During the last fortnight, two coronal holes (one on each solar hemisphere) transited the solar disk, as shown in this clip from GOES15/SXI imagery. Passing the disk's centre around 22-23 April, the high speed gusts of energetic particles created active geomagnetic conditions on 25 April and even a brief minor storm period on 26 April.
One can see the geomagnetic effects of the coronal holes (CHs) in the ACE-data underneath, topped by a daily GOES15/SXI image. These effects were felt only about 2 days after the CHs passed the central meridian. Indeed, first some denser (orange curve), slow moving material of the solar wind passed the Earth as it was being swept up by the fast stream of the CHs (24 April). The solar wind speed then continued to gradually increase (yellow curve), and was also becoming less dense and "hotter" (more energetic; green curve), marking the true influence of the CHs.
The CHs are recurrent ones, meaning they were also visible during the previous solar rotation. At that time, they were not so distinct but the northern CH still produced minor geomagnetic storm conditions lasting for several days late March. See the comments in this STCE Newsletter. The evolution of the CHs can be seen in this clip of the synoptic maps (whole Sun) created from SDO and STEREO-A and -B imagery as the CHs transited the Sun's backside.
Coronal holes usually cause only active geomagnetic conditions, with sometimes some minor storming. Based on geomagnetic data till mid-April from the Kyoto World Data Centre for Geomagnetism, there have been only 80 days with at least one 3-hours geomagnetic storming period so far this solar cycle. 31 of those were due to the influence of coronal holes (based on SWPC and SIDC reports), and the very large majority was minor storm only. Only on 2 days (2 May 2010 and 28 May 2011) there was a moderate geomagnetic storm due to a coronal hole.
Credit - Data and imagery were taken from GOES15/SXI, ACE, SDO, STEREO-A and -B, and the Kyoto World Data Centre for Geomagnetism.
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