long-term variations in uv and euv solar spectral...
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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).