feb 2006 aia/hmi science team meeting 1 solar spectral irradiance: from the sun to the earth, the...

AIA/HMI Science Team Meeting 1Feb 2006

Solar Spectral Irradiance:From the Sun to the Earth,the Calibration Connection

Frank Eparvier, EVE Project Scientist

[email protected]

303-492-4546

AIA/HMI Science Team Meeting Eparvier - 2Feb 2006

Irradiance: One Spectrum, Two Stories

Atmospheric Physics The interaction of the solar photon output and the atmospheres of

the planets

Solar Physics The solar sources of irradiance and the causes of its variability

Scientifically, we want to understand both stories. Operationally, we want to be able to nowcast/forecast

both ends of the chain.

Calibration is key to making the connection between the stories.


What is Irradiance?

Irradiance is the solar photon energy incident on a unit area at some distance from the Sun. Usually expressed in Watts/m2. Usually normalized to a distance of 1 AU. Occasionally called “full disk integrated flux” (not in favor).

Irradiance comes in two flavors: Solar Spectral Irradiance (SSI)

Watts/m2/nm or photons/cm2/sec/nm Total Solar Irradiance (TSI)

Spectral irradiance integrated over all wavelengths Watts/m2


Image courtesy J. Lean

The Solar Irradiance Spectrum

Spectrum looks like blackbody (~5700K) in visible TSI is integrated total (~1361 W/m2, depending on whom you ask) VUV accounts for <0.1% of TSI, yet can account for up to a third of TSI

variability.


VUV (0.1-200 nm) spectrum contains many emission lines and continua from various locations in the solar atmosphere

VUV

EUV FUVXUV

VUV Solar Spectrum and Solar Formation Regions/Temperatures


Solar Cycle (11-years)

Solar Rotation (27-days)

XUV 0-7 nm

H I 121.5 nm

Flares

Flares - seconds to hours Related to solar solar eruptive events due

to the interaction of magnetic fields on Sun

Solar Cycle - months to years Evolution of solar dynamo with 22-year magnetic cycle, 11-

year intensity (sunspot) cycle

Solar Rotation - days to months Beacon effect of active regions

rotating with the Sun (27-days)

Timescales of Solar Spectral Variability


Variability depends on source

Based on TIMED-SEE data

Large flares can cause VUV variability as large as solar cycle variability.


Spectral Irradiance Variability

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.


Center-to-Limb Variation

Optically thick emissions = limb darkening Optically thin emissions = limb brightening

Type of source emission changes rotational variability (e.g. active regions) Location of emitting source on disk changes impact (e.g. flares)

Flare ExampleWoods et al. (2004, GRL)

X17 flare on 28 Oct 2004 showed an increase of ~100% throughout the 27-110 nm EUV range.

But X28 flare on 4 Nov 2004 (~2 times bigger in the XUV) showed only an increase of ~60% in the EUV range because the flare was on the limb whereas the X17 flare was near disk center.

Solar Rotation ExampleCoronal emissions often have 2 peaks when AR is near the east and west limbs; whereas, chromospheric and TR emissions have single peak when AR is near disk center.


Measurements of Spectral Irradiance

EUV irradiance measurements have been sparse.

Accuracy, precision, cadence, and wavelength coverage have been less than ideal.


History of EUV Spectral Irradiance Measurements

Early Space Age: Spectral measurements with little or no absolute irradiance calibration

Late 1970s to early 1980s: Rockets and AE-E measurements (Hinteregger et al.)

Cadence sporadic (rockets) or daily Good precision, but dispute over accuracy, especially at shorter wavelengths, by as

much as factor of 4

Late 1980s to early 2000s: Rockets, SOHO-SEM, SNOE, TIMED-SEE (Woods et al., Judge et al.)

Cadence improved to sub-daily at some wavelengths Improved calibrations narrowing of disputed irradiances to < 50% Precisions of 1-30% (mostly < 10%) Accuracies 15-30% for bright EUV lines (daily averages), larger for XUV broad bands

Future: SDO-EVE (Woods et al.)

