Visibility of coronal mass ejections in SOHO/LASCO coronagraphs

We studied the detection efficiency of coronal mass ejections (CMEs) of the Large Angle Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). For this purpose LASCO/SOHO observations are compared with these obtained by the two Solar Terrestrial Relations Observatory (STEREO) satellites in quadrature in the period of time June-November 2011. These unprecedented observations enable us to the direct detection of CMEs that are not visible in LASCO coronagraphs (invisible events). Determination of these events allowed us to evaluate the detection efficiency of LASCO coronagraphs. We found that the total visibility function is ≈0.80. Having source location, from associated flares or other signatures observed in the corona, longitudinal variation of the visibility function was also found. It was demonstrated that invisible-to-LASCO CMEs are narrow (average width is only 20°), slow (average velocity is 328 km s−1) and originate from the disk center. We have shown that the detection efficiency of the LASCO coronagraphs with typical data availability is sufficient to detect all potentially geoeffective CMEs.