Long-Term Variations in UV and EUV SolarSpectral Irradiance

Linton Floyd1 Don McMullin2

1Interferometrics Inc. / Naval Research Laboratory

2Space Systems Research Corporation / Naval Research Laboratory

International Workshop on Solar Variability,Earth’s Climate and the Space Environment

Bozeman MTJune 1 - 6, 2008

Relevance of the Study of Solar EUV/UV Irradiance

Knowledge of the Solar UV irradiance helps us better understand:

I Solar mechanisms

I Earth’s atmosphere

I Earth’s climate

Solar EUV/UV and Solar Atmospheric Structure

Solar EUV/UV originates in:

I corona,

I transition region,

I chromosphere, and

I upper photosphere

where much of the solarirradiance variation occurs.

From: Fox (2004).

Above the temperature minimum, surface structures limit theusefulness of this 1-D model.

Relevance to Earth’s AtmosphereAbsorption of Solar EUV/UV

Solar UV absorption drivesatmospheric:

I constituent densities,

I thermal structure, and

I dynamics.

Solar UV is absorbed by:

I ozone (200–320 nm)

I molecular oxygen(140–242 nm)

Haigh, 2004

Solar UV and Earth’s Climate

I Climate and weather data shows connections to solar activity,e.g. QBO, NAO, and SST.

I Models show possible solar UV connections to dynamicalchanges descending from the stratosphere to the troposphere.

I Cosmogenic isotopes show correlations to climate over thepast two millennia, independent of Milankovich (orbital andterrestrial attitude) changes.

I Solar causal connections to climate are poorly understood.Solar UV variation is a leading candidate.

Solar UV/EUV Irradiance Spectrum

Solar Ultraviolet Irradiance Spectrum

100 200 300 400wavelength (nm)

-3

-2

-1

0

1

2

3

4Lo

g 10 S

pect

ral I

rrad

ianc

e (m

W/m

2 /nm

)

HeII

HeI

Lyedge

CIIILy β

Ly α

OI

CII

SiIV

CIV

SiII

Al edge

Mgedge

Mg IIMg II

K H

Ca II

EUV FUV

MUV NUV VIS

5778 K Blackbody

Thuillier et al. (2005)

Solar EUV/UV Irradiance Spectrum

I extends from 30 nm in the EUV to the visible (400 nm)

I spans roughly 5 orders of magnitude

I contains about 8.7% of the total solar flux

I shows exponential increase in FUV to Al-edge (208 nm)

I for increasing λ, the spectrum is characterized by:I strong emission lines (120–181 nm)I absorption lines (220–400 nm)I line-blanketed continuum

I continuum at ∼160 nm from solar temperature minimum

Measurements of the Solar UV/EUV IrradianceInstrumental Responsivity Calibration

Typical source of largest uncertainty is changing instrumentalresponsivity.

End-to-end calibration methods utilize measurements which are:

I stellar (e.g. SOLSTICE, SORCE),

I of lamps (e.g. SUSIM, SOLSPEC/ISS),

I “vicarious” (e.g./ NOAA-11/SBUV2, SEM), or

I redundant (e.g. SORCE, TIMED, SUSIM)

Measurements of the Solar UV IrradianceCoverage of Past Experiments

Temporal Coverage of Solar UV Irradiance Experiments

1965 1970 1975 1980 1985 1990 1995 2000year

0

100

200

300

400

wav

elen

gth

(nm

)

OSO3

OSO4

AE-C

AE

RO

S-A

AE

RO

S-B

OSO5

AE-E

SMEUARS

SUSIM,SOLSTICE

SM5

Nimbus-7/NOAA 9,11

GOME

SEM

SNOE

Key:

onboard cal

underflights

no in-flight cal

Measurements of the Solar UV IrradianceCoverage of Present and Future Experiments

2000 2002 2004 2006 2008 2010 2012year

0

100

200

300

400

wav

elen

gth

(nm

)

UARSSUSIM,SOLSTICE

SDO EVE

GOMEGOME-2

SEM

SNOE

NOAA-16,17

SORCESIM,SOLSTICE II

SORCE XPS

SEE/TIMED

SCIAMACHY

OMIAURA

NOAA-N’(?)

NPOES(?)

SOLSPEC

SOL-ACES

GOES (N-P)

?

