The cluster formed by active regions (ARs) NOAA 11121 and 11123,
approximately located on the solar central meridian on 11 November 2010, is of
great scientific interest. This complex was the site of violent flux emergence
and the source of a series of Earth-directed events on the same day. The onset
of the events was nearly simultaneously observed by the Atmospheric Imaging
Assembly (AIA) telescope aboard the Solar Dynamics Observatory (SDO) and the
Extreme-Ultraviolet Imagers (EUVI) on the Sun-Earth Connection Coronal and
Heliospheric Investigation (SECCHI) suite of telescopes onboard the
Solar-Terrestrial Relations Observatory (STEREO) twin spacecraft. The
progression of these events in the low corona was tracked by the Large Angle
Spectroscopic Coronagraphs (LASCO) onboard the Solar and Heliospheric
Observatory (SOHO) and the SECCHI/COR coronagraphs on STEREO. SDO and SOHO
imagers provided data from the Earth's perspective, whilst the STEREO twin
instruments procured images from the orthogonal directions. This spatial
configuration of spacecraft allowed optimum simultaneous observations of the AR
cluster and the coronal mass ejections that originated in it. Quadrature
coronal observations provided by STEREO revealed a notably large amount of
ejective events compared to those detected from Earth's perspective.
Furthermore, joint observations by SDO/AIA and STEREO/SECCHI EUVI of the source
region indicate that all events classified by GOES as X-ray flares had an
ejective coronal counterpart in quadrature observations. These results have
direct impact on current space weather forecasting because of the probable
missing alarms when there is a lack of solar observations in a view direction
perpendicular to the Sun-Earth line.Comment: Solar Physics - Accepted for publication 2015-Apr-25 v2: Corrected
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