U.S. #5607
2021 55¢ Sun Science – Coronal Hole
Issue Date: June 18, 2021
First Day City: Greenbelt, MD
Type of Stamp: Commemorative
Printed by: Banknote Corporation of America
Printing Method: Flexographic, Foil Stamping
Format: Pane of 20
Self-Adhesive
Quantity Printed: 18,000,000
The Sun's corona is a bright outer layer of plasma that reaches hundreds of thousands of miles into space. Temperatures measure over 1.8 million degrees Fahrenheit in most areas, but there are some spots with lower temperatures. When viewed under ultraviolet or x-rays, these areas appear darker than the surrounding corona and are thus nicknamed "coronal holes." (In actuality, coronal holes are still incredibly bright and cannot be distinguished under visible light.)
The Sun's corona is constantly shifting and changing, which means so are coronal holes. All of this is influenced by the Sun's magnetic fields. The first coronal holes were observed in the 1960s. They were viewed through x-ray imaging and also at radio wavelengths by the Sydney Chris Cross Radio Telescope. However, scientists still didn't know what the phenomenon was.
In the 1970s, Skylab made history by using x-ray telescopes to shed some light on coronal holes. This provided much greater data on the structure of the Sun's corona. Since then, scientists have found that the coronal holes move to different areas during different parts of the 11-year solar cycle. However, there is still much we don't understand about the science of the Sun's corona. This poses a fun challenge for scientists everywhere – and job security!