Using precise solar limb shape measurements to study the solar cycle

Despite 20 years of total solar irradiance measurements from space, the lac k of high precision spatially resolved observations limits definitive answe rs to even simple questions like "Are the solar irradiance changes caused s olely by magnetic fields perturbing the radiative flux at the photosphere?" More subtle questions like how the aspheric structure of the sun changes w ith the magnetic cycle are only now beginning to be addressed with new tool s like p-mode helioseismology. Solar 5-min oscillation studies have yielded precise information on the mean radial interior solar structure and some k nowledge about the rotational and thermal solar asphericity. Unfortunately this progress has not been enough to generate a self-consistent theory for why the solar irradiance and luminosity vary with the magnetic cycle. We ne ed sharper tools to describe and understand the sun's global aspheric respo nse to its internal dynamo, and we need to be able to measure the solar cyc le manifestation of the magnetic cycle on entropy transport from the interi or to the photosphere in much the same way that we study the fundamentally more complex problem of magnetic flux transport from the solar interior. A space experiment called the Solar Physics Explorer for Radius, Irradiance a nd Shape (SPHERIS) and in particular its Astrometric and Photometric Telesc ope (APT) component will accomplish these goals.