PICARD/PREMOS

PICARD/PREMOS

Solar UV Irradiance Experiments: Data ComparisonsData Comparisons: Ly-α

Upper panel shows the Ly-αirradiances from SUSIM andSOLSTICE.

Lower panel displays theirdifferences.

The two experiments showbetter relative than absoluteagreement (mostly).

UARS Solar Lyman α Irradiance

3

4

5

6

7

x1011

ph/

sec/

cm2

SOLSTICE V18

SUSIM V21r3

1992 1994 1996 1998 2000 2002Year

-0.8

-0.4

0.0

0.4S

OLS

TIC

E -

SU

SIM

Average = -0.36360 STD = 0.00277

Solar UV Irradiance Experiments: Data ComparisonsData Comparisons: 200–205 nm Integrated Irradiance

Upper panel are the200–205 nm integratedirradiances from SUSIM andSOLSTICE.

Lower panel displays theirdifferences.

The two experiments showbetter relative than absoluteagreement (mostly).

UARS Solar Irradiance 200 - 205 nm

42

44

46

48

50

mW

/m2

SOLSTICE V18

SUSIM V21

1992 1994 1996 1998 2000 2002Year

0

1

2

3

4

SO

LST

ICE

- S

US

IM Average = 2.57163 STD = 0.02891

Solar UV Irradiance VariationsSources and spatial distribution of UV radiance

Variations in received solar UVirradiance are caused by the emergenceand decay of active regions as theytransit the solar disk.

Active regions contain enhanced:I UV brightness (faculae and plages)

I localized enhanced magnetic fields

Upper right: BBSO Ca II k line brightness

Lower right: GONG Magnetogram (Sources: BBSO)

Solar UV Irradiance VariationsTime series characteristics

UV irradiance time seriesperiodicities dominated by:

I solar rotation (∼27 day)

I solar cycle (∼11 yr)

Variation at different wavelengthsare in phase.

Occasionally, the short-termbehavior can be quite differentamong various wavelength ranges(see figure).

SUSIM Irradiance Time Series

6.26.46.66.87.07.27.4

Ly α (mW/m2)

3.75

3.80

3.85

3.90

3.95170-175 nm (mW/m2)

42.8

43.3

43.8 200-205 nm (mW/m2)

291

292

240-245 nm (mW/m2)

0.258

0.260

0.262Mg II Core-to-Wing Ratio

JUN 94 JUL AUG SEP OCT NOV DEC JAN 95 FEB MAR APR MAY

EUV Irradiance Variation over the Solar Cyclewavelength dependence in the EUV (30–120 nm)

40 60 80 100 120wavelength (nm)

0

100

200

300

SC

var

iatio

n (%

)

HeI

I

HeI

CIII

Ly β

CIII

Estimates derived from TIMED SEE V9

Solar UV Irradiance Variation over the Solar Cyclewavelength dependence in the FUV (120–200 nm)

Variations shown are derivedfrom UARS SOLSTICE; similarresults have been obtained fromSUSIM and NOAA-11 SBUV2.

Relative FUV irradiancevariations are larger:

I for shorter wavelengths, and

I in emission lines (e.g. Ly-α,Cii, Siiv, Civ, and Siii).

Adapted from: Rottman, Floyd, and Viereck (2004).

Solar UV Irradiance Variation over the Solar Cyclewavelength dependence in the MUV (200–300 nm)

Relative variations are roughlyconstant up to about 263 nmwith larger variations inabsorption line cores (e.g. Mg II).

Above about 290 nm, thevariation is below experimentaluncertainties.

Adapted from: Rottman, Floyd, and Viereck (2004).

Solar UV Irradiance Variation over the Solar Cyclewavelength dependence in the NUV (300–400 nm)

Relative variations are:

I much stronger in absorption lines

I uncertain, but less than 1% overall

I estimates obtained through signal detection methods

Solar Cycle Variation from Synthetic Solar Model

From: Fox (2004).

Contribution of UV Irradiance Variationto total solar irradiance (TSI) variation

I UV energetic variation dominated by longer wavelengths

I larger relative variations below 200 nm are insignificant

I variation for 300–400 nm highly uncertain

I contribution of UV to TSI variation (0.1%) range from 17%to 60%

Solar Mg II Core-to-Wing Ratio Index

I irradiance ratio of the core of theMgII absorption feature (280 nm)and its nearby wings

I sensitive to the large solar variationin the core while effectivelyremoving instrumental effectswhich vary weakly with λ

I derived from the measurements ofmany experiments having uniqueinstrumental properties (e.g.resolution)

I various Mg II index series arelinearly related (r > 0.98).

