astronomy 142, spring 2013 31 january 2013dmw/ast142/lectures/lect_05p.pdf · astronomy 142, spring...

Astronomy 142, Spring 2013 31 January 2013

(c) University of Rochester 1

Today in Astronomy 142: the Sun

The structure of the Sun’s outer layers: convection zone, photosphere and corona.

Solar activity:

31 January 2013 Astronomy 142, Spring 2013 1

Solar activity: magnetism, sunspots, flares and CMEs.

Solar activity and Earth’s climate

Solar energy Multicolor ultraviolet image of the Sun, showing several sunspot-rich active regions (TRACE/NASA).

The Sun’s convection zone

The Sun’s interior, being fully ionized, has

Gas is unstable to convection if

Figure: Chaisson and McMillan, Astronomy Today

Hinode/NASA/JAXA

5 3.P VC C

1 2T dP


In the Sun this is true forthe Sun has a large convection zone. • See AST 111

regarding convection.

2 3 :R r R

.5P dT

Solar “granulation:” the tops of convection cells


Peter Sütterlin, U. Utrecht/Dutch Open Telescope



MSFC/NASA

The Sun’s radiative zone rotates like a solid body, but not the convection zone. CZ: faster at low latitudes than high. This differential rotation winds and

amplifies a “poloidal” solar magnetic field, turning it more “toroidal”

The solar dynamo and the solar cycle


g• …because magnetic field is frozen to

the ionized material by the Lorentz force.

Convection makes the field lines twistout of the surface and loop through the lower atmosphere, thereby creating sunspot pairs and prominencesconnecting them.

The solar dynamo and the solar cycle (continued)

The twisting and winding of the field lines eventually results in the production of a poloidalfield again, but with north and south switched. Then the process repeats This the process repeats. This repeating self-generation of magnetic field is called dynamo action.

For the Sun: 22 years between identical field configurations, 11 years between sunspot-number maxima.


Paul Charbonneau, U. Montreal

The solar photosphere

As we’ve noted, the spectrum of the Sun closely resembles a blackbody. From the total energy flux at

Earth (total solar irradiance [TSI], or “solar constant”):

6 1 2


we get the Sun’s luminosity,

TSI , L, and solar flux at most wavelengths, vary very little with time (see below).

At very long and very short wavelengths, though, flares can change the Sun’s brightness by huge factors.

6 -1 -21.366 10 erg s cmf

33 -13.826 10 erg s .L



The solar photosphere (continued)

In detail: absorption lines are also seen in the solar spectrum; they match up


they match up with many known transitions of atoms, ions and molecules.

Figure: the ultimate high-resolution spectrum of the Sun (Nigel Sharp, from data by Bob Kurucz et al. ( NOAO/NSO/Kitt Peak FTS/AURA/NSF))

The solar photosphere(continued)Spectral-line absorption by atoms and molecules is a hallmark of stars. Gases absorb strongly at the wave-

lengths of spectral lines (transitions between the quantum-mechanical states) of the atoms and molecules

Wavelength

Flux

Hot Coolerstates) of the atoms and molecules of which they’re composed.

Stars are heated from inside and are cooler on the outside. Thus to an outside observer, a star becomes opaque at a

higher altitude for wavelengths of spectral lines. One sees deeper into the star adjacent wavelengths. Because the deeper material is hotter, and hotter blackbodies are brighter, the star is brighter in between the spectral lines.


Hot Cooler

Chromosphere and corona

The corona is heated by magneto-acoustic noise from the boiling top of the convection zone, and by flares.


by flares. But its density is

so low that it can’t cool very well, so it reaches very high temperatures, > 106 K.Figure: Chaisson and McMillan, Astronomy Today



Corona during total solar eclipse


Miroslav Druckmüller, Brno Inst. Tech.

Sunspots and solar activity

Sunspots appear dark because they’re slightly cooler than the rest of the solar surface.

Surrounded by h h


hotter-than-average regions called faculae.

Zeeman effect measurements show that they are also maxima of magnetic field.(SOHO/NASA)

The 11-year sunspot cycle

The first sunspots in a cycle form near 30 latitude, and the last near the equator, producing the “butterfly diagram.”

