Lecture 13: Sunspots

Outline

1 Terminology and History2 Sunspot Structure3 Sunspot Evolution

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 1

Overview

Sunspot Terminology

DOT Blue Continuum Movie of AR10425

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 2

Basic Appearance

sunspots often appear in sunspot groupssunspots consist of umbra and penumbrapores are sunspots without penumbraeumbral structures: umbral dots and light bridgespenumbral filamentsumbra and penumbra have well-defined, structured boundariessunspot is much more stable than granulationbright penumbral grains move inward

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 3

Active Region Numbers

www.solarmonitor.org

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 4

History before Telescopes

www.hao.ucar.edu/Public/images/worcester.jpg

Theophrastus of Athens reports sunspots around 330 BC112 descriptions of sunspots in official historical records fromChina between 28 BC and 1638 AD8 December 1128, John of Worcester makes the first diagram ofthe Sun containing two large spotsKepler misinterpreted sunspot as Mercury transit in 1607

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 5

web.hao.ucar.edu/

public/education/sp/images/fabricius.html

Johann Goldsmid (Fabricius)... Having adjusted the telescope, we allowedthe sun’s rays to enter it, at first from the edgeonly, gradually approaching the center, until oureyes were accustomed to the force of the raysand we could observe the whole body of thesun. We then saw more distinctly and surely thethings I have described [sunspots]. Meanwhileclouds interfered, and also the sun hastening tothe meridian destroyed our hopes of longerobservations; for indeed it was to be feared thatan indiscreet examination of a lower sun wouldcause great injury to the eyes, for even theweaker rays of the setting or rising sun ofteninflame the eye with a strange redness, whichmay last for two days, not without affecting theappearance of objects.

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 6

galileo.rice.edu/sci/observations/sunspot_drawings.html

History after TelescopesOther contemporary observations by Galileo Galilei (Italy), ChristophScheiner (Germany), and Thomas Harriott (England)

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 7

Drawings by Christoph Scheiner

galileo.rice.edu/sci/observations/sunspots.html

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 8

Drawings by Samuel Langley (1834–1906)

www.ociw.edu/ociw/babcock/howardtalk.pdf

Why are these drawings comparable in quality to some of the bestpictures from modern telesopes?

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 9

Sunspots in Numbersdiameter: 10’000 – 100’000 kmintensity of umbra: 5% of quiet photosphere at 500 nmintensity of penumbra: 80% of quiet photosphere at 500 nmtemperature of umbra: 3500 K

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 10

Sunspot Field Strengths: 2000-4000 Gauss

www.noao.edu/image_gallery/html/im0972.html

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 11

Empirical Temperature - Field Strength Relation

Kopp & Rabin 1992

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 12

Theoretical Temperature – Field Strength Relation

magnetohydrostatic force balance

∇(

p +B2

2µ0

)− 1µ0

(~B · ∇

)~B = ~Fgravity

radial component in cylindrical coordinate system

∂

∂r

(p +

B2

2µ0

)− 1µ0

(Br∂Br

∂r+

Bφ

r∂Br

∂φ+ Bz

∂Br

∂z−

B2φ

r

)= 0

for untwisted field (Bφ = 0)

∂p∂r

=Bz

µ0

(∂Br

∂z− ∂Bz

∂r

)integrate from radius r to field-free region outside of spot

pe (z)− p (r , z) =B2

z (r , z)

2µ0+

1µ0

∫ ∞r

Bz∂Br

∂zdr ′

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 13

continuing ...

from previous slide

pe (z)− p (r , z) =B2

z (r , z)

2µ0+

1µ0

∫ ∞r

Bz∂Br

∂zdr ′

for simple magnetic field configurations can rewrite this as

pe (z)− p (r , z) =B2 (r , z)

2µ0(1 + f (r , z))

and using perfect gas law, write this as

T (r , z)

Te (z)=µ (r , z)

µe (z)

ρe (z)

ρ (r , z)

(1− 1 + f (r , z)

2µ0pe (z)B2 (r , z)

)

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 14

Empirical Temperature - Field Strength Relation

Kopp & Rabin 1992

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 15

Azimuthally Averaged Sunspot Quantities

Keppens & Martinez Pillet 1996

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 16

Wilson Depression

sunspot umbrae are lower than quiet photosphere (Wilson 1769)

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 17

Umbral Dots

Tritschler & Schmidt 2002

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 18

Umbral Dots Simulations

Schüssler & Vögler 2006

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 19

Evershed Effect: Radial Outflow

Penn et al. 2003

spectral lines show outflow of up to 10 km/s−1

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 20

Penumbral Filament Simulations (R.Schlichenmaier)

www.kis.uni-freiburg.de/ schliche/index-Dateien/video.html

thin magnetic flux tube in magnetohydrostatic sunspot modelsurrounding field-free area heats tube, it becomes buoyantcools off when reaching photospheresince field in tube decreases faster than background field, gaspressure builds up leading to outward flow

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 21

Sunspot Oscillations

DOT 2005-07-13 AR10789

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 22

Large Sunspot Region from early 2001

Best of SOHO Movies

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 23

Sunspot Region Evolution

www.noao.edu/image_gallery/html/im0605.html

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 24

Sunspot Evolution

Zhang & Tian 2005

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 25

Active Region Development

science.nasa.gov/ssl/pad/solar/dynamo.htm

sunspots occur in two belts, symmetric around and parallel to theequator at ± (5◦– 35◦) lattitudesunspots are areas of strong magnetic fieldssunspots often appear in pairs with opposite magnetic polarity(bipolar groups)polarity changes with hemisphere and solar cycle (Hale’s Law)sunspot groups are inclined with respect to the equator

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 26

Sunspot Groups

science.nasa.gov/ssl/pad/solar/dynamo.htm www.ociw.edu/ociw/babcock/howardtalk.pdf

sunspot groups are tilted with respect to equatortilt angle depends on lattitude (Joy’s Law)

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 27

Sunspot Lifecycleappearance and disppearance of sunspot groups is alwaysrelated to the same phenomena⇒ everything together makes upan active regionappearance of magnetic field within a few days, strong increase infaculae, magnetic field becomes bipolar, small pores and spotsappear, corona becomes bright above active regionmaximum activity takes 2 to 3 weeks, large bipolar groups form,active region expands to 200’000 km, flares occurdisappearance takes 6 to 9 months, active region expands to500’000 km, spots disappear, faculae of opposite polaritiesappear that are separated by a quiet prominence, slowdisappearance of these phenomenasunspot lifetime: days to months, most less than 11 days

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 28

Zürich Sunspot Classification: Evolutionary Sequence

Waldmeier (1955)

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 29

Rudolf Wolf

web.hao.ucar.edu/public/education/sp/images/wolf.html

Sunspot Numbersdiscovery of sunspot cycle byHeinrich Schwabe in 1843Rudolf Wolf at ETH Zurichcompares sunspot observationsfrom different observersinvents Sonnenflecken Relativzahlin 1847number of sunspot groups g,number of sunspots f

R = k(10g + f )

k ≈ 1: observer-dependentcalibration factorSunspot Number is a goodmeasure of fractional area of Suncovered by spots

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Solar Physics, Lecture 13: Sunspots 30