The Solar Corona: Arthur B. C. Walker
prepared by Ruth Howes
Marquette University
with support from the
Wisconsin Space Grant Consortium
Why Study the Sun?
The Sun is a very typical main sequence star that is
close to us.
The Sun influences climate on Earth,
Radiation from the Sun released during violent
solar events can pose a hazard to both people
and electronics in space as well as disrupt
radio communications.
The photosphere is the visible surface of the Sun.
The Sun’s atmosphere lies above the photosphere
and has 2 layers.
The chromosphere lies just above the photosphere:
thickness:
10,000 km
temperature: rises from 4500K at the photosphere to
50,000K
The corona lies above the chromosphere.
thickness:
greater than 1,000,000 km and variable
temperature: 1-3 million K in variable pattern
density 10-12 of photosphere so not much heat and
little radiation. Visible only during solar eclipse.
Layers of the Sun downloaded from
http://imagine.gsfc.nasa.gov/docs/science/know_l1/sun.html
Studying the Corona
Emission in short wavelength UV and X-ray
regions
Bright photosphere swamps dim corona
coronograph produces artificial eclipse
Atmosphere blocks short wavelength UV
Conventional lenses cannot focus UV or X-ray
The solar corona as seen during a total eclipse
downloaded from
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/corona.html
The Corona in Ultraviolet Light showing a coronal hole
over the pole and several flares
Image downloaded from http://solarscience.msfc.nasa.gov/index.html
Image from NASA and downloaded from
http://en.wikipedia.org/wiki/Image:Atmospheric_transmittance_infrared.gif
Arthur Bertram Cuthbert Walker
Discovered science as a child.
Attended Bronx High School of Science
Received bachelors degree from Case Institute of Technology
Ph.D. in photomeson production from the University of Illinois
Served at the Air Force Weapons Laboratory and discovered
space-based instruments
In 1965, joined Space Physics Laboratory of
Aerospace Corporation
Recruited to Stanford University in 1974 as a professor of
applied physics and worked there for 31 years
Arthur B. C. Walker
image from
http://news-service.stanford.edu/news/2005/may18/memlwalk-051805.html
Walker’s development of ultraviolet telescopes
1) Special materials developed that were created with
very uniform planes of atoms that behaved like
atomic planes
2) Materials made of different substances can be tailored
to reflect different wavelengths by varying the
spacing between planes.
3) A parabolic mirror coated with such a material will focus
normally incident ultraviolet or x-rays to form an image.
4) Satellites carried several telescopes tuned to different
wavelengths
When d is equal to λ/2, the waves will interfere
constructively. That wavelength of radiation will reflect.
EMR of Wavelength λ incident as a
parallel wavefront perpendicular to the
atomic planes.
Atomic plane spaced d apart
Materials with tailored layers of atoms can maintain d needed
To reflect ultraviolet and x-ray wavelengths.
Ultraviolet waves from
distant object are parallel
Mirror
Coated
With
tailored
film
Camera
Mirror bends them
to form and image at the
point where the receiver is
Getting ready to
launch the
Multi-Spectral Solar
Telescope Array
Image downloaded from
http://solarscience.msfc.nasa.
gov/MSSTA.shtml
Solar image taken by the Multi-Spectral Solar Telescope Array in
the UV corresponding to a temperature of 2 million K
http://solarscience.msfc.nasa.gov/MSSTA.shtml
Three Big Questions about the Sun
(Marshall Space Flight Center)
1. How does the corona get heated and maintain its heat?
2. How can we predict when solar flares and the coronal mass
ejections that accompany them will occur?
3. How can we predict the behavior of the sunspot cycle?
Seeking answers is the STEREO Project that uses some
of the technology Walker developed.
One of the STEREO satellites under construction.
Image from http://stereo.jhuapl.edu/
The Solar Terrestrial Relations Observatory (STEREO)
STEREO consists of two satellites which will be placed
into solar orbit.
One satellite will orbit 45o ahead of Earth. The other will
orbit 45o behind the Earth.
Instruments on the two satellites can form 3-dimensional
images of the Sun.
Each satellite will carry a suite of four instruments.
The mission’s goal is to understand and predict the
behavior of the Sun, its magnetic field and its
atmosphere as it goes through a sunspot cycle.
Coronal loops in image by MSSTA downloaded from
http://solarscience.msfc.nasa.gov/images/eit020.jpg
Sun Earth Connection Coronal and Heliospheric Investigation
(SECCHI)
Five telescopes that use Walker’s technique
will image the solar atmosphere and its features
in three dimensions.
In-situ Measurements of Particles and CME transients
(IMPACT)
Will examine the energies and behaviors of electrons and
ions emitted during a flare or CME and form images
based on these particles as well as watch their
interaction with the interplanetary medium.
The Plasma SupraThermal Ion Composition Investigation
(PLASTIC)
Study the distribution of different types of ion from
hydrogen to iron and how their distribution
changes during CMEs and flares
STERO/WAVES
Radio telescopes will monitor the interaction of the
material ejected from the Sun with the interplanetary
medium.
MSSTA image of a solar flare
Image from http://solarscience.msfc.nasa.gov/images/flare.jpg
Walker’s work set the stage for STEREO.
He also mentored 13 Ph.D. students including
the first U.S. woman astronaut, Sally Ride.
He constantly fought for more admission of more
minorities and women to graduate programs
in physics and astronomy.
He formed a group of African American faculty
at Stanford that supported young faculty
including political scientist Condoleezza Rice.