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 of photosphere so not much heat and little radiation. Visible only during solar eclipse.

Layers of the Sun downloaded from

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

The Corona in Ultraviolet Light showing a coronal hole over the pole and several flares Image downloaded from

Image from NASA and downloaded from

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

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

EMR of Wavelength λ incident as a parallel wavefront perpendicular to the atomic planes. Atomic plane spaced d apart When d is equal to λ/2, the waves will interfere constructively. That wavelength of radiation will reflect. 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 bends them to form and image at the point where the receiver is Mirror Coated With tailored film Camera

Getting ready to launch the Multi-Spectral Solar Telescope Array Image downloaded from gov/MSSTA.shtml

Solar image taken by the Multi-Spectral Solar Telescope Array in the UV corresponding to a temperature of 2 million K

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

The Solar Terrestrial Relations Observatory (STEREO) STEREO consists of two satellites which will be placed into solar orbit. One satellite will orbit 45 o ahead of Earth. The other will orbit 45 o 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

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

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 scientistCondoleezza Rice.