Overview of Objectives and Requirements of the Helioseismic and Magnetic Imager Investigation

The primary scientific objectives of the Helioseismic and Magnetic Imager (HMI) investigation are to improve understanding of the interior sources and mechanisms of solar variability and the relationship of these internal physical processes to surface magnetic field structure and activity. The HMI investigation is based on inferences to be made from measurements obtained by the HMI instrument as part of the Solar Dynamics Observatory (SDO) mission. The HMI instrument will make measurements of the dynamic motion of the solar photosphere on global and local solar scales to study solar surface oscillations and will make similarly scaled measurements of the polarization in a spectral line to study all three components of the photospheric magnetic field. HMI observations are crucial for establishing the relationships between the internal solar dynamics and related magnetic activity, leading to an understanding of the limits of and possible development of techniques for reliable predictive capability.

Specific scientific objectives of the HMI investigation are to measure and study:

1. Convection-zone dynamics and the solar dynamo;
2. Origin and evolution of sunspots, active regions and complexes of activity;
3. Sources and drivers of solar magnetic activity and disturbances;
4. Links between the internal processes and dynamics of the corona and heliosphere;
5. Precursors of solar disturbances for space-weather forecasts.

To enable accomplishment of the science objectives of the investigation, the HMI instrument will produce measurements in the form of filtergrams in a set of polarizations and spectral line positions at a regular cadence for the duration of the mission that meet these basic requirements:

1. Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arc-sec resolution at least every 50 seconds.
2. Full-disk vector magnetic images of the solar magnetic field with 1.5 arc-sec resolution at least every 10 minutes.

The HMI data completeness and continuity requirement is to capture 99.99% of the HMI data 95% of the time.

In addition, the HMI investigation will provide sufficient computing capability to convert these raw filtergram measurements into a set of observables and derived data products required for the HMI science objectives. The primary observables (Dopplergrams, longitudinal and vector magnetograms, and continuum intensity images) will be constructed from the raw filtergrams and will be made available at full resolution and cadence. Other derived products such as subsurface flow maps, far-side activity maps, and coronal and solar wind models that require longer sequences of observations shall be produced and made available. All HMI data products will be made available to all interested investigators.