National Aeronautics and Space Administration

Glenn Research Center

Kylee Underwood

“Solar Limb Darkening”


Solar limb darkening describes the way the intensity of radiation from the sun decreases closer to the edge, or limb, of the sun from the perspective of the observer.  This effect is different for each wavelength, effecting shorter wavelengths more than longer ones.  Therefore, when observing the limb of the sun, not only is overall intensity of incident radiation decreased, but the entire solar spectrum experiences a shift, or darkening.  The Solar Probe Plus will be approaching the sun to within 10 solar radii.  At this distance, care must be taken to protect the probe from heat and increased solar radiation while still allowing it to be powered by the onboard solar panels.  A thermal protection shield (TPS), under which the solar panels can be retracted, is employed to protect the on board equipment and regulate the amount of solar radiation entering the solar panels. When fully stowed behind the TPS, the solar arrays will only observe the very limb of the sun and thus be exposed to an extremely darkened spectrum.  In this research, the limb darkened solar spectrum is investigated as well as its effect on the performance of the solar cells employed on the arrays of the probe.  Results show that as the arrays are drawn in behind the TPS, their performance is affected. This includes, but is not limited to, a decrease in expected power generation and a change in the current limiting factors of the cells.