Continuous Observations of Solar Magnetic Fields from SOI/MDI on SOHO

J T Hoeksema, R I Bush, P H Scherrer 
 W.W. Hansen Experimental Physics Laboratory, Stanford University
C Heck, N Hurlburt, T Tarbell, A Title 
 Lockheed Martin Palo Alto Research Laboratory

The Solar Oscillations Investigation's Michelson Doppler
Imager instrument (SOI/MDI) on SOHO measures the
photospheric magnetic field over the whole disk nearly
every 96 minutes with 4" resolution and a noise level of a
few Gauss.  Beginning in April 1996, this unprecedented
continuous series of frequent, uniform quality magnetograms
provides a striking view of the continual emergence,
motion, evolution, and interaction of magnetic flux
everywhere on the visible solar surface near solar
minimum.  These evolving photospheric fields ultimately
drive the variations of the corona and solar wind that
affect the terrestrial environment.  Knowledge of the
rapidly evolving photospheric field provides a crucial
input for forecasting conditions in the corona,
heliosphere, and geospace.

A few magnetograms are available each day within hours of
observation through the SOHO web site at
http://sohowww.nascom.nasa.gov/.  These may be used for
planning and forecasting, e.g. to compute models of the
solar corona.  The remainder are generally available within
a few weeks.  Sometimes more focused campaigns provide
magnetic observations as often as once each minute for up
to 8 hours.  Campaigns can be run with either the full disk
resolution or with 0.6" pixels in a limited field near the
center of the disk.  The SOI project welcomes
collaborations.  More information can be found at
http://soi.stanford.edu/.