Past Research

Solar Bolometric Imager

Solar Imager

We have worked with the Applied Physics Laboratory of Johns Hopkins University on the modification for space flight of our Solar Bolometric Imager (SBI). The SBI is an innovative new kind of telescope that is able to image the Sun in broad- band light (Applied Optics 40,1138 (2001)

The human eye, photo film, and digital cameras all respond only to a restricted range of colors.  The SBI takes pictures in the total light from an object like the Sun. This includes all colors from the ultraviolet through the visible and well into the infrared. Images obtained with the SBI carried on a giant NASA balloon from New Mexico and from Antarctica, in 2003, 2006 and 2007, help us understand why the Sun’s brightness varies and whether such variation might influence global warming. (Ap.J Letts, 611, 57 (2004); Solar Phys. 279, 365 (2012)

Solar Luminosity Variation, Convection and Magnetism

MT Wilson Plates

Our study of this topic has included analysis of measurements of solar brightness made from satellites and observations using ground -based telescopes such as those at the National Solar Observatory sites in New Mexico and Arizona. We also used archival solar images obtained daily since about 1915 at the Mt Wilson Observatory near Pasadena, California. Our digitization of these thousands of photographic plates has enabled us to reconstruct the variation of the Sun’s brightness in ultraviolet radiation over the past century (Solar Phys. 255,229, 2009). The results from these and related ongoing studies provide  important inputs to global warming studies. (Ap.J. Letts, 733, 38 (2011)

Most Recent Publications


"A Study of Solar Photospeheric Temperature Gradient Variation Using Limb Darkening Measurements"
S. Crisuoli & P. Foukal, Ap.J,835,99 (2017)

"Dimming of the Mid=20th Century Sun" Ap.J, 815 9 (2015)
"Really Reaching the Public; Face to Face" EOS (Feb 18, 2014)
“An Explanation of the Differences Between the Sunspot Area Scales of the Royal Greenwich and Mt. Wilson Observatories, and the Soon Program."
Solar Phys. 289, 1517,DOI 10.1007/S 1 1207-013-0425-2 (2014)
"The Search for Magnetic Reconnection in Solar Flares."
Physics Today, 61, 8 (2014)
"Rewards and Risks for Physicist Entrepreneurs."
Physics Today 66 , 8 (2013)
"Follow Obama's Lead and Take A Pay Cut."
Nature 500 , 151 (2013)

Past Research Highlights

Solar Rotation

The Sun is a giant gaseous globe which rotates over 20% faster at the equator than at its poles. In the mid- 1970’s, we drew attention to evidence that this rotation rate increased with depth by about 5% in a shallow layer below the photosphere. With R. Jokipii, and later with P. Gilman, we suggested that the explanation of this acceleration lies in angular momentum conservation in the supergranular overturning layer just below the Sun’s visible surface (Ap. J. Letts 199, 71).

A shear layer of this depth has since been confirmed by helioseismic measurements, and our explanation is increasingly accepted. Due to this shear, spots (one here imaged at Big Bear Observatory) plow through their surrounding gas at about 200 mph!


Solar Brightness Variation

Measurements with satellite – borne radiometers beginning in 1978 showed that the Sun brightens by slightly less than 0.1%, around the peak of its 11- yr sunspot cycle. That is surprising since sunspots are dark. In the late 1980’s, we demonstrated with J. Lean that this output increase  was caused by bright magnetic structures called faculae that occupy magnetically active areas around the spots, and more than balance their dimming effect (Ap.J. 328, 347). We used this new understanding to reconstruct solar output variation back to 1874 (Science, 247, 556); a result that was widely used in global warming studies.

The reason why dark spots dim the Sun is not obvious: why doesn’t the blocked heat flow just pass around these magnetic obstructions, without causing a drop in solar luminosity?  To solve this puzzle, with M. Livshitz and L. Fowler, we carried out numerical simulations of time dependent heat flow around thermal plugs (see figure at right). These showed (in agreement with analytical calculations performed at the same time by H. Spruit) that the mechanism of solar luminosity variation lies in surprisingly efficient heat storage of the blocked heat in the solar convection zone (Ap.J. 267, 863). We showed that the brightening caused by faculae (which are heat leaks instead of plugs) has a similar explanation.

Click on the image for a larger version

Convection and Magnetic Fields

Understanding the brightness variations required improved techniques of imaging the faculae, and the surrounding photosphere. In the 1980’s and 90’s we took advantage of newly available electronic detectors to develop, mainly with T. Duvall and L. Petro, new techniques of  photometric imaging in the visible and near IR. Our findings provided the first evidence that small solar magnetic flux tubes affect the Sun's brightness more than expected. This increases the likelihood of Sun-climate influence. (Ap.J. Letts, 733,38 (2011)), (Ap.J., 815,9 (2015))

Our observations at the National Solar Observatories demonstrated the lower temperature gradient of faculae (Ap.J. 296,739), their darkness at deeper levels (Solar Phys.142, 35), and placed tight limits on changes in photospheric temperature gradient (Ap.J. 283,426). More recently, our SBI obtained the first bolometric images of the Sun (see figure at right). These provided the first broad band facular contrasts C


Chromosphere and Corona

Using the Skylab data (see figures at right), we showed that sunspots were the brightest solar emitters in EUV radiations formed around 105 K (or 100,000), and that their loop structures were 10 x cooler than coronal surroundings (Solar Phys.43, 327). These findings help constrain mechanisms of coronal heating. Ap.J. 223, 1046

Beginning in the 1980’s, mainly with T. Moran and B. Behr, we developed diagnostics to search for macroscopic electric fields and test for reconnection in solar magnetic structures. Our “electrograph” measurements placed limits on fields in prominence current sheets. Solar Phys. 156, 293

In 1971,using hydrogen light imagery from CalTech’s Big Bear Observatory, we showed that the horizontal component of magnetic field runs parallel to filament neutral lines  although the vertical component switches sign across these structures (Solar Phys.20,298). This result has played a key role in subsequent studies of magnetic field chirality by S.Martin, A.van Ballegoijen and others.

MT Wilson Plates