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Magnetic Flux Budget of a Decaying Sunspot

Magnetic Flux Budget of a Decaying Sunspot. Masahito Kubo 1 Collaborators: B. W. Lites 1 , T. Shimizu 2 , K. Ichimoto 3 , Hinode/SOT team 1: HAO/NCAR, 2: ISAS/JAXA, 3: Kyoto University. Lifecycle of sunspots. Sunspot lifetime: hours ~ months - Linear relation with maximum size

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Magnetic Flux Budget of a Decaying Sunspot

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  1. Magnetic Flux Budget of a Decaying Sunspot Masahito Kubo1 Collaborators: B. W. Lites1, T. Shimizu2, K. Ichimoto3, Hinode/SOT team 1: HAO/NCAR, 2: ISAS/JAXA, 3: Kyoto University SHINE Workshop 2009

  2. Lifecycle of sunspots • Sunspot lifetime: hours ~ months • - Linear relation with maximum size • (Gnevyshev-Waldmeier rule) • - Growing phase (hours ~ several • days) < Decaying phase • - Not clear transition from the • growing to decaying phase Moat region penumbra umbra “Japan” sunspot observed with Hinode SHINE Workshop 2009

  3. Moat region (cells) moat region moat flow • Large mature sunspots are usually • surrounded by the moat region. • A width of the moat region • is 10-20 Mm and roughly • proportional to the sunspot size. • (Brickhouse & Labonte 1988) • - Supergranule ~ 10 Mm • Radial outward flows (moat flow) • with 0.5-1 km/s are dominant. penumbra umbra small magnetic elements SHINE Workshop 2009

  4. Sunspot decay • Numerous small magnetic elements move radially away • from a mature decaying sunspot. • Moving magnetic features (MMFs, Harvey & Harvey 1973) G-band intensity (photosphere) Line-of-sight magnetogram (photosphere) SHINE Workshop 2009

  5. Moving magnetic features • Key to disintegration and transportation of the sunspot magnetic flux • Two types (e.g. Zhang et al. 1992, Shine & Title 2001) • Bipolar MMFs: • - more studied • - no contribution to sunspot decay • Unipolar MMFs: • - sunspot magnetic polarity (not all) • - important for sunspot decay • Relation to penumbral magnetic fields • (Kubo et al. 2007a) • - Horizontal component  bipolar MMFs • - Vertical component  unipolar MMFs or vertical horizontal penumbra moat region SHINE Workshop 2009

  6. Formation of bipolar MMFs BipolarMMFs correspondto the intersections of the solar surface with serpentinehorizontal fields extending from the penumbra(Sainz Dara & Martinez Pillet 2005, Kubo et al. 2007b&2008a, Sainz Dalda & Bellot Ruibo 2008). Magnetic flux density [103 Gauss] Line-of-sight “sea serpent”-like structure (Harvey & Harvey 1973) transverse penumbra moat distance from penumbral outer boundary MMFs Evershed flow position angle around the sunspot center SHINE Workshop 2009

  7. Formation of unipolar MMFs Outward convective motions in the outer penumbra are related to the disintegration of the sunspot magnetic flux. - against the stabilization of the sunspot hot plasma Bright features MMF Strong (vertical) penumbral field Kubo et al. 2008a SHINE Workshop 2009

  8. ー ＋ ＋ ー ー ＋ Magnetic flux “cancellation” • One magnetic polarity element collides with another element of the • opposite polarity in the photospheric magnetograms. •  Disappearance of the magnetic elements • The outer boundary of the moat region is one of major cancellation sites. • (Martin, Livi, & Wang 1985; Yurchyshyn & Wang2001; Chae et al. 2004; • Bellot Rubio &Beck 2005) SHINE Workshop 2009

  9. Magnetic flux budget of sunspots • How much magnetic flux is carried away from a sunspot through its • surrounding moat region? • How and where is sunspot magnetic flux removed from the photosphere? Sunspot (penumbra) Moat region Outside moat region moat flow MMFs Flux cancellation SHINE Workshop 2009

  10. Flux budget study from ground-based observations • The net magnetic flux carried by all the MMFs exceeds • the flux-loss rate of the sunspot(Martinez Pillet 2002, Kuboet al. 2007a). •  Day-by-day flux-loss rate of the sunspot • vs. • Flux transport rate by MMFs in a few hours Sunspot (penumbra) Moat region Outside moat region moat flow MMFs Flux cancellation Flux loss rate Flux transport rate SHINE Workshop 2009

  11. Flux budget study from Hinode/SOT • The Solar Optical Telescope (SOT) aboard the Hinode satellite • provide us, for the first time, accurate magnetic flux budget of • a decaying sunspot (Kubo et al. 2008b). • - No effect of atmospheric seeing, No long gap due to night. • - Comparison of simultaneous magnetic flux loss of the sunspot, • flux transport, flux cancellation. Sunspot (penumbra) Moat region Outside moat region moat flow MMFs Flux cancellation Flux loss rate Flux transport rate Flux cancellation rate SHINE Workshop 2009

