We see the brightest star in the Earth’s sky every day, yet much of the sun’s physics remains mysterious to us. The sun is a massive sphere of incandescent plasma, heated by nuclear fusion at its core. Understanding how this energy ultimately produces its million-degree atmosphere and drives powerful solar winds and flares is the focus of Stephen Bradshaw’s work in the Solar Astrophysics Research Group at Rice.
Commonly thought of as a giant ball of fire, the sun is better described as a structure with a central core and surrounding internal layers. At the visible surface, called the photosphere, the temperature is about 10,000 F. Above the surface, the first layer of the sun’s atmosphere, the chromosphere, reaches 36,000 F. Then there is a transition region, a very thin layer where temperatures can reach millions of degrees, before reaching the corona, where temperatures can exceed 10 million degrees. “Why is the sun’s atmosphere so much hotter than its surface? It is a long-standing mystery in astrophysics,” said Bradshaw. Elucidating the mechanism by which energy is transported into the solar atmosphere and then distributed as magnetic and kinetic energy, heat and radiation also improves our understanding of solar flares and related phenomena.
“My work is primarily computational and analytical,” Bradshaw explained. Astrophysicists like him build mathematical models to describe the plasma in a magnetized star like our sun; they test hypotheses by making predictions about its behavior that are checked against observations. Bradshaw uses imaging and spectroscopic data collected by instruments aboard satellites orbiting the Earth. “I mainly use satellite data from space-based observatories that look at the sun to validate the models, test hypotheses and inspire new ideas.”
Bradshaw's trajectory in the field of solar astrophysics has been closely tied with NASA. “NASA’s fleet of solar observatories is designed to look at the sun,” he said. Each satellite is equipped with specific instruments that observe certain wavelengths of light emitted by the star. “According to the wavelength, the sun can look very different; you observe different kinds of features,” Bradshaw explained. “If you look with visible light, it looks very plain, but if you look at the sun with ultraviolet light, then you see its atmosphere and it looks very different.”
One such satellite, the Interface Region Imaging Spectrograph (IRIS), observes ultraviolet light. “IRIS is designed to look at the very thin transition region between the chromosphere and the corona,” Bradshaw said. Using IRIS images, Bradshaw and his team confirmed predictions about the role of magnetic fields in the intense heating of the solar atmosphere. The group combined IRIS observations and computational modeling, enabled by Rice’s Center for Research Computing and its high-performance computing cluster, and demonstrated how loops of magnetized plasma in the transition region are heated by the rapid and turbulent conversion of magnetic energy.
Comprehending the physics of the sun’s atmosphere improves the models that astrophysicists use to predict the star’s emissions. “On Earth, we live in the atmosphere of the sun, constantly bathed in a strong outflow of plasma that we call the solar wind,” Bradshaw explained.
Periodically, the sun emits flares of superheated gas and releases magnetic clouds of energetic particles, coronal mass ejections, that sometimes collide with Earth. This radiation can damage satellites, present health risks with air travel and induce current surges in power lines that cause blackouts. “Understanding the solar progenitors of this ‘space weather’ will improve forecasting, providing more time to prepare and mitigate its harmful consequences for us on Earth.”
Beyond our own planet and star, predicting the behavior of other stars with orbiting exoplanets can improve the discovery of habitable planets. “If a planet’s magnetic field is strong enough to mostly shield it from what its host star is throwing at it, then it may potentially develop an atmosphere and become habitable,” said Bradshaw. If a star and a planet can exist in this magnetic harmony, sustaining life on that planet becomes a possibility. A major component of the Rice Space Institute’s research mission is to transpose our knowledge of the Sun-Earth system to answer questions about exoplanetary habitability and, ultimately, the existence of life elsewhere in the universe.
—Andrés B. Sánchez-Alvarado