Active region NOAA 2740 became visible on 3 May. It was the second act of this sunspot group, as it had already completed a transit of the solar disk as NOAA 2738 just 2 weeks ago. At the time, it was a somewhat larger-than-usual sunspot, symmetrical and simple, and thus not very flare active (see this STCE newsitem). This time however, it announced its second transit with some backside coronal mass ejections (CMEs) and some impressive limb activity, as can be seen in these extreme ultraviolet (EUV) images made on 4 May by SDO at relatively low temperatures (AIA/304; about 80.000 degrees).
As NOAA 2740 rotated further onto the disk, magnetograms made clear that the main spot had some small satellite sunspots to the north and northwest, which had an opposite magnetic polarity. One of these small spots was actually located at the rim of the main sunspot, forming a delta structure. The SDO/HMI images show the region in white light (left) and its magnetic structure (right). The blue color denotes magnetic field lines returning to the sun ("negative" polarity), the red color is for field lines coming out of the Sun ("positive" polarity). The delta structure can be clearly identified within the northwestern portion of the main sunspot early on 6 May. The reddish hue at the east limb of the spot is mostly a line-of-sight effect (see this STCE newsitem).
"Delta's" are notorious for their flare production, and this time it was not different. In particular on 6 May, NOAA 2740 produced no less than 7 C-class flares. The strongest of these was a C9.9 flare, making it almost into the M-class group ("medium"). The last M-class flare dates already back from 20 October 2017! SDO's EUV imagery (AIA 131, multi-million degrees) underneath shows the event near its peak time. Coronal dimming (to the southwest) and what appears to be a coronal wave (to the north and northwest) can be seen in the difference images (one picture subtracted from the foregoing picture) based on SDO's AIA/211 imagery (temperatures near 2 million degrees).
The C9.9 flare was the first of 7 C-class flares on 6 May. Most were low-level events, but there was also a C7.3 flare peaking shortly after noon (see image underneath). So far, none of the coronal mass ejections associated with the frenzy of flaring events in NOAA 2740 seems to be earth-directed. Clips were made with JHelioviewer.