Cadence: 10-sec accuracies to be < 25% at end of 5-yr mission for bright lines in full wavelength range

(0.1 - 105 nm)


Why do we care about irradiance at all?

Answer: Because we live with a star!


Spectral interaction with the atmosphere

Plot shows where the solar radiation is deposited in the atmosphere

Primary atmospheric absorbers are N2, O, O2, and O3

Ionosphere


(Solomon and Qian, JGR, 2005)Color bar scale is in units of log10(W/m4)

The highly variable solar EUV irradiance is absorbed in the Earth’s upper atmosphere causing: Ionization Dissociation Heating

These lead to photochemistry and dynamics

Variability in the solar irradiance leads to variability in the Earth’s atmosphere

Impacts communications, satellite drag, navigation, …

Upper Atmosphere Absorbs Solar EUV


(Lean, 1997)

Electron density varies by a factor of 10 over a solar cycle

Neutral density varies by more than a factor of 10

Temperature varies by a factor of 2 over a solar cycle

Solar Cycle Effects on Earth’s Atmosphere


(TIMED drag rate data courtesy D. L. Woodraska, 2005)

Satellite decay rate fluctuations match solar irradiance variability.

Solar Variations Cause Satellite Orbit Decay


Example: Sudden increase in the dayside density at low latitude regions due to the X17 solar flare on October 28, 2003

(E. Sutton, 2005)

Solar Flares cause:

• Increased neutral particle density in low latitude regions on the dayside.

• Sudden Ionospheric Disturbances (SIDs) which lead to Single Frequency Deviations (SFDs).

• Radio communication blackouts

• Increased error in GPS accuracy

GRACE daytime density (490 km)

Latitude (Deg)

2003 Day of Year

Flares Can Cause Sudden Atmospheric Changes


Models of Irradiance

Atmospheric scientists (and space weather operations people) need accurate EUV irradiance inputs for their models.

Irradiance measurements aren’t available at the time cadence or wavelength coverage needed.

Therefore, irradiance models are employed: Crudest models use 10.7 cm radio emission (F10.7) and scale

linearly between solar min and solar max reference spectra in 37 or so wavelength bins through EUV.

Most complex use images, mask analysis, and physics-based models source region emissions.

Punchline: Neither really matches measurements well, which is why we need SDO.


Solar Irradiance Model Definitions - 1

Proxy A solar measurement that is used to predict the irradiance at a different wavelength - e.g.

the ground-based 10.7 cm radio flux (F10.7) and the space-based Mg II core-to-wing ratio (Mg C/W)

Solar atmospheric layers (emission classes) Photosphere (phot): “surface” emissions near 5700 K - e.g. continuum > 260 nm Chromosphere (chrom): warmer region above photosphere, non-LTE and usually optically thick

emissions dominate in the UV - e.g. Mg I and II Transition Region (TR): warm-hot, narrow region between chromosphere and corona, also with non-

LTE and optically thick emissions - e.g. He II Corona (cor): hot, outer region with optically thin emissions - e.g. Fe XVI and the X-ray emissions -

region where Differential Emission Measure (DEM) technique works the best


Solar Irradiance Model Definitions - 2

Sources of Solar Variability (surface features) Quiet Sun (QS): quiet regions whose radiances are usually assumed to be constant, but

SOHO-SUMER observations have proven this concept wrong for the EUV and FUV emissions (Schühle et al., 2000, A&A)

Sunspot (SS): dark regions that appear in the photosphere (effects irradiance > 260 nm) Faculae (F): bright regions in the visible (> 260 nm) Active region (AR): very bright regions in the UV - above the sunspots; also called plage Active network (AN): slightly bright regions in the UV (similar to faculae) Quiet network (QN): even less bright regions in the UV Coronal hole (CN): dark regions in the corona emissions

