Konvektion und solares Magnetfeld

Transcription

1 Vorlesung Physik des Sonnensystems Univ. Göttingen, 2. Juni 2008 Konvektion und solares Magnetfeld Manfred Schüssler Max-Planck Planck-Institut für Sonnensystemforschung Katlenburg-Lindau

2 Convection & magnetism: closely related

3 Outline 1) ) Basic physics of convection 2) Numerical simulation of convection 3) Overview of solar magnetism 4) Surface magneto-convection 5) Deep convection zone field & dynamo

4 The solar convection zone 200 Mm thick layer in turbulent motion Velocities range from 100 m/s (bottom) to 10 km/s (top) Energy flux nearly completely transported by convective motion

5 What is convection? Flow driven by thermal buoyancy Convective instability Viewgraphs...

6 Granulation: Solar surface convection

7 Solar granulation

8 Granulation und laboratory convection

9 Granulation as a convective phenomenon

10 Supergranulation

11 Supergranulation and magnetic field: the Ca + network

12 Granulation, sunspots, & small-scale scale magnetic field

13 Realistic solar simulations elaborate physics: partial ionization, radiation, compressible, open box, transmitting boundaries, spectral line diagnostics (Stokes profiles) + : approximation to solar conditions + : direct comparison with observations : computational restrictions (box size, resolution) : Reynolds numbers much below solar values

14 Approach: Local simulation box including photosphere radiative energy transport convective energy transport

15 Computer-simulated convection Boussinesq model Rayleigh number: D, mesh wide box, aspect ratio: 10 (meso)granulation Cattaneo & Emonet (2001)

16 The MURaM code: equations Continuity equation Momentum equation Q rad = F = 4πρ κν ( Jν Sν ) dν Energy equation di ds ν = κ ν ρ ( Iν Sν ) Induction equation Radiative Transfer Equation

17 Computer-simulated convection realistic simulation ionization, rad. transfer 3D, mesh 6 Mm 6 Mm 1.4 Mm granulation Vögler et al. (2005) Emerging intensity

18 Computer-simulated convection realistic simulation ionization, rad. transfer 3D, mesh 6 Mm 6 Mm 1.4 Mm granulation Vögler et al. (2005) Vertical velocity (red: down, blue: up)

19 Computer-simulated convection Upper photosphere

20 Mesogranulation?

21 Simulated long-lived lived convective downflows Virtual corks are carried by the horizontal flow. They accumulate in downflow regions.

22 Averaged energy fluxes in a simulation of solar convection Stein & Nordlund, 2000

23 Simulation and observation Simulation (original) Simulation (smoothed) Observation

24 Change of downflow topology down up Stein & Nordlund, 1998

25 Simulated convection in a solar-like like spherical shell Miesch 1998

26 Magnetic fields on the Sun Sunspots A large sunspot group

27 What is the nature of sunspots? Smoke clouds? Holes? Tornadoes?

28 The magnetic nature of sunspots Sunspot with spectrograph slit Magnetically split spectral line

29 Magnetic variability Full-disk magnetogram Magnetic patterns on the rotating Sun

30 Hot plasma draws magnetic field lines...

31 Hot plasma draws magnetic field lines...

32 The solar magnetic field... continues into interplanetary space. Its variability in the course of the 11-year cycle and its long-term modulation... - affects cosmic rays, - perturbs the terrestrial magnetic field.

33 G-band observations KIS/VTT, Obs. del Teide, Tenerife

34 G-band observations Dutch Open Telescope, Obs. del Roque de los Muchachos, courtesy P. Sütterlin

35 What is magneto-convection? Interaction between convective flows and magnetic field in an electrically well-conducting fluid High Reynolds numbers: nonlinear dynamics, structure and pattern formation Interference with convective energy transport

36 Regimes of solar magneto-convection <B> increases: quiet Sun plage umbra sunspot umbra horizontal scale of convection decreases plage convective energy transport decreases quiet Sun T. Berger, SVST 12 May 1998, Obs. del Roque de los Muchachos Adapted from a figure by Thierry Emonet, Univ. Chicago

37 Good electrical conductors : frozen field Initially field-free free volumes remain field-free free Magnetic flux through a given volume remains constant

38 Frozen field in the Sun Magnetic flux is transported to the downflow regions of the convective flow patterns magnetic network

