Cool Stars 14 - Submitted Abstract # 200
This version created on 05 October 2006

Preparing for Kepler:  Understanding Solar Microvariability


Kurt Soto, UC Santa Barbara
Gibor Basri, UC Berkeley
Francisco Ramos-Stierle, UC Berkeley
Tristan Lewis, UC Berkeley
Ansgar Reiners, MPI für Sonnensystemforschung

The key to understanding total stellar irradiance variability is
understanding the mechanisms by which continuum contrast is affected
by magnetic flux.  The MDI instrument on the SOHO satellite has
obtained magnetograms along with contemporaneous continuum images
throughout Solar Cycle 23.  These pairs of images can be used to study
the correlation of brightness with field over the entire disk.  To
properly study these effects, MDI magnetograms must be corrected for
geometrical effects near the limb.  We derive the correction by
assuming that the distribution of true magnetic flux in the active
latitudes should not depend on viewing angle.  We examine a series of
106 image pairs from 2001, using both annular rings at various limb
angles, and following individual active regions over their full disk
passage.  We are able to find a simple law which successfully corrects
the magnetograms over the full disk (a big improvement over the naive
correction by 1/mu).  The relationship between continuum images (with
limb darkening removed) and contemporaneous corrected magnetograms at
all limb angles is used to produce a 3rd-order polynomial surface of
continuum contrast vs.  magnetic flux and disk position (similar to
the approach of Ortiz et al.  2002, A&A 388, 1036).  We show how well
this procedure reproduces actual solar features.  This methodology is
useful for predicting stellar microvariability, given assumed magnetic
configurations, or attempting the inverse problem.

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