SunspotSunspot magneticmagnetic fieldsfields WhatWhat areare Sunspots?Sunspots? Slag on lava? Clouds of smoke? Holes in the Sun ? Cyclones? Sunspot structure & dynamics UmbraUmbra PenumbraPenumbra GranuleGranule TTeff ≈≈ 45004500 KK TTeff ≈≈ 55005500 KK TTeff ≈≈ 58005800 KK Evershed effect Observation:Observation: PenumbraPenumbra seenseen atat μμ<1<1 showsshows on limb side: Doppler red shift on disc side: Doppler blue shift Interpretation:Interpretation: horizontalhorizontal Limb OUTflowOUTflow ofof materialmaterial fromfrom innerinner penumbrapenumbra toto outerouter LowLow resolution:resolution: 11--22 km/s,km/s, highhigh resolution:resolution: supersonicsupersonic bright: redshift, dark: blueshift Regimes of solar magnetoconvection Magnetic activity in cool stars is driven SunspotSunspot by the interaction of umbraumbra the magnetic field with convection, i.e. magnetoconvection penumbrapenumbra Sunspots allow us to probe magneto- plageplage convection for quiet stronger fields, on quiet larger scales than SunSun other magnetic features Sunspots, some properties FieldField strengthstrength:: PeakPeak valuesvalues 20002000--40004000 GG BrightnessBrightness:: umbra:umbra: 20%20% ofof quietquiet Sun,Sun, penumbra:penumbra: 75%75% SizesSizes:: LogLog--normalnormal sizesize distribution.distribution. OverlapOverlap withwith porespores (log(log--normalnormal == GaussianGaussian onon aa logarithmiclogarithmic scale)scale) LifetimesLifetimes:: ττ betweenbetween hourshours && months:months: GnevyshevGnevyshev-- WaldmeierWaldmeier rule:rule: AAmax ~~ ττ,, wherewhere AAmax == maxmax spotspot area.area. Magnetic structure of sunspots BB dropsdrops steadilysteadily fromfrom 20002000 –– 40004000 GG inin umbraumbra towardstowards boundary,boundary, BB((RRspot)) ≈≈ 10001000 GG AtAt centre,centre, fieldfield isis vertical.vertical. ItIt becomesbecomes almostalmost horizontalhorizontal nearnear RRspot RegularRegular spotsspots havehave aa fieldfield structurestructure similarsimilar toto aa buriedburied dipoledipole Magnetic flux tubes Sunspots are intersections of the solar surface with large magnetic flux tubes In CZ and in photosphere most magnetic R energy is in concentrated magnetic flux tubes (bounded by topologically simple surface=current sheet) B2 B2 Pressure balance: 1+PP = +2 B1 B2 8π 1 2 8π P1 P2 Thick flux tubes such as spots, R>HP, where HP is the pressure scale height, A display strong variation across their cross-section. Pressure balance valid Rump of a flux tube only across boundary. flux tube The Wilson effect Near the solar limb the umbra and centre-side penumbra disappear ÎWe see 400-800 km deeper into sunspots than in photosphere Correct interpretation by Wilson (18th century). Other interpretation by e.g. W. Herschell: photosphere is a layer of hot clouds through which we see deeper, cool layers: the true, populated surface of the Sun. Why do we see deeper inside sunspots, or what causes the Wilson effect? Darkness:Darkness: OpacityOpacity inin thethe solarsolar photospherephotosphere isis duedue toto thethe HH-- ion,ion, whichwhich dependsdepends stronglystrongly onon temperature.temperature. InIn sunspotssunspots temperaturetemperature isis lowerlower ÎÎ opacityopacity isis lowerlower ÎÎ wewe seesee deeper.deeper. ResponsibleResponsible forfor ≈≈½½ ofof observedobserved effecteffect MagneticMagnetic field:field: MagneticMagnetic fieldfield producesproduces aa pressurepressure ~~BB2/8/8ππ.. DueDue toto pressurepressure balancebalance withwith surroundings:surroundings: B2 +spot =PPPP → <<ρ → ρ << 8π spot surr spot surr spot surr OpacityOpacity inin spotspot isis decreased.decreased. ResponsibleResponsible forfor ½½ ofof observedobserved effecteffect Why are sunspots dark? Basically the strong magnetic field, not allowing motions across the field lines, quenches convection inside the spot. Since convection is the main source of energy transport just below the surface, less energy reaches the surface through the spot Î dark β >1 ββ <1<1 Why are sunspots dark? II WhereWhere doesdoes thethe energyenergy blockedblocked byby sunspotssunspots go?go? SpruitSpruit (1982)(1982) ÎShortShort diffusivediffusive timescaletimescale ofof CZ:CZ: blockedblocked heatheat isis redistributedredistributed inin CZCZ withinwithin 11 monthmonth –– 11 yearyear (at(at mostmost onlyonly veryvery weakweak brightbright ringsrings aroundaround sunspots)sunspots) LargeLarge heatheat capacitycapacity ofof CZ:CZ: thethe additionaladditional heatheat doesdoes notnot leadlead toto aa measurablemeasurable increaseincrease inin temperaturetemperature LongLong timetime scalescale forfor thermalthermal relaxationrelaxation ofof thethe CZCZ (Kelvin(Kelvin--HelmholtzHelmholtz timescale):timescale): 10105 yearsyears ÎÎ excessexcess energyenergy isis releasedreleased almostalmost imperceptiblyimperceptibly (KH timescale: how long can Sun shine using only its gravitational energy) Solar irradiance during passage of a sunspot group The Sun as a whole darkens when spots move across its disc I.e. the blocked heat does not reappear somewhere else on a timescale of days to weeks Why are sunspots so bright? SunspotSunspot umbra:umbra: 20% of photospheric radiative flux 2000-4000 G mainly vertical field SunspotSunspot penumbra:penumbra: 75% of photospheric radiative flux 1000-2000 G complex, more horizontal field ForFor both:both: normalnormal convectionconvection completelycompletely quenchedquenched (Gough(Gough && TaylerTayler 1966).1966). RadiationRadiation carriescarries <10%<10% ofof energyenergy fromfrom solarsolar interior.interior. Î SomeSome formform ofof magnetoconvectionmagnetoconvection mustmust bebe actingacting atat smallsmall scalesscales thatthat transportstransports thethe missingmissing energyenergy fluxflux Current view of fine-structure of penumbra Penumbra is bigger hurdle than umbra (75% of energy flux) and much more controversial Convection Zakharov et al. 2008, Rempel et al. 2008 MHD simulation of a sunspot Red box represents the simulation box overlain on image of an observed spot Detailed structure of a penumbral filament Continuum intensity at 630 nm: 0.13 ... 1.02 <I> Cuts perpendicular to the filament The filament is formed by a hot, sheet-like convective upflow that turns over and flows down at the sides of the filament -1 3 T [10 K] vz [km s ] 4.0 6.0 8.0 10.0 12.0 14.0 -2.0 -1.0 0.0 1.0 2.0 Cuts perpendicular to the filament The filament is formed by a hot, sheet-like convective upflow that turns over and flows down at the sides of the filament -1 3 T [10 K] vz [km s ] 4.0 6.0 8.0 10.0 12.0 14.0 -2.0 -1.0 0.0 1.0 2.0 .