/M

SFC


Dav

id H

atha

way

, NA

SA/



More solar images from 3/29/2001: He II at 30.4 nm (red), Fe IX at 17.1 nm (blue), and visible light (yellow).

Sunspots and solar activity (continued)

Note the X-ray bright faculae surrounding the sunspots: these regions add more to the Sun’s luminosity than the spots take away.


(SOHO/NASA)


X-ray emission from gas at T = 106 K in magnetic loops connecting sunspots on the limb of the Sun


(TRACE/ NASA).

Note the small flare about two seconds into the video.


Flares are driven by magnetic reconnectionwithin oppositely-directed tubes of magnetic flux. The “reconnected”

lines of B strongly lines of B, strongly curved at first, straighten out quickly.

The ions frozen to them impel other material outwards and inwards, like an arrow from a bow.


Chen et al. 2008




Large flares can lead to coronal mass ejection (CME) events, a.k.a. “solar storms”: blasts of ions accelerated to high energy (and speeds up to energy (and speeds up to ~0.3c) which expand into the Solar system. CMEs cause aurorae,

and wreak havoc on satellites and power systems on Earth.

STEREO/GSFC/NASA


Notable CMEs

September 1, 1859 (the “Carrington event”): strongest in history. Flares visible to the naked eye; aurorae brighter than full moonlight, and visible from very low latitudes (e.g. Hawaii); set fire to telegraph systems all over the world.

August 2, 1972: three strong CMEs in 15 hours, in between the visits to the Moon by Apollos 16 and 17. The astronauts got lucky; the radiation exposure would have been lethal radiation exposure would have been lethal.

November 4, 2003: second strongest CME in history, one week after the fifth strongest in history, and a few months after the launch of the NASA Spitzer Space Telescope and the two Mars Exploration Rovers took these satellites outside the Earth’s protective magnetosphere.• Damage to Spitzer’s detectors in a few seconds was equivalent to

that at five years of normal radiation levels.• One MER’s computer had to be rebooted 60 times before the

memory-repair routine finally worked.


250

300

350

1366

1367

1368

1369

t number

nce (W

m‐2)

The Sun’s luminosity – including flares and CMEs – varies in sync with the sunspot number, currently by about 0.1% over a solar cycle.


Total solar irradiance

0

50

100

150

200

1361

1362

1363

1364

1365

1366

1976 1981 1986 1991 1996 2001 2006

Annual sunspot

Total solar irradia

Year


(TSI) data from Claus Fröhlich; sunspot number from NGDC/ NOAA.



The Solar cycle and Earth’s temperature

As you know (e.g. from here), the Earth’s average temperature can be estimated from

2 4 2in out2

1 4

1 44

1

LP A R P T R

r

LAT

so a small change in luminosity leads to


21

16

AT

r

1 43 4

2

1 41 4 1

2

1 1 14 16

1 1 14 416

dT AT L L L

dL r

LA TL L L

Lr

The Solar cycle and Earth’s temperature (continued)

The 150-year average of the global mean ocean surface temperature (GMOST) is 16.0 C = 289 K, and the ocean’s heat capacity is such that it can respond to heating changes in about a month. (That’s why it’s usually colder on this date than it was on the first day of winter.) But a month is short compared to 11 years, so we expectcompared to 11 years, so we expect

per 1 W m-2 change in TSI, or “equivalently” per change by 100 in annual sunspot number.


289 1 K 0.053 K 0.053 C

4 1366.5T

Sunspots and climate on multi-solar-cycle scales

Sunspots have been counted systematically for hundreds of years. Over that span there have been big swings in the peak

sunspot number in solar cycles. There has been one extended period centered around the 17th century, called the Maunder Minimum during which sunspots were the Maunder Minimum, during which sunspots were practically absent.

Over that span there have also been big swings in Earth’s climate. There has been one extended period centered around the 17th century, called the Little Ice Age, during which Earth’s surface was dramatically colder than today.

These coincidences have not gone unnoticed.




