HMI Requirements Flow


PLEASE NOTE the requirements discussion is "living" document.

In order to obtain the most up-to-date version please use this page:

Rather than a paper "snapshot" made at any particular moment.


The HMI Investigation includes the HMI Instrument, data processing, and science analysis. There is a chain of objectives, goals, and requirements that flows from the top level science goals of LWS to specific requirements for performance of HMI and SDO space hardware. This list starts at the HMI objectives as "Step 1" and follows the chain to the spacecraft at "Step 9".


The following is a description of the flow of requirements and goals from the top level science goals down to detailed requirements on instrument and analysis performance and on the SDO spacecraft. There is a natural chain of logic with some high level requirements having direct impacts on lower level systems. More often the requirements on lower level systems follow only after a detailed analysis of the implications of the higher level requirements. This top-level discussion starts at the most general and proceeds to the specific, in outline form. The requirements of each level imply the requirements on the lower levels. The numbers are not intended to correspond to NASA terminology of "Level-1" etc requirements (except where noted).


  1. HMI Investigation: The top level objectives of the HMI Investigation flow directly from the science goals of SDO and LWS. These in turn flow from various NASA Roadmaps and NAS studies which represent the needs identified by the space science community as a whole and the solar physics community in particular. 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. Details of the HMI "Level-1" requirements and background documents are in the following list.


  1. HMI Science Objectives: The broad goals in step 1 are approached in a number of more specific HMI Science Objectives. These include study of such topics as solar differential rotation, sub-surface flows around sunspots, emergence of magnetic flux in active regions, etc. These goals are described in the HMI Investigation proposal dated April 2002. These objectives will be met by analysis of HMI and related data with the results published in journals, conference proceedings and dissertations. In most cases new theory, modeling, simulation, and analysis techniques must be developed to accomplish the science objectives. HMI should be able to obtain the data needed for these objectives however the HMI investigation is not expected (nor proposed) to have sufficient resources to see all of the science objectives through to results and publications. The Co-I team (and the instrument team after 2 years on orbit) must seek other resources to complete the investigation. A summary of the HMI science objectives and background documents are in the following list.


  1. HMI Science Data Products: The analyses in step 2 require specific data products as input to detailed analyses. These data products are not raw instrument observables but are rather often (usually for HMI) the result of substantial processing of time series of varying lengths. Examples are maps of subsurface flows, far-side active region maps, models of coronal fields, etc. A summary of these data products, a table showing the flow from science objectives to the data products and a chart showing the derivation of the data products from the processing pipeline are in the following list.
    • HMI Science_Data_Products : Summary of HMI science data products
    • Required Analysis/Processing Algorithm Development
    • HMI_Objectives-Data_table : Matrix showing tie between objectives to data products. Entries in this table show the observation sequence types required.
    • HMI science pipeline chart: Chart showing the flow of data from raw data to science data products to science objectives. Each arrow in this figure represents a matrix element from the above HMI Objectives-Data table. Also available as .ppt


  1. HMI Observations: The data products in step 3 require specific observation sequences providing data fulfilling specific requirements. These sequences are tailored to provide the data necessary for each of the data products above. Different data products imply differing requirements on observable, duration, completeness, noise, resolution, etc. This is the level at which time accuracy and continuity implications for the instrument and spacecraft data systems are first identifiable. Examples are 72d sequences of spherical harmonic amplitudes of solar surface velocity suitable for time series as source data for mode frequencies for internal rotation studies and 24h sequences of velocity in surface regions projected to particular mappings and tracked with time as source data for travel-time data-cubes suitable for sub-surface flow measurements. The "HMI Observations" are time series of maps of observables and intermediate products derived from them. Examples of observables are surface velocity and vector magnetic fields. In the tables below, the basic sequence parameters are identified apart from the observation sequences since some of the sequence types are used by more than one "HMI Observation" type. This distinction is made to allow identification of the requirements of the observation types apart from the continuity requirements for the sequences. The following list contains lists of HMI observation types, sequence types, and the chart showing the intermediate processing of the observation types.


  1. HMI Observables: The observations in step 4 are built from disk-image maps of basic HMI observables. The observables are built from short sequences of filtergrams made in various polarizations and tuning passbands across the spectra line. The basic observables are line-of-sight surface velocity from Doppler shift measurements, line-of-sight magnetic field from Zeeman split line components in circular polarization, vector components of magnetic fields from Stokes parameter measurements of the Zeeman split components of the line, and continuum brightness measurements. The cadence, sensitivity, linearity, acceptable measurement noise, image stability and related requirements for the observables are derived from the HMI observations requirements. The following list points to a description of the observables and a chart showing which data products are generated from which observables.
    • HMI Observables : Table of HMI basic observables. These observables are made from short sequences of filtergrams made in various polarizations and tunings across the spectral line.
    • HMI Filtergram Sequence: Chart showing the sequence of filtergrams needed to build the HMI observables.
    • Data_Analysis_Pipeline : Chart showing more detailed flow of data from raw data to science data products. The data products are in the rightmost column. Also available as .ppt


  1. HMI Instrument Data: The MDI observables in step 5 are built from sequences of filtergrams. The filtergrams must be made in various polarizations, tuning positions, sequence order, optical configuration (i.e. observing or calibration) etc., to meet the functional requirements of the observable calculations and must be made with sufficient accuracy, noise levels, repeatability, timing knowledge, wavelength knowledge, image registration, image orientation, etc., to meet the science requirements. The table linked below lists the basic requirements on the filtergrams.
    • HMI Observables: List of basic requirements of filtergrams is also in the Observables requirements table.


  1. HMI Instrument Concept: The required instrument data in step 6 will be obtained with the HMI instrument. The instrument concept was developed in an iterative process intended to result in a design concept which meets the requirements through step 6 and is achievable with real parts. The instrument is a system constructed of several subsystems. The requirements on the subsystems derive from the steps above with an understanding of the role of the subsystem within the instrument conceptual design.
    • HMI Instrument : The HMI instrument description from the April 2002 proposal
    • HMI Optics Layout: The HIM optical package plan. This diagram shows the locations of optics and mechanisms. This diagram shows what HMI is, not how to build it.
    • Sub-system requirements: Tables of implied requirements and estimated performance for subsystems including powered optics, image stabilization system, filter performance, tuning and polarization analyzer motors, shutter, camera system, and data delivery system.


  1. Sub-system design concepts:


  1. HMI to SDO Interface Requirements: The SDO spacecraft provides the home and environment for the HMI instrument. SDO should provide an environment and resources to enable steps 1 through 8 to be accomplished. The requirements on SDO have been encapsulated in the SDO Mission Requirements Document linked below. The top level requirements include support for HMI mass, power, telemetry needs as well as field-of-view, pointing direction and stability, thermal environment, etc.



  1. Ground System:


  1. Processing System:


  1. Data Distribution: