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We present a detailed examination of the features of the active region (AR) NOAA 10798. This AR generated coronal mass ejections (CMEs) that caused a large geomagnetic storm on 24 August 2005 with the minimum Dst index of −216 nT. We examined the evolution of the AR and the features on/near the solar surface and in the interplanetary space. The AR emerged in the middle of a small coronal hole, and formed a sea anemone like configuration. H α  filaments were formed in the AR, which have southward axial field. Three M class flares were generated, and the first two that occurred on 22 August 2005 were followed by Halo‐type CMEs. The speeds of the CMEs were fast, and recorded about 1200 and 2400 km s −1 , respectively. The second CME was especially fast, and caught up and interacted with the first (slower) CME during their travelings toward Earth. These acted synergically to generate an interplanetary disturbance with strong southward magnetic field of about −50 nT, which was followed by the large geomagnetic storm.

Evolution of the anemone AR NOAA 10798 and the related geo‐effective flares and CMEs