Magnetische Kopplung und Rekonnexion

Transcription

1 Magnetische Kopplung und Rekonnexion Solarer Magnetismus und photospherisches Magnetfeld Extrapoliertes Magnetfeld Netzwerk und Teppich Offener und geschlossener Magnetfluss Ideales Plasma - Eingefrorenes Magnetfeld Alfven Theorem Beobachtungen nichtidealer Prozesse Rekonnexion: Konzept, Modelle Anwendungen an der Sonne

2 Geschichte des Magnetismus (BC) China: Magnetism is known, and magnetic needles are used; sunspots observed; (BC) Greece: Magnetism known in Europe (BC) Theophrastus: sunspots observed -Galileo,Scheiner,Fraunhofer: i h optical Sun, spectra Zeeman: Spectral line splitting due to B fields Nobel prize with Lorentz for pre-quantum (wrong)explanation: spinning electrons emit light Hale: Magnetic fields in sunspots are strongest t Giovanelli: Flares and magnetic discharges Leighton: 5-minute photospheric h B oscillations

3 Zeeman-Effekt -> >Sichtlinien i B-Feld BF Komponente This movie shows the results of 1 month ( ) of observations by SOHO-MDI: Black and white: positive and negative polarities of the line-of-sight component of the photospheric magnetic field One sees 1.) a granulation 2.) active regions

4 Das Schmetterlings- Diagramm

5 Granulation: Aufsteigender B-Fluss White spots: Flux tubes, scale 100 km Flux tube simulation (Steiner, 1998) 35 Mm x 40 Mm Magnetic regions are seen in G-band near 430 nm) between granules

6 Granulation: Netzwerk von Zellen Magnetic cell 30 Mm SUMER CIII 977 Å full disk scan Little loops cross the network lanes [Peter, 2002]

7 Helle Bögen <-> Magnetfelder? TRACE

8 Magnetischer Teppich -> Extrapolation

9 Offeenes Feld koronaler Löcher closed open Linker et al., JGR, 104, 9809, 1999 Elephantenrüssel-Koronaloch

10 Geschlossenes und offenes Feld Nützliche, praktische Unterscheidung wegen der bevorzugten Bewegung von Plasma parallel zu B!

11 Ausgedehntes Koronafeld Dipolar, quadrupolar, current sheet contributions Polar field: B = 12 G Current sheet is a symmetric disc anchored at low latitudes! Banaszkiewicz et al., 1998 LASCO C1/C2 images (SOHO)

12 Zyklusänderungen des Korona-B-Felds minimum Model extrapolation: Potential field, Δ B=0 Force-free field, jxb=0 Solar cycle variation maximum Bravo et al., Solar Phys.,1998

13 Theoretische Behandlung: MHD -> Induktionsgleichung + Zustandsgleichung für P = n T, evtl. Energiegleichung

14 Induktionsgleichung -> Ideales Plasma From the MHD and Maxwell s equations follows the induction equation: B t Magnetic Reynolds Number ( v B ) + ( η B ) = 0 Due to Coulomb-collisions (Spitzer-Härm-theory!) -> for typical scales and velocities -> the coronal R m is about ~ 10 10, the coronal plasma is practically ideal

15 Ideales Plasma -> Alfven-Theorem

16 Alfven Theorem If the magnetic flux through a circuit of fluid particles of the solar stream vanishes initially, it must vanish at all times. On the other hand, if there us a flux at t=0 it moves together wit the plasma

17 Beobachtung: Plasmabeschleunigung OVI 629 Å Loop height: km Temperature: K Scale height: H = km Large shifts of up to 100 km/s How can cool plasma reach such heights?

18 Beobachtung: Explosionen, Ablösungen High-resolution TRACE (1999) EUV observations The loop dynamics is observed as brightening. i How much is this due to magnetic fields?

19 Beobachtung: Protuberanzen Prominences (cold plasma, magnetically confined) rest until they, suddenly, erupt -> magnetic field?

20 Beobachtung: Plasmaflüsse aus dem Netzwerk -> >Sonnenwind? Line-of-sight Doppler Ne VIII 770 Å velocity ( K) images September, 1996 North and midlatitude polar region Raster scan: 540" 300" Network in Si II 1553 Å ( K) Hassler et al., Science, 283, 810, 1999

21 Beobachtung: Plasma jets Evolution of a jet in Si IV 1393 Å visible ibl as blue and red shifts in SUMER spectra E-W stepsize1" size, Δt =5s Jet head moves 1" in 60 s Innes at el., Nature, 386, 811, 1997

22 Konzept: Magnetische Rekonnexion cb cb d d a a Giovanelli 1946: Magnetic discharges may take place at sun which might cause the observed excitation of atoms in the course of solar flares. Axford 1984: Generally, a change of magnetic connectivity through a region of non-ideal plasma that allows a most efficient release of free energy stored in the magnetic field and plasma. -> Reconnection needs nonideal coronal plasma

23 2D Rekonnexion - Prinzip Fast plasma outflow Magnetic Separatrices Magnetfeld Slow plasma inflow Fast plasma outflow Slow plasma inflow Nonideal plasma

24 Rekonnexion im Labor <- Yamada et al.: MRX-experiment, PPPL, Princetone University, 2000

25 Rekonnexion einfaches 2D Modell How fast can the plasma flow (u) to transform a maximum How fast can the plasma flow (u) to transform a maximum magnetic energy into plasma acceleration (V~Va)?

26 Stromschicht - Rekonnexion

27 Sweet-Parker Lösung:Langsam Inflow velocity Continuity it of plasma flow The z-component of Ohm's law yields Pressure balance along the length of the layer yields Further, with: -> one finds: In solar Flares, e.g. S=Rm(Va) ~ > M ~ > too small!

28 Petschek Lösung: Schneller, aber 2D

29 Übergang 2D -> 3D Rekonnexion In two dimensions: Plasma flow through a separatrix -> electric field perpendicular (Vasyliunas 1975) But what does this mean in three dimensions?

30 3D magnetische Rekonnexion Spiraling field line out of a X-type Null Separ surfac O-type Null Fan field lines out of a X-type Null Spine f line ou Xt X-type