the euv spectral irradiance of the sun from minimum to maximum

The EUV spectral irradiance of the Sun from minimum to maximum

Giulio Del Zanna

Department of Space and Climate Physics

University College London

Vincenzo Andretta

INAF – OAC

EUV spectral irradiance of the Sun

UCL DEPT. OF SPACE & CLIMATE PHYSICS SOLAR & STELLAR PHYSICS GROUP

Open questions

• What are the characteristics of the EUV irradiance?• What are its long-term variations? • Is the ‘quiet’ Sun constant in time?• What is the relation between irradiance, magnetic activity and

the various solar regions ?• Can we use indices of activity to model the EUV irradiance ?• Does the EUV irradiance affect the global climate ?

SOHO (1996-2005) gives us the first opportunity to study EUV irradiances



Global warming

Last 10 years probably the warmest in the last 8 000 years !



Global warming

(Eddy 1976)

"Winter Scene with Frozen Canal" by Aert van der Neer

During the Little Ice Age, London’s Thames River froze in winter in the 17th Century, a very rare event.

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1700 1730 1760 1790 1820 1850 1880 1910 1940 1970 2000

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Solar activity (e.g. Sunspot number)correlates with many things

(including temperature variations).

But to what extent ?



Solar vs. anthropogenic and other (e.g. Volcanoes) effects

(Lean 1997)



The total irradiance is one of the main drivers for global climate.

Variability exists but is small (0.2%) and insufficient to explain variations on global weather.

Fröhlich 2005, updated Fall 2004 AGU poster



The Solar spectrum

(Floyd 2005)

(Woods 2005)

UV

EUV



The EUV and UV

• UV radiation controls the amount of stratospheric ozone.•UV variations could be a significant driver of changes in the terrestrial climate (e.g. Haigh 1996).

•The EUV radiation creates the ionosphere and might also have an indirect effect.

(Lean 1997)



EUV `Hole’

(Floyd 1997)Very few EUV observations (below red line) until TIMED in 2002The EUV irradiance has been modelled by using proxies (until 2002).

The X-ray irradiance is still modelled.



TIMED: UV irradiance since 2002

(Woods et al 2005)Grey areas show the variability during 2002-2004

Variability increases toward the EUV



Variability in the X-rays

Well known to be largeYohkoh SXT 1991 – 1995 ~100x change in x-ray brightness (Acton et al 1999).But SXT was a broad-band instrument.



Variability in the EUV from SOHOSolar EUV Monitor (SEM)

… but SOHO/SEM produces broad-band count rates.We need EUV spectral irradiance to calibrate SOHO/SEM.

How much is the contribution from the active regions?



The contribution from active regions seems dominant.There are no obvious long-term variations of the ‘quiet Sun’ at chromospheric levels

Livingston, B. 2005, unpublished



OSO 3: Hall & Hinteregger (1970)

The EUV spectrum is dominated by emission lines



With the SOHO/Coronal Diagnostic Spectrometer we observe the 15-80 nm regionwith the Normal Incidence (NIS) and Grazing Incidence (GIS) channels (9)

NIS 2 GIS 3 NIS 1



SOHO/EIT vs. Magnetograms (SOHO/MDI)



GIS observations of the quiet Sun (1996-2005)

At first we have obtained GIS radiances of the ‘quiet Sun’ during 1996-2003 and used SOHO/EIT to infer irradiances

GIS spectra show a considerable increase in hotter lines



Histograms of the EIT intensity distributions



EUV Irradiances from GIS



EUV irradiances compared to previous records



NIS full-Sun spectra: 700-1000 single slit exposures sampling the whole Sun in 13 hours (1998-2005).

See Thompson & Brekke (2000) for details.

He I, He II



Transition region – O III, O V



Mg X – 1MK



Corona at 2-3 MKSi XII – Fe XVI



EUV Irradiances from NIS



EUV Irradiances from NIS compared to previous records



Conclusions

• The SOHO/ GIS, NIS have produced a valuable dataset (1996-2005) that allows, with the support of SOHO/EIT and atomic data (CHIANTI) to reconstruct the EUV spectral irradiance from the last solar minimum.

• Preliminary irradiances are in agreement with old measurements (work is still in progress on the instrument’s calibration).

• The irradiances show a clear increase with the solar cycle, more pronounced for lines formed at T > 1MK and after 1998.

• During maximum, the corona becomes hotter, and the characteristics of the `quiet Sun’ change.



Additional material

the euv spectral irradiance of the sun from minimum to maximum

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