Solar Mg II Absorption Feature

276 278 280 282 284wavelength (nm)

0

100

200

300

400

Irra

dian

ce (

mW

/m2 /n

m)

k

h

1.10 nm

0.15 nm

0.01 nm

(Hall & Anderson)

Composite Solar Mg II Core-to-Wing Ratio Indexcomponents of 2004 version

NOAA SEC Composite Mg II Core-to-Wing Ratio Index

1980 1985 1990 1995 2000 2005 2010

SC 21 SC 22 SC 23SC 23

Viereck et al. (2004)

1980 1985 1990 1995 2000 2005 2010

Solar UV Experiments

which produce

or will produce

a MgII Index

Nimbus-7

NOAA-9

NOAA-11

SOLSTICE

SUSIM

GOME

NOAA-16

SCIAMACHY

NOAA-17

SORCE

OMI

GOME-2

NOAA-N’ ?

Composite Solar Mg II Core-to-Wing Ratio Indexcurrent version (2008)

1975 1980 1985 1990 1995 2000 2005 2010year

0.260

0.265

0.270

0.275

0.280

0.285

0.290

MgI

I Ind

ex (

dim

ensi

onle

ss)

21 22 23

NOAA SWPC MgII Composite, May 2008

Comparison of Mg II Index with UV IrradiancesSUSIM Ly-α, 200–205 nm, and 235–240 nm

I Relative long-term variations ofthe UV irradiance(120–290 nm) are welldescribed by the Mg II index(within experimentaluncertainties).

I Above 290 nm, Lean et al.(1997) report that sunspotsalso contribute significantly.

I Uncertainty as a fraction of thesolar variation grows forincreasing λ.

6

8

10

12

mW

/m2

Ly-αr = 0.972

DataMg ΙΙ Fit

-1

0

1

resi

dual

s

-20-1001020

%

4344454647

mW

/m2

200-205 nmr = 0.952

DataMg ΙΙ Fit

-1

0

1

resi

dual

s

-4

-2

0

2

4

%

235

240

245

250

mW

/m2

235-240 nmr = 0.895

DataMg ΙΙ Fit

1992 1994 1996 1998 2000 2002Year

-4-2024

resi

dual

s

-2

-1

0

1

2

%

Solar Ultraviolet Irradiance ResearchTheoretical and modeling research

Composite Mg II Index (Viereck, Weber, and others)

Composite solar UV irradiances (Snow, DeLand, and others)

Solar cycle dependence of solar UV irradiance (Floyd, Pagaran, andothers)

Empirical past and predictive models of solar UV irradiance (Tobiska andothers)

Semi-empirical models of solar UV irradiance (Solanki, Krivova andothers; Morrill and others; Ermolli and others; Unruh and others)

Synthetic solar irradiance model (Fontenla, Kurucz and others)

Solar Ultraviolet (UV & EUV) IrradianceInteresting Questions for Further Research

I What are the detailed mechanisms of solar UV irradiancevariation?

I What is the connection between magnetic activity and UVirradiance variations?

I What is the contribution of UV variation to that of the TSI?

I How much does the solar UV vary over time periods longerthan the solar activity cycle?

I What was the solar UV irradiance during the MaunderMinimum?

I How well does the Mg II index describe relative irradiancevariations from the EUV to the visible?

Suggested and Planned Future Directionsfor solar EUV/UV irradiance research

I Continued UV spectral irradiance measurements especiallythose by instruments with in-flight end-to-end calibrations

I Improvements in long-term calibration of UV instruments(e.g. SORCE)

I Imaging in the UV (e.g. Picard) perhaps from differentdirections with simultaneous UV irradiance measurements

I Continued and improved measurements of solar activityindices (e.g., F10.7, Mg II, He 1083 EW, and even SSN)

Acknowledgementsmany thanks to organizers and sponsors

I thank the organizers, especially Judit Pap, Dibyendu Nandi, andDean Pesnell as well and the conference sponsors: NASA LWS,Montana State University, SCOSTEP/CAWSES, UMBC/GEST,and IHY.

This work was supported by a grant from the NASA Living withthe Star Program (contract NNH05CD10C).