Profile:

The Helioseismic and Magnetic Imager (HMI) has just started producing data that will help determine what the sources and mechanisms of variability in the Sun’s interior are. The instrument measures the Doppler shift and the polarization of the Fe I 6173 Å line, on the entire solar disk at a relatively-high cadence, in order to study the oscillations and the evolution of the full vector magnetic field of the solar Photosphere. After the data are properly calibrated, they are passed to a Milne-Eddington inversion code whose purpose is to infer certain aspects of the physical conditions in the Sun’s Photosphere, such as the full 3-D topology of the magnetic field and the line-of-sight velocity at the solar surface. Despite the trade-offs that an ambitious project that wants to measure "all the Sun, all the time" has to make (i.e. limited spectral resolution and polarimetric sensitivity), HMI data produce results that are quantitatively comparable to those of state-of-the art, high resolution instruments, like the Spectro-Polarimeter on board Hinode. The images show a rough comparison of the magnetic properties (from top to bottom: magnetic flux density, inclination and azimuth) of an active region as seen by Hinode's SP (left) and SDO/HMI (right).