Hale COLLAGE (CU ASTR-7500)
“Topics in Solar Observation Techniques”
Spring 2016, Part 1 of 3: Off-limb coronagraphy & spectroscopy
Lecturer: Prof. Steven R. Cranmer
APS Dept., CU Boulder
steven.cranmer@colorado.edu
http://lasp.colorado.edu/~cranmer/
Lecture 1:
Introduction, motivation, & history
Brief overview
Goal of Part 1: To introduce you to the world of off-limb
solar observation techniques & diagnostics
1.
2.
3.
4.
5.
Introduce basic concepts in radiative transfer
Coronagraphic occultation: why & how?
Off-limb visible continuum: formation & diagnostics
Off-limb emission lines: formation & diagnostics
(mini) Bridging “remote-sensing” and “in-situ” techniques
Goal of Lecture 1:
1. Give bird’s eye perspective on observing the Sun above the limb
2. Summarize history of these observations
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
August 1, 2008, Mongolia
Image: Miloslav Druckmüller, http://www.zam.fme.vutbr.cz/~druck/
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
photosphere
corona
solar wind
ISM
chromosphere
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
photosphere
corona
solar wind
ISM
chromosphere
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
photosphere
corona
solar wind
ISM
chromosphere
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Lecture 1: Introduction, motivation, & history
Voyager 1, 2
Pluto
Jupiter
Earth
Mercury
Big picture: the heliosphere
Hale COLLAGE, Spring 2016
Voyager 1, 2
Pluto
Jupiter
Earth
Mercury
Big picture: the heliosphere
protons
electrons
oxygen ions
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Big picture: the heliosphere
in situ
probes
remote
sensing
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Observation regions
techniques
Low/Inner Corona
• where most coronal
heating occurs
r ~ 1 to 1.3 R
direct imaging
Outer/Extended Corona
• where most solar wind
acceleration occurs
• transition from
collisional (MHD) to
collisionless (kinetic)
r ~ 1.5 to 10 R
eclipses,
coronagraphs,
Inner heliosphere
r > 60 R
heliospheric
imagers
in situ probes
radio sounding?
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Off-limb diagnostics
Direct:
• Plasma density
• Plasma velocity
• Plasma temperature
• Magnetic field
• Plasma fluctuations
(incl. elemental abundances, ionization states)
(vector)
(incl. non-Maxwellian velocity distributions)
(vector)
(waves, shocks, eddies, jets, solitons, …)
Indirect:
• Properties of (primordial?) dust
• Rates of { mass, momentum, energy } transfer
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
History: observing above the limb
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Total eclipses
• Sun and Moon have ~ equal solid angles;
coincidence? anthropic?
• Wondrous/terrifying events for all of
human history.
• First photographs: 1851, but drawings remained “better” for decades.
G. Tempel, 1860
Lecture 1: Introduction, motivation, & history
E. Holden, 1889
Hale COLLAGE, Spring 2016
Eclipse expeditions
Reminiscence
from Hendrick
van der Hulst
(1918-2000)
1898 Lick Observatory
expedition, Jeur, India
1919: Eddington’s validation
of general relativity
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
The chromosphere
“blood red streak”
“sierra”
“full lake red”
“flames, beams,
and streamers, as
transient as those of our
own aurora borealis.”
However… Lockyer’s
“chromo” color probably
refers to the full range of
bright emission lines!
“pink
transparent
gas”
Photos: Fred Espenak
“saw-teeth … for a circular saw”
Sketch: N. Lockyer
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Norman Lockyer & Pierre Janssen
• 1868: They independently took note of a strange line
in the solar spectrum: almost overlapping with
known lines… it was suggested to be a new element:
Sodium
Helium
• On closer inspection, off-limb eclipse
spectra contained several other unknown
emission lines… “coronium?”
• 1869: Lockyer founded Nature.
In 1st issue, editorialized that these lines were
“…bizarre and puzzling to the last degree!”
ApJ, 1921
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
The coronium heating problem?
• There was a proliferation of proposed elements, but atomic physics
eventually caught up to the observations:
Coronium:
“green line” at 530.3 nm
“red line” at 637.4 nm
“yellow line” at 569.4 nm
[Fe XIV]
[Fe X]
[Ca XV]
Aurorium / Geocoronium:
green line at 557.7 nm
red line at 630 nm
[O I]
[O I]
Nebulium:
blue-green lines at 495.9,500.7 nm
[O III]
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Mounting evidence for a hot corona
Alfvén (1941) summarized the evidence and concluded that T ~ 106 K
• “Coronium” lines are associated with high ionization states of
heavy elements (Grotrian, Edlén: late 1930s)
• Spectral line Doppler widths (both emission & absorption) are
consistent with high T
• If observed radial dependence of electron density is in
hydrostatic equilibrium, it demands high T (van der Hulst)
• During flares, Earth’s ionosphere is enhanced: UV & X-rays
(still unobserved in 1940s) must be coming from Sun.
What kind of source produces such photons? A hot one!
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Parallel history: the solar wind
• 1850–1950: Evidence builds for outflowing magnetized plasma from the Sun:
 solar flares  aurora, telegraph snafus, geomagnetic “storms”
 comet ion tails point anti-sunward (no matter comet’s motion)
1859
• Kristian Birkeland’s
1895 experiments
• concluded Sun emits
“cathode rays”
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Discovery of the solar wind
• 1958: Eugene Parker put the pieces together: the million-degree
corona has such a high gas pressure that it naturally expands!
• 1959-1961: Intermittent detection:
Russian Lunik, Venera; American
Explorer 10
• 1962: Marcia Neugebauer & colleagues
got continuous data from Mariner 2 on
its journey to Venus.
• Parker’s prediction was vindicated.
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Eclipses are nice, but we really want a more consistent view of:
• where the heating takes place
(later: where flares, CMEs, particle acceleration occur)
• where the solar wind is accelerated
Goal: Observe off-limb, outside of eclipses
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Invention of the coronagraph
• Despite some earlier attempts to block out the Sun in telescopes,
the first successful instrument was built by Bernard Lyot in the
1930s… and improved throughout the next decades.
L to R: 1930 … 1938 … 1941
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Space opens up the X-ray and UV
• 1940s: re-purposed German V-2
rockets at White Sands, NM
(Naval Research Laboratory)
• 1950s: rapid detector development
• 1960s: first dedicated satellites
• 1970s: John Kohl’s first UV+C+S
• 1980s-1990s: ISTP, SOHO, & beyond . . .
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016
Next time
• Define some basic properties of radiation
Then…
• Why is blocking the Sun needed? (quantitative)
• Physical optics… not just geometric ray tracing
Lecture 1: Introduction, motivation, & history
Hale COLLAGE, Spring 2016