39 Simulation of flux expulsion (Weiss, 1966) b: final state for Re m = 40 a: streamlines of the fixed velocity field c-j: time evolution for Re m = 1000 evolution of an initially vertical magnetic field under the influence of a fixed flow field kinematic, 2D the magnetic flux is expelled from the area of closed streamlines and concentrated in narrow sheets

40 Flux expulsion and intermittency N.O. Weiss (1964): first simulations (Hupfer, KIS Freiburg, 2001)

41 Flux expulsion and intermittency N.O. Weiss (1964): first simulations (Hupfer, KIS Freiburg, 2001)

42 B 0 = 200 G (plage): time evolution horizontal cuts near τ= km 6000 km 1400 km grid points up down vertical magnetic field brightness vertical velocity [kg] I/<I> [km/s]

43 B 0 = 200 G (plage): time evolution +2 kg -2 kg 6 Mm Vertical magnetic field component

44 B 0 = 200 G (plage): time evolution Brightness 6 Mm

45 Convective intensification Flux advection by horizontal flow (flux expulsion) Suppression of convection, cooling and downflow Evacuation, field intensification

46 The magnetically variable Sun Minimum 11-year cycle of magnetic activity and surface flux Maximum Maximum

47 The 11-year solar cycle Maunder-Minimum Solar magnetic activity varies with a period of roughly 11 years. Long-term variations are superposed upon this cycle.

48 14 C: Solar activity back to AD 1000

49 Butterfly diagram

50

51 Where do the surface fields come from?

52 Origin of sunspots Ω 1 Convection zone

53 Origin of sunspots 2 Ω Overshoot layer

54 Origin of sunspots 3 Magnetic flux tube Ω

55 Origin of sunspots 4 Parker instability Ω

56 Origin of sunspots 5 Magnetic buoyancy Ω

57 Origin of sunspots 6 Magnetic buoyancy Ω

58 Origin of sunspots 7 Tube expansion and decreasing field strength Ω

59 Origin of sunspots 8 Eruption at the solar surface Ω

60 Origin of sunspots 9 Ω Formation of a bipolar sunspot pair/group

61 Magnetic buoyancy of a flux tube Pressure equilibrium Pa = Pi + B 2 /8π B 0 Pi < Pa ρi <ρa buoyancy Pa,ρa external pressure, density Pi,ρi internal pressure, density Pa ρa Parker instability B Pi ρi

62 Magnetic buoyancy of a flux tube Pressure equilibrium Pa = Pi + B 2 /8π B 0 Pi < Pa ρi <ρa buoyancy Pa,ρa external pressure, density Pi,ρi internal pressure, density Pa ρa Parker instability B Pi ρi

63 Magnetic buoyancy of a flux tube Pa ρa B Pi ρi

64 Magnetic buoyancy of a flux tube Pa ρa B Pi ρi

65 Magnetic buoyancy of a flux tube r Pa ρa B Pi ρi

66 Generation of magnetic flux requires an electrically conducting medium plasma (ionized gas)... requires fluid motion for induction convective flows (differential) rotation... how is the field maintained against dissipation? (self-excited) excited) dynamo process

67 The induction principle Conductor moving in a magnetic field perpendicular electrical field and force electrical current new magnetic field Lenz s rule! (no perpetuum mobile)

68 A simple dynamo Initially weak seed field Rotation induces electrical field between axis and edge Current closed by wire Current generates a magnetic field which amplifies the seed field Sun: no isolated wires homogeneous dynamo

69 Local dynamo Vögler & Sch. 2007

70 Differential rotation generates azimuthal (toroidal) magnetic field

71 Internal rotation of the Sun as determined by helioseismology Convection zone rotates similar to surface Core rotates nearly rigidly Steep transition at the bottom of the convection zone; width ~2% R sun Region of strongest shear Dynamo!

72 Internal rotation of the Sun as determined by helioseismology

73 The solar dynamo (1) Winding up of the field by differential rotation strong toroidal field Dipol field in the convection zone

74 The solar dynamo (2) Twist of the erupting field by Coriolis force reversed dipole field Rise and eruption of magnetic flux tubes sunspots

75 Twisting of a field line in a rising & expanding convective flow by the action of the Coriolis force (Parker, 1955)

76 Reversal of the meridional field

77 Reversal of the meridional field

78 B = 10 Tesla B = 1 Tesla

79 The end...