Outreach talk given to the Renfrewshire Astronomy Society on 26 January 2012

1. Solar spectroscopy
Solar spectroscopy
Dr Nicolas Labrosse
School of Physics and Astronomy
University of Glasgow

2. Solar spectroscopy
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3. Solar spectroscopy
Important dates
• Newton (1704) observed the dispersion of light by a prism
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4. Solar spectroscopy
Important dates
• Herschel (1800) detects infrared radiation using thermometers
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5. Solar spectroscopy
Important dates
• Wollaston (1802) notices dark lines in the spectrum of the Sun
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6. Solar spectroscopy
Important dates
• Fraunhofer (1817) describes the dark lines in Sun’s spectrum:
spectroscopy is born!
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7. Solar spectroscopy
Important dates
• Solar chemical composition in the 1860s
map of the solar spectrum published
in 1863 by Kirchhoff, showing the
identification of a large number of
spectral lines with various chemical
26/01/2012 elements
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8. Solar spectroscopy
Important dates
• Lockyer (1868) revealed the presence of an unknown element: helium
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9. 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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10. 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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11. Solar spectroscopy
High resolution solar spectrum (type G2)
700 nm
400 nm
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12. Solar spectroscopy
High resolution spectrum of Procyon (type F5)
700 nm
400 nm
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13. Solar spectroscopy
High resolution spectrum of Arcturus (type K1)
700 nm
400 nm
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14. 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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15. 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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16. 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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17. Solar spectroscopy
• Constitution
What do we know? – 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 17

18. Solar spectroscopy
Courtesy G. Doschek
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19. Solar spectroscopy
Courtesy G. Doschek
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20. Solar spectroscopy
Courtesy G. Doschek
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21. Solar spectroscopy
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Courtesy H. Peter Dr Nicolas Labrosse - Talk to Renfrewshire Astronomical Society 21

22. Solar spectroscopy
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23. Solar spectroscopy
Multi-wavelength view of the solar atmosphere
(SDO/AIA)
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24. 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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25. Solar spectroscopy
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26. Solar spectroscopy
Line intensity and width (Hinode/EIS obervations)
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27. Solar spectroscopy
Hinode/EIS observations of solar flares
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!
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28. 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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29. 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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