Ca II K line6/03/1992

Map• Black: quiet-Sun• Purple: active network• Lined: active region (AR)• White: brightest AR

Example of identifying surface features from a solar image - from Worden et al. (1998, ApJ)


Existing Solar Irradiance Models

Advantages Some proxies are easier to measure than the

EUV directly Careful selection of proxy for each wavelength

/ band can lead to accurate estimate of irradiance

Proxy models are empirical relationships to measurements

DEM models are physics-based models for optically thin emissions

Disadvantages Proxies do not always behave exactly as other

EUV emissions Accuracy of model is driven by accuracy of the

original measurements No irradiance models for flares exist today Proxies need to have same time cadence of

EUV measurement requirement Daily F10.7 and Mg C/W are not useful for EUVS

concepts

HFG (SERF1, EUV81) proxy model - Hinteregger et al. (1981, GRL) Original proxies were the H I 102.5 nm (TR) and Fe XVI 33.5 nm (cor), from AE-E Substitute proxies are the F10.7 and <F10.7>81

EUVAC proxy model - Richards et al. (1994, JGR) Proxy is P10.7 and is the average of the F10.7 and <F10.7>81, re-analysis of AE-E

SOLAR2000 proxy model - Tobiska et al. (2000, JASTP) Proxies are the F10.7, H I 121.6 nm, and Mg C/W, based on lots of measurements

NRLEUV DEM-proxy model - Warren et al. (1998a, 1998b, 2001- all in JGR) Original proxy are QS, AR, and CH solar features identified from solar images Substitute proxies are the F10.7 and Mg C/W

FISM-proxy model - Chamberlin et al. (2005 PhD dissertation) Proxies for chrom, TR, cor, and flares, based on TIMED-SEE data


How are the models doing?

TIMED SEE measurements are considered the most accurate measurements of the solar EUV irradiance, but only the SOLAR2000 model has been updated to include the SEE results

The existing solar EUV irradiance models have absolute differences by a factor of 2 or more at many wavelengths

Model comparisonfor day

2002/039


How are the models doing? pt 2

Relative changes, such as solar rotational and solar cycle relative variations, tend to agree better between the existing models and the TIMED SEE measurements Nonetheless, there are significant differences for some of the models

Example comparison in the 5-25 nm range indicates that the SEE solar irradiance is much higher and that solar rotational variations are larger than the NRLEUV and EUV81 model predictions

Comparison in the 50-75 nm range indicates: SEE solar irradiance is lower and similar solar

rotational variations as compared to the EUV81 model

SEE solar irradiance is about the same level but larger solar rotational variations as compared to the NRLEUV model


Improving Solar Irradiance Models

Calibration is the key to improving models! Accuracy of the solar EUV irradiance measurements

Historical rocket and AE-E measurements have been used for 20+ years New, more accurate measurements from TIMED SEE are already

improving solar EUV irradiance modelling Additional improvements expected from the future SDO EVE and GOES

EUVS measurements Accuracy of the solar source region measurements

Image analysis is the wave of the future in modelling Need accurate radiances from sources and absolute calibration of images

Accuracy of physical modelling Lacking in atomic line parameters for physics based models Even deeper, true understanding comes from modelling of magnetic

structures and the emissions from the plasmas contained in them.


SDO: Tying it all together

Attend Session S2 in Bayview after the break We’ll be having an interactive discussion (gloves off and

sleeves rolled up) of inter-calibration of AIA and EVE and of SDO with other missions. What are the goals and the current plans? What are the steps needed for implemention? What are the pitfalls and shortcomings?

Ask not what SDO can do for you, ask what you can do for SDO!

feb 2006 aia/hmi science team meeting 1 solar spectral irradiance: from the sun to the earth, the...

Documents

solar sources of irradiance

solar image

solar measurement

photosphere effects

planets solar physics

solar photon output

quiet regions

bright regions