  A filament eruption on 2010 October 21 observed from three different
viewpoints by the Solar Terrestrial Relations Observatory (STEREO) and the
Solar Dynamic Observatory (SDO) is analyzed with invoking also data from the
Solar and Heliospheric Observatory (SOHO) and the Kanzelhoehe Solar
Observatory. The position of the filament just before the eruption at the
central meridian not far from the center of the solar disk was favorable for
photospheric magnetic field measurements in the area below the filament.
Because of this, we were able to calculate with high precision the distribution
of the coronal potential magnetic field near the filament. We found that the
filament began to erupt when it approached the height in the corona where the
magnetic field decay index was greater than one. We determined also that during
the initial stage of the eruption the filament moved along the magnetic neutral
surface.
