Solar spectroscopy

Solar spectroscopy

Dr Nicolas Labrosse

School of Physics and Astronomy

University of Glasgow

Solar spectroscopy

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Solar spectroscopy

Important dates

• Newton (1704) observed the dispersion of light by a prism

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Solar spectroscopy

Important dates

• Herschel (1800) detects infrared radiation using thermometers

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Solar spectroscopy

Important dates

• Wollaston (1802) notices dark lines in the spectrum of the Sun

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Solar spectroscopy

Important dates

• Fraunhofer (1817) describes the dark lines in Sun’s spectrum:

spectroscopy is born!

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Solar spectroscopy

Important dates

• Solar chemical composition in the 1860s

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map of the solar spectrum published

in 1863 by Kirchhoff, showing the

identification of a large number of

spectral lines with various chemical

elements

Solar spectroscopy

Important dates

• Lockyer (1868) revealed the presence of an unknown element: helium

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Solar spectroscopy

Important dates

• The 1879 eclipse revealed a coronal green line at 530.3 nm

– Origin unknown for 50 years: was this “coronium”?

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Solar spectroscopy

Important dates

• 1939: Edlén showed this line is emitted by highly ionised iron (Fe XIV,

which has lost 13 electrons)

– Corresponding to

temperature over

1 million K!

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Solar spectroscopy

High resolution solar spectrum (type G2)

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400 nm

700 nm

Solar spectroscopy

High resolution spectrum of Procyon (type F5)

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400 nm

700 nm

Solar spectroscopy

High resolution spectrum of Arcturus (type K1)

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400 nm

700 nm

Solar spectroscopy

Spectral lines

• Lines characterised by intensity, position, and width

• In solar spectroscopy, width affected by (among others)

– Instrumental profile

– Temperature

– Collisions

– Unresolved motions

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Solar spectroscopy

Spectroscopic instruments

• Needed to obtain physical parameters, such as

– Temperature

– Magnetic field

– Flow speed

• Measurements are multi-dimensional:

– Two spatial dimensions

– Wavelength

– Time

• At present, detectors only record 2 dimensions at a time.

– Filter instruments record 2D images at a fixed wavelength

– Slit spectrographs record 1 spatial dimension and a certain wavelength

range.

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Solar spectroscopy

What do we know?

The Sun is a huge ball of plasma – a gas which is not

neutral but contains free electric charges

– Although one speaks of the solar surface, the Sun has neither

solid or liquid matter anywhere inside it.

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Solar spectroscopy

What do we know? • Constitution

– Sun’s energy output comes from nuclear reactions at centre

– Energy transported outwards through radiative zone then convective zone

– 70% hydrogen, 28% helium (by mass)

• Photosphere

– Temperature ~ 5800 K

– Sunspots

• Chromosphere

– Temperature ~ 20 000 K

• Corona

– Temperature ~ 1 million K (!!!)

– Visible only during solar eclipses with the unaided eye

– Holes

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Solar spectroscopy

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Courtesy G. Doschek

Solar spectroscopy

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Courtesy G. Doschek

Solar spectroscopy

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Courtesy G. Doschek

Solar spectroscopy

26/01/2012 Dr Nicolas Labrosse - Talk to Renfrewshire Astronomical Society 21 Courtesy H. Peter

Solar spectroscopy

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Solar spectroscopy

Multi-wavelength view of the solar atmosphere

(SDO/AIA)

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Solar spectroscopy

X-ray and EUV spectroscopy

• Visible to ~1900 Å: Dominated by the continuum, mostly absorption lines

• 1700 – 1100 Å: The photosphere, chromosphere, lower transition region. A

few coronal lines for above the limb (no coronal disk observations)

– Temperatures from 10 000 K to about 250 000 K

• 1100 Å – 500 Å: the lower and upper transition region but limited coronal

access for disk observations, some forbidden lines for flares, e.g., Fe XVII,

Fe XVIII, Fe XIX, Fe XXII

– Temperatures from 250 000 K up to about 1 million K

• 500 Å – 170 A: the corona and flares (some transition region lines)

– Temperatures from about 800 000 K up to about 20 million K

• Below 170 Å : flare allowed lines of Fe XVIII through Fe XXIII between about

90 Å and 140 Å.

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Solar spectroscopy

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Solar spectroscopy

Line intensity and width (Hinode/EIS obervations)

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Solar spectroscopy

Hinode/EIS observations of solar flares

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Data from a major solar flare that occurred near Sun center on 18 February 2011.

The strong signal at 192.0 Å shows that the temperature in the flare has reached

15 million degrees!

Solar spectroscopy

Summary

• Solar spectroscopy tells us how the solar atmosphere is

structured...

• ... and points to what we need to work on in the future

– Solve the puzzle of the hot corona

– Identify mechanisms behind energy transport, bulk flows, particle

acceleration, ...

• A rich discipline relying on complex quantum mechanics

calculations to predict the spectra emitted by atoms, ions,

and molecules

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Solar spectroscopy

Additional information and resources

• K. M. Harrison, "Astronomical Spectroscopy for

Amateurs". Patrick Moore's Practical Astronomy Series.

Springer, 2011. ISBN 9781441972385

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