos with exterior CMEs? Warnecke, Brandenburg, Mitra (2011, A&A, 534, A11)
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24 Strong fluctuations, but positive in north Warnecke, Brandenburg, Mitra (2011, A&A, in press) Shell dynamos with ~CMEs
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To carry negative flux: need positive gradient Brandenburg, Candelaresi, Chatterjee (2009, MNRAS 398, 1414) Sign reversal makes sense!
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26 (iii) How deep are sunspots rooted? Solar activity may not be so deeply rooted The dynamo may be a distributed one Near-surface shear important Hindman et al. (2009, ApJ)
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27 Two alternative sunspot origins Theories for shallow spots: (i) Collapse by suppression of turbulent heat flux (ii) Negative pressure effects from vs B i B j Kosovichev et al. (2000)
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28 Negative effective magnetic pressure instability Gas+turb. press equil. B increases Turb. press. Decreases Net effect?
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Self-assembly of a magnetic spot Minimalistic model 2 ingredients: –Stratification & turbulence Extensions –Coupled to dynamo –Compete with rotation –Radiation/ionization 29
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Imposed vs. self-assembly Appearance of sunspot when coupled to radiation Can be result of self- assembly when ~1000 G field below surface 30 Stein & Nordlund (2012) Rempel et al. (2009)
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31 Conclusions Interest in predicting solar activity Cyclonic convection ( helicity) Near surface shear migratory dynamo Bi-helical fields, inverse cascade Solar wind also bi-helical field, but reversed Formation of active regions and sunspots by negative effective magnetic pressure inst.
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