Program in Solar and Terrestrial Physics (STP)

A joint program between the Yale Geology/Geophysics and Astronomy departments to study the physical mechanisms common to the Sun and Earth that govern their changes over time.

rom our Earth-bound perspective, the weather may always be changing, but the sun seems a reliably burning orb. We imagine the Earth's magnetic field to be a steady constant available to guide our way, and climate to be the same for us as it was for earlier generations. But whether or not we notice, all of these things change over time in erratic and mysterious rhythms. The Yale STP program strives to understand the causes and consequences of these rhythms. This requires understanding the physics, fluid mechanics, and chemistry of both the Earth (including its atmosphere, oceans, ice sheets, and interior) and the Sun.

Unresolved questions range from long-standing paradoxes in Earth history, to pressing problems of the present day. For example, our best current theories of stellar evolution predict that the Sun was about 30% weaker early in Earth's history than today. Theories of Earth's climate indicate that with so little solar heating, the Earth should have frozen over. Yet somehow the Earth's climate has remained, if not constant, at least equable for life for nearly four billion years. We currently do not have a satisfying explanation for this "faint young sun" paradox. We are also struggling with a problem on much shorter timescales---the prediction of how climate will change in the coming decades due to human alteration of the composition of Earth's atmosphere. Terrestrial climate is a rich and very challenging problem that requires deep understanding, but also creativity, to overcome the obstacles of studying a bafflingly complex, dynamical system. Fluid dynamics is fundamental both to Earth's weather and to the response of its climate to various influences.

One potential driver of past climate change is variability in solar radiative output. This potential has been recognized relatively recently, and is still hotly debated. One of the chief stumbling blocks is our poor understanding of what kinds of variability a Sun-like star is actually capable of. Our current knowledge is tied directly to observed sunspot counts. Sunspots result when solar magnetic field lines erupt at the solar surface due to the interaction of electromagnetic forces in the solar plasma with solar turbulence. Long-term variations in solar behavior, if they exist, can probably be explained only through basic understanding of solar fluid dynamics.

The solar magnetic field itself is thought to be created by the interaction of charged particles with circulations deep within the solar interior in what is known as a "dynamo." A similar dynamo operating in Earth's core presumably generates our own magnetic field. Both fields change in time: the Earth's is currently waning and has reversed on many occasions throughout the planet's history, while the Sun's flips about every 11 years. Some believe that changes in Earth's magnetic field have led to past extinctions or climatic changes; changes in the solar field are clearly fundamental to variations in solar radiative output.

Owing mainly to faster computers and better observing capabilities, methods used to attack problems in climate change, stellar magnetohydrodynamics, and the circulation of planetary interiors have experienced a recent convergence. For example, theoretical flow calculations on computers have not only come to the fore in all fields, but have been moving toward first principles or full "Navier-Stokes"-type approaches in which similar equations apply in each system. Better data have caused a similar convergence in seismic methods for analyzing solar and Earth interiors. Rapidly building paleoclimate records on Earth now document solar cycles and terrestrial climate variability far more completely than only a few years ago, and are coming to the point of providing real tests for state-of-the-art theories and models of climate. Basic mathematical-physics approaches like dynamical systems theory continue to be useful in understanding variability across all of these systems.

The joint STP program is an unique collaborative effort between a space sciences and Earth sciences department, poised to exploit the growing convergence between key elements of the study of the Sun and Earth. The core mission of this program is to attract and educate new graduate students who will have the fluency across disciplines to generate the new discoveries that will become possible.

The program is founded on four key study areas: fluid mechanics, computational methods, radiative transfer, and thermodynamics. Aside from fulfilling coursework in these fundamentals, students will have wide flexibility in designing a curriculum to suit their particular area of interest. Additional courses are offered by both departments in many topics including solar physics, atmospheric waves and convection, continuum mechanics, Earth's climate history, and others.

For more detailed information on current research topics, please visit the web pages of the joint program faculty (although exciting new topics may not yet appear here!) For more information on application procedures and program requirements, click here.

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