  12. Hinode/SOT observations of AR NOAA10972 • SP scans (fast map)  Magnetic flux • - Cadence ~ 1-2 hr except for • two 4 hr gaps • - Vector magnetic field with the • Milne-Eddington inversion code • FG line-of-sight magnetogram (Na D line) •  Horizontal velocity • - Cadence: 2 minutes • Small bipolar sunspots almost disappear • in two days (6-Oct-2007 ~ 8-Oct-2007). • Many moving magnetic features are • observed around the sunspots. • Magnetic flux cancellations are observed • around the center of the active region. Upper: continuum intensity Lower: flux density of vertical fields SHINE Workshop 2009

  13. Inner area: 0” < r < 20”  sunspot & inner moat region Outer area: 20” < r < 40”  outer boundary of the moat region Following sunspot & Moat Region Continuum intensity Magnetic flux density [103 G] r: radius from the sunspot center SHINE Workshop 2009

  14. Evolution of total magnetic flux inner area outer area Positive Negative • Magnetic flux normal to the solar surface SP observations θ : heliocentric angle f : filling factor B : magnetic field strength γ : inclination w.r.t the local vertical *Projection effects are corrected. SHINE Workshop 2009

  15. Magnetic flux change rate (dF/dt) inner area outer area Positive 15hr Negative Flux change rates in the period between two red lines (06:15 – 21:26 on Oct 7) - The sunspot significantly decayed (but still survived). - No significant flux emergence (form visual inspection) SHINE Workshop 2009

  16. Radial flux transport rate (Ft) • Magnetic flux passing through the circle with a radius • of “r” from the sunspot center per unit time r2 r r1 vr r1, r2 : r ± 1 arcsec Magnetic flux averaged over the region between r1 and r2 Cross-section F(r) : Magnetic flux density normal to the solar surface (SP data) vr (r): Radial component of horizontal velocity (The local correlation tracking technique is applied for line-of-sight magnetograms with FG) SHINE Workshop 2009

  17. Magnetic flux budget Magnetic flux density Inner area Outer area Ft = 7.4 Ft = 0.8 dF/dt = - 8.0 dF/dt = + 2.7 Positive polarity Ft = 0.4 Ft = 1.2 dF/dt = + 0.3 dF/dt = - 2.3 “Ft” is averaged over the flux transport rate obtained in the same period as “dF/dt”. Negative polarity Unit : [1015 Mx/sec] The flux transport rate of positive polarity is much larger than that of negative polarity at the outer boundary of the inner area.  In terms of magnetic flux, the bipolar MMFs are less important. SHINE Workshop 2009

  18. Transport of sunspot magnetic flux Magnetic flux density Inner area Outer area Ft = 7.4 Ft = 0.8 dF/dt = - 8.0 dF/dt = + 2.7 Positive polarity Unit : [1015 Mx/sec] • The flux transport rate at the outer boundary of the inner region is • similar to the flux decrease rate in the innerarea.  Most of magnetic flux disappeared in the sunspot and the inner moat region aretransported to the outer boundary of the moat. SHINE Workshop 2009

  19. Where is sunspot flux removed from photosphere? Magnetic flux density Inner area Outer area Ft = 7.4 Ft = 0.8 dF/dt = - 8.0 dF/dt = + 2.7 Positive polarity Unit : [1015 Mx/sec] • The transported magnetic flux is larger than • - increase of positive magnetic flux in the outer area • - flux transport rate at the outer boundary of the outer area  The transported magnetic flux mostly disappear from the photosphere around the outer boundary of the moat region. SHINE Workshop 2009

  20. Mutual flux loss at the moat boundary • Magnetic flux loss rate: Observations Outer area - Positive: 3.9 [= -2.7 + 7.4 - 0.8] - Negative: 3.9[= -(-2.3) + 0.4 + 1.2] Ft = 7.4 Ft = 0.8 dF/dt = + 2.7 Positive polarity The fluxloss rates of positive and negativepolarities balance each other in the outer area.  Magnetic flux cancellation! Ft = 0.4 Ft = 1.2 dF/dt = - 2.3 Negative polarity SHINE Workshop 2009

  21. Conclusion • Most of the magnetic flux disappeared in the sunspot (and inner moat • region) is transported to the outer boundary of the moat region • as moving magnetic features. • The transported magnetic flux is removed from the photosphere by • the magnetic flux cancellation at the outer boundary of the moat region. Sunspot (penumbra) Moat region Outside moat region moat flow MMFs Flux cancellation SHINE Workshop 2009

  22. Thank you! SHINE Workshop 2009

  23. Solar Optical Telescope aboard Hinode • Largest aperture (50cm) solar telescope • flown in space •  High resolution and continuous • observations of magnetic field vector • in the photosphere • - Fitergraph (FG): Large field-of-view with high cadence • - Spectropolarimeter (SP): Highly accurate magnetic field vector • More detailed information: http://solar-b.nao.ac.jp/sot_e/ SHINE Workshop 2009

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