One of the instruments on SDO is the Helioseismic and Magnetic Imager, HMI for short.
What does the HMI do?
"
HMI makes measurements of the motion of the solar photosphere to study solar oscillations and measurements of the polarization in a specific spectral line to study all three components of the photospheric magnetic field."
And what does this data help us to work out?
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It also produces data to enable estimates of the coronal magnetic field"
And how does HMI acquire the empirical (observational) data?
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The HMI instrument design and observing strategy are based on the highly successful MDI instrument, with several important improvements. HMI will observe the full solar disk in the Fe I absorption line at 6173Å with a resolution of 1 arc-second. HMI consists of a refracting telescope, a polarization selector, an image stabilization system, a narrow band tunable filter and two 4096 pixel CCD cameras with mechanical shutters and control electronics. The continuous data rate is 55Mbits/s."
In some more detail:
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HMI will provide the first full-disk continuous observations of solar magnetic fields in all orientations. Prior measurements (e.g. MDI) measured only the component of the field along the line of sight to the observer.The new measurements should improve our understanding of the 3-D structure of the evolving field.We can only measure the fields in the layer of the atmosphere where most all of the light originates (photosphere) and we can then compute estimates of the field in the upper atmosphere where AIA observes the effects of the fields."
And there's this, tantalizing, statement:
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Examples of science data products from SOHO/MDI. Improved versions of these can be made with HMI observations:"
And what's on that list? I'll choose just four:
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E. MHD model of the magnetic structure of the corona.
F. Synoptic map of the subsurface flows at a depth of 7 Mm
G. SOHO/EIT image and magnetic field lines computed from the photospheric field.
I. Vector field image showing the magnetic connectivity in sunspots."
Now as MM knows extremely well, being a keen student of Hannes Alfvén, once you know
B and
v (which the SDO HMI data products will provide), you can work out
E and
j!
Or, in simple terms, SDO will provide the raw data from which robust estimates of mass flows, ion mass flows, and current flows can be derived.
(
Source for the above. If anyone's interested in published papers describing how things like MDH models can be built - complete with upside-down-triangly thingies, drunken letter d's, and so on - just ask, and I'll provide you with some references).