100

120

140

160

180

200

sunspot n

umber

Sunspot number for the last 360 years

More historically harsh winters (e.g. novels by Dickens, Hugo, …)

Muir tracks Sierra Nevada glaciers

LittleIce Age

0

20

40

60

80

1650 1750 1850 1950

Annual s

Year


Source: National Geophysical Data Center (NGDC/NOAA)

Maunder minimum

Earth’s global surface temperature

To match up with the sunspots and their record of the total solar irradiance, we have several ways of monitoring global mean ocean surface temperature, GMOST. We have global ocean-surface temperature measurements

taken with real thermometers which go back to 1850.W h t llit t f th l t f d dWe have satellite measurements for the last few decades.We also have ways to recover oceanic temperature over

much longer timespans: • water-isotope abundance measurements in ice cores

and in tree rings.• tree-ring width (e.g. Mann & Jones 2003).

All agree with the historical record for last two millennia.


0

0.1

0.2

0.3

0.4

0.5

200

250

300

350

400

n temperature ‐16 C

sunspot n

umber

The last 1800 years of T and sunspots

Decline of Roman, Han empires

Climatic optimum: Norse in Greenland, etc.

LittleIce Age

Global warming

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

0

50

100

150

200 700 1200 1700

NH mean

ocean

Annual s

Year


Mann & Jones 2003 and NGDC



Sunspots and climate (continued)

For much of the last 400 years, the temperature and peak sunspot number track each other: eras with high (low) peak sunspot number go with eras with high (low) temperature.One might think this suggests that variation in solar activity produces variation in climate. There are at least three big problems with that suggestion:There are at least three big problems with that suggestion: Ice ages are even colder (-8C) than Little Ice Ages (-0.4

C), as we saw in AST 111 (see here). Sunspot number <0? The correlation deteriorates continuously through the past

century; it vanished a few decades ago. Some of the more recent temperature drops are much

better explained by volcanic activity: absorption of sunlight by high-altitude ash and H2SO4.


0

0.1

0.2

0.3

0.4

0.5

100

120

140

160

180

200

220


of ejecta, km

3

Tambora

Taupo

Mt. Rinjani

Baekdu

The last 1800 years of T and volcanic explosions

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

0

20

40

60

80

200 700 1200 1700

NH mean

ocean

Volume

Year

Kuwae


Mann & Jones 2003 and USGS

‐0.3

‐0.25

‐0.2

‐0.15

80

100

120

140

160


sunspot n

umber

TamboraKrakatoa

T, solar cycle and volcanic explosions, 1650-1900

‐0.45

‐0.4

‐0.35

0

20

40

60

1650 1700 1750 1800 1850 1900

NH mean

ocean

Annual s

Year


Mann & Jones 2003, NGDC, and USGS



0

0.1

0.2

0.3

0.4

0.5

320

340

360

380


2concentration, ppmv

0

0.1

0.2

0.3

0.4

0.5

150

200

250

300


sunspot n

umber Pinatubo

T, solar cycle and volcanic explosions, 1900-

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

260

280

300

1800 1850 1900 1950 2000

NH mean

ocean

Atm

ospheric CO2

Year

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

0

50

100

1900 1920 1940 1960 1980 2000

NH mean

ocean

Annual s

Year


Mann & Jones 2003, NGDC, and USGS

Sunspots and climate (continued)

So solar activity doesn’t seem to be a contender to explain long-term, substantial changes in global temperature, neither global warming nor the cooling episodes. Better: Atmospheric CO2 increases since the beginning of the

Industrial Revolution almost certainly have caused the recent global warming (IPCC 2011; see also Muller 2012recent global warming (IPCC 2011; see also Muller 2012and references therein).

Large volcanic explosions in the tropics seem at least as promising as an explanation of the Little Ice Age as the low solar activity of the Maunder Minimum…

… and such explosions seem certainly to explain some of the more recent and more confidently-characterized global cooling events.


0

0.1

0.2

0.3

0.4

0.5

320

340

360

380



The last 1800 years of T and atmospheric CO2

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

260

280

300

200 700 1200 1700

NH mean

ocean

Atm

ospheric CO2

Year


Mann & Jones 2003, Ethridge et al 1998, and NGDC



0

0.1

0.2

0.3

0.4

0.5

320

340

360

380



T and atmospheric CO2, 1800-


‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

260

280

300

1800 1850 1900 1950 2000

NH mean

ocean

Atm

ospheric CO2

YearMann & Jones 2003, Ethridge et al 1998, and NGDC

Solar activity and climate on solar-cycle time scales

How about smaller temperature changes on shorter times? Sunspot number, TSI and

temperature are also correlated on time scales shorter than the solar cycle.

Thi lt i t 100

150

200

250

300

350

1364

1365

1366

1367

1368

1369

nual sunspot n

umber

l solar irradiance (W

m‐2)

This results in a tempera-ture change of 0.15 C per W m-2 change in TSI, over the 160-year thermometer record. Results on the 35-year TSI record yield a similar result, 0.11 C per W m-2 of TSI. Either way it’s significantly more than the solid

expectation of 0.05 C, calculated a few pages back. 31 January 2013 Astronomy 142, Spring 2013 32

0

50

100

1361

1362

1363

1976 1981 1986 1991 1996 2001 2006

An

Total

Year

0 15

0.2

0.25

0.3

0.35

s correlation

122 2

j i i ji

T T N N

0 15

0.2

0.25

0.3

0.35

s correlation

Solar activity and climate on solar-cycle time scales (continued)

0

0.05

0.1

0.15

0 10 20 30 40

Cros

Delay (years)

Global mean ocean surface temperature ‐ sunspot number

11‐year cycle, 2‐year lag


22 2

i ii i

T T N N

0

0.05

0.1

0.15

0 10 20 30 40

Cros

Delay (years)

Global mean ocean surface temperature ‐ sunspot number

11‐year cycle, 2‐year lag

Data from NGDC/ NOAA




‐0 2

0

0.2

0.4

0.6

ean

ocean

surface

ature anomaly, C


T= 0.15 C W‐1 m2

r = 0.29p(156,r)=0.02%

‐0.8

‐0.6

‐0.4

‐0.2

0 50 100 150 200

Global me

tempera

Annual sunspot number, year‐2


This “amplification” of TSI changes is puzzling; it cannot be accounted for by the current generation of climate models. It would be a worry even without global warming! The observed T in phase with the solar cycle can’t be

produced without a factor of 5-7 more heat than the h i TSI (Sh i 2008 l M hl t l 2009) change in TSI (Shaviv 2008; see also Meehl et al 2009).

Climate models fail many simple tests like this. Like the climate itself, they are extremely complex, and just don’t work perfectly yet. The models will get better over time. • But this should worry you. We can’t really predict the

size of anthropogenic effects, if we can’t correctly predict the size of temperature modulation from the tiny solar-cycle modulation.


Energy and the sun

Hydrostatic equilibrium and ideal-gas behavior ensure that the center of the Sun is very hot, and energy (in the form of light) is radiated from the center.

The high opacity of the Sun to light determines the rate at which the energy leaks out. As we have seen, it takes a long time for photons to diffuse from center to surface.


This cannot go on forever, without the Sun cooling down, or replacement for the energy that leaks away.

We know that the solar system is about 4.6109 years old (from many radioisotope abundance measurements on meteorites), and that life has existed here for at least 3109

years. Thus the Sun must have had close to its present luminosity for billions of years.



How long would the Sun’s present heat last?

Energy density at center of Sun: energy density of the electron gas there, considered again to be an ideal gas:

Energy density of light (borrowed from PHY 227), which is about to leak away:

32e eu n kT

4


We showed last class that it takes about t = 31000 years for a photon to leak from center to surface, so the heat lasts

Much less than the Sun’s age; some process must be replacing the energy that leaks away.

uc

fT

cr 4 4 4

7

4 33 3

3 10 years.2 84

e ee

e r p

u u c kcn kT t t

du dt u t mT T

astronomy 142, spring 2013 31 january 2013dmw/ast142/lectures/lect_05p.pdf · astronomy 142, spring...

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