=== Sources of Solar Irradiance Variability ===
===== Harry Warren =====
===== NRL =====

 . The physical conditions in the Earth's upper atmosphere, such as the particle density or the state of ionization, are largely determined by the magnitude and variability of the solar irradiance. As our presence in space has expanded, so has our need for timely and accurate information on the upper atmosphere. Atmospheric density measurements, for example, are needed to forecast satellite drag and predict the motion of space debris. Information on the ionosphere is needed to assess the accuracy of GPS measurements. One of the primary goals of the SDO mission is to understand the sources of irradiance variability using combined observations from EVE, AIA, and HMI. In this talk we will discuss the relationship between photospheric magnetic fields, solar features in the chromosphere, transition region, and corona, and changes in the solar radiative output at UV, EUV, and soft X-ray wavelengths. At the most basic level, changes in the solar irradiance can be understood in terms of changes in the total unsigned magnetic flux. Understanding irradiance variability in detail, however, requires determining how various solar features contribute to the radiative output of the Sun.

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