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, R Shine, 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/.